This web page last updated 15 Sept 2021
Methods of Decelerating a Spacecraft Through Atmospheric Re-entry Using a Shuttlecock-like Design
Abstract / Pre-Summary
This sub-page to www.rocketslinger.com describes how payload fairings for an upper stage could be modified to remain attached to the satellite(s) deployment mechanism(s), and to one another, and also (optionally but highly desirable) to the entire interim upper stage (fuselage, fuel and oxidizer tanks, and rocket engines, etc.) as well. Whereas a commonly used design today involves the use of 2 (two) clamshell fairings, which are jettisoned before satellite deployment, the base design here envisions “N” fairing-segments, or “flower petals”, or simply “petals”. The petals can also be thought of as “feathers” in a feathered badminton shuttlecock. Each of 2 clamshell fairings are now subdivided (sliced) into smaller segments, with each shell-slice being vaguely reminiscent, not only of a flower petal, but also of the skin-slices of a sliced orange. For this document, “N” will be set to “4”, with one (marked) exception.
Each petal remains attached to the base of the assembly (ideally but not always, this assembly is the entire upper stage), at the base of the petal, by hydraulic actuators. During re-entry, the petals are sometimes more-retracted (less deployed) and sometimes more-deployed (less retracted), as one travels radially around the periphery of the bulk (center of mass) of the “shuttlecock”. “N” = 4 petals is enough to control the descent path (vector), as controlled by the hydraulic (or otherwise-powered) actuators. Unlike the grid fins used by Space X on Falcon 9, these petals will not rotate on an axis that skewers the centerline of the fuselage, but rather, will be retracted to be almost vertically parallel with the fuselage when fully retracted, and nearly perpendicular to the fuselage when fully deployed. This is the “degree of freedom” which is needed already anyway, to protect the payload on ascent (“flower” is in the “bud” stage; payload = pollen not yet deployed, so to speak). The “bud” must turn into a “flower” to deliver the payload.
“Petal control” is a bit complex, but highly important! Not only are the petals adjusted to control the descent path, but also, from one side of the center of mass (of the fuselage or capsule) to the other, the deployed petals are more-deployed on one side than on the other. This lop-sided deployment slowly spins around the center of mass. Thus, the (usually cylindrical) center of mass tilts, and does a slow “rotisserie roll” to spread the heat of re-entry across all of its surfaces. Each petal also gets to spend some time partially protected from the highest re-entry heat as well. A given fuselage-surface (or petal) element will slowly alternate time in the heat v/s time in the relatively sheltered slipstream. The probably-ideal flight path might best be described as a corkscrew. This will prolong the travel distance and time, providing more time to shed heat.
Solid petals would gather too much force from the hot air or plasma during re-entry, requiring excessive mass (for both petals and actuators) for strength, to avoid being sheared off. This is why Space X uses “grid fins” rather than planar solids. But we need solids to fully protect the payload during ascent. “Having your cake and eating it too” is obtained here as follows: The petals are manufactured as grids, but are filled in (grid-holes plugged) by optimized plastics or ceramics (or other lower-melting-or-burning-point materials) formulated to melt, burn, and-or fall away during the heat of re-entry. This means more work for refurbing the petals before re-using them (if they are re-used at all), but this is a small price to pay for recycling an entire upper stage.
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 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. Here is a random gathering of a few web sites which one can use to familiarize oneself with some of the basics, or, of the current state of the art, in common practice today: Start here, perhaps, with https://finance.yahoo.com/news/stoke-space-stakes-claim-launch-202145047.html “Stoke Space stakes its claim in the launch industry’s rush to fully reusable rockets”. This is a sample of a potential user (or target audience) for the design ideas here listed. Another one (skimpy root page here) is https://www.rocketlabusa.com/rockets/neutron/ (another potential user).
