From (by) RocketSlinger@SBCGlobal.net (email me there please)… This is a sub-site to main site at www.rocketslinger.com …

This web page last updated 02 Dec 2022

 

Propulsion Designs Using Novel Nested COPVs and the Astron Omega 1 Engine

 

 

Abstract / Pre-Summary

This sub-page to www.rocketslinger.com is duplicated at www.ResearchGate.net .  Here, we describe COPVs (Carbon-Over-wrapped or Composite-Over-wrapped Pressure Vessels) that are long and slender (“tubular”), and bundled together into a larger assembly of many tubes, for large capacity.  Other novel COPV designs are described as well, to include the use of Kovar and borosilicate glass.  They are designed and arranged to collect (in-flight-collected) ambient air that has been compressed by modified Aston Omega 1 engines, or other sources of compressed air.  Since the air is inherently (unavoidably) heated during compression, the COPVs are designed for optimally shedding heat into the ambient airstream.  An option for boosting the rocket or aircraft at take-off is to pre-fill the COPVs with pure compressed (possibly even liquefied) oxygen.  With this option, the COPVs can then be re-filled in-flight with compressed air, for later burning, saving take-off mass.  The probably-optimal application is for an aircraft that launches a rocket, in the style of the “Stratolaunch” Model 351, or “Roc”.  However, the ideas described here could (possibly but unlikely) also be applied to rockets, especially strap-on booster rockets that are used only at lower altitudes.  The ideas described here could be described as ingredients for a “poor man’s air-breathing vehicle”.  Some other ideas are described as well, but do not deserve to be mentioned here.

Frankly, the author considers one of the attempts to design novel aspects of pressure vessels to be a failure, here.  The ideas aren’t practical, with currently available materials.  But the details are described anyway, and the faults are explained.  Readers are invited to find fixes!  The failed ideas involve the use of borosilicate glass “pressure domes”.  A much-more-plausible alternate COPV design described here is a “nested” design.  Nested designs of low-pressure “gas bags” are also discussed.

            The body of this document contain far more details, and sometimes-implausible variations.  However, the above covers the most important basics.

            As with other sub-pages of www.rocketslinger.com, the intent here is to “defensively publish” propulsion-related (and “misc.”) ideas, to make them available to everyone “for free” (sometimes called “throwing it into the public domain”), and to prevent “patent trolling” of (mostly) simple, basic ideas.  Accordingly, currently-highly-implausible design ideas (usually marked as such) are sometimes included, just in case they ever become plausible, through radical new technology developments (often in materials sciences).

 

 

Introduction / Basics

 

            Please read the abstract above… Some of those basics may not be thoroughly (completely) repeated here below.  (OK, that’s “boilerplate”.  I think that pretty much everything above is repeated below.)

            Dear Reader, excuse me as I will often slip out of stilted formal modes of writing here.  I have no boss or bosses to please with these writings, so I’ll do it my way!  I’ll often use a more informal style from here on in, using “I”, “we”, “you”, etc.  “We” is you and me.  “You” are an engineer, manager, or other party interested in what’s described here.  Let’s thwart the patent trolls, and get ON with it!

            The “Stratolaunch” Model 351, or “Roc”, can easily be researched on the internet.  All that I will bother with here is to cite https://en.wikipedia.org/wiki/Scaled_Composites_Stratolaunch ...  From there…”It will require 12,000 ft (3,700 m) of runway to lift-off.  It should release its rocket at 35,000 ft (11,000 m).”

            Some of my objectives here are to improve on the above, adding additional take-off thrust (shortening the runway), and increasing the operating altitude (allowing rocket-release at a higher altitude).  That’s part of the “big picture” here.

            Another variant on Stratolaunch (see https://theconversation.com/virgin-orbit-launched-a-rocket-from-a-plane-heres-how-154207 for example) is “Cosmic Girl”, a modified Boeing 747 which can launch the “LauncherOne” rocket for Virgin Orbit, at 6.8 miles.  6.8 miles is about 36,000 feet.

For air-breathing rocket engines, the following is relevant:  “Air Breathing Rocket Engines and Sustainable Launch Systems”, at https://www.researchgate.net/publication/258586259_Air_Breathing_Rocket_Engines_and_Sustainable_Launch_Systems  Quoted from there…

“An air-breathing engine gets its initial take-off power from specially designed rockets, called air-augmented rockets, that boost performance about 15 percent over conventional rockets. When the vehicle's velocity reaches twice the speed of sound, the rockets are turned off and the engine relies totally on oxygen in the atmosphere to burn the hydrogen fuel. Once the vehicle's speed increases to about 10 times the speed of sound, the engine converts to a conventional rocket-powered system to propel the vehicle into orbit.”

I’m not sure exactly how much or how little of that will precisely conform to what could be done with the ideas that I will describe further below.  But there you go, for what it’s worth…  Along similarly might-be-useful (wasn’t much so for me) lines, I also give you this:  https://en.wikipedia.org/wiki/Air-augmented_rocket …  That source is relevant to very high speeds, which aren’t very “friendly to” most of what I will describe below.

            Concerning “Reaction Engine’s Synergetic Air Breathing Rocket Engine (SABRE)”, we have this:  https://physicsworld.com/a/air-breathing-rocket-engines-the-future-of-space-flight/ .  From there, “The precooler can cool high mass-flow airstreams from temperatures above 1000 °C to ambient in less than 50 milliseconds, within a compact and low-weight design. It is formed from over 42 km of tubing, the walls of which are thinner than a human hair – thereby providing an enormous area for heat-transfer between the air and cooling medium.”  Thinner than a human hair!  HOW is this going to be affordable, robust and highly reliable?!?!  How will it withstand “FOD”?  FOD = Foreign Object Damage.  Color me skeptical!  This is why I propose design ingredients (here) for a “poor man’s air-breathing vehicle”.

            While we’re providing links to basic “tech” ingredients, how about that them thar Aston Omega 1 engine, eh?!?  See https://www.autoevolution.com/news/omega-1-is-a-game-changing-internal-combustion-engine-too-good-to-be-true-179507.html (that site calls it “Astreon” but it should be called “Astron”) and https://www.visordown.com/news/general/ultra-light-rotary-engine-makes-160bhp-and-weighs-15kg for short summaries of general interest.  https://www.motortrend.com/features/might-new-concept-rotary-range-extender-fly-technologue/ shows this engine being used for aircraft propulsion (in a video), using a large ducted fan.

            But best of all (IMHO, In My Humble Opinion) is Astron’s site, at https://astronaerospace.com/ , which includes an EXCELLENT video, at 10 minutes long.  If anyone wants to understand this engine, I highly recommend studying the video, repeatedly!  In their 10:00 minute video, go to 9:50 for a one-page bulleted summary.  One of their points that I’d like to emphasize is “ultra-high altitude capability”, meaning that it can operate in very thin air.  Just what we need!  (Just HOW high of an altitude, I’ve not been able to find out.)

            For uses in aircraft or rockets as I want to detail here, some uses are obvious… We can locate these engines anywhere which is convenient and sensible, and we can use them to power fuel-injector, oxygen-injector, or air-injector pumps or turbo-pumps (to assist jet or rocket engines).  We can use them to generate on-board electricity.  We can ESPECIALLY use them to power air compressors!  This engine already has a built-in air compressor.  See the blue “intake compressor paddle rotor” and (also blue) “isolator rotor intake compressor rotary valve” at 3:25 and at 4:25 in the 10 minute video.  Light blue air is uncompressed air and darker blue (or purple) is compressed air.

            To add an air compressor, one could do some general research and maybe start with https://www.bigrentz.com/blog/air-compressor-types … Select a type and go!  But that’s not what I would do!  I would take the existing Astron Omega 1 design and leave alone all elements currently there already, but add a SECOND set of the blue-colored elements as described above.  This second set of rotors would NOT kick compressed air along the spin axis, over to the combustion rotors, but would, instead, kick (pulsed) compressed air outwards and away to hoses (more likely pipes) to our compressed air “accumulators” (tanks), or bundles of COPV tubes, which will be described further below.  Adding a one-way flow valve (gate) for the pulsed, compressed air may or may not be needed.  I suspect that such a one-way valve will be needed ONLY if one ever wants to turn the engine / pump combination totally OFF, while still storing the (tanked, accumulated) compressed air.

            Ask Astron for help… If you are a large manufacturer (customer or would-be customer of theirs, unlike an idling fan-boy speculator like me), I bet that they’d be happy to help you!  Further suggestion:  Ask them to name their / your new derivative engine (with built-in compressor, with common elements and simple BOM, Bill Of Materials, for engine-compressor v/s external-load compressor, based off of their “Omega 1”) to be named the “Omega 13”, in honor of the movie “Galaxy Quest”!

I do NOT believe that designing (or asking for) a larger (scaled up) version of the Omega 1 is at all a good idea!  It is for the same reason why large-diameter COPVs aren’t practical, either, and the reason why will be detailed shortly below.  If you want more power, STACK the existing Omega 1 engine in series (add one to the other along the spin axis of the twin rotating hollow shafts internal to the engines).  This is as is shown at the middle of https://astronaerospace.com/ …  Use search-string “THIS IS MODULAR TECHNOLOGY” (on that site) to make sense of this.

            Note also that another site at new link  https://contest.techbriefs.com/2022/entries/automotive-transportation/11821   says that the Omega 1 “…can run on both liquid and gaseous fuels. It is particularly well suited to hydrogen and HCCI operation…”

 

 

Pressure-Wall Thickness V/S Vessel Sizes for Hoses, Pipes… And Engine Chambers and COPVs!

 

            For aerospace applications, we MUST keep things lightweight!  This is highly relevant to much of what I describe here.  Why (below) do I add all sorts of complexity, and fancy songs and dances, to get large-capacity storage COPVs?  Why not just a very few, very large COPVs?  Because the walls get entirely too thick and heavy, very quickly!  They otherwise MUST do this to retain usefully high pressures!

To get started, see https://www.hoseassemblytips.com/common-considerations-for-hydraulic-tubing/ which says…  “It’s a mistake to assume any given wall thickness will offer the same pressure holding capacity across all tube diameters.  Hydraulic pressure acts at perpendicular force vectors against tubing wall, and larger diameter tube has more surface area for pressure to act against.”

In my own words, I would say, “The larger the diameter of a pressure-containing hollow wall (be that a hose, pipe, COPV, or engine cylinder), the longer that the linear length of this pressure-containing barrier becomes.  The cumulative stretching (vessel-bursting) force expands proportionately as this linear length (exactly diameter times “pi” if circle-shaped) of the wall expands (lengthens).  Bigger pipe?  Pressure held constant?  Need thicker walls!”

