Expander cycle

Expander cycle engines use energy from cryogenic fuels going from a liquid to a gas state to drive turbines necessary for the operation of the rocket engine. In many engines some of the fuel us combusted in a preburner to supply the power to these turbines. In expander cycle engines the same fuel that will be used in the main combustion chamber is warmed to make the fuel go from a liquid to a gas, and the expansion of the fuel is then captured by the turbines before the fuel is burned to produce thrust for the vehicle.

Because there is no need for the "preburner" the engine is less complicated and ligher, which results in more useful load capacity.