Fuel Requirements

Several different fuel combinations have been used in heavy lift lauch vehicles. The earliest ones were simply liquid hydrogen and liquid oxygen, which, when combined, produces a very significant amount of thrust, and whose only combustion byproducts are heat and water vapor. This approach was used for the upper stages of the Saturn V, and is still used for the main engines on some HLLVs, such as the Ariane 5, Delta IV and the Space Shuttle. It is also often used for upper-stage motors, due to its high specific impulse.

Other fuel options include those used by solid-fuel rockets, combinations of various liquid fuels such as RP-1 and liquid oxygen as used in the Atlas 5, and hypergolic fuels, such as unsymmetrical dimethyl hydrazine and nitrogen tetroxide, used in the Proton rocket.

The advantages of liquid hydrogen and liquid oxygen include the largest thrust to mass ratio (which equates to the highest specific impulse), and the absence of any toxic byproducts. On the other hand, both liquid hydgrogen and liquid oxygen must be constantly replenished as they boil off, and the time the rocket can remain on the launch pad is short due to the cryogenic handling required.

Solid-fuel rockets are generally unthrottleable, but recent advances in catalyst triggers make hybrid rockets as throttleable as liquid-fuel rockets.

The main advantage of fuels such as dimethyl hydrazine and nitrogen tetroxide is that they require no cryogenic handling, and can thus sit on the launch pad for long periods of time. Their main problem, however, is that they're highly toxic, and thus require special handling.