Google and the internet failed me here!  I could NOT find a simple drawing to show the above (VERY important in our context) concept, and so, in the service of optimal clarity, I now give you an illustrative drawing:

 

Figure #1

 

So this is why hoses, pipes, COPVs, and internal combustion engine components (compression chambers and combustion chambers) need much thicker walls for larger sizes, with pressure held constant.  Our COPVs, compressors, and engines are size-limited if we want to keep weight down for aerospace uses.  As side notes, the same will apply to combustion chambers and nozzles in rocket engines.  Thus, Space X appears to have been wise to use many smaller rocket engines instead of fewer larger ones.  Also, many smaller vessels will have more surfaces (per volumes contained) for shedding heat.

 

Internet Research on Current State of the Art for COPVs, and Their Walls and Over-Wrap Binding Materials

 

Here are some links and comments concerning COPVs, and materials for fabricating them.  After this, we’ll move on to probably-mostly-novel proposals for design variations (design elements) for said COPVs.

COPV BASICS…  See https://en.wikipedia.org/wiki/Composite_overwrapped_pressure_vessel for starter basics.  From there and from other sources (see https://astforgetech.com/composite-overwrapped-pressure-vessels-copv-ultimate-guide/ ), we know that aluminum and steel are of interest to us (for liners or cylinder-walls), as are carbon, composite, or Kevlar (or blends of the same) for material to be used in the over-wrap layer.  To that, I want to add some other materials for more-novel COPV designs.  Those are high-temperature (ceramic or other) gasket materials, Kovar, and borosilicate glass.  Note that borosilicate glass is often sloppily called “Pyrex”, and that it closely matches Kovar for the thermal coefficient of expansion.  This will be important for us below.  From the above “astforgetech” link, note that they list a type of COPV that is composite-walled.  These will NOT be of interest to us here, since they won’t be very thermally conductive.  We will need to shed heat (to the ambient-air) from compressed air, for our solutions discussed here.

Somewhat relevant “FYI” links on COPVs:  https://ntrs.nasa.gov/api/citations/20110015972/downloads/20110015972.pdf   says 5,000 psi can be safely (human-rated) held, and note 2 tank sizes in table 1; a 26 inch (diameter) tank can hold more pressure than a 40 inch tank…  This should be no surprise to us, after I explained wall thickness v/s pressure (wall thickness penalty associated with larger vessels) as is associated here with my Figure #1 above.  NASA’s 5,000 psi here is 5,000 / 14.7  =  340 bar, with “bar” being a common unit of pressure, especially with respect to COPVs.  “Ask The Google” and “The All-Knowing Google” will quickly tell you that… “Bar is atmospheric pressure Atmospheric air pressure where standard atmospheric pressure is defined as 1013.25 mbar, 101.325 kPa, 1.01325 bar, which is about 14.7 pounds per square inch.”  https://en.wikipedia.org/wiki/Bar_(unit)    COPVs (presumably small ones) can hold some ridiculously high pressures.  See https://www.osti.gov/biblio/1604170  =  “THERMALLY CONDUCTIVE 700 BAR COMPOSITE TANK FOR HYDROGEN STORAGE”  (Emphasis mine).  Wow!  This is composite-based, though, so won’t be of much interest for us.  And the overwrap materials won’t withstand high temperatures either, I’d bet, at a glance.

See https://www.researchgate.net/publication/335445233_Investigation_of_interlayer_hybridization_effect_on_burst_pressure_performance_of_composite_overwrapped_pressure_vessels_with_load-sharing_metallic_liner/figures?lo=1 also, as being relevant.  And this one as well: https://static1.squarespace.com/static/5ca8e523ebfc7fd95c0c4fc1/t/5fb2cbbe44ad9171878b4010/1605553092670/COPV_Design_Report.pdf  Table 2.1 shows space-travel related (rocket related) parametric profiles of interest to COPV designs… It also shows internal “common bulkhead” designs for ONE COPY tank to contain TWO kinds of fluids, and anti-slosh internal floats for liquids v/s gasses, and at the bottom it shows that a LONG (skinny) cylinder is a practical shape for a COPV.

I will resist the temptation to include too much clutter with too many more links on this particular topic.  Part of my intention here is to facilitate ”public domain propulsion science and engineering”, which is why I include links of marginal interest to the immediate tasks.  “Pursue your own thing” though…  And these extra links may be of help to you!  Email me at RocketSlinger@SBCGlobal.net if needed, for YOUR preferred flavors of clutter…  But for now, we move off to MORE of what may be clutter!

HANDY SITES FOR CONVERTING UNITS…  These are easy to find, but I list a few of them here that I’ve found useful when building this web page right here.  Units conversion density = https://www.calculatorsoup.com/calculators/conversions/density.php .  Thermal Conductivity = https://converter.eu/thermal_conductivity/ .  Tensile strength =  https://www.westyorkssteel.com/technical-information/tensile-yield-strength-unit-conversion-calculator/ .

SITES FOR MATERIALS PARAMETERS…  These may be less easy to find… Let’s get ON with it!  https://web.mit.edu/course/3/3.11/www/modules/props.pdf provides a table of different materials types…  With costs!!!  And densities, strengths, etc., as well!  https://precision-ceramics.com/materials/properties/density/ is excellent, as are also the various other sub-pages there, of https://precision-ceramics.com/materials/properties/ for other properties of ceramics.  https://www.engineeringtoolbox.com/thermal-expansion-metals-d_859.html is handy for metals (for thermal expansion of course).  https://amesweb.info/Materials/Density-of-Metals.aspx is likewise useful.  Also https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html .

            For borosilicate glass, which is of special interest here, see https://en.wikipedia.org/wiki/Borosilicate_glass and https://www.imetra.com/borosilicate-glass-material-properties/ .  This type of glass isn’t all that terribly much different than soda-lime glass, other than being a bit lighter (less dense), and FAR more tolerant of temperature shocks and temperature-differences stresses.  For our purposes, I don’t think that the (affordable, practical) SIZES of the glass parts that we will need, will be a problem.  For details on that, see the immediately-above-cited “wiki” site, plus https://www.foodmanufacturing.com/home/video/20856852/company-makes-worlds-largest-glass-bottle and https://faourglass.com/six-trends-in-commercial-jumbo-glass/ .

            Veering almost-off-topic just a TINY bit, spacecraft windows (for the Space Shuttle, for example) and bathyscaphes windows have SOME application similarities to what I plan to describe further below.  The Shuttle windows used quartz, which is likely to be too expensive for us.  See https://www.nasa.gov/centers/ames/research/2007/faq-shuttleglass.html  and https://www.qsiquartz.com/difference-between-glass-and-quartz/ .  For bathyscaphes windows, Plexiglas is used, which isn’t high-temperature-tolerant enough for us.  See https://en.wikipedia.org/wiki/Trieste_(bathyscaphe) .

For Kovar, which is of special interest here, see https://www.mindrum.com/metal/kovar/ and https://www.edfagan.com/controlled-expansion-alloys/kovar-astm-f-15-pernifer-nilo-rod-bar-sheet-plate/kovar-properties/ and https://www.kovaralloy.com/kovar-properties.php .  Also see

https://psec.uchicago.edu/thermal_coefficients/kovar.pdf , where a table is given for variation in this figure (of thermal expansion for Kovar), per exactly what (differing, varying) temperature range we are dealing with.  This kind of variation will happen with (all or almost all?) ALL metals, actually, if you read the fine print!  While in the neighborhood, also check out https://psec.uchicago.edu/thermal_coefficients/ and then https://psec.uchicago.edu/thermal_coefficients/metal.pdf , for reference, as well.

            We will need Over-Wrap Fibers.  For these, we’d like for them to be extra thermally conductive, to shed the heat generated by in-flight-compressed air.  https://carbonfibergear.com/blogs/carbonfiber/is-carbon-fiber-conductive says that “When it comes to thermal conductivity (the ability to conduct heat), carbon fiber is average. Relatively high-quality carbon fiber has about as much heat conductivity as silicon, and even top-of-the-line carbon fiber has less conductivity than aluminum.”  So we’d like to improve on that.  Maybe we could co-weave in there, strands of copper, which would reduce strength, but add thermal conductivity.  Or spiral-wrap the copper around the carbon or Kevlar fiber-bundles.  And of course we could add Kevlar and-or other fibers.  This paper will probably get to be quite long enough, without going in-depth here.  But the following definitely seems to deserve to be mentioned: https://www.researchgate.net/publication/344763745_Surface_Pretreatment_and_Fabrication_Technology_of_Braided_Carbon_Fiber_Rope_Aluminum_Matrix_Composite “Surface Pretreatment and Fabrication Technology of Braided Carbon Fiber Rope Aluminum Matrix Composite”  This also adds cooper, and appears to be HIGHLY likely to be useful to us.

            For ceramics, which are of special interest here, see https://dobsongasket.com/products/high-temp-materials for High Temp Gasket Material…. Super-wool (Ceramic paper) is rated for up to 1,300 degrees C…  And ceramics that can bend, at https://www.popularmechanics.com/technology/a19300/new-flexible-ceramic-could-make-its-way-into-electronic-devices/  From there, “New Flexible Ceramics Could Make Bendy Gadgets a Reality” and “The material apparently has properties similar to paper while retaining the high heat-resistance of ceramics.”  At further length, ‘Eurekite's "flexiramics" ostensibly retain the positive properties of ceramics while being flexible rather than brittle. In a video from Eurekite, CEO Gerard Cadafalch holds a piece of the material over a flame and it doesn't catch fire.  The material can reportedly withstand heats of at least 1,200 degrees Celsius—about 2,190 degrees Fahrenheit—the hottest temperatures Eurekite's lab can achieve.  The company says they can make the material in thickness ranging from "a few micrometers to over a millimeter," according to Phys.org.’

            The above (perhaps even in combination) can serve as gaskets for us.  Also note that non-electrically-conductive (non-metallic) gaskets are useful not only for pressure-sealing (flow-sealing) joints, but also for preventing or slowing down galvanic corrosion when joining (bolting, for example) two different kinds of metals together.

            Note that copper (a soft metal) can be used to create gaskets.  See https://coppergaskets.us/ for example.  “Ceramic paper” type materials (as above) could be used, in alternating thin layers, to add bulk (thickness, compliance) to gaskets for our use.  One would want to start and stop with ceramics layers, to avoid galvanic corrosion, when creating a “sandwich gasket” to be placed between different types of metals.  A sandwich gasket might be ceramic-copper-ceramic-copper-ceramic, for example.  Other soft metals (lead, for example) could be used in place of copper.

            Also see https://www.researchgate.net/publication/265606808_Strong_lightweight_and_recoverable_three-dimensional_ceramic_nanolattices “Strong, lightweight, and recoverable three-dimensional ceramic nanolattices”.  If this stuff is affordable, we may want to use it to create what I will call “tethered puff balls”, to be discussed further below.  This sponge-like material could be used for “padding” here and there, to prevent unwanted inter-materials scratching, rubbing, and chattering (shock-and-vibrations-induced).  For a brief discussion of the exact same material, see http://www.sciencedaily.com/releases/2014/09/140911135450.htm    “Ceramics don't have to be brittle: Incredibly light, strong materials recover original shape after being smashed”.

