Gas-generator rocket cycle. Some of the fuel and oxidizer is burned separately to power the pumps and then discarded. Most gas-generator engines use the fuel for nozzle cooling.
The gas-generator cycle, also called open cycle, is one of the most commonly used power cycles in bipropellant liquidrocket engines. Part of the unburned propellant is burned in a gas generator (or preburner) and the resulting hot gas is used to power the propellant pumps before being exhausted overboard, and lost. Because of this loss, this type of engine is termed open cycle.
The gas generator cycle exhaust products pass over the turbine first. Then they are expelled overboard. They can be expelled directly from the turbine, or are sometimes expelled into the nozzle (downstream from the throat) for a small gain in efficiency.
The main combustion chamber does not use these products. This explains the name of the open cycle. The major disadvantage is that this propellant contributes little to no thrust because they are not injected into the combustion chamber. The major advantage of the cycle is reduced engineering complexity compared to the staged combustion (closed) cycle.
Examples
RD-107, RD-108—Soviet engine type developed in the 1950s, used on R-7 family vehicles including the active Soyuz-2.[1]
F-1—RP-1/LOX engine used on the first stage of Saturn V. Most powerful single combustion chamber liquid-fueled engine ever flown.[2]
J-2—Upper stage LH2/LOX engine developed in the 1960s and used on Saturn V.
RS-27A—American RP-1/LOX engine first flown in 1990.[3]
Vulcain—A family of European first stage engines using LH2/LOX flown on Ariane 5 and planned for Ariane 6.[4]
