Originally, I had specified that there are three types of rocket propellants: solids, storable liquids, and cryogenics. After further evaluation I have discovered three additional types of propellants, hybrids of solid and liquid propellants, nuclear propulsion, and experimental propellants. These three new propellants, however, do not have enough of a service history for me to consider them for my project.

    Optimally, the best propellant would be storable propellants. This would circumvent the issues surrounding the use of cryogenics, which are profoundly cold and can cause ice to build up on the fuel tank. Unfortunately, most storable propellants are highly toxic, so much so that several of them are known to cause cancer.

    There is one storable propellant that is not a severe health risk – kerosene, or ordinary jet fuel. The Russians have used this as a propellant for several decades, nearly perfecting it and even selling their rocket engines to large American corporations such as Boeing.

    Kerosene does not solve the entire problem. Since kerosene is only a fuel, an oxidizer is needed. There are several high performance storable oxidizers, including nitrogen tetraoxide, but there are significant health and handling risks associated with it. As my research continued, I realized that there are not any rocket engines that both use storable propellants and provide enough thrust for my vehicle.

    Alas, there is oxygen, the best oxidizer there is. Liquid oxygen is a cryogenic fuel, being over two hundred degrees below zero. This has been the Russian's oxidizer of choice for their kerosene-fueled engines, provided dozens of possible engines to choose from. None of these kerosene/oxygen engines provided enough thrust and burn time to send any crew transfer vehicle to the International Space Station.

    The Russians foresaw this problem, and in the late eighties started development of a rocket engine that used kerosene, liquid oxygen, and liquid hydrogen, providing both the benefits of storable propellants and the power of cryogenics. This engine is the RD-701 engine, the very same engine used for the MAKS program that I originally based my launch concept on. However, the fall of the Soviet Union halted development of MAKS, stranding the RD-701 in the experimental stage.

    The RD-701 shows great promise, using all three propellants at first, creating more thrust than either hydrogen or kerosene could develop on its own. Later, after the kerosene has been consumed, the engine switches to only hydrogen and oxygen, which is better in the latter part of a launch since hydrogen has a very low mass, allowing more of the thrust to be focused on the acceleration to orbital velocity.