Alternatives for Translunar Cycling Spaceships

Adding the return-trip hydrogen in the mass landed on the moon plus the tanks to hold the hydrogen causes the mass calculation to diverge, so there's no solution without an additional step in the transportation system.

We have several ways to fix this:

• One-way tankers carrying hydrogen from Terra to Luna instead of just from Terra to LEO
• Extraterrestrial hydrogen available on the moon (from asteroids?)
• Mass driver for the boost off Luna
• Mass driver delivering tanks of O₂ to LEO
• Aerobraking to LEO (Probably won't pay off because of the additional weight of the aerobrake plus additional structure)
• On-orbit ferry from lunar surface to lunar orbit (saves more than half the delta-V)
• Nuclear propulsion
• Hybrid boosters, such as aluminum-LOX, made entirely from lunar material

Any of these might work. All of them cost money. Darn.

Using I_sp=444.4 sec and a 5:1 mixture ratio (which matches the RL-10 rocket engine), we get:

                  LEO to Luna        Luna to LEO

     Delta V       19,754            19,885        ft/sec
     Mo/Mf           3.986             4.023

So, for chemical propulsion between LEO and the surface of Luna, we can use M_o/M_f = 4 for most conceptual design activities. The overall mass ratio is improved if we assume the higher performance of engines such as the Space Shuttle Main Engines, but using a mass ratio of 4 gives us a more conservative analysis.