Phase II Final Report - NASA's Institute for Advanced Concepts

Chapter 3.0 Vehicle Design
3.5 Fuel Storage and Production
piston, similar to that of a conventional internal combustion engine. However, there are a number
of issues associated with the used of this type of fuel. Mainly, the solid metal oxides will
condense within the combustion cylinder and potentially clog the engine and be a source of wear
on the piston.
The main products of the combustion reactions listed in Table 3-18 are condensed metal oxide
and CO. Of these, Mg is the easiest to ignite and has the highest burn rate, which is necessary to
produce the required gas pressure for operation of the vehicle.
Magnesium oxide, which makes up about 7.8% of the soil on Mars, is present in significant
enough quantities to potentially mine the soil for the magnesium that is needed. If the magnesium
can be effectively separated out of the soil, it will probably need to be dissolved in solution
to make it usable as a propellant [278, 279]. One potential candidate would be methanol
(CH 3 OH). However, the use of this type of fluid would require a supply of hydrogen as well as
the ability to separate out carbon and oxygen from the atmosphere. This diminishes the attractiveness
of a system that utilizes the CO 2 directly out of the Mars atmosphere. Based on results
given in References 278 and 279, there are other significant issues with using methanol or any
other fluid as a carrier for the magnesium. The magnesium would tend to settle out of the mixture,
requiring frequent mixing. Also, the carrier fluid would need to evaporate before ignition of
the magnesium would take place. There may be other carrier fluids that would better than methanol;
however, a different approach using a gas as the carrier might work.
A gas would eliminate the problems of evaporation and mixing, as well as the issues associated
with the production of the carrier fluid. The ideal gas to use would be the Mars atmosphere
itself. It may be possible to devise a mixing chamber on board the vehicle that would be used to
mix the magnesium and atmosphere (CO 2 ) prior to injection into the combustion chamber. The
atmosphere could be pumped in at a rate that would stir up the magnesium particles and form a
suspension of magnesium power within the tank. The magnesium could be gravity-fed into this
mixing chamber at a rate that would maintain the correct concentration of magnesium within the
chamber (similar to sand falling through an hourglass). The rate of magnesium power that enters
this mixing chamber could be controlled by changing the size of the orifice through which the
magnesium power passes. This suspension could then be injected into the combustion chamber.
This scheme would not require any gas production and would utilize a fairly simple control
scheme of adjusting the atmosphere injector and opening to the magnesium power tank. The
mixing chamber would need to be large enough to allow the magnesium to be suspended at the
correct mixture ratio prior to being injected into the chamber.
The design and evaluation of a CO 2 burning engine is beyond the scope of this effort; however,
the concept has some potential benefits and may be worth evaluating in further detail during any
future effort.
3.5.3 Propellant Production and Storage
To evaluate the tradeoff between carrying hydrogen and producing fuel on Mars or just carrying
the fuel directly, the overall mass of a system that produces fuel on the surface will be estimated,
and this will be compared to the amount of fuel that can be carried directly from Earth utilizing
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