Propellants The term liquid propellant is used to define both liquid oxidizers (liquid oxygen, liquid flourine, nitric acid, etc.) and fuels (RP-1, alcohol, liquid hydrogen, etc.). The choice of propellant is critical to system design since it affects performance, and component design criteria. Propellants can be classified as monopropellants or bipropellants. Monopropellants - may be a mixture of oxidozer and fuel or a single compound that can be decomposed. Monopropellants simplify tankage, feed plumbing and injection, but most monopropellants (e.g., hydrogen peroxide) have relatively low performance. Bipropellants - require separate tanks for an oxidizer and fuel. While leading to more complex feed systems, bipropellants are the most commonly used today because of a good combination of performance and safety. Cryogenic propellants - liquified gases with very low boiling points (- 230ì F to -430ì F) at ambient pressure. The most common cryogenics are liquid oxygen, liquid hydrogen, liquid flourine and oxygen diflouride. Hypergolics - bipropellant combinations that ignite spontaneously upon mixing. These give greatly simplified ignition but pose the hazard of explosion due to tank or other hardware failures. The RL-10 engine is an LO2, LH2 engine with the hydrogen tank on top and the oxygen tank beneath. The RL-10 uses hydrazine (N2H4) for attitude control and burns the residual hydrazine at the end for extra lift. In general, missions don't fly to depletion of the fuel. If one engine shutsdown first - tips the vehicle.