Gas-generator cycle (rocket) |
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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 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. For this reason it is an open cycle (because something is "thrown away") and 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; 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 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.
The Merlin rocket engine is a recent example of a gas-generator engine.
The Vulcain engine of Snecma Moteurs is also an example of a gas-generator engine. It is used on the ESA Ariane 5 rocket.
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Published in July 2009.
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