For a last item on ceramics, the following may be of passing interest to us: https://www.technologyreview.com/s/411301/ceramics-that-wont-shatter/ .

For Glass Compression Domes, which are of special interest here, see https://rayoteksightwindows.com/knowledge-center/optical-dome-capabilities/flanged-molded-glass-dome-capabilities-build-to-order.html#:~:text=Glass%20domes%20can%20go%20to,degradation%20of%20maximum%20operating%20pressure.  From here, “Glass domes can go to over 50,000psi. However, dome mounting is the limiting factor in pressure systems. The maximum operating pressure is determined by how the dome is mounted. Over time mounting may contribute to degradation of maximum operating pressure.  External Pressure only (convex side).  Never operate with internal, concave side pressure.”  (Emphasis mine).  Yes, they use Borosilicate glass.  And note that 50,000 psi (divided by 14.7) is 3,401 bar!

            And now for some comments about WHY one should use domes in compression… Especially domes built out of materials that are good in compression, but not in tension, stress modes, such as rock, concrete, ceramics, glass (a type of ceramics), and eggshells, see a video about compressing an egg between your hands, at https://www.youtube.com/watch?v=_B--gsSQmF0 .  As I learned many years ago in mechanical engineering at the US Air Force Academy (but can’t find succinctly explained on the internet), a flat material (plate), when loaded from the top, “wants to” be compressed on the top parts of the plate, and expanded at the bottom parts.  Compression-mode at the top, with tension-mode on the bottom.  Since our brittle materials here can’t stand the tension, our plate will quickly break under heavy loads.  By bulging the material upwards (against the load or pressure), the bottom parts of our bulge have “nowhere to be stretched towards”, so they don’t (can’t) go into tension mode.  The molecules of our brittle material are “constrained” against stretching (have nowhere to go).  ALL parts of our bulge are now in compression, not in tension.

While I’m dispensing “mechanical engineering folk wisdom” here, it is important to understand CONSTRAINED V/S UNCONSTRAINED materials behavior.  Once again, the internet is letting me down!  Under higher temperatures, does a material (especially a metal) have a fairly empty space into which it can expand?  If so, the material is “unconstrained”.  If, on the other hand, the material does NOT have much room to grow into (it is contiguous with much more of the same type of material, or is firmly pressed, bonded, or fastened to another type of hard material), then it is “constrained”.

In more detail, if there’s a hole drilled in the middle of a large flat sheet of metal, the hole will NOT expand or contract significantly as the metal is heated, since the surrounding metal is “constrained”.  In this case, the heated metal can NOT expand towards the hole or void, so the hole-size will NOT shrink much, as the temperature goes up.  One can think of molecules or crystals (grains) of metal, all trying to shoulder aside their neighbors, trying to expand towards the hole.  They can’t do it to a significant degree, because they are all “fighting each other”, in all trying to all do this at the same time.

If the surrounding metal is NOT constrained, then think of a ring (or a washer, in a nut-bolt-washer arrangement).  The limits to the surrounding metal disc around the hole are not much bigger than the hole, so we have a ring (AKA torus or doughnut).  When heated, the entire ring (including the hole) will expand outwards, since there are fewer close-by molecules (or grains or crystals of metal) that are all cramming in together, all around.  That is, the entire ring has room to freely expand (and the hole will get larger).  This can also be partially true of a hole drilled too close to the edge of a flat sheet.  If one can intuitively understand this “where am I free, or not free, to expand towards” aspect of materials behavior, from the perspective of the bouncing, being-heated molecules of materials, we’ll have an easier time of it, when trying to design a sensible (practical) pressure vessel.

While dispensing more “mechanical engineering folk wisdom” here, note that we MUST keep in mind, ease of manufacturing, or even, can this thing be manufactured at ALL?  I can easily describe a thing that can exist, stably, indefinitely, at room temperature and pressure (gravity, etc.), but can NOT be built by us mere mortal humans!  For example, use nothing but unbroken, contiguous, homogenous materials, no fasteners allowed, build me a sold sphere of plastic explosives, encased in a solid sphere of wood, inside a sphere of clear diamond, inside a sphere of clear glass, inside an eggshell!  Add some more at will!  (Well, I thought it was funny.  In any case, I may need to remark on practical aspects of actually building our “stuff” here below, from time to time.)

Let’s now add some FINAL GENERAL INTRODUCTORY NOTES before diving in.  I may sometimes repeat some of the above-cited links when it is important to do so.  At other times, I may simply state facts (density of one material v/s another, sometimes in a ratio) that can be derived from the above links.  If you need help, or find mistakes, and care to do so, as before, then feel free to email me at RocketSlinger@SBCGlobal.net .

Here, borosilicate glass is selected for clear (Ha!  Clear as glass!) reasons, including that it mates well to Kovar, is strong in domed compression, and is MUCH less dense than Kovar or steel.  For this glass, feel free to substitute some other type of glass or of ceramic… In the drawings, I may just label it as “glass” or “b-glass” to cut the clutter.  Kovar is selected to mate to the b-glass and provide a machining-friendly interface (but note that Kovar isn’t as strong as steel; Yield strength of strong 4340 steel is at 1240 MPa, Kovar is at 276 MPa, so 4340 steel is 4.5 times as strong as Kovar).  Steel is also affordable.  Feel free to make substitutions of course…  Aluminum is lighter than steel, but not as strong, and has a worse (higher) rate of thermal expansion and contraction than does steel.

Aluminum yield strength is 500 / 1240 or …  Steel is (1240 / aluminum 500) 2.5 times as strong as aluminum, but aluminum is less dense.  On the other hand, aluminum has a higher rate of thermal expansion, at about twice as high as steel.  Steel density 7.8 / aluminum 2.7, so steel is 2.9 times as dense as aluminum.  These kinds of things are easily derived from my above links, and (along with the designs of others) guide the design choices that I’ve made below.

            For JOINING MATERIALS, do as you wish, of course… I’ve (perhaps arbitrarily) chosen bolts, often.  There’s brazing, soldering, gluing, riveting, welding, and who-knows-what-all-other choices out there.  Let’s not get side-tracked down a rabbit hole here.  I’ll just give you one link that may be of interest, on bonding metal to ceramic …  https://s-bond.com/blog/ceramic-metal-bonding-part-one/ says these things:  “When considering braze attachment of ceramics to metals, the issue of CTE is limiting since brazes melt over 840°F (450°C) and upon cooling the solidified joint stresses can fracture or distort a part. Many times brazed ceramic-metal joints will require the use of a low CTE metal such as Kovar®, Invar® or Molybdenum.”

            Now finally let’s get ON with it!

 

 

Novel Design Elements for COPVs

 

Here’s a starting drawing for a compression glass dome, for the inner side (high-pressure side) of a COPV cylinder.  A cylinder is easier to manufacture, and contains less material per volume contained, than does a 4, 5, or 6-sided (etc.) container with many flat faces.  300 sides approaches a cylinder in practical reality, but… Let’s not get side-tracked!

 

Figure #2

 

A few thoughts come to mind as we view the above drawing.  For one thing, perform a physical experiment.  Take a large cylinder (a bucket, for example) to simulate our over-all pressure vessel.  Take a smaller cylinder with an open mouth (a drinking glass, or a tin can) to simulate our glass pressure dome.  Now place the mouth of the smaller cylinder insider the bucket, and butt it up to the bucket wall.  You will instantly see that the mouth of the smaller cylinder (our glass dome) will need to assume a “saddle” shape.  This isn’t obvious from cross-sectional drawings as above.  The “saddle” shape will need to be imposed on the glass and-or Kovar parts above (possibly both can share the load of being partially saddle-shaped).  Deforming the steel pressure-vessel wall-hole to assume any kind of a saddle shape (other than ”what it wants to naturally be” as part of this larger cylinder shape) would make no sense at all, IMHO.  This is especially true from a perspective of ease of manufacturing.

Also note that, just as we made a “sandwich” working downwards (outwards), of glass-Kovar-steel, we could easily “grow” the sandwich to instead be glass-Kovar-steel-aluminum.  Making the very outermost layer of the pressure vessel be aluminum instead of steel would reduce mass for us.  However, aluminum is only 2.7 / 7.8 or 35% as strong as steel, so this isn’t a very good trade-off for us.  Also, layering together (via bolts, or other methods) more and more materials types with differing CTEs (Coefficients of Thermal Expansion) is just begging for trouble!

            The “tapered gap for ease of fit” comment in Figure #2 above shows that we’ll have to place the glass dome in the cavity of the surrounding Kovar.  Force-fitting this isn’t compatible with getting a good gasket seal.  That is, during the forced fit, the gasket would get pushed out of place.  So this (a tapered fit as shown) is about our only sensible choice.  Pressure inside the vessel will later on (after assembly) do our final tapered “force fit” and compress the gasket into place.  With good luck, this will stay seated after the pressure vessel’s pressure returns to ambient.

            But what if we do NOT have good luck, and shock and vibrations knock the glass dome loose when the pressure vessel is empty?  It might even fall out of place!  That’s why we repeat the above drawing (minus some clutter) with some “jug ears” added to the Kovar.

 

Figure #3

 

To keep the glass dome in place where the pressure vessel has no pressure, we take rope or string (say, Kevlar or carbon fiber bundles, for space-age durability and heat tolerance) and tie them in a “star” pattern to the jug-ears.  Jug-ears are spaced out in a clock-like manner around the Kovar periphery of this saddle-shaped b-glass dome-shape of a clock face, facing the glass from inside the vessel.  If there are (for example) 6 clock-arranged jug-ears, tie your string to station 1, then loop it through 4, then 2, then 5, then 3, and finally tie it off at number 6.  Across the convex (inner) face of the glass dome, that is.  Nice and tight, but no more…  This is a low-cost, simple, and lightweight solution to make sure that the glass dome stays in place.  The jug-ears could be turned 90 degrees from what is shown.  Or the jug-ears could be dispensed with, and slanted holes could be drilled through the Kovar instead, at these same locations.  Other fixes are possible, but most (if not all) alternate “fixes” would require more mass.

            Now Dear Reader, I’ve already ‘fessed up to this in the abstract, that this design effort will fail (IMHO), so this isn’t much of a “spoiler”.  But the bigger picture here is that I’m trying to substitute b-glass (borosilicate glass) for steel, since steel is about 3.5 times as dense as b-glass.  B-glass in compression domes should perform just fine, strength-wise.  BUT THE NOW-REDUCED MASSES OF THE STEEL NOW MUST TAKE ON ALL OF THE TENSION LOADS!!!  So now the Swiss-cheesed steel full of holes (the lattice of remaining steel) needs to be much thickened up to bear the additional tension stresses of a filled pressure vessel!  This is the basic problem summarized.

            I’ve tried many “fixes” for this, and all of them fail my “smell test”.  Please prove me wrong, or find fixes for this!  I will cheer you on!  Or help YOU write YOUR paper on this!  But let’s walk through some details and my non-viable “fixes”.  And of course, WHY the fixes aren’t practical, affordable, and effective.

