SSTO Concept Design Project Team
Section 6.8.7.
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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:

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

Using Isp=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 Mo/Mf = 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.

SSTO Concept Design Project Team

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