Gas-generator cycle (rocket)
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The gas generator cycle is a power cycle of a bipropellant 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. Because something is "thrown away" this type of engine is also known as open cycle.
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; this also 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 than a staged combustion cycle.
As in most cryogenic rocket engines, some of the fuel in a gas-generator cycle is used to cool the nozzle and combustion chamber. Current construction materials cannot stand extreme temperatures of rocket combustion processes by themselves. Cooling permits the use of rocket engines for relatively longer periods of time with today’s material technology. Without rocket combustion chamber and nozzle cooling, the engine would fail catastrophically. [1]
Examples of gas-generator engines are the Merlin rocket engine, the Vulcain engine of Snecma Moteurs used on the ESA Ariane 5 rocket, and the more recent J-2X engine to be used on the Ares I and Ares V rockets as part of NASA's Constellation Program.
