Ascent Vehicle
Section 4.2.4.
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Fuel Options for the Ascent Stage

A topic of frequent debate is that of whether to use low-efficiency hypergolic fuels or use cryogenic fuels for launching the ascent stage.

The current baseline for the ascent stage uses hypergolic UDMH (unsymmetrical dimethylhydrazine) and N2O4 (nitrogen tetroxide). This yields an isp of only 287s; however, it is easily stored for the duration of the mission. It is also very dense and reliable.

The other main option is to use cryogenic liquid hydrogen and loquid oxygen, which are much more energetic then the baseline mission's fuels, with an isp of 450s or more. However, boiloff of the fuels, especially the liquid hydrogen, is a concern. Also, ascent from the lunar surface demands extreme reliability, and cryogenic fuels are far less reliable than hypergolics, involving more parts and operations for a successful engine start.

Boiloff for this option, or the mass of insulation required to prevent it, reduces the attractiveness of it considerably. Also, as the burn is short and the mass moved is small, the needs for the highest available performance are significantly reduced.

There are other options, such as using a non-cryogenic alternative to LH2. It is not at all hard to shade LOX tanks enough that the equilibrium temperature is less then the boiling point of oxygen. The story is completely different for liquid hydrogen, where boiloff rates would be unacceptable due to hydrogen's very boiling point.

A typical isp for a hydrocarbon alternative fuel would be 390s, if used with liquid oxygen. Although it would have the same reliability concerns as the LH2/LOX option, it would not have significant boiloff issues and the fuel would be fairly dense.

Assuming a total delta-V of 6131 ft/s, and margins of 3%, the hypergolic craft (isp 287s) would have a mass-ratio of 1.98. If a higher-efficiency fuel was used with an isp of 390s, then the mass ratio would fall to 1.65. With a total dry mass of the ascent stage being 1627 lbs, the hypergolic craft would require about 1600 lbs of fuel. The hydrocarbon/LOX vehicle would only require 1050 lbs of fuel, saving 550 lbs of mass which can be used on the lunar surface.

If nothing else, it would allow substantially more lunar samples to be returned then the 200 lbs planned. It would also permit 550 lbs more outfitting to be carried to the lunar surface, in addition to the 10,000 lbs already planned.

However, an additional 550 lbs is only a 5% larger lunar surface habitat, and there is a need for absolute reliability and simplicity in the ascent stage. The insulation and tank mass would also eat significantly into the 550 lb mass saving, considering the heavier ascent stage needs to be transported back to LEO.

All considered, using the hypergolic fuels makes a good deal of sense in terms of reliabiity, and acceptable performance. If there is a need to bring 550 lbs more lunar habitat to the Moon, it would be a better solution to simply buy a larger launcher.

Ascent Vehicle

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