Thursday, May 31, 2012

Fuel comparison

Which for a crewed mars mission is a better fuel choice: Kerosene or Hydrogen?

While I usually use 5.8 km/s for the mars mission, this paper (link provided by Frank) uses 6.1 km/s so I'll go with that. Let's say we are sending a 60mt ship. Trent's calculator is handy for these results...

LH2 using Rocketdyne SSME Isp=453s. We need about 3.95 times ship mass for fuel. Hydrogen boil off should be less than a ton after 9 months according to the referenced paper. This would require 5 FH fuel flights for $500m.

RP-1 using Merlin Vacuum: Isp=342s. We need 6.17 times ship mass for fuel. This would require 7 FH fuel flights for $700m.

By itself this would suggest LH2 has a 40% cost advantage, but that isn't so. Hydrogen will require more expensive, heavier and voluminous tankage.

How much will the unfueled ship cost? How many flights to assemble it in orbit? How much to add crew and supplies? Assume all of that can be done for $500m. That reduces the cost advantage to 20%. Less considering the tankage differences.

What if the ship has to stay in mars orbit unmanned for an unspecified amount of time? Boil off becomes a bigger issue doesn't it? I think it's a wash but I'd go with kerosene. Just look at the hydrogen tanks on the space shuttle which dominate the structure at launch.

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