shuttle to the moon?

[cjameshuff]

Are the materials and processes available to synthesize methane on the Moon?  I've already heard about manufacturing methane on Mars, but my understanding in that the process uses carbon dioxide from the Martian atmosphere.  If you know of a process that will work on the Moon, I'm interested in hearing more about it.

The same process will work anywhere you can get water and CO2, and the deposits at the poles appear to be rich in both, as well as methane itself, carbon monoxide, ammonia, and various other volatiles. A good 20% of what LCROSS kicked up was volatiles other than water.

[ka9q]

Thanks guys, this is what makes hanging around this crowd so worthwhile...

Bob, I knew that the performance of nuclear thermal propulsion depended on the atomic weight of the exhaust but I had no idea it would be so bad for oxygen. Add the materials problems of attack by hot oxygen and I guess it's a non-starter. I guess it'll have to be hydrogen, or something containing lots of it. You mention disassociation for hydrogen; how much would there be for methane or water at those temperatures?

Abaddon, I'm going to have to research the delta-V requirements associated with L1. I know that the GRAIL spacecraft used it as a less expensive way to get into lunar orbit. Judging from the animations, it looks like it greatly reduces the delta-V needed to decelerate into lunar orbit. I should also look at the delta-V requirements to get to and from L1 from the earth and directly from the moon. I would not necessarily prefer L1 to lunar orbit for mere orbital stability as L1 is an unstable Legrange point and would also require stationkeeping. With the precision lunar gravity models soon to come from GRAIL we might be able to find lunar orbits that require minimal stationkeeping to be long-lived.

Chew, yes, the moon is practically at infinity for delta-V purposes. The eccentricity of the Apollo translunar trajectory is about 0.97. 1.0 is a parabolic escape trajectory, and >1 is hyperbolic (excess velocity at infinity). That says it's not much more costly to get to L1 than to the moon's vicinity.

cjameshuff, you make a good point about the weight of tanks. It just occurred to me that you might someday even make them out of lunar materials. Aluminum, magnesium and titanium are plentiful on the moon. And if you fabricate them outside you wouldn't need shielding gases to weld them. 

I didn't know that the volatiles at the poles found by LCROSS included so much stuff other than water. Carbon is another rare element on the moon, so if there's lots of CO₂ at the poles that could be very useful.

raven, aluminum/oxygen as a fuel has occurred to me too. After all, much of the exhaust of a solid rocket motor is aluminum oxide. I even thought of using finely ground raw lunar materials (which includes alumina) as the propellant in a nuclear thermal rocket, but the problems of high molecular weight and of erosion by solid refractory particles probably makes it a non-starter.

You could even use metallic aluminum or magnesium. That would avoid chemical attack on the rocket parts and both have atomic weights less than diatomic oxygen. Ideally you'd have to heat them to their gaseous phase. At STP aluminum boils at 2519 C (!) but magnesium at (only) 1091 C but they'd be significantly higher under pressure. Hmm. has anyone ever considered metallic propellants in any kind of rocket, thermal or electromagnetic?

This is fun. Thanks for all the ideas and analyses.

[cjameshuff]

raven, aluminum/oxygen as a fuel has occurred to me too. After all, much of the exhaust of a solid rocket motor is aluminum oxide. I even thought of using finely ground raw lunar materials (which includes alumina) as the propellant in a nuclear thermal rocket, but the problems of high molecular weight and of erosion by solid refractory particles probably makes it a non-starter.

The obvious issue here is that the combustion product is solid. You need to use excess oxygen to provide a fluid to drive the expansion, and the mix of solid particles and expanding gases won't be as efficient...a lot of energy will be left in the exhaust as different velocities of gas and particles, heat not transferred from particles to gas, etc.

You could even use metallic aluminum or magnesium. That would avoid chemical attack on the rocket parts and both have atomic weights less than diatomic oxygen. Ideally you'd have to heat them to their gaseous phase. At STP aluminum boils at 2519 C (!) but magnesium at (only) 1091 C but they'd be significantly higher under pressure. Hmm. has anyone ever considered metallic propellants in any kind of rocket, thermal or electromagnetic?

There you'd have a similar problem with the exhaust condensing and ceasing to provide propulsive force while still containing a substantial amount of heat energy.

