From RocketSlinger@SBCGlobal.net
(email me there please)… This is a sub-site to main site at www.rocketslinger.com …
This
web page last updated 06 August 2019
Designs for Recycling Rocket Exhausts on the
Moon or Mars
Abstract / Pre-Summary
This
sub-page to www.rocketslinger.com is
meant to describe methods for recycling rocket exhausts. “Volatiles” (such as water, carbon dioxide,
carbon monoxide, and other gasses as might be found in rocket exhaust) are
precious on Mars, and even more so, on the Moon. Methods described here could be used to
recover some significant percentage of such “volatiles” (gasses) in the exhaust
plumes of landing (or launching) rockets, on any airless or near-airless
“heavenly body”. After collecting such
cooled-back-down (liquid or solid) volatiles, they can be (with the input of
energy) recycled for uses such as rocket fuel, drinking and sanitation, and
industrial and agricultural uses.
Two primary versions are described here: Revising an existing cave (such as a “lava
tube”), or building an exhaust-recycler “from scratch” (digging or building your
own caves or tunnels). In either case,
the rocket exhausts are caught in cold voids shaded from the heat of the
sun. There, they are allowed to cool,
condense, and be collected for recycling.
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”, and to prevent “patent trolling”
of (mostly) simple, basic ideas.
Introduction
The virtues of recycling rocket exhausts hardly needs to
be expounded upon (at a remote moon or planet which lacks many volatiles, that
is). The Earth’s Moon is a first and
primary target of the discussions here, but Mars would work as well. The Moon lacks significant amounts of carbon
(unlike Mars, where carbon can be harvested from the thin carbon dioxide atmosphere). So on the Moon, capturing
carbon-dioxide-containing exhausts would be especially advantageous. This would apply, for example, to rockets
that use methane fuel, such as SpaceX’s “Starship”.
Recycling volatiles in not the only advantage of the
designs presented here… The other main
advantage is eliminating (or at least suppressing) the problem of rocket
exhausts blowing dust all over everything and everyone, close to the landing /
launching pad. For details about that,
please see, for example,
https://www.theverge.com/2019/7/17/18663203/apollo-11-anniversary-moon-dust-landing-high-speed , “Apollo taught
us that landing on the Moon is a dusty nightmare.”
Re-Purposing a Cave (Lava Tube)
Links concerning caves (empty lava tubes) on the Moon and
Mars can easily be found. See, for
examples, https://www.futurity.org/lava-tubes-moon-2107272-2/ , https://phys.org/news/2019-07-humans-lava-tubes-moon.html , and https://en.wikipedia.org/wiki/Lunar_lava_tube . Sure, we could live in caves, protected from
micro-meteors, radiation, and temperature extremes. But why not also use caves for recycling
rocket exhausts?
OK,
so, then, time for some drawings! We
COULD take a natural opening (sky-light style) to a cave, and build a bridge
(suspension-style or otherwise) over the hole, and put a landing (or launching) pad in the middle of the bridge. A metal grid (with plenty of holes or voids)
could be used for the landing-pad. If
one fears for the metal grid melting or otherwise degrading over time, under
the mechanical and heat-assaults of the rocket exhaust from many-many rocket
landings, then the top-most surface of the metal grid could be clad with
high-temperature-tolerant ceramics.
Ceramics should be able to be easily sourced locally on the Moon or
Mars, and replaced as they wear out or break.
Simple,
yes, but to be sure, here is a drawing of the “grid” surface of the landing
grid (surface of landing [or launching]
bridge)… “Open grid steel decking” is a
Google search-string that will tell you a LOT about associated matters! Also see http://www.baileybridge.com/grating_01.html for example. The
metal that we’d use for our high-temperature-resistant application might be
titanium (somewhat expensive), or stainless
steel (310S alloy to be specific), for example.
If good-enough high-temperature-tolerant metals are used, a ceramic
top-cladding material would probably not be needed.
On, to the basic drawings!