See the following also: Ruag Space is a leading manufacturer of fairings. See https://www.ruag.com/en/products-services/space/launchers/launcher-fairings-structures . Then the below are simply listed, without any comments from the author. https://en.wikipedia.org/wiki/Payload_fairing . Also http://sicsa.egr.uh.edu/sites/sicsa/files/files/projects/ices-2019-113-leonardo-guzman.pdf “Payload Fairing Geometries as Space Stations with Flexible “Plug and Play” Rack System”. Also https://apmonitor.com/do/uploads/Main/Report_rocket_landing2019.pdf and https://en.wikipedia.org/wiki/Grid_fin. Also https://www.spaceandscience.fr/en/blog/grid-fins#:~:text=Falcon%209's%20first%20stage%20is,ascent%20and%20deployed%20during%20reentry.&text=Grids%20Fins%20acts%20like%20classic,can%20find%20on%20an%20airplane.
Now Dear Reader, please excuse me as I often slip away from the stilted use of third-person writing. I will much more-so 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 interested party in what is described here. Let’s get ON with it!
As here envisioned here, the relative orientation of the upper stage / payload deployment (payload delivery) device will stay fairly constant (ignoring a bit of tilt in varying directions), through launch, ascent, and descent. The top will stay the top, and the bottom will stay the bottom. “Bottom” = aft end on ascent (directional-of-travel-wise), but = the “fore” end on descent, and vice versa. So, to reduce confusion, from here on in, “top” = “top” (or tip(s)), and “bottom” = “bottom” (or base). Clear enough?
Applying to all of the below, if I haven’t provided enough drawings to clarify what I have written about, please email me at RocketSlinger@SBCGlobal.net. I can always provide more drawings for clarification. Generally, more-plausible design variations will be listed first, and less-plausible variations will be listed last.
OK then, let’s REALLY get ON with it, now!
It is possible for the four “petals” (AKA orange-slice skins-segments) to be sharp-pointed, and meet each other at the very top-most tips. I judge this to be HIGHLY impractical! On ascent, these tips would be highly stressed (buffeted) by prevailing winds, and hard to keep secured to one another. On descent, the petals (to include sharp petal-tips) would be far longer than is needed. Examine (for example) the relatively stubby (short) grid-fins on Space X’s Falcon 9, Falcon Heavy elements, and now, also the “Starship’s” booster. As we turn the “petals” into “feathers” for a feathered shuttlecock (AKA into grid fins), so to speak, these petals need to be (usually fairly largely) shortened anyway.
So the first drawing (below) shows a top-down view of the tips of the petals, with a large carve-out for what will here be called a “discard tip” of the fairings (AKA “flower bud”). The “discard tip” most likely will be cast away (discarded entirely, and not recycled in any way), in this design.
If Figure #1 (above) can be called the flower “bud” before the flower opens, then the next drawing shows the flower after it has opened, and the payload has been deployed. For simplicity, no satellite(s) deployment mechanism(s) is/are shown. Note, however, that per one design option, the “discard tip” is shown as being attached (via a tether) to the tip of one of the petals. The discard tip could be equipped with a cold-gas (or other) thruster(s) to ensure that it is pulled out of the way during payload deployment.
Figure #2 (above) shows the discard tip as being attached (via a tether) to the tip of one of the petals. Some various options are available here:
‘A) Put a cold gas (or other) thruster on the discard tip, skipping the tether option, and maneuver the tip away, add a de-orbit burn, and trash it. A variation here is to add a 'Terminator Tape'… See https://www.space.com/space-junk-removal-terminator-tape-satellite-test.html “'Terminator Tape' did its job in space-junk test — and it will be back”. Presumably, the tether, if properly designed (with the bulk of it left attached to the discard tip), could double up as a “terminator tape”.
‘B) To save mass on the above-mentioned thruster, skip the thruster, and add the tether. Count on the heat of re-entry to burn away the tether during re-entry. This may be dangerous, in that the discard tip might “bang around” uncontrollably during the de-orbit burn, or early re-entry, damaging the petals or even the spacecraft. So a sub-option is to add a pyrotechnic (or other) cutting device to cut the tether after the de-orbit burn has at least started, to pull the discard tip out of orbit.
