Gas-generator cycle (rocket)
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The gas generator cycle (also known as open cycle) is a power cycle of a bipropellant rocket rocket engine. Some of the propellant is burned in a gas-generator and the resulting hot gas is used to power the engine's pumps. The gas is then exhausted.
There are several advantages to the gas generator cycle over its counterpart the staged combustion cycle. The gas generator turbine does not need to deal with the counter pressure of injecting the exhaust into the combustion chamber, this allows the turbine to produce more power and increase the pressure of the fuel and combustion chamber, thus increasing specific impulse or efficiency; also this reduces wear on the turbine, increasing its reliability, reducing its production cost and increasing its operational life-span (particularly advantageous for reusable rockets).
The main disadvantage is lost efficiency due to discarded propellant, though this efficiency loss can be outweighed in production engines by the higher chamber pressure's increase in net efficiency. Even so a gas generator cycle tends to have lower specific impulse then a Staged combustion cycle.
As in most cryogenic rocket engines, some of the propellant in a gas-generator cycle is used to cool the nozzle and combustion chamber, increasing efficiency by allowing higher engine temperatures.
The Merlin rocket engine is a recent example of a gas-generator engine.