Figure
#1
Figure #1 (above) needs no more comments, so let’s move
right along to #2…
Figure
#2
Moving right along to Figure #3, let us draw a side view
of a cave (lava tube). The idea of using
a bridge (covering part of a natural sun-roof-type opening at the top of the
cave, with a rocket landing-launching pad, with a flame-permeable grid)
scarcely needs a separate drawing… So
the below drawing shows an artificial hole has been cut into the cave, for the
landing-launching pad. A natural opening
(if one exists) can be used for other uses, such as human access. Or, a single natural or human-cut opening can
be double-used.
Figure
#3
So, fairly obviously, the exhaust gasses will be retained
in the sun-shaded cooling temperatures of the cave, where they will condense
into liquids and even solids, falling out as they cool. They can then be collected and recycled.
The following comments will apply, whether the cave is
natural or artificial: Recycling the
condensed exhausts will be made easier by making the cave surfaces (or at the
very least, the cave bottom) lined, such as to be impermeable by the exhausts
(whatever their phase-state may be). One
way to affordably fashion such impermeable surfaces (on the Moon especially)
may be to gather fine Moon dust (regolith), cover the surface to be lined, and
then microwave it. See http://ssi.org/2010/SM14-proceedings/Building-a-Vertical-Take-Off-and-Landing-Pad-using-in-situ-Materials-Hintze.pdf , for
example. 3-D printing methods might also
be used, for the manufacture of blocks for pavements or other uses. See http://blogs.discovermagazine.com/d-brief/2018/11/20/lunar-regolith-moon-dust-3d-printing/#.XTYMVetKiUk for example.
“Mooncrete” (AKA lunarcrete) could
also be used (for sealing the landing surfaces, and cave walls, ceilings, and floors).
See https://en.wikipedia.org/wiki/Lunarcrete . Perhaps such versions of concrete might be sprayed
on cave rock-walls and ceilings in a manner similar to the spraying of
pool-wall “gunite”.
See https://www.riverpoolsandspas.com/blog/gunite-vs-shotcrete …
Artificial
barriers inside the cave can be used to separate areas, one from the other, for
exhaust recycling v/s human uses. The
area (volume) for exhaust recycling will need to be optimally sized. The attributes of such an area (volume) could
be validly compared to what has been learned on Earth, with launch-pad “flame
trenches”. See https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100031698.pdf for example. If the volume (for exhausts) is too low, the
gasses will “blow back” out of the launching / landing pad, going to waste, and
possibly even endangering the spacecraft.
The
Moon’s (or Mars’s) outermost surface surrounding the landing-launching pad
would ideally be smooth-paved (via microwaved regolith, or other methods) to
reduce having dust being blown all over the local area (around the pad, by
rocket exhausts). But such ideas have
already been described at http://ssi.org/2010/SM14-proceedings/Building-a-Vertical-Take-Off-and-Landing-Pad-using-in-situ-Materials-Hintze.pdf . Optimally, such pavement would not only
prevent regolith (lunar dust) from being blown all around, it would also keep
our being-captured exhaust gasses from seeping through the rock, and escaping.
Above
and beyond that, though, here’s another idea:
The volatiles (gasses) that we’re trying to capture here, might escape
from our recycling efforts, at the uppermost stretches of the opening above our
cave, whether artificial or natural. The
volatiles might escape through cracks in the rocks, especially closest to the
surface (where the exhaust gasses are hottest, and where residual heat energy
from the sun lingers in the rocks and dirt, even at night). So therefor, for at least a few yards or tens
of yards downwards, we might want to surround the landing pad with drilled
holes, and seal the cracks in the rocks with a “grout curtain” (injected
concrete). For details about what a
“grout curtain” is, use “grout curtain” as a search string, “Google” or
otherwise. (I’m not finding very highly
relevant descriptive links here, for “grout curtain”, so I’m giving up for
now).
In
other words, and in more detail, the uppermost rocks all around the landing pad
(in a doughnut-shape or torus or annular ring) will almost certainly be like a
porous sponge, with cracks and voids that will absorb the gasses that we’re
trying to capture and recycle. To
prevent their escape, we’ll pave (or “paint” with mooncrete
gunite, or some such) the surrounding surfaces, to
seal them as best as we can. The
uppermost surfaces of the vertical hole (right underneath the landing grille or
flame grid) may be subjected to such frequently-repeated highly destructive
temperatures, that sealants such as mooncrete gunite might not hold up very well. That’s why we back off a
few yards radially outward, and drill down some holes into the “doughnut” here,
and squirt in a “grout curtain”.