‘C) Do “B” above, but spool the tether out WAY long during the de-orbit burn, to vastly reduce the “banging around” hazard.
‘D) Don’t discard it at all… Bring it out of orbit separately, perhaps with a parachute descent at the tail end of the ride. I have no special ideas or comments to add to this item.
‘E) Be irresponsible, and leave the discard tip in a stable orbit! This isn’t at all recommended!
The above drawing shows a warning label about “Gaps!” left between the bases of the petals, and the center of mass (fuselage or re-entry capsule). The reason why, here, is that there’s no plausible way to make a strong hinge which is flexible. A hinge must have a straight-line actuation (rotation) axis. A single-point hinge (as where a straight line hits the outer rim of the circumference of the cylindrical fuselage at a tangent or point) is clearly an absurdity.
So this leaves us a few options, none of which are totally ideal.
‘A) At 4 points, for 4 powered hinges where petal-bases meet the fuselage, change the profile of both fuselage and petals to assume those straight lines, with these straight-line hinges entirely outside of the circular periphery of the cylindrical rocket body (fuselage). This adds manufacturing complexity. It also means that our fairings design has now partially changed to a “hammerhead” design (meaning that the girth of the fairing exceeds the girth of the rocket body). This means more aerodynamic drag (and aerodynamic instability) during ascent. This is probably still the best option.
‘B) At these same 4 points, add a complex powered mechanism to, first, shove the hinges up and-or away from the mating circle, and THEN splay the petals outward (with details left unspecified here). This is beyond my expertise! I will not begrudge you for your well-earned patent(s) for fleshing this out!
‘C) As a version of the above, have relatively simple powered hinges mate petals to the fuselage, with these hinges buried (or partially buried) inside the fuselage circumference. Comes open-up-the-flower-bud-time, these hinges are thrust straight outwards from the vertical centerline of the fuselage, on sliding rails (say, 2 per petal) or a sliding plate (1 per petal), well away from the fuselage, before the hinges are activated.
Note that options B and C above will allow (presumably superheated) air to flow between the petals and the fuselage (no sheltered “slipstreams” to be had there). The hinges or other actuators will need to be designed to withstand high-speed winds. For option “C”, the sliding plate should thus (most likely) be a grid rather than solid-planar.
‘D) Keep the cylindrical profile for the petals and fuselage (do not go to the partial “hammerhead” design). Keep the hinge-lines buried inside the fuselage. Before the hinges are activated, however, the bottoms of the petals (the parts that would otherwise bang into the fuselage when opening up the petals) are blasted away (discarded) by pyrotechnic device, or otherwise cut away. Complexity and-or debris left in orbit, are difficult problems here.
As usual, email me at RocketSlinger@SBCGlobal.net for more details or drawings if needed.
The “joints” where one petal meets another, or the fuselage (or capsule or base of the deployment stage), or the “discard tip”, can be made secure (airtight), and then broken free from one another, according to what I understand is standard industry practice today. That is, presumably use some sort of gasket material with some flexible “give” (compliance), for the air-sealing function. Mechanical rails and mechanical or electro-mechanical latches provide the retention-release functions. Pyrotechnic separation devices may also be used at selected locations (such as, at the base of the fairings).
It might be possible to add pressurized (inflatable) tubing in there as well, but that’s an implausible idea. Such tubes might be able to help suppress shock, vibration, and sound. When pressurized (inflated), the tubes would tend to push parts apart from each other, though, and so, retention latches would still be needed anyway. I will add no more details about this implausible idea at this time.
The below drawing shows (via cross-section) that, during re-entry, imbalance from one side to another, in degree of deployment (v/s retraction) of the petals will cause a “falling bias” or vector in the vehicle’s path. If we slowly roll this imbalance around the petals, the imbalance vector will travel in a circle, controlling both the direction of travel and which side of the vehicle (and petals) are most subjected to heat, alternating with being allowed to cool down a bit, on the lee side (in the sheltered slipstream). Thus, we do a slow “rotisserie roll” to distribute the heat-pain! We also have a method of flight-path control. Both can be done at the same time, if the planned flight path describes a cork-screw route. These 2 goals are complementary.
