In-Situ Propellant Production

In-Situ Propellant Production, or ISPP, refers to manufacture of rocket fuel from local resources, a subset of In Situ Resource Utilization (ISRU). Production of rocket fuel from lunar resources would be a great boost towards self sufficiency for any lunar colonization effort, eliminating the need for costly imports of a substance which would be needed in large quantities.

Oxygen makes up nearly half the mass of the lunar crust, and is expected to be a major byproduct of industrial operations on the moon. As oxygen comprises much of the mass of currently used propellant systems (as much as 80%), its production alone would cut down the amount of propellant that would have to be imported by a large factor. Manufacture of the remaining fraction from lunar resources is hampered by the fact that most of the substances used in the manufacture of terrestrial propellants are rare or nonexistent in the lunar environment. Hence, this issue has produced a number of unconventional proposals.

Hydrogen

Hydrogen-Oxygen rockets have two main advantages in a lunar environment. First, the specific impulse (essentially the amount of thrust gained per unit of fuel burned) is listed as 450 seconds, the highest of any chemical rocket ever flown, meaning less fuel mass is needed compared to other fuel types. Second, hydrogen-oxygen rockets have been used since the first days of spaceflight, and as such the technology is well developed.

The biggest disadvantage of this approach is the scarcity of hydrogen from lunar sources. Hydrogen is present at the poles in the form of water ice, as well as being available in the regolith in low concentrations (see Volatiles). The mining of water ice in the polar regions is complicated by very cold (100 K and below) temperatures. There is also concern about the depletion of these resources, as the exact amount available is not known. Extraction from the lunar regolith is an extremely energy intensive process, requiring the processing of massive quantities of lunar material at high temperatures. There is a great deal of doubt that these processes can supply the needs of lunar colonization.

One way to address the great expense of extracting hydrogen from the lunar surface is to recycle the rocket exhaust of a rocket-sled to orbit.

Methane

Methane (CH₄) has also been proposed as a fuel for lunar use. Carbon is present in the lunar regolith in concentrations several times that of hydrogen, and heating the regolith to extract volatiles would result in some methane being produced, along with carbon monoxide and dioxide (which could be converted to methane by reacting with hydrogen). Burning methane with oxygen would give a specific impulse of around 300 seconds, requiring more fuel than a hydrogen-oxygen rocket. However, methane is only about 25% hydrogen by weight, and using methane as fuel results in about a 50% reduction in the amount of hydrogen needed for a given launch mass.

Silane

Another alternative is to combine lunar obtained hydrogen with silicon to create silane (SiH₄), which when burned with oxygen would produce water and silica as exhaust. The specific impulse of silane is slightly less than methane, and would use slightly less hydrogen for a given launch weight.

Silane holds an advantage over methane as silicon is vastly more plentiful than carbon (nearly 25% of the moons crust). In addition, technology for using silane as propellant is currently utilized in supersonic ramjets, where it is used as a starting propellant since it ignites spontaneously in air. As such, technology for handling and injecting silane into a combustion chamber is already developed.

One disadvantage of this approach is the complexity of producing silane. The process used terrestrially for silane production is long, rather complex, and requires a number of reagents that are quite rare on the moon. Methane and hydrogen production are quite straightforward by comparison.

Sulfur

Another proposed solution is to use sulfur as a propellant, in what is sometimes referred to as a "Brimstone Rocket". Sulfur melts at about 115 °C, which could be easily achieved by preheating the fuel tank before launch. Burning this molten sulfur with liquid oxygen would produce sulfur dioxide as exhaust, with a specific impulse of around 285 seconds. Sulfur is present in the lunar regolith in much higher quantities than both hydrogen and carbon, some mare soils containing as much as .27% by weight.. In addition, unlike hydrogen and carbon, sulfur compounds may be extractable by magnetic benefication rather than heating the regolith, greatly reducing both the complexity and energy requirements of gathering them.

Aluminum

Is is proposed that aluminum could be used as a fuel. This would have the advantage of virtual inexhaustability, as aluminum makes up a significant percentage of the moons crust. One downside is aluminum's high melting point(compared to other propellants), which would make conventional bi-propellant fuel processes difficult.

One proposed solution to this problem is to mix finely powdered aluminum with liquid oxygen, adding a small amount of fumed silica to the mix. The result would be a gelled monopropellant which would provide an estimated specific impulse of 285 seconds, the same as with sulfur. This approach has been tested on a small scale, and was determined to be reasonably stable.

One potential issue with this approach is the production of dust. As the exhaust cools, particles of aluminum oxide would form, which could be an issue for heavy use. Proper design of the engine could mitigate this however. If the combustion was complete and all products were entirely vaporized upon ejection, the resulting dust should be quite small, perhaps microscopic in size, and traveling at sufficient speed to allow for wide dispersal. Current spacecraft are already designed to handle dust of this size, and its generation should not endanger their use.

Silicon

Lunar silicon could possibly be used in the same manner as aluminum, as they are similar in both atomic weight and potential energy, and hence could have similar specific impulses. Silicon has been utilized in test mixtures, powdered and mixed in a liquid oxygen gel as with aluminum. As silicon dioxide is the most common component of the lunar crust (nearly half by weight), it's use in this manner is attractive.