            The mental exercise of the “Swiss cheese full of holes” (for an outermost steel wall of a pressure vessel) and the additional tension-stress that this adds to the remaining hollowed-out skeletal steel lattice should be obvious, but an additional drawing (for this) is supplied anyway.  Imagine our retained gasses “pressing down hard” into the page as you view it.

 

Figure #4

 

However, we COULD make the circular (actually saddle-shaped) holes be long and skinny ovals instead.  This simplifies our cross-sectional drawings, and, I suspect strongly, the manufacturing processes as well.  The engineering principles (concerning “handling the pressure”, stresses, compression, tension, etc.) remain the same, though, whether the glass domes are roughly circular, or elongated arches.  The engineering principles also remain the same, even if our pressure vessel was a sphere instead of a cylinder, as well.  Using a sphere would much complicate manufacturing processes, by the way.

            So let’s take it to the logical extreme and turn those oval holes in the steel cylinder into gaps-in-the-steel that run ALL of the way to the ends of the cylindrical pressure vessel.  Our (my) goal here is to create a lightweight but LARGE pressure vessel here (keep that in mind please).  See https://faourglass.com/six-trends-in-commercial-jumbo-glass/ and see that they tell us (us being their potential customers) that they can cast glass up to 144 to 300 inches (12 to 25 feet) long on the longer dimension.  So the “trough” length of a stretched-C-shaped “trough” (cast out of glass) could conservatively be set to, say, 10 feet long.  This will much complicate the ENDS of our being-designed pressure vessel here, but, as a (doomed in my opinion… Our pressure domes are doomed domes!) design effort, let’s try to design a 10-foot-long uniform-on-the-long-axis large pressure cylinder.  Keeping the same color schemes as before, the below is what ANY point in the design might look like, in cross section (excluding the end-caps).

 

Figure #5

 

The above drawing is conceptual only.  Not to scale, and so forth.  The glass parts (B-glass or borosilicate glass) should be much larger, and the metal parts (Kovar, steel, bolts, and eye-bolts) should be smaller, or else we won’t get our desired weight reduction.  And the eye-bolts (for the tie-backs) could be mass-reduced by replacing them with integrated-into-the-steel “jug ears” as were shown in the Kovar in Figure #3.

The above design has its virtues (reduced mass in the glass, compared to steel).  The glass conducts heat poorly, but the steel will do the trick for us, there.  One could secure aluminum finned heat sinks to one or both sides of the steel to improve that, if we wanted to.

But this design will NOT hold much pressure, no matter WHAT fixes (that I can think of) that we can throw at it!  As stated before, the reduced (remaining) steel now has to bear all of the outermost-vessel-wall tension forces.  Our tethers (tie-backs to opposite-wall tie-back loops) would have to be very many and-or very strong.  Tethers are shown in BROWN above.  They’d have to be many-many (many more tie-back loops, all behind the other eye-bolt holes as shown), which adds mass and detracts from storage volume.  Very thick and strong tie-back cables or ropes would do the same (add mass, subtract from storage volume).

An outer jacket of overwrap (as in conventional COPVs) added to the above won’t help us much.  It would be difficult to manufacture, and contain stress points for the over-wrap, due to the “scalloped” nature of the design (plus the bolts, if they are used).  Besides that, it won’t help us design LARGE but lightweight tanks, for reasons previously discussed far above, here in this paper, centered around Figure #1.

            A brief examination of the strengths of tie-back materials may be in order, though.  Steel (for the outer tank wall, now much reduced) and for steel cable (if used for the tie-back cables) might serve as a good benchmark.  If our tie-backs are MUCH stronger than steel, we MIGHT be in luck!  Steel yield strength is about 1,240 MPa for strong (4340) steel, for a benchmark for comparison.

https://en.wikipedia.org/wiki/Kevlar says that “When Kevlar is spun, the resulting fiber has a tensile strength of about 3,620 MPa (525,000 psi), and a relative density of 1.44 (0.052 lb/in3).”  3,620 / 1,240 is 2.9 (Kevlar roughly being 3 times as strong as steel, then).  In view of ALL the large magnitudes of accumulated forces at the outer walls of the pressure vessels, I think we’d need too many Kevlar tie-backs!  But I don’t know how to model or calculate that to PROVE it, to be honest.

Carbon fiber yield strength (in composite form apparently) according to https://www.matweb.com/search/datasheet_print.aspx?matguid=39e40851fc164b6c9bda29d798bf3726 is 3,220 MPa, which is roughly comparable to Kevlar.  It won’t help us much either.

Now let’s try carbon nanotubes yield strength… https://www.researchgate.net/publication/334371954_Strength_of_carbon_nanotubes_depends_on_their_chemical_structures “Strength of carbon nanotubes depends on their chemical structures” says that “Single-walled carbon nanotubes theoretically possess ultimate intrinsic tensile strengths in the 100–200 GPa range, among the highest in existing materials.”  This is what we need!  We would STILL have to deal with MANY-MANY tie-back end-points!  And how large and strong would those tie-back end-points have to be?  (This is a VERY important question to ask, and presents what I believe to be a fatal weakness here in our design.)  Conventional overwrap designs distribute the tension loads at the outer wall of the tank uniformly and widely.  Our tie-back end-points would concentrate these same forces into small areas, something terrible!  I think we’d STILL be in trouble with this design, even if carbon nanotubes were readily available and affordable, which they are not.

            “For grins”, let’s see if carbon nano-tube over-wrapped pressure vessels are available.  https://www.researchgate.net/publication/293801198_Assessment_of_Carbon_Nanotube_Yarns_as_Reinforcement_for_Composite_Overwrapped_Pressure_Vessels   “Assessment of Carbon Nanotube Yarns as Reinforcement for Composite Overwrapped Pressure Vessels” is clearly relevant.  https://www.sciencedirect.com/science/article/abs/pii/S1359835X16000609  has the full paper (exact same paper), written in 2016.  A quote from there says “… the mechanical properties of bulk forms of CNTs have not been sufficient to justify their consideration in structural applications.”  (CNTs being Carbon NanoTubes, and this being written some years ago, by now).  Google “buy bulk carbon nanotubes” or similar, or search using terms for CNT COPVs, and I find nothing suitable and affordable for sale, for our use here for tie-backs, nor do I readily find such-wrapped COPVs for sale.  I would think that such COPVs will be one of the VERY first practical uses for CNTs.

            I did, however, find this, which is highly relevant:  https://www.spaceflightinsider.com/organizations/nasa/newly-developed-nanotube-technology-revolutionize-spaceflight/  “Newly Developed Nanotube Technology Could Revolutionize Spaceflight”.  This web pages directs us to https://www.huntsman.com/products/detail/344/miralon as a source for CNTs, where we can then go to https://huntsman-pimcore.equisolve-dev.com/Documents/Miralon%20Yarn%20_US_e.pdf and see that their best available yarn yields 1,024 MPa… Which isn’t good enough for us.  My moderate-length Google search seems to tell me that we’ll still have to wait a while for what we need.  Other reasons also prevent our design (as described) from working well, anyway.  See a few paragraphs above, highlighted.

            As temperatures vary inside the pressure vessel, we’d also have to worry about CTE (Coefficients of Thermal Expansion) of the tethers v/s the overall tank.  The tethers will relax as they expand.  Unless we use some “negative CTE materials”, which are actually a “thing”!  Such materials could compensate our tethers-expansion for us.  See https://www.researchgate.net/publication/352740638_Negative_thermal_expansion_in_magnetic_materials ...  “Negative thermal expansion in magnetic materials”.  I seriously doubt that this is any kind of a suitable “tech” ingredient for our use here, but I leave further research about this to YOU, Dear Reader!  And ditto for worrying about CTE on the tethers as well!  I think our efforts here are doomed already, anyway!

            Our most fatal design flaws are already described well, I think.  Other solutions are just “putting lipstick on a pig”, so to speak, IMHO.  We could put a “gas shock” (or “gas spring”) inside our pressure vessel, to yank on the tie-back tethers harder and harder as the internal vessel-pressure goes up.  To understand that better, see https://www.lesjoforsab.com/technology/gas-springs/ , which has a nice drawing.  For our use, during manufacturing, with regards to the “cylinder” to the right of the gas spring?  We’d fill that with ambient air, and place the piston to the far left (maximally extending the entire gas spring).  We would make the seal (to the far left) be open (porous) to the gas pressure inside our pressure vessel, unlike the air inside the “cylinder”, to the right.  We’d tie the whole gas spring snugly (just moderately tightly) into our tether.

            Now as the pressure-vessel fills up (outside-of-the-gas-spring pressure increases), pushing rightwards onto the piston and hence onto the “cylinder”, and so the gas-spring “cylinder” contracts, pulling more strongly on the tether, compensating for thermal expansion of the tether, AND yanking inwards on the outer walls of the pressure vessel as well!  Proportionately to internal tank pressure!  What a BRILLIANT idea, I told myself, at first!

            Yes, it would work…  But not NEARLY well enough for it to be worthwhile!  Gas-spring force here would be proportional to the left face of the “piston” in the middle, MINUS the surface area taken up there, by where the “piston rod” meets this piston.  This small amount of area?  It would compensate ONLY for a measly equal amount of area on the outer surface of the pressure vessel!  We would need absurdly large, and-or absurdly many, gas-springs for this idea to work well!

            I think that we’ve already beaten this dead horse thoroughly into the ground.  However, a few more mental engineering exercises will strengthen our brains!  These include exercises in “intuitive finite element analysis”, if you will.  Let us think like some structurally stressed molecules!  And last but not least, it allows me to draw another picture!  (FYI, this is something that I enjoy doing, but only if it might conceivably be useful to someone somewhere, some sunny day.)

 

Figure #6

 

First, let’s start off by asking a question.  Can we “pre-stress” this pressure vessel (in the same or highly similar manner compared to “pre-stressed concrete beams”; see http://www.dot.state.mn.us/historicbridges/prestressed-concrete.html ).  OK, let’s not get side-tracked!  …  Can we “pre-stress” the cylinder by putting significantly large amounts of tension onto the tie-back tethers, during tank manufacture in ambient pressures?  To strengthen the vessel? (The “cheat” or look-ahead answer is an emphatic “no!”)  All of the tie-backs would have to be simultaneously tensioned roughly to the same degree as all of the others, or we will warp, twist, and break our toy!  But it just MIGHT be possible to do this (equally applied pre-tensioning), so let’s look deeper.

            The short and “intuitive” way to understand why this won’t work, is to say that pre-tensioning the tie-backs is mechanically analogous to the forbidden practice of placing “negative pressure” onto the top of a compression dome made out of brittle materials (or positive pressure added to the bottom of the dome).  We have a mechanically simulated vacuum inside the vessel, with ambient pressure outside the vessel.  We’ve done this by pulling inwards on some elements of the outer wall of the vessel, which is the same thing as what happens with a vacuum inside the vessel, and higher pressures outside.  The only difference (of significance) is that our inwards-pulling forces are NOT evenly distributed on the vessel-wall.