We COULD alternate between modes of path-control v/s “rotisserie roll”, and have the planned path be less-so a corkscrew, and more-so a straight line. This is far less desirable! One reason why is, what if one or more actuators (powered hinge or other) fails, or becomes crippled? The more-natural “corkscrew” path-method involves less stress on the actuators (is more resilient), and so, is more likely to attain mission success, than a more-straight-line-descent method, if one or more petals and-or actuators becomes compromised.
OK, so, then, finally the drawing:
The rotisserie roll will most likely cause the entire vehicle to spin a small amount. However, excessive spin is not likely to be a problem. Why? Because of the inertia of the ever-thicker air which the vehicle will descend through. This inertia will damp excessive spin. Another way to think about this would be, WHEN would be the only case where the preceding statement would NOT be true? Answer: When the vehicle is descending down the middle of the funnel of a tornado or other wind-storm! If I’m wrong, and spin IS a problem, then add more air-flow control (at the added costs of additional mass and complexity)… Such as the ability to rotate one or more petals, out at the tip perhaps, or, add a separate airflow-control fin.
As previously mentioned in the abstract, solid planar “petals” here would collect too much force from the flow of air or super-heated plasma. The is why Space X uses grid fins, not planar solids. “Beefing up” the petals and actuators to withstand such high forces would add probably-prohibitive mass penalties. So on our way down, we will want to convert our planar petals to short grid-fins. We could do this by one or both of two methods (or perhaps more, that I can’t think of, that may be possible or even plausible):
‘A) The core structure of the petal can be made of strong, durable material, with a metal being the only candidate that sounds plausible to me. The metal is arranged in a grid. The metallic grid is surrounded by fastened-in blocks, or surrounded by cast “sacrificial” material, which will burn, ablate, or fall away, early in the process of re-entry. Candidate “sacrificial” materials are plastics, fiber-and-epoxy material similar to “FR4” circuit-board materials, inflated small airbags, ceramics, or even wood (see https://arstechnica.com/science/2020/12/wooden-satellites-an-intriguing-idea-but-wont-solve-space-junk-problems/ ). As far as ceramics are concerned, see more details about ceramics formulated to absorb “heat work”, in a different propulsion context, at www.rocketslinger.com, where “ceramic turbine blades” are discussed. The ceramics-firing industry already has a handle on fairly precisely formulating “ceramic cones” to deform with varying amounts of “heat work” absorbed. See http://en.wikipedia.org/wiki/Pyrometric_cone. Other “sacrificial” materials may be plausible… And using a mix of different sacrificial materials may be possible as well.
‘B) It seems to me that this following idea is quite plausible: The metallic core-grid of the petals could be staggered materials. The innermost part could be made of (highly heat resistant) titanium. This portion is fastened (welded, bolted, etc.) to one or more less-heavy, less-heat-resistant metals travelling towards the tip. Stainless steel, less expensive plain steel, and even aluminum could be used. The further away from the fuselage (closer to the tip) that we go, the less durable the metal might become. The tip-most metal grids, then, can be regarded as being “sacrificial” as well. Aluminum tips could even catch fire during descent, without adding excessively great dangers, in some contexts.
Note that the petal actuators being very flexibly controllable, with respect to tilt, can compensate for large imbalances from one petal (or grid fin) to another, in the magnitude of airflow-force gathered. That is, where the degree of degradation in “sacrificial materials” (at a given instance in time) varies greatly, from one petal to another, we have plenty of control available, to compensate for that.
Putting all of that together, then, we could, for example, make grid fins out of titanium (innermost) bolted to aluminum (towards the tips). Then we could place the grid fins in large molds, and inject a suitably formulated plastic material all around the grid fins. Cure the plastic, pop them out of the molds, add the trimmings, and now we’re done!
