A bipropellant rocket is a rocket that uses separate fuel and oxidizer propellants. Bipropellant systems are more efficient than monopropellant systems, but they tend to be more complicated because of the extra hardware components needed to make sure the right amount of fuel gets mixed with the right amount of oxidizer (this is known as the mixture ratio.)

Thousands of combinations of fuels and oxidizers have been tried over the years. Some of the more common and practical ones are:

The best mixture oxygen and hydrogen suffer from the deep temperature (20K (-253°C)) which is needed to store the fuel. Also the low density (70 kg/m3), which makes the need for large and heavy tanks, is a disadvantage. The use of lightweight foam, to insulate the cyrogenic tank, proved do be dangerous as seen in the Columbia (STS-107) accident. For storable intercontinental rockets or interplanetary space crafts the cooling is an unsolvable problem. Because of this the mixtures of hydrazine and its derivatives in combination with nitrogenoxides are used for these rockets.

see also: spacecraft propulsion,Tripropellant rocket