Now our system might still not work quite perfectly… Nothing ever does… But we’ve made several hopefully-practical and
affordable improvements. Parts of the
“doughnut” might soak up some gasses, but once saturated, they should soak up
no more… And allow very few gasses to
“sneak around the corners” to escape!
Further below, see a repeated drawing (from above) with changed labels,
to clarify these matters.
Preferentially
landing and launching in the middle of the night (with regards to the Moon or
Mars or otherwise, but especially with regards to the Moon, with a 2-week, very
long and cold night) will keep temperatures low, and will help you to recycle
exhaust gasses. That’s pretty obvious,
but let it at least be mentioned here…
Done!
Figure
#4
For
any more clarification (or drawings) on the above (or anything else here),
please email me at RocketSlinger@SBCGlobal.net .
The above are the
at-least-vaguely-plausible suggestions.
Before we move on, to describing totally custom-cut (artificial) “caves”
for recycling exhaust gasses, let us briefly veer off, into the less-plausible
(IMHO, In My Humble Opinion) options. But,
as a reminder, we’re here to fend off the “patent trolls” by “defensively
publishing” ideas, here, so that they can NOT be patented! ALL ideas (including implausible ones) are
potentially to be “freed up” from the patent trolls, here!
“).
Also https://phys.org/news/2016-09-thermal-metamaterial-waste-heat-harvesting-technology.html
.
Also,
it MIGHT (perhaps just barely?) make sense to “heat pump” the heat out of the
cave, into the cold night, on the Moon, or on Mars, to cool or “pre-prep” the
cave, to do a better job of cooling down the exhaust gasses. The thin (Mars) or non-existent (Moon)
“atmosphere” wouldn’t help us much via “heat convection”, but “heat radiation”
might do the trick. I have no exotic or
brilliant ideas to offer here. See
“Peltier effect” for grins. See https://en.wikipedia.org/wiki/Thermoelectric_effect for that... Alternately, we could heat-pump the heat from
the cooled volume (gasses capture volume) of the cave, to human-habitation
areas (volumes), when heat is needed in the habitation volumes.
OK,
on, then, to describe what things might look like, if we custom-carve a “cave” (or
set of caves) for this purpose, of collecting and recycling exhaust gasses…
Custom-Cutting Caves for Exhaust Gasses
Recycling
Let’s first describe a “deluxe” custom-cut hole or
cave. We’re assuming here that we use
tunnel-boring machines, automated (roboticized) as
much as possible. Bigger holes, and more
holes, of course, will cost more. A
“deluxe” hole might be very wide and very deep, with perhaps one smaller,
slanted access hole, for getting to the bottom and retrieving (or pumping out)
the gathered (condensed) volatiles. Also
shown here is an optional (probably powered, not passive) one-way shut-off
valve. This would be located far enough
down to let the exhaust gasses cool off a bit, first, before hitting this large
one-way gas-flow valve (super-hot gasses will endanger our valve). Open the valve for launches and landings, of
course, but then shut it down after the gasses are captured (prevent excess back-flow
and waste of the gasses).
Figure
#5
So
then, dropping back from the “deluxe” system (assumed to be very expensive, and
impractical for a long time, for all but the busiest spaceports), let us
describe some less-expensive implementations and options. I assume here that smaller-diameter tunnels
and holes are more affordable. I do know
that more-gentle fluid-flow turns (whether we are turning the flow of liquids
or gasses, matters not) will impede fluid-flows less drastically. What I don’t know, is what kinds of tunnels
can be bored most cost-effectively. So several options are shown here.
First off, let’s show what the top layer of a more-affordable
“gas recycling” spaceport might look like.
Assume that we start with a fairly flat surface (of the Moon or Mars or
other). We pave or otherwise seal a flat
landing-launching surface, without bothering with ANY kind of (or with very
minimal) provisions for gently re-directing exhaust gasses from the mode of
being propelled downwards, to the mode of being propelled at a right angle from
downwards (that is, to then being directed towards outwards from the pad-center). After being re-directed from travelling
downwards, the exhaust gasses will have turned a right angle, and travel into
artificial “caves” directed outwards, radially all around the pad. Each cave-entrance will be constructed out of
blocks fashioned out of “in situ” gathered materials, mostly.
