Subject: Space-tech Digest #88 Contents: John Roberts Re: Lunar volatiles Paul Dietz Re: Lunar volatiles Bill Davidsen Re: Lunar volatiles Henry Spencer Re: Lunar volatiles Karl Dishaw Re: Lunar volatiles Paul Dietz Tubular Carbon Microfilaments ------------------------------------------------------------ Date: Tue, 19 Nov 91 12:09:30 EST From: John Roberts Disclaimer: Opinions expressed are those of the sender and do not reflect NIST policy or agreement. To: space-tech@cs.cmu.edu Subject: Re: Lunar volatiles >Date: Mon, 18 Nov 91 20:55:17 EST >From: dietz@cs.rochester.edu >To: space-tech@cs.cmu.edu >Subject: Lunar volatiles >There was a short discussion on sci.space about the ever-popular >lunar polar ice question. >The question arises: how would one get the lunar ice off? Mass >drivers were suggested, but these tend to be large and need power in >short bursts (which would require massive power conditioning >equipment). More likely, and more obviously, early ice shipments >would be by rocket, with material derived from the ice used as >reaction mass. What fuel should the rockets use? >The ice on the moon, if a product of lunar impact of volatile-rich >objects, is likely contaminated with a wide range of impurities, >including hydrocarbons, alcohols, clathrates of various kinds, >sulfurous compounds, halogenated compounds, and so on. So, any ice >processing system is going to have to be able to purify the resulting >water. However, some of the compounds may be useful. >Electrolysis can be avoided entirely if nuclear thermal rockets are >used to launch from the moon. The fuel then could be simply water (or >the CO2 waste from the steam reforming process); hydrogen could be >produced by thermochemical reactions as described above for use in >high-Isp thermal rockets in space. > Paul F. Dietz > dietz@cs.rochester.edu I think there are some excellent ideas here, but I also think at least three more factors have to be added to the consideration: 1) Escape from the moon is going to be far easier than escape from Earth. Optimizing launcher performance is therefore far less important than it is for a launch from Earth. 2) If there's any lunar ice there, it's a rare and valuable resource until some way is found to get hydrogen to the moon inexpensively. One might imagine a lunar colony starting out in a polar region near a deposit of ice, and the price of hydrogen and oxygen being comparable for a while, but the price of hydrogen gradually rising to 20 or 50 times the price of oxygen, or perhaps far more. It might be nearly analogous to burning diamonds for fuel for an earth launch. 3) The chronology of the development of a large-scale lunar launch system is very important. The very first native-fuel launchers might be as you describe, but for reasons (1) and (2) above, I don't think they'll stay in business very long. If that is the case, then it doesn't pay to work terribly hard at optimizing them beyond the point at which they work pretty well, since they're going to to be replaced pretty soon by more long-range designs. I think the long-term designs may well use mostly the resources that are abundant on the moon: energy (solar or imported nuclear), metals (aluminum, iron, etc.), ceramics, and oxygen. One such device would be the linear launcher, but as you describe, that would be a huge construction project, and require extensive power conditioning, so it would probably not be the first employed. I have thought of another approach (posted about 2 years ago), but don't have the thermodynamic training to evaluate its practicality - the aluminum-burning rocket. There could be at least two varieties of aluminum rocket. The first would simply combine the aluminum and oxygen in a high-temperature reaction, and eject the aluminum oxide as exhaust, with or without additional oxygen as reaction mass. Obviously, this could not be done with any existing engine technology. Potential problems would be oxidation or melting of engine parts, and accumulated deposits of aluminum oxide clogging up the nozzle and other engine openings. A second variety would use the combination of aluminum and oxygen to produce heat, and use the heat to drive a reaction mass of pure oxygen. Bringing along the waste aluminum oxide might not be as wasteful as it sounds - it could well be a valuable material in earth orbit, and could thus be regarded as part of the payload. Again the possible problems would be melting and oxidation. I'm not sure just aluminum and oxygen would be the best choice. Perhaps something akin to thermite (which could also be made on the moon) would be useful as an aid to combustion, though