A slight variation on the above would be to very deliberately plan on having the aluminum (or other less-robust metal) part of the grid fin separate away from the inner titanium (or other more-robust metal) part of the grid fin, at a reasonably-well-controlled time in the flight profile. This could be done with pyrotechnic devices, of course, but they might add unneeded extra cost, complexity, and danger. Perhaps “heat worked ceramics” fasteners could be devised… See the above mention of “ceramic cones”. Such a ceramic or partially ceramic fastener (perhaps made of both metals and ceramics) could replace pyrotechnic devices. The degree of control of “break timing” would not be as precise, but, as remarked above, precise control isn’t really needed in this case (petals actuators control makes precision break timing superfluous). The brittleness of ceramics does, however, present obstacles here.
The inner surfaces (upper or downwind surfaces during descent) of the petals could be lined with “terminator tapes” (see https://www.space.com/space-junk-removal-terminator-tape-satellite-test.html). As an alternate to such tape, artificial feathers (or any other plausible lightweight drag-adding devices) could be used. These would “grab” at passing air, to add drag. That would be at cross-purposes to having the petals turn into grid fins to reduce drag. However, in the very earliest phases of re-entry, adding these would PERHAPS make sense, in terms of reducing the amount of reaction mass expended during the de-orbit burn, and spreading more of the re-entry heat into the earliest part of re-entry. In any more-plausible scenario, the feathers or tape would tear or burn away later during re-entry.
In the not-very-plausible-at-all category, blocks of sacrificial material in the holes in the grid fins could be replaced by one-way air-flow gates (in the grid-gaps). See http://rocketslinger.com/Var_Cfg_Rck_Nzl/ Figure #2 and Figure #3 to envision such air-flow gates. In your mind, the “Direction of Exhaust Flow” arrows should be replaced by “Direction of Air Flow” arrows (when viewing those drawings). These one-way air-flow gates could be buried inside the sacrificial materials (which, of course, are burned away early, during re-entry heating). The “grid fins” approach is preserved during the time that the one-way flow gates are exposed after sacrificial material burn-off, by designing the one-way flow gates to pass air upwards, but not downwards, from one side of the petal to the other.
Now after entering the lower atmosphere, with way-excessive speed and heat having passed away already, the bases of the petals could be rotated 180 degrees (the ability to provide any of this rotation is otherwise pretty much totally unneeded). Now, the gates no longer permit upward airflow. By blocking upward airflow, they form a parachute function better than grid fins alone would provide. Such one-way airflow gates could replace SOME (not all) grid-fin holes, and still be useful. I consider this idea to be impractical but possible.
Solid Planar Petals Option
The below is fairly highly implausible, but is included in the interests of completeness. It is enough different from the above far-more-plausible scenario, that it deserves a fairly complete, separate description.
The above scenario envisions higher-pressure air being retained inside the fairings (“petals”, flower-bud-stage here) during ascent. This allows clean-room-quality air to be retained with the satellite(s), inside the fairings. Many satellites can be contaminated or damaged by dirty air. Clean air on the pressurized inside helps prevent dirty air from leaking in. On the other hand, higher-pressure air conducts noise (and shock and vibration, which, in air, are “noise” as well) more intensely. Rockets make a tremendous amount of noise, early on during launch, and this noise is, to a surprisingly large degree, a major hazard to delicate satellites. Ideally, to reduce this noise, we’d fill the air-filled voids inside the fairings with a vacuum… The better the vacuum, the less noise-energy that we conduct to our delicate payload!
A VERY serious, major downside (above and beyond the hazards of inward-leaking dirty air) for having an encapsulated vacuum here is that “nature abhors a vacuum”, and will squeeze our fairings, just as water pressure squeezes a deep-sea bathyscaphe. Like a bathyscaphe, our pressure-resisting fairings-walls will now have to become heavier in order to be stronger. The mass penalty on a rocket vessel will be a steep price to pay. The actuators will need to be “beefed up” as well, here, if the petals stay planar or partially planar.