One
example of such cave-entrance construction might look like the classical “arch
with keystone” design, with insulating top-fill between (and on top of) the tops of the arches. Note that grout curtains and sealants (mooncrete gunite or other) could
apply to some of the drawings below, but have already been discussed and
diagrammed, and so, are omitted (to cut clutter) from here on down.
So
here’s what we might see, if we were sitting in the middle of the pad, looking
towards the cave entrances…
Figure
#6
A top-side view or
bird’s-eye view (OK, no birds are expected on the Moon or Mars, so a
rocket-drone’s view, then) of the radially arranged gas-capture caves might
look like this:
Figure
#7
Your humble author here hasn’t a clue about the following: How expensive will it be to carve artificial caves (tunnel-bore) under the Moon or Mars? How will that compare to the costs of building caves almost purely above the surface? How far do we have to go down (or how thick do we have to cover our artificial surface caves?) to get the cold temperatures that we need? How expensive is it, to tunnel-bore sharply-turned tunnels, either underground, or at the surface?
Depending
on the answers to the above, any number of configurations could be used. The tunnels (artificial caves) shown above,
could run parallel to the surface, for whatever total length is needed to cool
the gasses. This is so simple, as to not
be diagrammed. Or, the tunnel could go
out a short ways, and then take a SHARP turn (or a SHALLOW turn) as it dives
down deep under the rock and dirt, then comes back
out. Like this… Note, only one of many outgoing “octopus arm
tunnels” is shown, going out from the central landing pad, for simplicity, but
the ideas should be clear…
Figure
#8
Simply for
illustrating some fairly obvious ideas, in the name of completeness, let’s provide
a few more discussions and drawings. The
below is a view from above, of one of many radial surface tunnels built around
the landing-launching pad. The first
(innermost) tunnel-or-cave chamber should be long, to
make sure that there’s plenty of volume to capture the gasses (and to prevent
wasteful and dangerous blow-back). The
entire tunnel could be straight, but is shown here as sinusoidal, partly just
to fit it on the page.
While we’re
building a tunnel (or cave) anyway, we might as well be efficient, and put it
to multiple uses. After the cooling-down
(large volume) compartment, we place a wall, and then a greenhouse. If practical (or when) cooled-down gasses are
ever sufficiently dense for this to make common sense energy-expenditure-wise,
the gasses can be pumped (presumably from lower pressure to higher pressure)
from the cooling chamber to the greenhouse chamber. If the gasses can be collected from the floor
of the cooling chamber in LIQUID form (after condensing), they could be pumped
to the greenhouse more conveniently.
Periodic excursions (by robots and-or humans) to collect solid-phase
condensates, and move them through an airlock to the greenhouse chamber, may
make sense, also.
Figure
#9
The following may
be obvious, but let’s add it for completeness:
8 or 12 or “N” tunnels or caves entrances arranged
radially around the space-port pad makes sense, for not immediately impeding
hot gasses from flowing freely. The
“plumbing” details are highly variable, though.
8 or 12 separate greenhouses and human-use areas may make little
sense. Tie the chambers together and
combine them to smaller total numbers may make a lot more sense (but still preserving
the radial arrangement of at least the first parts of the cooling chambers, for
not impeding the flow of the high-speed, high-pressure exhaust gasses). The above is illustrative only.
Running the gasses through a greenhouse, first, before
human use, allows plants to pull out excess carbon dioxide (and convert it to
oxygen). Greenhouses can be maintained
by a mix of human and robot labor.
Humans (in the greenhouse) may need to wear oxygen breathing gear if the
carbon dioxide (or other gas levels, or smell) is dangerous or unpleasant to
humans. Note that not only plants, but also
soil bacterial, fungi, and assorted microbes can clean toxins and bad smells
out of air. This is especially true of
the air is forced (pumped) through soil beds deliberately. Biosphere II showed this to be true. I have not looked for the very-very best link
to show this, but here is one for starters:
https://www.jstor.org/stable/1312123?seq=1#page_scan_tab_contents .
