Engine for Trans-Lunar Shuttle

I based the reference mission hardware on using the RL-10 engine for trans-lunar propulsion even though it was a bit more powerful than we really need to do the job. Here are some specs for the folks working on spaceship designs:

     RL-10 Engine:         RL-10-A-3-3A

     rl_10_eng_wt          305       lb
     rl_10_mixture           5       5:1 O:H wt
     rl_10_thrust       16,500       lb/engine
     rl_10_isp             444.4     sec
     rl_10_ch_press        475       psi
     rl_10_max_firings      25       max
     rl_10_eng_time          1.25    hours
     rl_10_eng_diam         39.54    inches
     rl_10_eng_len          70.1     inches

     Reference: NASA Publication NP-133 "Accessing Space," NASA 1990		

Based on the DC-X experience, the RL-10 is a bit more robust than even its specifications indicate; but we'd expect the specifications to be a bit conservative.

About the Ascent Stage: Besides the decision to use hypergolic fuel for that one stage, you can see why I didn't want to use an RL-10 to lift a 1500-lb vehicle off the moon. The RL-10 comes in a throttleable version, but 10 g's is a tad bit more acceleration margin than we need.