it could lower specific impulse somewhat. Aluminum rocket engines could also be used in conjunction with linear launchers, for course corrections, earth orbital insertion, etc. As for the problem of engine oxidation - can't ceramics theoretically be employed that won't oxidize even in the presence of superheated oxygen? A really good nuclear thermal rocket would have to be made of ceramic anyway, so perhaps nuclear rockets could also use the abundant oxygen as reaction mass. Again, maximizing specific impulse is nowhere near as important as for earth launches. So - does anyone know enough thermodynamics and math to predict whether these approaches would be even theoretically possible? In other words, could an optimized aluminum-burning rocket of either type achieve enough delta-V to escape from the moon? As for purifying the water, I would suggest distillation as a first step, no matter what else is done. I wonder what the optimum ambient pressure would be. John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ Date: Tue, 19 Nov 91 13:28:17 EST From: dietz@cs.rochester.edu To: henry@zoo.toronto.edu Subject: Re: Lunar volatiles Cc: space-tech@cs.cmu.edu >Electrolysis can be avoided entirely if nuclear thermal rockets are >used to launch from the moon. The fuel then could be simply water (or >the CO2 waste from the steam reforming process)... Note, however, a nasty design complication: both water and CO2 become oxidizing agents at high temperatures, which presents materials problems for a nuclear-thermal rocket. Hydrogen or hydrocarbons are better here too. Another good fuel could be methanol. Assuming there is water and some kind of carbon at the lunar pole, methanol can be made from purified synthesis gas. It is easily stored (some heating may be needed to prevent it from freezing). At high temperature it decomposes (endothermically) to carbon monoxide and hydrogen. Completely decomposed, it would have an average molecular weight of 15 -- better than methane (assuming no dissociation of the latter) -- and should not oxidize engine surfaces. Some coking might occur, but that might be desirable in some places. A mixture of methanol and water is another possibility. With enough methanol, the mixture should be reducing rather than oxidizing. More generally, mixtures of water and generic reduced carbon compounds should react in a high temperature engine to form CO2, CO and hydrogen. Enough of the latter make the mixture reducing. Ammonia would be still better; if decomposed to nitrogen + hydrogen the average molecular weight is only 11, and the nitrogen is unreactive. But nitrogen is likely to be scarce. Paul ------------------------------ Date: Tue, 19 Nov 91 17:58:28 EST From: davidsen@crdos1.crd.ge.com To: space-tech@cs.cmu.edu Subject: Re: Lunar volatiles Reply-To: davidsen@crdos1.crd.ge.com Sender: mnr@DAISY.LEARNING.CS.CMU.EDU > (Aiming the beam up the nozzle avoids the window problem, but creates > new problems of being able to thrust only along the beam axis and having > to keep a beam in focus through the exhaust plume.) Why a window? Most of the materials suitable for buing a (fairly) high pressure bioler are also pretty good conductors of heat. To call it boiler plate may be a simplification, but the mass to be heated is so much greater than the mass of the boiler that heating the whole thing would seem to be an option. Your point on focus worries me more, and another thought comes up, that if the reflector were in orbit, the boiler could be at the far end of the payload, if need be. That adds weight and cost, but running high pressure pipe is pretty low complexity. Maybe a shallow boiler of large diameter with a small nozzle bump. ________________ | | |=== heat the entire | volatiles | |=== surface area | | > | | | \_ nozzle |______________|_|=== Boy is that ugly, a trashcan with a hole in the back. Still, the boiler and nozzle could be detached and returned to Luna carrying only fuel to be heated for landing. I don't see this as manned, so structural strength is the limiting factor to acceleration. Didn't someone work up a design for a parabolic reflector providing boost for a light sail type probe or something? If you were going to use light sails for probes a big mirror could supply a lot of delta-v early on for cheap probes of the outer system. That could allow some prospecting, if you could make the probes cheap enough to send bunches of them. Something like say, $10k/probe? Perhaps that's another thread. ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 19 Nov 91 20:00:05 EST To: space-tech@cs.cmu.edu Subject: Re: Lunar volatiles > Why a window? Most of the materials suitable for buing a (fairly) >high pressure bioler are also pretty good conductors of heat. To call it >boiler plate may be a simplification, but the mass to be heated is so >much greater than the mass of the boiler that heating the whole thing >would seem to be an option. I think this runs into trouble with heat transfer, unless your boiler is very thin along the beam axis. Heat will be transferred from the exposed surface inward only by conduction, which is not very effective compared to the rate at which flowing propellant removes heat. Even with a thin boiler, our materials now have to be excellent heat conductors. This is a particularly troublesome requirement if the chamber environment is oxidizing and hence we can't use metals. A more fundamental problem is that now the chamber structure has to be as hot as the exhaust. Hotter, in fact, since we aren't talking about equilibrium, not in a high-thrust engine. This is bad news; in a normal rocket most of it runs much cooler. This is going to either make the materials problem rather worse or limit exhaust velocity badly, I think. The exhaust velocity of orthodox nuclear-thermal rockets is actually worse than chemical rockets if they have to use the same propellant, because their core has to be hotter than their exhaust and this limits temperatures; they win only because their exhaust can be hydrogen. (Exhaust velocity is proportional to square root of molecular weight, so hydrogen [MW 2] is about 3 times better than the next-best stable molecules [MW 16-18 for methane, ammonia, water]. The penalty for using anything other than hydrogen in a thermal rocket is *huge*. Alas, there's nothing useful in between. Helium [MW 4] already takes a 40% penalty, and is rare and a pain to handle. Lithium, beryllium, and boron hydrides are liquids or solids that decompose on heating; to add to the fun, lithium is highly flammable, beryllium is extremely toxic, boron is great at forming refractory deposits on engine nozzles, and all three are rare elements for quite fundamental reasons. We're not likely to find any of them concentrated on the Moon.) Most of the schemes I have seen for numerically-viable propulsion along these lines -- laser launchers, radiative heat transfer from gaseous fission, etc. -- have worked hard to get the photons absorbed by the propellant itself, not by structural materials. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ Date: Wed, 20 Nov 91 09:53 GMT From: Karl Dishaw <0004244402@mcimail.com> To: space-tech Subject: Re: Lunar volatiles Any volatiles you find on the moon are too valuable to waste as propellant! They're real scarce--life support and manufacturing will have dibs. The propellant sources you want are stuff you can get from the lunar rock--oxygen and aluminum (oxygen 40%, aluminum 5% by weight). Pure oxygen can be used for a cold-gas or thermal rocket. You can boost the Isp above cold-gas by burning aluminum powder with the O2, which can give an Isp of ~120 sec. The biggest problem there is a fuel injector for the Al powder (my senior project at MIT). Either way, you'd probably use the rocket to circularize the orbit of something launched from a mass driver, or perform other maneuvers. A mass driver can't get a unpowered payload to a target by itself. A remote laser would be good for an oxygen fuel thermal rocket, but you'd have to have a laser in reach of all the points where you perform maneuvers. A lunar mining/manufacturing facility would want to export its product with locally produced fuel and engines. Not too many products would be worth more than the volatiles to export them. :-) Karl kdishaw@mcimail.com ------------------------------ Date: Tue, 19 Nov 91 15:29:41 EST From: dietz@cs.rochester.edu To: space-tech@cs.cmu.edu Subject: Tubular Carbon Microfilaments The most (?) recent issue of Nature has an interesting article on a new result concerning carbon allotropes. Some Japanese scientists have found that they can make short (~ 1 micron) carbon fibers that consist of multiple concentric layers of graphite bent into tubes. Kind of like the fullerenes, but very elongated. The fibers grow on one of the electrodes in an arc used to make fullerenes, and are aligned with the local electric field. The ends of the tubes are capped with polyhedral or hemispherical carbon layers. These fibers, if they can be made longer, should be much stronger than conventional graphite fibers. Interesting implications for high strength composites and high tensile strength cables. Paul ------------------------------ End of Space-tech Digest #88 *******************