After
lingering in the greenhouse for a while, excess air and-or water can be
removed, cleaned and conditioned some more, and used for humans, of course. Food grown in the greenhouse will need to be
monitored for contaminants. Only fairly
clean-burning rocket fuels should be allowed to feed this kind of scheme!
Some
of the drawings above show a gated one-way valve to prevent backflow, fairly
close to the hot exhausts. This is
probably the most plausible and practical idea.
However, other drawings show the tunnels or caves as completely
open. This is also possible (as is also,
a mix of the two types of tunnels, even at the same spaceport).
If that latter idea is used, then (probably best) very close to the
bottom-most part of the cave, it might be wise to place two types of “rubble
piles” (or permeable filters)… One made
of potassium hydroxide or lithium hydroxide, or
other suitable chemical for capturing carbon dioxide, and another pile (of
salts or other desiccants) for absorbing water.
The rubble piles would impede the out-flow (and
waste) of the gasses, at the other end of the tunnel or cave. These rubble piles (or filters) would also be
periodically re-processed to extract the volatiles.
Now let’s
make an at-first-apparently-irrelevant detour, but then circle back around and
show its relevance. There’s a very-very
thin atmosphere on the Moon, if we can even call it that. However, as human activity ramps up on and
around the Moon, it will get thicker.
Here are the sources:
‘1) The solar wind
contains atomic nuclei, especially of hydrogen and helium, but other elements
as well. Some of these bounce around the
moon a while, and can get caught in “cold traps”, whether man-made or natural. (They will also capture electrons from the
solar wind, or from the surface of the moon, to become
mostly-neutral atoms).
‘2) Impacting asteroids
and comets deposit volatiles as well.
‘3) Future rockets,
satellites, and “space tugs” will emit gasses as well. Including conventional chemical propulsion,
of course! Nuclear-thermal tugs may emit
hydrogen, and cold-gas maneuvering rockets may emit nitrogen (for example) if
they originated on Earth. Moon-made
cold-gas rockets and satellites (made using “in situ” locally sourced
materials) might opt to use oxygen instead…
Nitrogen is near-non-existent on the moon, but oxygen can be separated
from very common silicon, aluminum, iron, titanium, and etc. (other metal)
oxides.
‘4) Heavy wheeled moon vehicles (see https://www.nbcnews.com/mach/science/toyota-s-moon-rover-concept-high-tech-six-wheeled-lunar-ncna982956 for example) will
transport humans, moon rocks, and supplies to and from the Moon’s “outback”
regions (as the Moon is explored for scientific, mining-prospecting, and other
reasons), and in between bases. They
will scrub their carbon dioxide out of their air, using potassium hydroxide or lithium
hydroxide, or other scrubbers. They may
not want to bother to bring all of their spent carbon dioxide back home… They may want to carry more rocks and
supplies instead. If practical, they
will re-process their scrubbers instead, and dump their carbon dioxide
overboard.
‘5) Another method of
exploring the Moon might use rocket-based “hoppers”. When departing from our spaceport, we can
recycle rocket exhaust as usual… So burn
a near-perfect match of, say, methane and oxygen, when departing… We’ll capture most of it back anyway! Get your highest “specific impulse” as is
practical, out of the fuel, when departing. When “hopping” around in the outback, though,
we might settle for lower specific impulse (burn cooler), and burn way
oxygen-rich, because oxygen is far more available on the Moon (for reaction
mass). So we’ll be adding more oxygen to
the Moon’s way-thin “atmosphere”, if we do that.
Also
plausible idea (when in rocket-hopping mode) is this: Start your burn with a normal mix of methane
and oxygen. (When starting a retro-burn
to come out of free-fall, this will help “settle the masses” in your tanks,
including a tank of finely powdered aluminum…
Read on…). Now throttle back on
the methane, but keep the flame lit so that the next (other, supplemented) fire
will stay lit also. Dump un-oxidized metallic
finely-ground aluminum powder (extracted locally) and excessive amounts of
oxygen into the fires (rocket’s combustion chamber). Burn oxygen-rich again, here, is probably
quite wise.
