COA 3: During the same Earth-Mars launch window, one unmanned ship carries dual ISRU plants to Mars; a second “mostly cargo” ship follows closely behind, with a crew of 12. If the Refinery Ship lands safely, the “mostly cargo” MCCS ship lands a few days later. If the Refinery Ship meets an untimely end, the manned ship aborts, executing a burn or free return to Earth.
There are two primary reasons to consider this COA—first, after considering whether to send a pair of cargo ships one launch window earlier, we conclude there are too many unknowns about autonomous unloading, site setup, etc. Second, we conclude that we can’t afford two additional ships.
There’s also a third reason—what if something goes wrong with the first pair? They might not land successfully; there could be a problem with unloading; there could be critical equipment failures, etc. It’s at least worth considering what the picture looks like without those precursor ships.
In this COA, we have one ship with ISRU plants, making fuel for the return trip. The other ship, a MCCS, has cargo to build a base, and enough supplies to last 4 years. It has life support for 24; the base it carries cargo to build is sized for 24. For this COA, let’s assume Refinery 1 is based on a developmental ship, i.e. a MCCS, so it has life support, as well (although perhaps just for 12—a question for detailed design). At this point, we have complete life support redundancy without requiring the crew to live atop the ISRU plant. They still can’t return to Earth without propellant, but even if the ISRU plants were to fail, resupply is available in two years. The resupply ships could even bring another ISRU plant, if necessary. Since the ships are traveling together, our MCCS won’t need to carry a third ISRU plant, freeing up another 50 tons for other supplies.
The big “con” of this COA is that fuel isn’t available prior to crewed landings, but COA 2 had that, too. The big “pro” of this COA is that we aren’t entirely reliant on autonomous unloading, setup, and mining, which COA 1 required.
This isn’t quite what SpaceX is currently proposing, but it seems like a workable solution.
A fun historical note: based on Musk’s comments, this “second ship,” the crewed ship, already has a name. “Heart of Gold” will be the first manned ship on Mars. Refinery 1 is a bit undramatic for the ISRU ship, so I’ll make a recommendation—it’s hauling a refinery, so how about “Nostromo”? Sure, that one didn’t work out so well, but do you have a better name for a spaceship/refinery? (I mean, Deepwater Horizon? Really? Nah. Besides, if the worst happens and it blows up, there’s always gallows humor, “What did you expect with a refinery ship named Nostromo?”) Note to artists: be sure to include the hull number and use the Starfleet font.
A couple quick details on this COA, so we keep track of them later—the trajectories will have to be carefully planned, to ensure the manned ship has either the fuel to execute a return-to-Earth burn or is proceeding on a free-return trajectory. Probably, the best option is to use a free-return trajectory that starts with a 3.34 km/s Earth departure velocity, 250 day transit to Mars, and 3 year return to Earth. That maximizes the cargo available. Option 2 is a free-return with a 5.08 km/s Earth departure velocity, 180 day transit to Mars, and 2 year return to Earth in the event of an abort. The downside is this reduces our cargo by about a third, to 100 tons. (See, “The Case for Mars,” pp. 83-84 and Musk’s 2016 IAC presentation, slide 37.)
Further constraining this requirement, the Refinery Ship can’t unload itself, so it is reliant on batteries or a successful ship-based solar panel deployment to stay alive until the manned ship arrives. That means the separation window will be narrow—probably not more than a couple of days. That, in turn, will probably affect the amount of cargo each can take. Such is planning.