Gas-generator cycle



The gas-generator cycle, also called open cycle, is one of the most commonly used power cycles in bipropellant liquid rocket engines.

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.
 * F-1—RP-1/LOX engine used on the first stage of Saturn V. Most powerful single combustion chamber liquid-fueled engine ever flown.
 * 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.
 * Vulcain—A family of European first stage engines using LH2/LOX flown on Ariane 5 and Ariane 6.
 * Merlin—RP-1/LOX engine developed by SpaceX for Falcon 9 and Falcon Heavy, used on both first and second stages.
 * RS-68—LH2/LOX engine built in the 1990s by Aerojet Rocketdyne. Largest hydrogen-fueled rocket engine ever flown.
 * CE-20—Indian LH2/LOX engine developed in the 2010s for use on the LVM3 launch vehicle.
 * YF-20—Chinese N2O4/UDMH engine developed in the 1990s and used on Long March 2, 3, and 4.
 * TQ-12—LCH4/LOX engine developed by LandSpace. First flew in 2022 on Zhuque-2.