Ascent Fuel Comparison

David Burkhead

With a 1000 psi chamber pressure and expanded through a nozzle to 1 atmosphere, alcohol/LOX has an Isp of about 279 seconds as compared to 390 for hydrogen/oxygen under the same conditions (Source: "Rocket Propulsion Elements"). With an RL-10 engine, alcohol has an Isp of 334, as opposed to 424 for LH2/LOX.

In terms of mass ratio requirements, with a velocity of 1630 m/s for low Lunar orbit and allowing a 20% delta v allowance, this means a total delta v of 1956 m/s. With the ascent stage weighing 1600 lbs dry, we have:

Between the first two there's not much difference, actually, and if mass ratio for the ascent stage vs. avoiding hydrogen were the only considerations alcohol/oxygen would probably weigh heavier in this comparison. However, they are not the only considerations.

Another important consideration is that we are looking at the integrated stages option, where the ascent stage uses one of the landing stage engines (saving some weight). In this case, that difference in mass ratios makes for a pretty significant change in mass on the moon's surface, very important to our overall mission. Also, if storability is our goal, then alcohol/LOX is not the way to go. The hydrazine derivatives in combination with N2O4 or RFNA are both higher Isp and higher density (lower tank weight).

Also, with hydrogen/oxygen, the reserves, if unused as reserves, can go to fuel cells, water-making, breathing (in the case of oxygen), and other uses that can extend our stay on the moon.

As for adding aluminum to the alcohol, that would increase Isp somewhat, but at a pretty high cost. This technology still has a problem with clogging fuel lines, damaging pumps, clogging fuel injector ports, and aluminum settling out of the fluid, among other undesirable things. There's a lot of work that needs to be done yet, vs. something like the RL-10, which is off the shelf.

It appears the best bet would be hydrogen/oxygen. After that would be hypergolics.