However, for completeness, let’s consider some of the added ramifications. We could then have the stronger petals stay planar (solid) or partially planar for more of the descent. Regardless of whether we go completely planar, or partially planar and partially “grid fin” here, the planar petals or planar petal portions will add far more “shade from the heat” (compared to grid fins, which block very little of the airflow) for the other petals, which could at least spend some of their time in the heat-shielded leeward or slipstream-protected wake of the heat-shielding petals.
Whereas the first (more plausible design above) has 4 clamshell fairings or petals, this design variation here envisions “Set A” and “Set B” (each consisting of a low number “N”; from here on in set to “3” and “3” for simplicity, while also retaining re-entry-steering functions) of “flower petals”, or simply “petals”. The petals can also be thought of as “feathers” in a feathered badminton shuttlecock. Each of 2 clamshell fairings (in a currently-common design not involving actuated, attached “petals”) are now subdivided (sliced) into 3 segments, with each shell-slice being vaguely reminiscent, not only of a flower petal, but also of the skin-slices of a sliced orange.
Each petal remains attached to the base of the assembly by actuators, just as in the first-described design. We are moving from a nominal 4 to a nominal 6 total petals, so that “Set A” and “Set B” sets of 3 petals each, can spend time in the heat, v/s time in the shade (to shed heat), while alternating deployed v/s retracted, while 3-each deployed petals will still provide robust steering capabilities. During re-entry, the petals are interleaved, alternating deployed-retracted-deployed-retracted-deployed-retracted, as one travels radially around the periphery of the bulk (center of mass) of the “shuttlecock”.
The above “short and sweet” description should be enough. In case it is not, here below I repeat myself, in slightly different words. A deployed set of petals (3 of them) is enough to control the descent path (vector), as controlled by the actuators. Call that “Set A”, while “Set B” petals remains relatively retracted, out of the reach of at least some of the superheating air or plasma as “Set A” is currently exposed to. That is, “Set B” is currently cooling down, in the sheltered slipstream above the descending center of mass of the “shuttlecock”, plus deployed petals. After “Set A” petals become heated, they are retracted for a “cooling break”, while “Set B” is deployed, and takes its place in the heat. Sets A and B take turns, being heated and shedding heat, eliminating or much reducing the need for heat-shielding materials. With “N” set to 3 or higher, thorough steering control is preserved at all times.
“Petal control” is even more complex now, but highly important! Not only are sets A and B alternately deployed v/s retracted for heating v/s cooling breaks, but also, from one side of the center of mass to the other, the deployed petals are more-deployed on one side than on the other. This lop-sided deployment slowly spins around the center of mass. Thus, the (usually cylindrical) center of mass tilts, and does a “rotisserie roll” to spread the heat of re-entry across its surfaces. A given surface element will slowly alternate time in the heat v/s time in the sheltered slipstream. The probably-ideal flight path might best be described as a corkscrew. This will prolong the travel distance and time, providing more time to shed heat. The preceding is a bit of a repeat from before, but is retained to emphasize that all of these functions are still kept from the earlier design, so that our new design is yet more complex in terms of avionics (actuators) controls, but is still plausible.
There are no major differences here (for these design ideas) that differ from what has so far been described, for human-rated vehicles. If I have missed some, please feel free to email me at RocketSlinger@SBCGlobal.net.
One obvious difference is that the petal-to-petal joints (and any other joints, such as to the base, or to the discard tip, if it exists) will no longer need to be airtight. The petals could be grid-fin-like from the git-go (from the beginning, at launch), without added sacrificial filler materials or blocks. Our passengers obviously can’t be exposed to vacuum when the “flower bud” opens up to become a flower! So they will have their own, permanent, separate air-pressure structural walls.
So then our “discard tip” isn’t needed any more, and we’re left with very little other than the grid fins as used by Space X, except that we tilt (instead of roll) our grid fins.
Where the petals join one another (at “joints”) in the satellite-ferrying version previously described, we can leave gaps. These gaps leave space for maneuvering thrusters, which will be needed for many or most human-rated vehicles.