For these
reasons (and probably more, especially as human activities ramp up), the Moon’s
very thin atmosphere may become a wee tad richer. Human-added oxygen atoms will snag hydrogen
atoms from the solar wind, adding weight to the atoms (making water), which
will be retained in “cold traps” more efficiently. So the below-listed ideas might JUST BARELY
start to make sense, as time goes by.
If you’re
going to bore a tunnel down and back up, we might as well use the other
(non-rocket-pad) end of the tunnel to capture some of the Moon’s thin
atmosphere, especially at night. It
might even make sense to spend a tiny bit of extra money at this far end, and
carve a mini-funnel or mini-crater to improve the “cold trap”. Especially in the middle of the lunar night,
if and when the bottom-of-the-cave cold trap is truly cold (there’s
not hot or warm gasses in there cooling down, right now), we open up the
far-end gate. See the drawing below.
Figure
#10
The far-end
gas-flow gate needs to (when shut) prevent gas-flow in both directions,
obviously. Over a long-long time, the
funnel-bottom may collect volatiles outside of the far-side gate. These can be collected with some extra
trouble, every once in a great while.
Or, we could put TWO gates at the far end, so as to capture volatiles at
the far end, even while cooling down a batch of hot gas at the bottom.
The entire
spaceport and tunnels and-or caves should ideally be located inside a moderate
to large-sized crater. This (the crater
walls) will protect the rest of the Moon (or at least, other nearby bases or
settlements) from any escaping rocket-blast-blown Moon dust (regolith).
Excavating the
far-side funnel is probably more trouble than it is worth, but it is documented
here in the name of thoroughness. If
these funnels are dug, the resulting extra “fill” debris could be used to patch
any low spots in the surrounding crater walls.
Falling into the same category of
probably-not-worth it, we could erect, around the rim of the enveloping crater,
“snow fences”. As snow fences on Earth
slow down the wind, causing snow flakes to drop out,
our “snow fence” (made of slats or poles, and voids) could slow down the
extremely diffuse “atmospheric winds”, and help us catch stray volatile
molecules. Structural integrity to
countervail against gravity isn’t as important on the Moon as on Earth, and the
“winds” will be weak to the vanishing point, so we can heavily tilt our “snow
fence” around the crater rim. Like this:
Figure
#11
Here
are concluding remarks for this sub-page, some of which should be obvious: Recycling rocket exhausts makes sense, only
on fairly highly-trafficked, well-developed spaceports. These kinds of ideas won’t likely be
practical till (?) the late 2030s at the soonest, or some
time after that. The ideas here
will be more useful, the further away a Moon spaceport is located away from the
Moon’s poles, where there strongly appear to be deposits of volatiles. And SOME of the ideas listed here, could
apply to processing lunar-pole deposits, in the same manner as (or combined
with) the processing of recycled rocket exhausts.
If the
ambient-gasses-collecting funnels (see above) and “snow fences” are used, then
one more step or set of steps MIGHT make sense as well: Some sort of electric charges, magnetic
fields, or combination of both might be set up, to attract more charged
particles from the solar wind (to neutralize and capture them for recycling, of
course). That might be a bad idea,
though, for increasing electromagnetic noise for electronics, and radiation
hazards for humans.
Also,
I really have no idea what such an arrangement might look like. A Moon-orbiting
electromagnet that is ONLY turned on at the right time and place, perhaps? That’s one guess of mine! If you have information or ideas along these
lines, please send them to me at RocketSlinger@SBCGlobal.net …
However,
this scheme DEFLECTS rather than COLLECTS the solar wind. So in our scheme, I think we might want to
put TWO electromagnets in orbit around the Moon. Turn them both on, only when our
collection-base is caught in between the two electromagnetic satellites (and
also, only when there are no humans on the Moon’s surface, exposed to extra
radiation). Now, between the TWO
electromagnets, more solar wind will be “shepherded” down for collection. That’s my best stab at it…
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.
Once again, comments or questions (or idea contributions) are welcomed
at RocketSlinger@SBCGlobal.net …
Stay
tuned… Talk to me! RocketSlinger@SBCGlobal.net
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to main site at www.rocketslinger.com