Metallic propellants have been used in ion thrusters...mercury, cesium, bismuth, and lithium are ones I'm aware of.

[Bob B.]

You mention disassociation for hydrogen; how much would there be for methane or water at those temperatures?

There's not much dissociation with water either, but with a molecular weight of 18, it's better than oxygen.

The computer program I use to perform these calculations tells me that methane breaks down mostly into diatomic hydrogen and solid carbon.  The carbon doesn't contribute anything to the pressure, but is does produce thrust due to its momentum.  The method I used to account for the solid phase is that developed by Richard Nakka in his Experimental Rocketry web site.  (Also presented in my Rocket Thermodynamics page.)  Richard's method assumes the gas and condensed-phase particles flow at the same velocity, thus it represents an upper limit on performance.  For the same set of conditions I stipulated in post #26, I calculate a specific impulse of 550 s for methane.

[smartcooky]

How about Ozone for nuclear rocket reaction mass? Would that be better or worse than Oxygen?

[ka9q]

I'd think ozone would be much worse. It has a higher molecular weight and is far more corrosive than even oxygen (O₂). It would probably decompose before it left the engine, though.

[ka9q]

Metallic propellants have been used in ion thrusters...mercury, cesium, bismuth, and lithium are ones I'm aware of.

Xenon seems to have become the favorite propellant in ion thrusters. I wonder why; it's rare and expensive.

Metallic propellants can also be used in electromagnetic rail guns. Problem is you need either a very long barrel or extremely high power to get a reasonable exit velocity. Rail guns have frequently been proposed for launching stuff off the moon; it may turn out to be the most practical solution in the far future.

[armillary]

Hi guys,

This is a little off-topic, but I was at a recent lecture by Alan Bond, who is chief designer for a British initiative called Skylon.

Basically, the concept is an air-breathing spaceplane with a special turbine providing oxygen to a rocket engine, and a small O2 tank for speeds over Mach 5.5, capable of launching to LEO with 12-15 tons payload and to ISS with around 10. While this is too small to allow any meaningful manned exploration, a few launches will get enough mass into orbit to allow for pretty impressive missions. They're doing tests of the engine subsystems at the moment, so it's well under way.

If (and that's a big if) this comes off, cost to orbit might drop to a few thousand dollars per kilo within 20 years.

Plus, the thing looks like a sci-fi fan's dream...

http://www.reactionengines.co.uk/space_skylon.html

I'd like to hear what your thoughs are on this concept.

[cjameshuff]

Xenon seems to have become the favorite propellant in ion thrusters. I wonder why; it's rare and expensive.

It doesn't cost that much in comparison with the rest of the stuff (consider how much a kg of copper, fiberglass, and ultra-high-purity etched silicon can cost), ionizes easily, is non-corrosive, easily storable as liquid in small pressurized tanks, and the high atomic mass improves thrust (at the cost of specific impulse, but they can get high specific impulse in spite of that).

Metallic propellants can also be used in electromagnetic rail guns. Problem is you need either a very long barrel or extremely high power to get a reasonable exit velocity. Rail guns have frequently been proposed for launching stuff off the moon; it may turn out to be the most practical solution in the far future.

Railguns require heavy pulsed power sources whose components are severely stressed with each shot, and have major issues with rail erosion. For mass driver propulsion, I favor other approaches. For example, while I doubt the practicality of Lofstrom loops for spacecraft launches, they could be used on a much smaller scale as pellet accelerators for propulsion. Or you might take a similar approach and spin up loops of fine maglev-supported chain, breaking and dumping the chain directly rather than electromagnetically transferring momentum to a propulsive pellet.

[cjameshuff]

Plus, the thing looks like a sci-fi fan's dream...

That's exactly what it is. And an engineering nightmare, motivated by basic misconceptions about spaceflight and a cargo-cult pattern of thinking that reasons that making a rocket more like an airplane will make it cheaper.

Propellant isn't what makes spaceflight expensive, certainly not liquid oxygen. Skylon carries extra liquid hydrogen so they can collect oxygen from the atmosphere. Hydrogen is low-density, expensive and difficult to handle, requires huge well-insulated tanks and increases the size of the vehicle, while LOX is cheap and compact, making this a poor trade.