            The steel walls (plus Kovar) will transfer the inwards-pushing forces of the pre-tensioned tethers onto the glass domes, adding to the inwards-pushing forces on the glass arches.  This would be compression, which would be fine, if the glass was also being pushed outwards, with equal or greater force.  But we’ve not applied equal (or more, as is desired for such arches) additional forces to the innermost surfaces of the glass arches!  Compression is greater on the outer surfaces, and less on the inner surfaces, which puts the inner glass surfaces into forbidden tension modes.

The tops (insides) of the arch-tops will go into tension (this is NOT good) because of our imbalanced forces, in ambient air, with the tethers pulling all elements together (albeit in an imbalanced manner).  The small arrows on the tops and bottoms of the arch-tops are place there to remind us that air will press equally on all sides, in an empty tank, all else being equal.  But with pre-tensioning, all things are NOT equal!  We have added compression to the bottom (outside away from vessel-center) of the glass arch, while putting relative tension on the top of the glass!  This is forbidden, and the glass will crack, with the crack most likely starting at the top of the arch.  In other words, “what is the innermost surface of the glass dome pushing against, to countervail against the pre-tensioned tie-back tethers which push inwards, to keep all parts of the glass in compression?”  The answer is, NOTHING is pushing back outwards on our glass, to keep the balance!  Nothing but ambient air, which is in balance all around us.  We have net force imbalances on the glass, then, in a manner which violates the design principles of “pressure domes”.

            Well, I’m not sure that all of my above babbling will make sense to all readers.  Looking at it differently, go see the “jug ears” in Figure #3 far above, and the associated comments about adding some strings across the inner faces of the glass domes, to make sure they don’t fall out.  With pre-tensioned tie-backs, the Kovar would be squeezing together on the glass domes, like squeezing a wet bar of soap between your hands.  The glass would be getting squeezed (inwards) out of there (where it is held by Kovar and gaskets) without gas pressure in the tank.  Our strings would hold the glass in place, sure.  But the glass would be getting squeezed (and most likely moved, or wiggled) back and forth as we filled and emptied the tank, which would play havoc with our gasket seals.  All of this is true, whether our glass is roughly hemispherical, or trough-shaped, by the way.  Havoc on our seals would be the least of our problems!  If the pre-tensioning of the tethers was strong enough, then even if our glass didn’t break, it would be squeezed out of there (like our wet bar of soap) so strongly, that the inner-face-of-the-glass-domes retention strings would now have to be “beefed up” as well, adding more mass.  We can’t win, it seems!

Speaking of adding mass, adding mass to the glass (as is shown by the dotted green lines in the above drawing) is an option (at an obvious price) for fighting against our cracked glass.  It might help, but not much, if we add very much pre-tensioning to our tie-back tethers.  The same (relative futility of thickening the glass) will be true for the forces-imbalances problems to be described below.  I don’t know how to (effectively) babble much more about WHY “pre-tensioning the tethers” is a BAD idea, so I’ll quit now!  If I’m wrong, or you have a sensible “fix”, please let me know!  (Let’s move on now.)

Another perhaps-useful mental exercise is to think about forces distributions.  We can stop thinking now (please!) about the pre-tensioning of the tie-back tethers, and start thinking about the concentrations of forces where the tie-back end-points are going to “fight against” the internally confined gas pressures which want to “blow up” (blow apart) our “balloon” of a pressure vessel.  Some of the same (above-discussed) problems will be exposed, showing our entire design to be fundamentally flawed.  This will be shown as being true, whether we pre-tension the tie-backs, or not.  We will now commence “intuitive finite element analysis”.  Start thinking like solid structural molecules!  Thus I command you!

The “short and sweet”, intuitive thing to do here, is to think of a balloon.  At a glance, we THINK of solids (steel, Kovar, glass, and tie-back tethers) as solids.  However, under great stress (strain, pressure, and differential or pushing, shoving, and squeezing pressures especially) our “solid” materials will deform, like the VERY thick liquids that they actually are.  Metals more so, glass less so, are deformable, even as so-called “solids”.  But stressed enough, they will ALL deform a bit, before breaking.  This is true, even well below their melting points.  So we have (in our pressure vessel) a balloon, of sorts.  Also, as we all know, “nature abhors a vacuum”.  If our vessel contains usefully high internal pressures, we have a strong vacuum on the outside.  Nature now hates us!

Nature hating us shows up here in the form of the molecules (of metal, glass, etc.) all trying their best to run away in abject horror, away from the pressures, migrating (deforming) towards any adjacent lower-pressure areas.  All of our “solid” molecules want to deform our elaborate “scalloped” tank-shape into a “better” cylinder or sphere shape, which distributes all of the blow-us-up forces more evenly, and across a wider surface area.  This means that our glass domes want to “reverse” themselves, or “pop out”.  And as often-enough mentioned by now, that puts us into tension (on the outermost surfaces of the glass), which will break the glass.  This will be true, even if “miracle occurs here”, and we find or invent super-strong tie-back tethers and tie-back end-points.

In a bit less “folksy” (but in a more lengthy and detailed) manner, let’s imagine internal stresses inside the metal and glass, all concentrated most intensely at the tether tie-back end-point, and at the bases of the glass domes.  This is why the above Figure #6 shows differently-sized inwards-pointing black arrows, which represent the inwards-pointing forces (ultimately coming from the tethers) that are supposed to help our balloon to “keep it together” here, fighting against the tank’s internal pressure.  Heavy (thick) black arrows mean more force, and vice versa (thinner means weaker).

Force is intensely concentrated at the tie-back end-point.  The steel there “wants to” migrate (bend, deform) towards the tank-center, more strongly than the surrounding steel.  In other words, SOME of the concentrated tie-back forces are “used up” in deforming the surrounding steel (temporarily or permanently, ether way; see “yield stress” v/s “ultimate stress”), and are no longer available to “push in on” our pressured gas.  The tie-back end-point will form an inwards-peaked steel dimple, and inwards-pushing forces imparted by the steel will diminish as we get further away from the dimple-center.

Material stresses (internal pressures; internal strains and “pain” in the materials) could be envisioned as a three-dimensional “pain sphere”, with the red-hot “pain center” at the end-point.  First, we simplify (mentally) and flatten it into two dimensions.  Now we have a “pain rainbow”. Red in the middle, weakening down to orange, yellow, green, etc.  “Pain” and “rainbows” don’t go well together in emotional connotations, I know!  So let’s call this a materials-stresses “painbow”.  I hope so see “painbow” in the dictionaries very soon now!

Liquids (fluids in general) can’t paint a painbow for us very well, and stiffer materials (like glass) don’t absorb or dissipate our structural forces as well (don’t deform as much) as metals do, so our “painbows” will assume some strange shapes for us here.  We also have to keep in mind that our structures will STRONGLY want to migrate towards unsupported edges, where materials touch ambient air.  The middle of ONE of our painbows is centered at the tether end-point, radiating and weakening away from there, in circles.  Where steel hits the hole in the steel (ambient air, under the footer of the glass arch), though, the steel will bend outwards, away from the center of the vessel, fairly strongly.  This will “want to” tilt the inwards-pointing Kovar “wall” (upwards-pointing Kovar wall in Figure #6) that surrounds the glass.  Now we have left a “fitting gap” there between the Kovar and the glass, so maybe, maybe not, the shrinkage (inwards-travelling) in this gap will save us.  Shrinkage in the gap, caused by stressed materials expanding towards the gap, and the tilting of the steel footer underneath the Kovar, that is.  That is, this gap will shrink, for sure, but it may or may not save us from squeezing the glass dome too hard (towards the middle of the glass dome).  Kovar is fairly soft, though, so it can’t squeeze very hard.  We’re probably OK here.  Again, all of this would have to assume that the steel-wall end-point of the tether is considerably “beefed up”, though.

For how my brain works, at least (I don’t know about yours), it is now (as we tackle this intuitively, painting painbows of materials stress) time to stop thinking about the red-hot pain-point centered at the tether end-point.  That particular painbow has run its course, dispersing out through steel and Kovar, much of which is mostly placed into a “bending mode”, where the outermost wall elements are compressed, and the innermost (close to vessel center) elements are stretched (tension mode, which is OK for metal, but not glass).  But at the footer of the glass dome or arch, where tremendous glass pressure meets metal, I think it is best to change our focus, as we think about painbows.  Painbows only, now…  No thinking about ponies, fairies, and unicorns will be allowed!

A simple and straightforward analogy should help.  Our glass dome or arch is similar to rock arches that have been built since the times of the ancient Romans, and before them, even.  See https://en.wikipedia.org/wiki/Arch , of course!  But, now, WHY can’t we built arches as tall as, say, the Andes mountains?  Simply because, every step of the way, the overburden and the materials of the arch-walls themselves add more, more, and yet more pressure, caused by the weight (mass) of the arch materials, and gravity.

In our design, gravity is replaced by internal tank gas pressure.  Without any countervailing relief, starting from the top of the arch, the glass is more and more compressed (outwards away from vessel center).  Glass doesn't deform much, in order to absorb or soften the painbow.  At arch-top, the painbow is (relatively) purple or blue.  But as the gas-pressure loads increase (or rather, accumulate) from there, transferred through the glass arch down towards the footer (where glass meets metal), our painbow graduates up to red-hot.  We’ll have to (much!) beef up the steel, down (out) there, especially if (as intended per the design) we’ve had to cut a hole in the steel there, for a mass reduction.  Pay me now, or pay me later!

It may be useful to think about “pay me now, or pay me later” in simple terms.  We’ve taken a simple design of a steel-walled vessel and knocked holes or slots in the steel.  Where the holes or slots are now, ALL of the load that WAS taken by the steel, is now taken up by the glass dome or arch, instead…  And ALL of this load is now transferred (and concentrated) to the steel edges of the hole.  Hence, the edge-steel there needs to be thickened up in proportion to how large the glass-borne load is.

I believe that the (weakly supported at the bases of the glass domes, as shown in the above drawing) tremendous pressures supported by the glass domes will all be transferred (via Kovar and then steel) to the tether end-points (but only if we thicken up the steel underneath the glass dome footers).  Tie-back tethers are ALL that keep the metal parts from flying outwards!  (Well, that’s true for a fully slotted design, but not fully true of a full-of-Swiss-cheese-holes steel wall with glass domes instead of troughs…  There, remaining steel would STILL need to be greatly thickened, and-or tied back.)  And then the glass would go with it all, as well, of course (flying outwards).  At the tether end-points in the steel, AND at the bases of the glass domes, then, the concentrated forces will require us to “beef up” the steel so much, that we’d lose the mass advantages that we obtained by substituting glass for steel, in the first place.  This is all AFTER we have assumed (“miracle happens here”) that we can find, design, or invent strong-enough tie-back “strings” or cables, and tie-back attachment methods.  All of this combined looks bleak to me!  Please correct me if I’m wrong, of course.