That’s all that I have (here) for now…
Converting Petals to Landing Legs
I consider this below idea to be fairly implausible as well, but I include it, once more, in the name of completeness.
Suppose the petals actuators are designed to not only swing through almost 90 degrees of actuation (as described above), but through almost 180 degrees of actuation, instead. Now, they can double up as landing legs! If the center of mass (of the “shuttlecock”) is inside a sharp-rimmed re-entry capsule, the locations of the actuators may remain fixed, at the outer capsule rim, during the entire journey.
If the center of mass is inside a cylindrical rocket body, the actuators will need to travel from the upper tips (rim) of the rocket’s cylindrical body, down to the lower rim, close to the rocket engines. These actuators could be forced through a powered journey down the outside of the fuselage, down some (cogged?) tracks. The tracks could double up as structural stiffeners (Space X apparently calls them “stringers”) for the rocket’s fuselage. The actuators will be able to complete the last half of their approximate 180 degree rotations, only after having been forced down the tracks. I trust that no drawings will be needed, but please advise me to the contrary if this is the case.
Such a transformation (latter half of the 180 degree swing, and track-travel if applicable) will need to happen right before landing. The time spent dangling under a parachute, late in the journey, would be a good time to do it.
Whether the excess complexity and mass needed to do all of this is worth it, or not, is an open question, but my best guess is “no”. Sensible landing legs include shock absorbers. Adding shock absorbers here (perhaps above and beyond the “crushability” of the petals) sounds prohibitive to me.
Power-Supplies Concerns and Options
Near-constantly adjusting and readjusting the petals is going to require plenty of power. It’s even possible that the total amount of power required here makes this entire set of design ideas implausible. Power density per mass and volume carried to orbit is the gating item here, quite clearly. However, sensible options for powering the actuators would include batteries and fuel cells. Probably less sensible would be flywheels, turbines, or internal combustion engines. (Nuclear or solar? Forget it!). Fuel-burning turbines or internal combustion engines would require not only fuel, but also, compressed air or oxygen to be carried to orbit (in COPVs, or Composite Overwrapped Pressure Vessels, for example), to carry them through start-up and the first parts of the descent, at the very least (far more likely, for all of the trip). Power density here might be better than with batteries, though. After the vehicle enters thicker atmosphere, it might even be possible to “live off of the land” (AKA, use in-situ resources) by compressing ambient air for the turbines or engines. This (compression) wouldn’t be needed in the lower-lower atmosphere at all, where the air is really thick, but that’s not where the bulk of our power needs are likely to reside.
We need lots of power during high-speed descent through thin air. There, we can scoop up super-heated air on the bottom of the vehicle, but, using it to burn fuel in a turbine or engine won’t work well at all… We need a temperature and-or pressure differential, high on the intake side, low on the outlet side, to extract useful power (work energy). And if we capture lower-pressure, lower-temperature air on the lee side (top side) of the descending vehicle, we’d have to compress it before using it! And compressing any gas heats the gas, and to where will we shed that extra heat? All of this goes nowhere fast, but is added here in the interests of completeness.
Let the spit-balling (brainstorming) continue! Perhaps thermoelectric materials could extract power for the heat differential between the heated (windward) sides and the cooler (lee) sides of the petals. See https://www.science.org/news/2021/08/cheap-material-converts-heat-electricity for example. Or perhaps such materials could also be used elsewhere in the vehicle. Of course, they won’t work without a heat differential.
Perhaps the voids (holes) in the grid fins could be filled with small “inverse propellers” that spin in the wind, and power electric generators in so doing. This is implausible to the point of absurdity, but let’s spell it out anyway. They could be embedded in the sacrificial filler materials, which is absurd, but then they’d also have to withstand tremendous heat and pressure after the sacrificial material is cast off, which is also absurd. Or they could be sheltered inside the fairings (petals), and then rotated out later, after payload delivery, individually, or in long strips that look like the wings on these VTOL aircraft here: https://lilium.com/jet ... (Take the ducted propellers that use electricity on the Lilium design, and invert them to become mini-turbines (that burn no fuel), powered by airflow, basically, to generate electricity instead). Or these mini-turbines could be NOT encased in the sacrificial materials in the grid-holes, but rather, air-flow-plugged during ascent, using some other method (while still residing in the grid-gaps). But all of this is absurd, using technology that we have today, in transonic and supersonic flight regimes. Such turbines can’t be built small, lightweight, and strong enough, today, for our particular needs, clearly. Still, we’re brainstorming here!