Air breathing requires drag-producing inlets, gets you oxidizer diluted 4:1 with nitrogen, and directly reduces thrust because much of the stuff you're pushing out the back started off at a high speed relative to you. You're basically trying to increase the speed of an already-supersonic airstream to produce thrust. Rocket propellant starts out at rest with respect to the vehicle, sidestepping this issue and allowing rockets to reach much higher velocities. Lower thrust means longer time spent in the atmosphere, wasting energy fighting drag. And it's not at all free: Skylon doesn't even have the sense to use staging, so all the equipment you have to carry to compress and near-liquefy the air before feeding it into your engine has to be carried all the way into orbit and back.

On top of all that, you require a custom, specially-reinforced runway to support the vehicle, one located in such a location that a crashing Skylon doesn't obliterate a city, kept super-clean so ingested debris doesn't damage those delicate heat exchangers. This is where the real cargo-cult thinking comes in, somehow equating horizontal takeoff and landing with airline-like operations. Airliners aren't giant flying liquid hydrogen fuel tanks wrapped in insulation and exotic ceramic thermal protection systems. You aren't going to make a rocket cheaper to use by tacking on wings, jet engine intakes, exotic cryogenic air coolers, landing gear, and so on to make it look like a jet plane.

If they get prices down to a few thousand dollars/kg, they'll be facing tough competition from companies like SpaceX who'll have been matching that price for years and will have a much broader range of capabilities. I doubt they will, they have a hugely complex system that'll be expensive to build, will require expensive infrastructure, and will be expensive to operate.

[Noldi400]

Under the influence of Dr. Schweiger's Orbiter sim, this was the part that caught my eye:

The external shell (the aeroshell) is made from a fibre reinforced ceramic and carries only aerodynamic pressure loads which are transmitted to the fuselage structure through flexible suspension points. This shell is thin (0.5mm) and corrugated for stiffness. It is free to move under thermal expansion especially during the latter stages of the aerodynamic ascent and re-entry.

(Red bolding mine.)
. . . .

During re-entry, which occurs at an altitude between 90 to 60km the heat is radiated away from the hot aeroshell. Heat is prevented from entering the vehicle by layers of reflecting foil and the low conductivity shell support posts. Liquid hydrogen is evaporated in the main tanks, passed through thermal screens to intercept the small residual heat leak and then vented overboard.

Not being in the industry I don't know about the latest tech, but is there a "fibre reinforced ceramic" that in a 0.5 mm thickness can manage reentry temps and then be ready for another launch without extensive maintenance?  I know there are some remarkable materials out there, but that claim seems a little... optimistic, to say the least.

^{Quotes taken from Reaction Engines Ltd website.}

[cjameshuff]

Ah, I'd forgotten about the excess liquid hydrogen used for active cooling...even more of the stuff to carry, and a bunch of plumbing as well.

Not being in the industry I don't know about the latest tech, but is there a "fibre reinforced ceramic" that in a 0.5 mm thickness can manage reentry temps and then be ready for another launch without extensive maintenance?  I know there are some remarkable materials out there, but that claim seems a little... optimistic, to say the least.

Probably part of why why they put its first flight a safe 20 years and $12 billion (before they discover some really quite minor issues that will take just a few billion more to solve) away.

[Bob B.]

How about Ozone for nuclear rocket reaction mass? Would that be better or worse than Oxygen?

In a chemical engine ozone performs a little better than oxygen, but the difference isn’t enough to make it worth the hassle.

In a nuclear thermal rocket (NTR) I see no advantage to ozone.  Granted, I haven’t performed any calculations, but I have to believe that at high temperature it’s just going to breakdown into diatomic oxygen, making its perform the same or similar to oxygen.

To improve the performance of a NTR we want a fuel that is going to decompose into products with a low molecular weight.  This means we want a fuel with a lot of hydrogen and not many heavy atoms.

Clearly hydrogen is the best fuel, but its extremely low density is troublesome.  The problem isn’t as bad in a chemical engine because there is about 5 to 6 kilograms of LOX for every kilogram of LH2, which lowers the average density to something more manageable.  But in a NTR the fuel is 100% LH2, so the low density is a bigger problem.

