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I've been musing over the possibility of an ITS booster mated to an ITS spaceship powered by RS-25D's, perhaps in an Octaweb arrangement or something similar, same diameter as Elon Musk's spaceship and perhaps a slight growth in length.
NASA is currently working on long duration cryogen storage in space and Boeing has developed composite LH2 tanks that don't require autoclaving. If ITS landed on Mars and the only thing the colonists had to do to refuel ITS with LOX/LH2 was water electrolysis, how does this compare to the Sabatier reaction in terms of energy required? The loaded mass of the spaceship will be much lighter, the specific impulse of the RS-25 is much higher, the RS-25 has a perfect performance track record, and the lesser mass of the upper stage should either enable greater payload or fewer refueling launches or both.
Any thoughts?
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Nasa was working on the active cooling for use of LOx/LH2 to which the papers have been posted before here as the concern for the boiloff to get to mars let alone the return using it was in question so going with the recently rehab'd Rs25's would make sense in a way as once on mars that same active cooling could be put to use as part of the process reducing energy needs on mars.
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According to Elon Musk, the ITS Spaceship has 12m diameter tanks, like the ITS Booster, and the ship is 49.5m in length.
The SLS core stage holds 196,000 gallons of LOX and 537,000 gallons of LH2 and each RS-25 consumes approximately 226 gallons of propellant during each second of operation at full thrust. It won't be possible to store that much propellant without increasing the length of the Spaceship or decreasing habitable volume, but the following fuel tank dimensions should be roughly equivalent to what the SpaceX graphics currently show.
1 US gallon of LOX weighs 4.322kg
1 US gallon of LH2 weighs .2679kg
LOX Tank - 12m diameter cylinder 5.7m in length would hold approximately 167,311 (448,157kg) gallons of oxidizer
LH2 Tank - 12m diameter cylinder 15m in length would hold 448,157 (120,061.26kg) gallons of fuel
I estimate the overall length of the propulsion section would be about 28m in length.
The redesigned Spaceship would use 7 RS-25's that produce 15,953kN of combined thrust vs 21,000kN of combined thrust for the 6 Raptor Vac engines. Mass of the LOX/LH2 NASA variant of the Spaceship would be approximately 978t vs 1,950t for the LOX/LCH4 SpaceX variant. LEO-Ken payload performance for both variants appears to be nearly identical, at about 300t.
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Someone out there is probably wondering why we'd design an ITS Spaceship with more or less identical payload performance to the original ITS. The answer is that you'd only have to launch the ITS tanker twice to completely refuel the ITS Spaceship. Three ITS launches per TMI seems a bit simpler than six launches per TMI.
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