Parenthetically, let’s add an afterthought here:  Punch more holes in the outer vessel walls, all around, and add washers on the outsides of the tie-back attachment spots.  Scatter MANY-MANY smaller tie-backs all around, to distribute the (must-be-balanced) tie-back forces all around, better, and more evenly distributed.  Including holes in the glass domes!  Yes, that’s a “fix”!  Pretty absurd though, isn’t it?  Yes, I thought so, too!

Our efforts to contain all of those pressure-bars have failed, it seems!  My (our) design is the worstest, bar none!  We’ve set the bar VERY low, butt I’ve failed to squeeze my butt in there, below the limbo-bar!  So now I’ll go to the bar, to drink away my sorrows, and hum a few bars of a sad, sad, song!  “My wife left me, my mule got lame, lost my money in a poker game, my tractor broke, and my gas molecules escaped!”  HOW could it get ANY worse?

            Well, it COULD get worse if we give up!  Never retreat, never surrender!  Let’s march on, and tackle less radical, but more plausible, methods of storing a LOT of pressurized gasses in a relatively small (space-saving, but still large) volume.  Hopefully with good heat-shedding capabilities, too.

 

Nested (Matryoshka or “Russian Dolls”) COPVs

 

I did an internet search for this, and all that I came up with is this: https://www.researchgate.net/publication/237562185_Conceptual_Design_of_Space_Efficient_Tanks .  This isn’t of much help to me, for what I’m thinking of.  So here’s a starting drawing for “nested COPVs” as I envision them for our use here.

 

Figure #7

 

The heavy vertical black bar at the left represents the “ports end” of the nested COPVs, and will be detailed further below.  But the basic idea here is that the innermost COPV has the highest gas pressure, with gas pressures taking steps lower and lower as we work towards the outermost walls.  This means that we’ll need to have a reasonably “smart” gas-pressure-regulating system, to always keep it that way, as the tanks are loaded and unloaded.  Well, the one exception will be when all the tanks are empty, or at ambient pressures.

The above drawing is “conceptual only”.  In reality, the tank-ends to the right would be far more tapered (rounded, not flat-faced).  The 300 bar, 150 bar, and 50 bar pressures are just made-up numbers.  The real numbers (pressures that could be attained) will vary, depending on MANY variables.  The basic ideas, though, are that by step-wise graduating these pressures, the innermost tanks will NOT need to be as strong-walled as they’d otherwise have to be, since they’re being “squeezed on” by the next layer (next, further-out tank).  The very outermost tank will have low pressure (but hopefully still-useful pressure), without ridiculously thick walls, of course, because of what we discussed way-way above, associated with Figure #1.  With a decent design, we’ll still contain a LOT of gas in a relatively small (but still large) volume.  Who knows, outermost tank diameter here might reach 6, 8, or even 10 feet.  The LENGTHS of the tanks could be quite long!  That’s yet another “conceptual drawing only” aspect to the above…  The tanks are shown as short and squat, but should be MUCH longer, for the purposes that I envision for them.

A fairly clear disadvantage of the above design is, how well (how quickly) can we dissipate heat created by filling it with what WAS recently ambient-temperature and ambient-pressure gas, but has now (very recently) been compressed (pressurized)?  As we all know, the high-pressure gas will now unavoidably be HOT!  The over-wrap will be a fairly poor heat conductor, and we now have several layers of said over-wrap.  We can improve the thermal conductivity of the over-wrap, perhaps.  https://www.researchgate.net/publication/344763745_Surface_Pretreatment_and_Fabrication_Technology_of_Braided_Carbon_Fiber_Rope_Aluminum_Matrix_Composite deserves to be mentioned here, along those lines.  I cited this previously, far above, where I mentioned thermal conductivity of the over-wrap in only slightly more detail.  Use the above link (or segment of it) for a search-string, if desired.

            Getting rid of the excess heat more quickly could be done at the price of more complexity, and reduced storage volume.  We could, for example, route cold fluids into there and back out (through more access ports, and internal metal pipes).  This could be ambient air, or it could be super-cold liquids that need to be heated anyway, in the same sense as we often see liquid hydrogen or liquid methane piped around the outside of a rocket nozzle (before being burned).  Think “internal-to-the-tank” heat exchanger.  This whole idea would be too complex, and even dangerous, IMHO.  But I thought I’d mention it anyway!

            The ports (access) ends of the nested tanks are located at the far left (heavy black bar) end in the drawing above.  They’ll be made of metal, and so there, at least, we’ll be able to shed some serious heat.  Maybe we’d need to add some finned aluminum heat sinks there, wherever we can find space.  Or we could even add Peltier devices there; see https://en.wikipedia.org/wiki/Thermoelectric_cooling ... But please be advised that these need lots of electrical power (currents).

            This entire idea (nested COPVs) brings to mind the classic difficulties in building a “model ship in a bottle”.  A difficult task, yes, but it’s not impossible.  We’ll have to keep that in mind, and NOT design something that is IMPOSSIBLE to build!  “Ability to assemble it” can’t be ignored!  Which brings to mind, the “puff balls” in the above drawing.  These would need to be fastened (glued?) to the metal liner of the outer tank (in a given gap-layer), not the overwrap layer of the inner tank, with the design of the access-ports end that I have in mind, for example.

            The “puff balls” are there to prevent shock and vibrations from causing the unsupported ends (far right above) of the internal tanks from rattling around too much, which could play havoc with the integrity of the left (ports) ends of the tanks (where tanks are mounted to ports-caps).  “Resonant frequency” shaking would be especially troublesome.  The core of a “puff ball” might be built out of thin aluminum, in short pipes (cylinders).  That’s not the best choice, IMHO, because sharp edges there (at pipe-ends) could scratch the over-wrap of the next inner (already-wrapped) tank as the inner tank is installed.  Better would be a highly perforated aluminum (or other) sphere.  Think “wiffle ball”; see https://www.forelle.com/baseball-softball/balls/training-balls/benson-wiffle-plastic-baseball-white/1272/ , for example.  In just the right spots on the outside of the wiffle ball, we now add (glue?) a soft, sponge-like material, such as https://www.researchgate.net/publication/265606808_Strong_lightweight_and_recoverable_three-dimensional_ceramic_nanolattices , which was previously mentioned here.  If this is readily available, and not too expensive, it would (will?) be an EXCELLENT choice for outer surfaces of our “puff balls”!  Run some computational simulations, and fasten (glue?) these puff-balls in the best locations (probably close to the far-right end as is shown above).  We’ll highly likely not need as many of them as are shown in the above conceptual-only drawing, by the way.

            The only other words of wisdom that I have concerning these “puff balls” are as follows:  During the manufacturing process, he or she who supervises the proper placement of these puff balls should be known as the “Puff Daddy” or the “Puff Mommy” At Large and In Charge!

 

Figure #8

 

So here are our “nested end-caps” with access ports.  Variations are clearly possible, but I present only the one design that I think is best.  No variations that I can think of, make much more sense to me.  I chose to transition from aluminum (for most of the long, long length for our application, chosen for low weight) to steel, for strength, and the ability to hold a good thread.  Note that where there are paired bolts (in two cases, one for mating aluminum to steel, and one for mating steel “collar” to steel “port-cap”), I SHOW them as being side-by-side, but in reality, they should “walk” around the circumference of the cylinder…  Alternating closer to the port-end, then further away, closer, and back and forth, for spreading the mating-forces of the bolts.  Also note that gaskets (in red) are shown only where we can get them in there, in the first place.  We can’t do that on force-fitted surfaces.

So the order of assembly should look something like this (ignoring the puff-balls, since they’ve already been discussed, and aren’t shown above): Start with the innermost COPV.  Force-fit the aluminum and steel collar together first.  Note that the aluminum is thickened there for good strength.  Now bolt the aluminum and steel collar together.  If needed, the holes here could be drilled AFTER the force-fit, for properly aligned holes.  We have full access, so nuts, bolts, and washers could be used.

            Next, force-fit the steel collar onto the port-cap.  Drill and tap (thread) the holes AFTER the force-fit is probably (definitely?) best, for proper alignment.  Note that recessed holes have been left for the bolt-heads here, PLUS clearance for a driver socket.  Note that all bolt-heads are recessed, in order not to hinder the overwrapping process.

Search for search-string “ceramic paper” far above for two possible candidates (materials sources) for the following: Optionally, to help leak-proof the whole assembly, apply some glue to one side of “ceramic paper”, and turn it into tape.  Now “tape up” the bolt-heads around the entire circumferences on the outsides of where bolts join aluminum to steel, and steel collar to port-cap, and nearby areas (especially over the ends of the gaskets that join metal to metal).  This tape will next be forcibly retained there by the overwrap.  If the overwrap is very porous (prone to leakage), this tape made of “ceramic paper” should help prevent leaks.

            Next, over-wrap the whole assembly. I’m not an expert on how to do this, but I’m not aware of anything being called for, here, being impossible.  Performing the wrap close to the threads in the port-cap might present troubles, since all of the wraps here are forced to run parallel to the port-cap surface, and no wraps (with any significant width to the wrap) can be wrapped diagonally.  After the over-wrap is done, it will need to be cured.  Glued and heated, as I understand.

            I hope that the above drawing is enough.  If not, please let me know.  There’s an inner thread shown (twice) for the innermost COPV cylinder, and the next-moving-outwards (much longer in circumference) thread is shown only once.  The innermost cylinder is inserted into the next-out cylinder, and then one torques (threads) the two together, for a tight seal.  Protrusions or handles (not shown) will need to be added to the outside surfaces of the port-caps in order to apply the torque forces needed for this application.

            Let’s address potential leaks once more.  If leaks are a problem (test them by pressurizing each tank after curing the overwrap), then there are additional steps that we might take.  Reach inside the tank with a paintbrush or other applicator (with gloved hands or with specially created tools), and apply paint (perhaps epoxy-based paint) or epoxy (or other) glue, where metal meets metal (at the aluminum-to-steel joint and the collar-to-ports-cap in the designs as shown above).  Fairly quickly, before the paint or glue cures, close and pressurize the vessel.  Spin the vessel slowly to keep the paint or glue from pooling at the bottom, and look for leaks.  Leaks will be indicated by the paint or glue coming “spitting out” under pressure.  Note where the leaks are, and give it some curing time (also maybe use epoxy that cures in, say, 1 hour).  Open the vessel back up, and perhaps take a heat gun (or other such device) and add some more speed-curing heat-work time.  Repeat the process till leaks are gone.

            With the design as is shown above, the internally-applied-glue leak-proofing method won’t be practical, for the “model ship in a bottle”-type troubles, for the very innermost of the nested cylinders.  Unless we add threading to that very innermost cylinder port-cap, just for these kinds of purposes.  I do NOT think that this would be worthwhile, since slow leakage from the innermost cylinder will be caught by the next-out cylinder anyway.