Although any other propellant we choose is going to perform worse than LH2 in terms of specific impulse, one with a higher density might come close to matching LH2 in terms of Δv because of the higher mass ratio attainable with the denser fuel.

One way to compare potential fuels is to simply calculate the average molecular weight of the exhaust products.  The lower the better.  At temperatures below 3000 K there is very little dissociation, so for a quick comparison we can assume gases will be in their diatomic form. 

Fuel                 Exhaust Products         Mass   Moles  Mass/Mole

Hydrogen, H2         1 H2                     2.02    1        2.02
Methane, CH4         1 C(s) + 2 H2           16.04    3        5.35
Propane, C3H8        3 C(s) + 4 H2           44.10    7        6.30
Pentaborane, B5H9    5 B(s) + 4½ H2          63.12    9.5      6.64
Ethanol, C2H6O       2 C(s) + 3 H2 + ½ O2    46.07    5.5      8.38
Ammonia, NH3         1½ H2 + ½ N2            17.03    2        8.52
MMH, CH6N2           1 C(s) + 3 H2 + 1 N2    46.07    5        9.21
UDMH, C2H8N2         2 C(s) + 4 H2 + 1 N2    66.10    7        9.44
Water, H2O           1 H2O                   18.02    1       18.02

This gives us only a rough comparison because there’s more that goes into calculating exhaust velocity than just molecular weight.  The calculations are particularly complicated when there is condensed species (liquids and solids) in the exhaust.  Nonetheless, this simple calculation allows us to see which fuels warrant a closer examination.

Pentaborane is a compound that I dug up just by looking for something that appeared to meet the criteria – low exhaust molecular weight and liquid through a good temperature range.  However, someone pointed out to me that boron is a neutron absorber; therefore it might adversely affect the function of the reactor.  I’m also concerned about the carbon containing compounds because they might foul the engine with soot, particularly if the engine is to be restartable or reusable.

If you can find any other potential fuels, please share them with us.

The situation changes at higher temperature, say >6000 K.  At these temperatures, hydrogen and oxygen exist mainly in their monatomic forms.  Water will also breakdown into predominately H and O.  Nitrogen, however, will remain mostly in its diatomic form.  At high temperature we have,
 

Fuel                Exhaust Products         Mass   Moles  Mass/Mole

Hydrogen, H2         2 H                      2.02     2       1.01
Methane, CH4         1 C(g) + 4 H            16.04     5       3.21   
Propane, C3H8        3 C(g) + 8 H            44.10    11       4.01
Pentaborane, B5H9    5 B(s) + 9 H            63.12    14       4.51
Ammonia, NH3         3 H + ½ N2              17.03     3.5     4.87
Ethanol, C2H6O       2 C(g) + 6 H + 1 O      46.07     9       5.12
MMH, CH6N2           1 C(g) + 6 H + 1 N2     46.07     8       5.76
Water, H2O           2 H + 1 O               18.02     3       6.01
UDMH, C2H8N2         2 C(g) + 8 H + 1 N2     66.10    11       6.01

[ka9q]

Okay, then what about ammonia? Its molar mass is almost as low as methane but it liquifies at much higher temperatures and is about 3 times as dense. Nitrogen, assuming it doesn't dissociate, is pretty inert. And you won't get any soot fouling up your engine.

[cjameshuff]

Okay, then what about ammonia? Its molar mass is almost as low as methane but it liquifies at much higher temperatures and is about 3 times as dense. Nitrogen, assuming it doesn't dissociate, is pretty inert. And you won't get any soot fouling up your engine.

Nitrogen's a bit harder to come by than carbon. There's apparently some in the lunar ices, but I don't know how much. Perhaps you could scoop it from Earth's atmosphere, or extract it from the atmosphere on Mars. Water and CO2 seem more abundant than ammonia, at least in the inner system (not sure about the contents of any iceballs that formed further out where ammonia is more stable).

Sooting could be solved by the addition of oxygen. This could be a brief procedure done while shutting down to prevent accumulation or burn off any deposits that have formed, as well as something done when higher thrust is needed. Lift off on LOX-augmented CH4, burn to your desired orbit on plain CH4, and use a brief shot of LOX on shutdown to clean the engine out. I favor just using LOX/CH4, though...I think reactors are better put to stationary uses doing things like producing propellant.