Propellants & Concrete
When a base is finally established on the moon, it will need to be as self sufficient as it can with respect to a variety of needs such as food, water, oxygen, energy, propellant and building materials. Our company has focused on the need for propellant and building materials. In the 1980's, NASA was using liquid oxygen (LOX) and liquid hydrogen as a baseline propellant system. While the oxygen could be obtained by processing lunar soil, the liquid hydrogen would have to come from earth. This would be a major logistical undertaking and expensive. It has been more recently speculated that water may be present on the moon, which could be used to make hydrogen and oxygen for rocket propellant. However, it seemed to us that if water was present on the moon that using the water as propellant would be a tremendous waste of the water, as it would forever be eliminated from the moon. A better use would be for life support where it could be recycled over and over. If water is not used for propellant and hydrogen is not brought from earth, what could be found on the moon to serve as a fuel with LOX? Our company looked at the results from the analyses of the lunar soil samples brought back from the Apollo missions and identified several potential fuel candidates. They were phosphorus, sulfur, magnesium and aluminum. All of these fuels could be obtained by processing the lunar soil.
It is well known how to get LOX into a rocket engine combustion chamber under pressure, but how would it be possible to get any of these fuels into a combustion chamber? Phosphorus and sulfur could actually be the easiest as they both melt and could be fed as liquids. Phosphorus melts at around 111o F. It has the advantage of auto-igniting with oxygen. Sulfur melts at 239o F, but if heated above 482o F, it will also auto-ignite with oxygen. For either fuel, electrical heaters would preheat the propellant tank, feed lines and valves prior to using the engine. Once the engine was operating, heat from the combustion process could be used to continue heating the fuel so that it would stay molten. Regardless of the propellant combination being used, it is envisioned that a small amount of LOX will be kept in a separate tank. This LOX will be circulated through the combustion chamber and nozzle walls to provide cooling before entering the combustion chamber to be burned with the fuel. The use of LOX for cooling liquid rocket engines has been demonstrated by other companies.
For aluminum and magnesium fuels, they would be in the form of a powder as their melting temperatures are too high. They could be injected into the combustion chamber with an inert carrier gas. We have built a rocket engine using carbon dioxide and magnesium powder for use on Mars that blows the magnesium powder via nitrogen gas into the engine's combustion chamber. This approach could also be used with magnesium or aluminum powder for a LOX-aluminum or LOX-magnesium rocket engine. An additional option available with aluminum is to suspend the aluminum powder in gelled LOX to form a monopropellant. This option is not available for magnesium as it is shock sensitive in LOX and will detonate.
Our company has made LOX-aluminum monopropellant and determined that it is not shock sensitive. It burns in a controlled manner such that it could be fed into the combustion chamber without the flame in the chamber flashing back up into the propellant tank. The viscosity of the LOX-aluminum monopropellant was no higher than 300 cps and decreased to 100 cps with increasing shear rate. A centrifugal pump with backward leaning blades could be used to pump the monopropellant. This type of pump is commonly used to pump slurries. One of the technical issues to resolve was whether the aluminum powder would settle in the gelled monopropellant. Our tests showed that there was no indication of settling in a six to seven hour period, which was the time limit of the tests.
As part of our research, we made a small rocket engine fueled by the LOX-aluminum monopropellant. The propellant tank was surrounded by a liquid nitrogen bath to keep the LOX from boiling off. The propellant feed lines ran through a liquid nitrogen bath on their way to the combustion chamber. A piston pushed against the propellant to feed the propellant into the rocket engine chamber. While the thrust was only about a pound, the engine was started and stopped several times without a flashback of the combustion flame front into the propellant tank. The tests showed that this approach was feasible in larger engines.
From these tests and our results with magnesium powder and carbon dioxide rocket engines, there are two demonstrated approaches to making a lunar soil propellant rocket engine, suspend the fuel in LOX or inject the metal powder into the rocket chamber with an inert carrier gas. This technology is now ready for the next step which is the construction of a scaled up rocket engine complete with propellant tanks, feed lines, valves and control systems.
Another area of research our company has been pursuing is the use of lunar materials to make buildings on the moon and in space. Dr. Lin of Construction Technology Laboratories first explored the concept of using lunar concrete as a building material. The use of lunar concrete to build a lunar base and other support structures would eliminate much of the need to bring materials from earth. Research has indicated that lunar soils can be used to make Portland cement, but this approach has one major drawback. It requires water. If water is on the moon, using it for lunar concrete may not be one of its best uses as the water would be lost forever in the formation of the concrete. Our company has chosen to pursue the development of cement based on a polymer, which does not require the use of water. Unfortunately, typical polymers that can be used as a binder or cement require hydrogen and carbon atoms to form the required binder molecule. Neither of these atoms is available in significant quantities on the moon. However, lunar cement might be made with silicates. Our current research is focusing on the use of a sodium silicate type adhesive mixed with fibers to serve as a concrete binder or cement. The lunar concrete properties can be tailored with the amount of fibers in the cement. The fibers can be silica, aluminum, iron or magnesium. This type of cement can be made with materials found on the moon.
The use of concrete requires building forms. Our company is looking at the possibility of using the concrete to form structural members such as tees, channels, beams, rails, flat sheets and columns through an extrusion process. While the extrusion of concrete shapes is not uncommon on earth, the shapes are somewhat limited. With the lower lunar gravity and a thicker concrete due to the addition of fibers, the extrusion of these more complicated shapes should be possible.
The use of lunar concrete flat panels and beams can be used to make hexagonal rooms, which can be stacked on top of each other to form a large habitat. The building is sealed to eliminate air leakage by spraying the inside walls with a polymer material. The air pressure inside the building trying to blow the walls out can be counter balanced by piling lunar soil on top of the building. This also provides radiation protection to the inhabitants.
While lunar concrete is commonly mentioned for use in buildings, our company is looking at its use to make rails for a lunar railroad. The top and inside portion of the rail would be sheathed with iron from the moon. The railroad ties would also be made out of concrete. While many envision flying transportation systems on the moon, the use of a railroad would require the least amount of fuel to move passengers and cargo from one lunar base to another or some other site on the moon. The energy for the locomotives engines could come from the burning of LOX and one of the fuel candidates mentioned for lunar rocket engines.
Another application for lunar concrete may be to build orbiting structures in space. These could be in lunar or earth orbit. This approach could be particularly attractive for building a GEO space station around earth. The building materials from the moon could be shot into space through a rail gun located at the lunar base. They would not have to be taken from earth up to orbit where the propulsion requirements are enormous climbing out of earth's gravity well. Our company has been investigating the use of an inflatable concrete form to make these orbiting structures. The inflatable form consists of an inner and outer membrane with netting just under the outer membrane. The netting provides tensile reinforcement as well as gives it shape. It also gives a structural redundancy so that catastrophic failure is less likely. The inner membrane of the form is the inner surface holding the concrete as it cures. It also serves as a sealing agent when the structure is pressurized for habitation.
While the development of the first lunar base is still some years away, we have been working on some of the technologies that will be required to make the base self sufficient as well as allow it to contribute to the further exploration and settlement of the solar system through the resupply of propellants to spacecraft and the building of large space structures.