I’m not sure if someone has already run the analysis, but I’m kind of curious about which ISRU-derivable propellant combination is better for locations like Venus or Mars where there is plenty of CO2 available in the atmosphere, but limited water.
Assume for a second that water isn’t available in useful quantities. I’m not sure yet if the concentration in the Venusian atmosphere is high enough to be useful, and it’s not yet clear that there’s easily accessible water at most places on Mars–there might be, but it’s far from clear1. Assume for now that you have to bring your hydrogen with you, from Earth.
A few quick observations:
- One immediately obvious point is that if you have to BYOH2 you’ll probably want to bring the hydrogen as LH2, not water. While everyone complains about how hard it is to store LH2 for long durations in space, water is still ~89% Oxygen, which is a material you can get almost anywhere, especially if you have CO2 available, so for every kg of hydrogen you bring tied up in water, you’re lugging around an unnecessary ~9kg of oxygen. You can definitely make a dewar with active cryogenic cooling that masses far less than 9x the mass of LH2 you want to bring with you–it may be “harder” to do it that way, but is far, far more efficient.
- A typical O/F ratio for LOX/CH4 is probably around 2.8-2.9:13, which means that about ~6.5% of your propellant mass is hydrogen for LOX/CH4. For LOX/LH2, you’re probably looking at an O/F ratio of around 5-6 typically, which would yield ~14-17% hydrogen. So for every kg of hydrogen you bring along, you could get4 ~15.4kg of LOX/CH4, or you could get 6-7kg of LOX/LH2.
- If you’re limited by hydrogen you can bring, rather than dry mass, or volume, or other things, it’s not yet clear which of those will result in more payload in orbit, since the two have significantly different bulk densities and Isp values. That’s the analysis that would be fun to run. My guess is a lot will depend on the required delta-V5, whether you’re looking at 1, 2, or 3 stages, if you assume on-orbit refueling before the earth-return, etc.
- One way to cheat a little with LOX/LH2 would be to use a LOX-rich Thrust Augmented Nozzle (TAN). Basically, you have a core running at the more traditional 5.5-6:1 O/F ratio, while initially running the afterburning portion of the engine at a much higher mixture-ratio, possibly even higher than stoichiometric! As the rocket accelerates, you could throttle down this element and then shut it off. This is probably more useful for Venus ascent than Mars, but would allow you to get not only a much higher engine T/W ratio than you could realistically get normally with LOX/LH2 engines, but also give you more propellant per kg of brought hydrogen, because you’re shifting your mission-averaged mixture ratio to a very, very lean range.
I honestly don’t know the answer, and don’t have time yet to run the numbers, but I’m genuinely curious. If you have a fixed supply of hydrogen, which ISRU propellant method (using a Sabatier reactor to convert H2 and CO2 to LOX/CH4, or using a solid electrolysis cell to crack O2 out of the CO2 to make LOX/LH2) actually yields the most mass delivered to orbit or to an Earth return trajectory from Mars or Venus? Has anyone already done this analysis? If not, I may try to find some time at some point to run the numbers.
[Update 9:58pm on 9/5/2016: in case you’re curious what brought this on, I was thinking about Venusian Rocket Floaties again, and was wondering whether a Venusian launcher first stage would want to be LOX/CH4 or LOX/LH2.]
- Except to Mars optimists who always assume the most favorable definition of maybe…
- Bring Your Own Hydrogen
- This is based on a ton of assumptions about chamber pressure, expansion ratio, etc. I used this chart, and eyeballed 1500psi with a decent expansion ratio.
- Assuming perfect chemical conversion efficiency and no losses along the way
- More delta-V like a Venus return will probably favor hydrogen more relative to methane, less delta-V like just getting to Mars orbit would likely favor methane more relative to hydrogen.