For long duration missions such as ones to mars what type of propellant would be best suited?
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According to current information Starship is only capable of using one pair of propellants -- liquid methane and liquid oxygen. Some sketches suggest special tankage (a tank inside a tank) to help keep the landing propellants cold on Mars missions.
answered Feb 26, 2019 at 7:28
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1$\begingroup$ It's unlikely it wouldn't use some other propellant like hydrazine or LN2 for RCS, but it's correct methalox is meant to be the main propellant and to be used for landing at the end of long missions despite being a cryogenic bipropellant. $\endgroup$– SF.Commented Feb 26, 2019 at 9:02
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2$\begingroup$ @SF.: One of the major goals of Starship's design is to reduce the number of fluids required, and enable those fluids to be obtained by ISRU. The cold gas thrusters could be fed by boiling either of the main propellants. Hypergolics would be difficult to produce from local resources and even nitrogen would require dedicated storage tanks and more extreme cooling for liquid storage (lower boiling point than either O2 or CH4), and any separate RCS propellant leaves you with the possibility of running out of that propellant. $\endgroup$ Commented Feb 26, 2019 at 13:15
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$\begingroup$ Yep, current design calls for RCS on Starship (and I think also on SuperHeavy) to be either cold-gas pressurized methane (or possibly oxygen?), or pressure-fed methalox bipropellant (much more efficient but requires more complex valving and reliable ignition) $\endgroup$ Commented Feb 27, 2019 at 9:23