From time to time I get really excited about in-situ resource utilization on the Moon and Mars. Lately, it's been one of those times. I've been filling out my NASA application and it's making me think about why I really want to be an astronaut. And in a nutshell it comes down to Mars.
Current Mars ISRU information on the web seems a little hard to come by. It can be separated into two major categories: atmosphere processing and regolith processing. You could substitute polar ice cap processing for regolith, but it'll probably be a few missions before we are in a position to do both.
The best reference I can find on atmospheric ISRU for Mars is Zubrin's paper on the Reverse Water Gas Shift reaction. Here's a summary of the science:
The Martian atmosphere is about 95% CO2. The energy requirements to separate out anything else are too great for smaller missions. There is a process for dissociating the carbon dioxide directly into carbon monoxide and oxygen. The carbon monoxide can be frozen into propellant grains and then burned with the oxygen in a cryogenic hybrid rocket motor. However, this motor technology is not mature and the dissociation process, known as zirconia electrolysis, requires fragile components and is not considered robust enough for flight.
Unfortunately, the alternatives to zirconia electrolysis require hydrogen. There is hydrogen available in the ice caps, but let's assume it's out of reach for now. Since we have to send a vehicle to Mars to perform the ISRU anyway, it might as well contain some hydrogen [at the cost of some launch mass].
The goal, then, is to maximize the hydrogen usage on Mars. The technique of choice is called the Sabatier electrolysis reaction. The reaction is as follows:
It is highly exothermic and has an enormous (109) equilibrium constant driving the reaction to the right. It will occur spontaneously in the presence of a proper catalyst (nickel - cheap, or ruthenium - more efficient) at temperatures above 250 C. The efficiencies of test reactors have been between 90% and 96%. A condensor can easily separate the water and methane products.
Now we have methane, a reasonable fuel, and water which can be electrolyzed in accord with:
The oxygen is kept as an oxydizer (or for CELSS in manned scenarios) and the hydrogen is recycled back into the Sabatier reactor. However, there is only half as much hydrogen being recycled as was put in (assuming 100% retention) because methane contains hydrogen. For every 1 kg of hydrogen brought to Mars, this method can produce 4kg of methane and 8kg of oxygen.
The reactor is vastly superior to the zirconia device because it is mechanically simple and very robust. The water hydrolysis technology is very mature, rugged, and quite efficient (90%).
Now the disadvantage: the ratio of methane to oxygen is far from optimal. The process described above creates 2kg of oxygen for every kg of methane. However, the optimal burn mixture ratio for O2 and CH4 is 3.5:1. Add to this that we may want to use some of the oxygen for CELSS and it is clear that we need an oxygen producing reaction in addition to the Sabatier process. The only alternative is to throw away some excess methane but that's rich in hydrogen which we are trying to conserve at all costs.
We could use the zirconia method to make oxygen, but we've already ruled it out as too flimsy. Instead, we employ a reaction called the Reverse Water Gas Shift (RWGS) given by
The carbon monoxide is given off as exhaust while the water is condensed and fed into the same water electrolysis device that the Sabatier reaction is feeding. In essence, this process boosts the amount of water created by the Sabatier, which in turn gives a boost in oxygen output per unit methane. All of the hydrogen used in the RWGS process can be recycled (theoretically) so it doesn't eat significantly into the hydrogen stock for the Sabatier reaction.
The only problem is that the RWGS technology is not very mature and it is a difficult reaction to drive to completion. Getting this process efficient is the one key technology remaining in this scheme. Assuming it can be done, then combining the three reactions given above leads to
The overall reaction is exothermic, requiring no net input to operate! This assumes proper heat management to get the heat from the strongly exothermic Sabatier reaction to the other two endothermic reators. Furthermore, the net output is 4kg of methane and 16kg of oxygen for every 1kg of hydrogen imported. This is a net propellant leverage of 20:1 and a mix ratio of 4:1. This leaves some bonus oxygen left over for CELSS!
The Zubrin paper goes on to talk about some alternatives to the SE/RWGS reactor that produce ethylene or methanol instead, but I'm rambling so I'll save that for later.
When I was at the NASA Centennial Challenges conference in DC last summer, I tried to get them to think about simple uses of in-situ resources-
sandbags. Meteorite shield, thermal mass and insulation, radiation shielding- all from a few nearly massless plastic bags. The response I got back was "We were thinking of using solar energy to sinter regolith into bricks." An inflatable igloo covered with sandbags is too simple, I guess- even as an anteroom to deal with space suits and dust. Oh, well.
CG