Perhaps more plausibly, build turbines and generators that aren’t ducted at all. Imagine a long, perhaps tapered, screw-like (or narwhal-tusk-like) rotating device that points upwards, up out of the base of the fairing-cavity. This “screw-propeller” now resides exactly where our satellite(s) payload wants to live, which is a big penalty! But perhaps we can elongate the fairings and cavity, or perhaps we can store the “screw-propeller” elsewhere, and electro-mechanically, robotically, or otherwise move it to where it belongs, after payload deployment. (Far better, see further below).
The “screw-propeller” would be made of strong, durable metal (perhaps titanium or stainless steel) and point upwards, out of the center of the base of the fairings cavity, partially protected from the worst of the heat of re-entry, but still being free to rotate in the wind, and imparting rotational energy to a generator at its base. But now we add “spin” to the descending vehicle, in so doing. The fix? Use (not just one) but TWO counter-rotating “screw-propellers”, egg-beater-style, to cancel this imparted “spin”. In general, use an even number of such screw-propellers, and balance their numbers of spin directions. It might also be necessary to do electrical load balancing (from one generator to another), to help with spin-balancing the vehicle’s descent path. More electrical load added to a given screw-propeller (screw turbine) should add more airflow-resistance for that given turbine, that is. If we have (as postulated in most of all of these above notes) four (4) “petals”, then perhaps we can place 4 of these “screw-propellers” (screw turbines), in a balanced fashion, close to where the 4 petals meet in “joints” (and away from the center-line). Now, the “screw-propellers” won’t need to be moved, and they’ll not get in the way of the payload, so much. Perhaps this scheme is actually plausible! It would generate power, while also helping to decelerate the falling vehicle.
This deserves a drawing for clarity. The below is a top-down view of the flower petals partially opened, with the payload already deployed. The drawing shows what I consider to be the most plausible design, which is the “partial hammerhead” design, where the powered hinges reside outside of the rocket fuselage’s circular profile. The 4 “screw turbines” spin counter-clockwise v/s clockwise, in sets of 2 and 2. These turbines are located close to the joint-lines between the petals, but still inside the fairings (petals) during ascent. Here, the turbines can catch upward-flowing air that forced to flow nearby, by the hinges, petals, and fuselage. The turbines also don’t (much) obstruct the payload space, when located here.
The above describes what I think to be the best design options here. The screw-turbines are simply and directly coupled to generators, which can be sheltered within the base of the deployment stage, or the upper part of the rocket body. There, the turbines are partially sheltered from the wind, yes, but that helps protect them, while they should STILL be exposed to plenty of wind that “whips around the corners” of the upper rim of the rocket body.
The below, I consider to be far-less-plausible options. After payload deployment, the turbines could be thrust out, away from centerline, to catch more wind. This adds complexity and mass, however it is done. And now, we will have to expose the generators to the wind (an absurdity), or somehow (belts, chains, gears?) transfer mechanical spin energy to sheltered generators, which is only slightly less absurd.
Screw-turbines could be replaced by something else, resembling, perhaps, the old paddle-wheels on old steamboats, or short vertical-shaft windmills (with the vertical windmill spin axis rotated 90 degrees for our application), such as are shown here: See https://blog.arcadia.com/vertical-axis-wind-turbines-advantages-disadvantages/ and https://en.wikipedia.org/wiki/Vertical-axis_wind_turbine. In my opinion, such devices wouldn’t be rugged and durable enough here, in our harsh environment..
I have no special expertise or any more plausible 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 …
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