            Is the above drawing (design) optimal?  What are the easiest, cheapest, and best ways to manufacture and assemble the parts?  Would we be better off integrating the steel “collar” and the steel “port-cap” into one solid piece?  I don’t know…  That would involve fewer assembly steps and fewer bolts.  It could be done with obvious adjustments having to be made.  How bad would our leaks (pressure losses) be?  Would we be better off bringing the aluminum all the way up to the steel port-cap, and bolting it together there?  What about thermal expansion mis-matches between types of metal?  I would think that the design as illustrated above distributes those (deformations from the thermal expansion mis-matches) fairly well, but I’m not an expert here.

            Would our threads (in the steel or other-metal port-caps) warp and distort under heat?  To include the heat of curing the overwrap?  Would we be best off pre-assembling these threaded concentric port-caps, so as to keep them stable during over-wrap curing?  Could we complete ALL of the over-wrapping, on all concentric COPVs, before curing them?  If we did this, obviously we’d want to remove the port-plugs themselves, I would think, during curing, so that the being-cured overwraps can “breathe” during curing.  Also, during repeated cycles of use, will these threads eventually permanently lock?  Or could we pull the assembly back apart, after prolonged use, for inspection, repair, or parts replacement?  I have many more questions than I have answers here, clearly…

            I’ve not addressed the ports themselves in detail, at all.  I’m thinking that this part of pressure-vessel design is well understood, and that I have no ideas to add here.  A paper can only get so long!  But if I’m missing something significant here, please let me know.

            Note also that the above drawing implies that the port-plate (end cap) for the highest-pressure (arbitrarily set to 300 bar) innermost COPV is set to the same thickness as the next-out port-cap, which holds (again arbitrary) 150 bar.  If this design were to be fully fleshed out and-or computationally simulated, the port-plates could be made less thick as we move outwards, for mass reductions.

 

Nested Gasbags

 

Next, let’s explore nested gasbags.  Internet research first…  Sierra Nevada makes inflatable habitats; see https://www.space.com/sierra-nevada-inflatable-habitat-moon-gateway.html , which says “The outside of the prototype LIFE habitat is comprised of a urethane pressure bladder, a nylon liner and a woven Vectran fabric restraint layer.”  Bigelow Aerospace actually deployed an inflatable habitat to the ISS; see https://bigelowaerospace.com/pages/beam/ for example.  Bigelow is currently out of business.  NASA had/has projects variously named HIAD and LOFTID, which are also relevant.  https://www.nasa.gov/feature/nasa-inflatable-heat-shield-finds-strength-in-flexibility says “The rings are woven from a synthetic polymer that is, by weight, 10 times stronger than steel”  Well OK…  What IS the polymer, and how much pressure can it hold?  https://core.ac.uk/download/pdf/76424956.pdf says “A pressure of 2 psi for the outer torus provides sufficient structural stiffness for the aerodynamic loads near the shoulder. The rest of the stack would likely be maintained between 15 and 20 psi which has been demonstrated to carry the required peak aerodynamic loads from previous HIAD tests.”  So with these kinds of pressures, we won’t hold much gas in a smaller volume, as we can with a COPV.

See also https://www.nasa.gov/mission_pages/tdm/loftid/index.html and https://www.nasa.gov/offices/oct/game_changing_technology/game_changing_development/HIAD/irve3-success.html , which says that HIAD uses Kevlar airbags.

            So what would nested airbags (gasbags) look like?  Has anyone designed any, built any, or patented any?  In a casual internet search, I didn’t find any descriptions of nested gasbags, with multiple concentric pressurized areas.  So, keeping in mind that one of my primary missions here is to prevent “patent trolling”, I want to “babble at the keyboard” some more, about this idea.  Parenthetically, this may be a good place to mention this: A funny (funny IMHO, at the very least) web page about “pre-emptive defensive publishing” can be found at http://www.rocketslinger.com/Near_Universal_Defensive_Publication/ .

            A nested gasbag would contain MUCH lower pressures than a good COPV, and the ports (access spots) could then be MUCH larger (as in inflatable habitats; the ports can be large enough for people and gear to move in and out).  Conceptually, we can take Figure #8 (above) and repeat it and modify it, and we still (on a highly similar basis) can build a multi-port ports-cap, with higher pressures inside, and lower pressures outside.  The COPV walls (of aluminum and overwrap) get replaced by flexible bags (of Kevlar, etc.).  We have no need for “puff balls” any more, and no need for heat (and glue) and a curing process for the overwrap.  The steel collar can go, and we very tightly wrap cable (or a hose-clamp) around each airbag, into indentations circumscribed all around concentric built-up “shoulders” on the inner side of the ports-cap.  Concentric airbags have to be secured starting from the inside and working outwards, of course.  Concentric threads on the ports-cap are no longer needed, either.  Those were added above in order to facilitate nested assembly at the highest level, WITHOUT knocking holes in the over-wrap, in order to secure (bolt) already-overwrapped areas to the ports-cap!  THAT would be a horrible idea, IMHO, because it would destroy the pressure-holding attributes of the overwrap!

            But back, now, to low-pressure nested gasbags…

 

Figure #9

 

Some of the following will be obvious, but let me write them down anyway.  The above is “conceptual only”, not to scale, and so on.  The LOW pressure figures (in PSI this time) are again made up.  The same logic applies as before, though…  The “need thick walls” problems with large vessels (details associated with Figure #1 far above) can again be “cheated on” a bit, with nested walls and graduated (stepped) pressures.  The one large hole at the central gas-bag (access port) could be large.  So could the next one moving outwards, to the next gas-bag!  The arrangement needn’t be concentric circles (where the hose-clamp secures the gasbags); they could be concentric ovals or egg-shapes…  Or they could be circles.  Ovals would enable each layer having one large port-hole per layer.  Circles would enable adding many smaller port-holes arranged in higher and higher numbers of ports, per each circle moving outwards.  These kinds of things can be adjusted per our application.

In passing, note this:  See https://www.kimballmidwest.com/50764 , which shows a spring-loaded hose clamp, with spring loading on the bolt.  As the cited link says, this “Automatically compensates for thermal expansion of hose or fitting connections.”  Simple but clever, eh?  Maybe we could use these, or something similar…

What are possible applications for this?  Brainstorming goes here…  Multiple different air pressures tightly spaced together for experiments in low gravity comes to mind.  Boeing uses airbags for landing its “Starliner” capsule; see https://phys.org/news/2022-05-boeing-capsule-earth-space-shakedown.html .  If we wanted a REALLY soft landing for VERY delicate cargo, we could use a multi-lined multi-pressure airbag.  The outermost bag hits first, and a pressure-relief valve (or several) pop loose on landing, bleeding out this air.  Then ditto for more bags working inwards.  The inner bags gas relief would also have to work its way outwards in a “baffled” manner, through outer layer relief valves, yielding a VERY soft landing.  This could be done with entire capsules, or with smaller “return to Earth” packages.  We could possibly even save on the mass of the gas needed to inflate our airbags, by “popping our chutes” (parachutes) as early as possible, and gathering “free ambient air” to compress and use, as we descend.  Astron’s “Omega 1” engine (modified to add an air compressor for us) is supposed to work at high altitudes (just HOW high, I wasn’t able to find out).  It would be an excellent fit for this!  Search for “Omega 1” in this document.

            We already know that inflatable airbags can make space habitats.  With a multi-layered approach, we could (for examples) fill the outermost between-bags layer with epoxy (or some other glue or concrete, mooncrete, asteroid-crete, or other in-situ or low-gravity-nearby-area scrounged-materials-based) filler which will harden, yielding additional strength to ward off micro-meteor hits.  A layer of water could be added to protect against radiation.  A simple layer of gas will provide thermal insulation.  Am I missing anything significant?  Maybe…

            If one is worried about distortion in internal space-layers between the layers of our air-tight bags (Kevlar or other-material-based), as we fill them, then think of your mattress, with internal tie-back strings and button-dimples (buttons being “washers”, if your will, like on bolts), holding layers in restraint, preventing too much bulging.  We can do the same thing here, of course.

            In a low-gravity environment (like the Moon), gravity may still be strong enough to fight us, as we fill the outermost gap with mooncrete, for example.  For solving this problem, we could stage the filling and curing steps.  Tip the ports end up a bit, with the whole multi-bag structure laid on its side, and with the inner bags inflated with air.  Fill only the BOTTOM of the outer bag with mooncrete, let it cure, and then roll it a bit.  (Add air to the rest of the outermost gap-area, after adding the liquid mooncrete, to enforce the proper overall shape, and remove this air, or most of it, before the next mooncrete-adding step.  Otherwise you’ll have to fight air pressure in order to inject the liquid mooncrete, which MIGHT also be a valid choice).  Repeat till done around the entire periphery and bottom of this outer bag.  This is an example where concentric rings (not ovals) with many port-holes would be best.  One may or may not want (as a last step) to roll the entire assembly to place the ports-end straight up, and mooncrete-fill this end as well.  Oh, and we could also build a conforming-shaped “cradle” to hold our bags in place against gravity, on the bottom, to reduce deformations, and therefor, reduce the number of mooncrete-filling steps.  Build one cradle for building many mooncrete domes!

            When all filled and cured, it will be heavy, even in moon-gravity!  We’ll need a lifting crane or equivalent, and grappling spots somehow added to the outermost bag.  I may be getting carried away by now, but there ARE potential applications here, for a nested gas-bag…

            Let’s get back to “poor man’s air-breathing vehicles”, now, shall we?  Using COPVs?

 

A Plain Bundle of Skinny COPVs

 

So glass domes or arches of borosilicate glass do NOT sound like a wise idea for inclusion in high-pressure pressure vessels, and nested COPVs might work, for “poor man’s air-breathing vehicles” (with shedding of the heat of compressed air being a possibly serious problem).  Low-pressure nested gasbags could be useful for other purposes, but won’t hold much compressed air for our vehicles (and their large sizes would be impediments as well, catching a lot of air resistance).  What else do we have?

We have a plain old bundle of conventional COPVs!  As previously mentioned, https://static1.squarespace.com/static/5ca8e523ebfc7fd95c0c4fc1/t/5fb2cbbe44ad9171878b4010/1605553092670/COPV_Design_Report.pdf (see for example Figure 4.18 page 93 there) tells us that long and skinny shapes are just fine for us.  And that’s the skinny here, folks!

So we could just tie a large bundle of these together, and tie the bundle, missile-style, slung on the bottom of the wing of an aircraft, for example.  Such COPVs would be far less “bespoke” (customized) for a specialized use, here.  And air-breathing aircraft for rocket launching-pads are few and far between!  We have to watch our custom-engineering costs budgets, of course.

We may or may not need to protect (or protect very intensely) the surfaces of our long-and-skinny COPVs, from the forces of air whipping around our vehicles.  We want SOME air for cooling, but not so much that it tears our COPVs apart.  So tie together our COPVs with a shell or some cables… Whatever works best, in the balance.

