This story keeps getting more interesting as I trawl around the Mars-Soc web-site. Bob Zubrin discusses the plan in more detail…
2. Technical Alternatives within the Mission Architecture
a. MAV and associated systems
In the plan described above, methane/oxygen is proposed as the propulsion system for the MAV, with all the methane brought from Earth, and all the oxygen made on Mars from the atmosphere. This method was selected over any involving hydrogen (either as feedstock for propellant manufacture or as propellant itself) as it eliminates the need to transport cryogenic hydrogen from Earth or store it on the Martian surface, or the need to mine Martian soil for water. If terrestrial hydrogen can be transported to make the methane, about 1.9 tons of landed mass could be saved. Transporting methane was chosen over a system using kerosene/oxygen for Mars ascent, with kerosene coming from Earth and oxygen from Mars because methane offers higher performance (Isp 375 s vs. Isp 350 s) than kerosene, and its selection makes the system more evolvable, as once Martian water does become available, methane can be readily manufactured on Mars, saving 2.6 tons of landed mass per mission compared to transporting methane, or about 3 tons per mission compared to transporting kerosene. That said, the choice of using kersosene/oxygen for Mars ascent instead of methane oxygen is feasible within the limits of the mass delivery capabilities of the systems under discussion. It thus represents a viable alternative option, reducing development costs, albeit with reduced payload capability and evolvability.
b. ERV and associated systems.
A kerosene/oxygen system is suggested for Trans-Earth injection. A methane/oxygen system would offer increased capability if it were available. The performance improvement is modest, however, as the required delta-V for TEI from a highly elliptical orbit around Mars is only 1.5 km/s. Hydrogen/oxygen is rejected for TEI in order to avoid the need for long duration storage of hydrogen. The 14 ton Mars orbital insertion mass estimate is based on the assumption of the use of an auxiliary aerobrake with a mass of 2 tons to accomplish the bulk of braking delta-V. If the system can be configured so that that Dragon’s own aerobrake can play a role in this maneuver, this delivered mass could be increased. If it is decided that the ~1 km/s delta-V required for minimal Mars orbit capture needs to be done via rocket propulsion, this mass could be reduced to as little as 12 tons (assuming kerosene/oxygen propulsion). This would still be enough to enable the mission. The orbit employed by the ERV is a loosely bound 250 km by 1 sol orbit. This minimizes the delta-V for orbital capture and departure, while maintaining the ERV in a synchronous relationship to the landing site. Habitable volume on the ERV can be greatly expanded by using an auxiliary inflatable cabin, as discussed in the Appendix.
c. The hab craft.
The Dragon is chosen for the primary hab and ERV vehicle because it is available. It is not ideal. Habitation space of the Dragon alone after landing appears to be about 80 square feet, somewhat smaller than the 100 square feet of a small standard Tokyo apartment. Additional habitation space and substantial mission logistics backup could be provided by landing an additional Dragon at the landing site in advance, loaded with extra supplies and equipment. Solar flare protection can be provided on the way out by proper placement of provisions, or by the use of a personal water-filled solar flare protection “sleeping bag.” For concepts for using inflatables to greatly expand living space during flight and/or after landing, see note in Appendix.
…which gratifyingly echoes my own thoughts. Landing a Dragon directly on Mars has a great appeal and as a Mars Descent Vehicle it’s a good system, given the modifications Zubrin outlines. But is it a Mars Habitat? The Inflatable extensions make it viable and I was wondering if Bigelow, SpaceX and Mars-Soc couldn’t combine forces on a design. Zubrin argues for eventual extensions of the architecture itself, calling for eventual Heavy Lift systems able to throw 30 tonnes to Mars, but IMO the Falcon Heavy Tanker modification is sufficient to launch ~24.7 tonne payloads now and with an LH2/LOX Stage II it might easily launch ~30-40 tonnes. Alternatively two FHTs can be ganged to launch 55-60 tonnes directly now. However such modifications are deployed is perhaps irrelevant. What’s needed is the political will to commit to Mars Colonization, not just a one-off stunt. All the good ideas to improve how we get there are irrelevant until we actually do…