Up to now, we have assumed mining in-situ water, which is probably the most difficult part of this process. Allow me to recommend two online resources: http://www.nasa.gov/sites/default/files/atoms/files/mars_water_isru_planning-hangout-5-24-16.pdf and https://planetaryprotection.arc.nasa.gov/file_download/90/29-Sanders.Mars.ISRU.PP_Sanders.V2.pdf as a homework assignment, and then let’s talk about Zubrin’s Mars Direct fuel plan. Both of these resources walk through variety of research done by NASA on what it will take to mine water on Mars. If you’re wondering what sort of technology development today’s NASA is capable of, grab a big cup of coffee (if it’s morning) or a big bourbon (if it’s evening) and relish this work.
The summary version from both presentations is that if we’re going to rely on surface water, then we need to be absolutely sure we can access it and get it to the ISRU plant. This will require orbital reconnaissance, ground surveys, and precursor missions to verify technology… all of which requires time, technology development, and money. We’ve already concluded that’s very problematic, leading us to the uncomfortable situation of putting manned ships on Mars with no proven way to get them home.
The primary problem isn’t LOX; that’s trivial. Power plus CO2 plus equipment equals oxygen. The problem is methane, CH4. There’s plenty of carbon available on Mars (see: CO2), but hydrogen…not so much. Hence the need to mine water from regolith or grab it directly from Mars’ atmosphere, both with a single objective: hydrogen. Hydrogen as feedstock for our ISRU plant; hydrogen to make methane.
Now, consider an alternative. What if we could put off mining water, and instead take our own hydrogen to Mars? As Zubrin points out, the mass penalty for bringing your own hydrogen still results in an 18:1 improvement over no ISRU. What would this look like?
At a first approximation, round figures, 61 tons of hydrogen would be enough feedstock to completely fill Nostromo’s 240 ton fuel tank. (Our friends at http://www.wolframalpha.com can help with this one, “hydrogen in 240 tons of methane.) That doesn’t seem like very much, considering that our ships are all capable of transporting at least 150 tons of cargo. Unfortunately, hydrogen presents a serious design problem—even in liquid form, it is extremely low density. In fact, if you take the entire pressurized volume of a BFR (825 m3, about the same as a 747-8 freighter), that’s what it takes to store 58 tons of liquid hydrogen. Sure, 58 tons and 61 tons are close, but the point is that liquid hydrogen takes 14 cubic meters per ton. That means that to fill a ship’s tanks with methane for the return voyage, the hydrogen required will consume the entire available payload of another ship. Not its cargo mass, not the volume of its holds—it needs a specialized tanker whose sole purpose is to transport LH2, replacing the entire pressurized compartment.
On the one hand, this is a real problem. An obvious workaround to the problem of in-situ water mining has always been the idea of bringing along the necessary hydrogen. Now, we realize that doing so actually requires another ship whose sole purpose is bringing that hydrogen. But, with the downsized SpaceX plan, we’re actually just looking at a specialized tanker travelling with Nostromo and Heart of Gold. In fact, Musk even mentions in his 2017 Reddit AMA that some of the optimized tanker designs look pretty weird…suddenly, bringing along hydrogen seems feasible, at least for the first manned landing. It brings up a number of problems, of course, starting with the infrastructure needed to transfer hydrogen from the special tanker to Nostromo. But the risk it buys down is tremendous. And, if we need to continue the pattern, additional hydrogen tankers in our architecture seem a small price to pay to ensure we can get the crews home.
Of course, sending an extra hydrogen tanker one-way for every MFR or MCCS going to Mars doesn’t seem sustainable long term, but it certainly gives us some valuable planning flexibility in the near term. The first couple of manned missions are where we really need to buy down risk.