The below (perhaps fairly obvious?) end-view of our bundle is probably not needed for many readers, but is easy enough to generate, so here it is:

 

Figure #10

 

Working inwards, the “over-over-wrap” label (in purple) should be considered “tongue in cheek”.  True (classical as understood in COPVs) over-wrapping here would be difficult and would make maintenance access become a nightmare.  The purple shape is (implied by the drawing to be) MUCH heavier here than it really deserves to be (it implies a thicker, stronger layer than what is most likely to be needed).  Optimally, it might best be two bolted-together aluminum half-shells.  Also likely to be perforated in a compromise between protecting the COPVs from the “wind” of ambient air as this assembly is slung under the wing of an aircraft…  Cruising speed of and an airliner is 550 mph or so… And the ability of the COPVs to shed heat.  The “over-over-wrap” might be as little as some (steel?) cables to hold the COPVs (plus “compliance shapes”) together, in a light… Over-over wrap!  Yes, that’s it!  But it could be made out of ANY suitable material.  Fiberglass or some sort of composites, perhaps.

“Any suitable materials” will likely work for the “compliance shapes” also, as shown above.  They will be hollow for cooling airflow, and likely perforated, like a wiffle ball, for mass reduction.  They may not even need to be contiguous… They could be segmented, with gaps in between them, as we “travel down into the paper” view above.  One will need to secure them to prevent them from “sloshing” or sliding back and forth, of course.  With the complex shapes being called for, here, these might be PRIME candidates for being 3-D printed.

Many weird-triangle shapes (not shown) of “compliance shapes” could also be added between the COPVs (shown in blue and brown above).  Adding these (many, and much smaller) “compliance shapes” would be overkill, IMHO.

The top-most “compliance shape” could be much “beefed up” (compared to the other ones) for facilitating under-wing structural mounting of the entire “bundle” assembly.

 

Using These COPVs in an Aircraft

 

So how could we use large-capacity COPVs in an aircraft, especially in an aircraft used to launch rockets into space and-or orbit?  We could pre-load the COPVs with oxygen (on the runway before takeoff).  Oxygen could be as pure or as impure as desired, and it could be pressurized and cooled gas, or it could be liquid oxygen, per other design parameters, as desired.  Now I know that custom development is expensive, and the market here isn’t very large, yet.  But perhaps it will become larger.  Military or natural (think exceptionally severe solar storms) disasters can wipe out many satellites, and then there will be a quick push to replenish the in-orbit satellites quickly.  So, development costs CAN be justified here.

This paper is getting long, so I’ll try not to meander off too far.  High-speed SCRAM and RAM jets are most often what people think of when they hear of “air-breathing rockets and jets”, and I can find next to nothing about slower-speed jets and rockets burning compressed air.  I’ll not babble about these things, or about turbo-pumps, or provide you with links, about these things.

But suppose we spent the money to develop some jet and-or rocket engines to make a Stratolaunch-style launcher that doesn’t come equipped just with plain old jet engines stolen from an airliner design.  We add jet engines (ideally just jet engines, but perhaps with separate rocket engines added as well) which can burn supplemental oxygen OR extra, added compressed air from our on-board COPVs.  Oxygen can be injected (via turbo-pumps or simply by high stored pressure) to jet engine combustion chambers and-or afterburners.  This would get us up off of the (shortened) runway QUICKLY!  Deplete most of your extra (COPV-carried) oxygen, and be quickly on your way!

Then while up in the air for a while… Long travel distances may often be required anyway, to get you to an ideal rocket-launching spot, with good weather, over the ocean for safety, closer to the equator for less rocket-launching-energy required, etc. … While en route, fire up your compressors, and re-fill the COPVs with compressed air, and let them (the COPVs) cool down as you fill them up.  Astron “Omega 1” engines modified to add an integrated compressor has already been mentioned here.

Then when your launcher-aircraft is getting towards the top of its normal-mode operational “service ceiling”, you bleed off your compressed air COPVs, and feed the gas to your engines (jets alone, or maybe with a custom-designed extra set of wing-mounted rocket engines), along with some extra fuel, and you exceed your normal “service ceiling” as best as you can, as best makes sense.  THEN you launch your rocket(s) and satellites.  That’s about it!

Some subset of your COPVs could carry fuel instead of oxygen.  This would make little sense to me, since normal (petrol or petrol-like) flavors of jet fuel don’t require pressurization.  But if we wanted to carry fuel (methane or hydrogen, for example) this way, to help us get off of the runway quickly, this might be possible (sensible is another question).  We COULD even carry compressed inert gas (nitrogen for example) in separate tanks, and run the inert gas through the emptied fuel-gas tanks, to purge them of combustibles.  All that (“trash gas”) could be dumped to the engines, even though it won’t burn well, for extra reaction mass.  HOW MUCH inert purging gas would we need to run through there to make the tanks safe for compressed air?  I sure don’t know.  But afterwards, we’d now have some more storage room for compressed air, in what were the fuel-gas and inert-gas tanks.  Possible?  Sure!  Sensible?  Maybe…  I just thought that it was worth mentioning, is all.  Fend off the patent trolls!

To be sure, if we DID tackle the above design idea, we’d have to add port accesses to BOTH ends of any COPVs that are going to carry fuel, then be purged with inert gas, and then carry compressed air.  Figure #7 shows access ports at only ONE end of the COPV.  We need to add mass and complexity for ports on both ends, if we want end-to-end flow for the purging process.  Otherwise, the purging process become unworkably inefficient.  Anti-shock-and-vibe internal “puff balls” wouldn’t be needed if we added ports to both ends, most likely.

Keeping in mind that one of my prime missions here is fending off “patent trolls”, I mention implausible ideas from time to time, in case they ever do become plausible.  We COULD take the Aston Omega-1-style engine, and add another stage to it, similar to what it has now, where a chamber compresses air, and then injects fuel into the compressed hot air, to start an explosion… And then the explosion is simply vented to the outside world!  The explosion isn’t used to extract rotational work, but rather, expelled through a small rocket-style nozzle.  For propulsion, of course.  This would be similar (as I understand it) to the NAZI “buzz bomb” of WW II.  I don’t see much use for doing this.  But there it is… For whatever it is worth!

 

 

Using These COPVs in a Rocket

 

This wouldn’t make much sense to me.  Rockets are already well optimized.  If we wanted to make a booster (strap-on or otherwise) using compressed air, a current-style jet engine would do, already, so long as the thrust-to-weight ratio of the added jet assembly is significantly larger than “one”.  That complexity isn’t worth it, for the short duration of time that a rocket spends in air that is sufficiently dense (and low-speed enough with respect to the rocket) for this to yield much of a pay-off.  The same is true of any “Robe Goldbergian” scheme of adding any sort of compressors, air-storing COPVs, or drop tanks.  We should also be thinking about the “cube square law”, see https://en.wikipedia.org/wiki/Square%E2%80%93cube_law# ), which tells us that a few LARGE tanks (not many small ones tacked together) will generally carry more fuel and oxidizer per money and mass spend building the surfaces of the tanks.  This is true of both COPVs and of low-pressure tanks.  Strap-on boosters aren’t really optimal.  Space X is smart to design the “Starship” plus booster the way that they’ve already designed it (without side-strapped boosters).

For any reader trying to research such things in more depth… Good luck to you!  The following might help:  As previously mentioned, https://static1.squarespace.com/static/5ca8e523ebfc7fd95c0c4fc1/t/5fb2cbbe44ad9171878b4010/1605553092670/COPV_Design_Report.pdf Table 2.1 shows space-travel related (rocket related) parametric profiles of interest to COPV designers… It also shows internal “common bulkhead” designs for ONE COPV tank to contain TWO kinds of fluids, and anti-slosh internal floats for liquids v/s gasses, and at the bottom it shows that a LONG (skinny) cylinder is a practical shape for a COPV.

            Now finally, I want to show a table to help us all think about just HOW quickly a rocket gains altitude and speed.  Any strap-on (or other) assistant booster or drop tank has to deal with ALL of these kinds of conditions, as well as adding to your “thrust to weight ratio”, in order to make the effort worthwhile.  It is a TOUGH problem!  I have no worthwhile “fixes” here at this time…

 

Time v/s speed profile for a rocket…  

https://www.nasa.gov/pdf/466711main_AP_ST_ShuttleAscent.pdf has one for a space shuttle… At table #1…  Translated here by Yours Truly…

Time             Altitude                        Speed

(s)                         feet                        MPH

0                               0             0

20                      4,080                        311

40                    17,363                        667 (Mach 1 is 767 MPH)

60                    38,103                        970 (Mt Everest top 29 K ft)

80                    65,180                        1,534

100                 103,031                      2,298

120                 146,701                      2,865

140                 188,259                      3,076

160                 222,689                      3,338

180                 254,151                      3,660

200                 280,971                      4,032

220                 303,338                      4,449

240                 321,453                      4,908

260                 335,547                      5,414

280                 345,452                      5,938

300                 352,432                      6,530

320                 356,270                      7,175

340                 357,330                      7,876

360                 …  Fill in the rest for homework! Note that

380                 from here on in, altitude fluctuates for a

400                 while as we gain speed for orbit, instead

420                 of climbing, so much, any more.

440                

460                

480                

500                

520                

 

Table #1

 

Note, 40,000 feet airliner cruising altitude is reached in about 1 minute here…  The Space Shuttle was rather gentle for not pulling too many “Gees” during the launch profile.  Many other rockets will be more aggressive (faster).

 

I have no special expertise or any more plausible (or implausible) ideas concerning any associated matters here, so I will sign off at this time.  This concludes my ideas as of this time.  Comments or questions (or idea contributions) are welcomed at RocketSlinger@SBCGlobal.net …

 

Well, one last quick set of additional research notes:  Led Zeppelin has some massively rocking tunage, to include the “Misty Mountain Hop”, at https://www.youtube.com/watch?v=n6fBQRaygeo for the music video.  Lyrics at https://genius.com/Led-zeppelin-misty-mountain-hop-lyrics ... Towards the end, you will find these lines…

 

Folk down there really don't care, really don't care Don't care, really don't Which, which way the pressure lies

 

Well, I, for one, don’t want to challenge Led Zeppelin, or their music writers, in their domain, here.  I’ll “stay in my lane” for this.

HOWEVER, I would dare to say, for COPV designers, we really, really, must, MUST care, which, which way the pressure lies!

 

Stay tuned…  Talk to me!  RocketSlinger@SBCGlobal.net

 

Back to main site at  www.rocketslinger.com 

 

References

 

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Kim, Jae-Woo & Sauti, Godfrey & Cano, Roberto & Wincheski, Russell & Ratcliffe, James & Czabaj, Michael & Gardner, Nathaniel & Siochi, Emilie. (2016). Assessment of Carbon Nanotube Yarns as Reinforcement for Composite Overwrapped Pressure Vessels. Composites Part A: Applied Science and Manufacturing. 84. 10.1016/j.compositesa.2016.02.003.

 

Song, Yuzhu & Shi, Naike & Deng, Shiqing & Xing, Xianran & Chen, Jun. (2021). Negative thermal expansion in magnetic materials. Progress in Materials Science. 121. 100835. 10.1016/j.pmatsci.2021.100835.

 

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