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Expander cycle (rocket)
Additional recommended knowledge
The expander cycle is a power cycle of a bipropellant rocket engine meant to improve the efficiency of fuel delivery.
In an expander cycle, the fuel is heated before it is combusted, usually with waste heat from the main combustion chamber. As the liquid fuel passes through coolant passages in the walls of the combustion chamber, it undergoes a phase change into a gaseous state. The expansion of the fuel can create pressures of over 1000 psi (7 MPa) , which is used to drive turbines that in turn supply more fuel and oxidizer to the rocket engine. After leaving the turbines, the fuel is then burned to produce thrust for the vehicle.
Because of the necessary phase change, the expander cycle is thrust limited by the square cube rule. In bell shaped rockets there is a point beyond which there isn't enough nozzle area to heat up enough fuel to drive the turbines and hence the fuel pumps. In bell shaped rockets the expander cycle is limited to engines with up to 300 kN thrust. In aerospike shaped engines the exhaust effectively sticks to the aerospike, and can achieve a much higher heat transfer, and hence achieve greater thrust. Both engine types need to use a cryogenic fuel such as hydrogen, methane, or propane, which can easily reach their boiling point.
Expander cycle usually used a gas generator of some kind to start the turbine and run the engine until it heats up enough for phase change pressure to take over.
In an open cycle, or "bleed" expander cycle, only some of the fuel is heated to drive the turbines, which is then vented to atmosphere to increase turbine efficiency. While this increases power output, the dumped fuel leads to decreased efficiency. A closed cycle expander engine sends the turbine exhaust to the combustion chamber (see image at right.)
The expander cycle has a number of advantages:
Some examples of an expander cycle engine are the Pratt & Whitney RL-10 (rocket engine) and RL60  and the Ariane 5 ESC-B .
Expander cycle engines include the following:
Expander cycle engines have been used in:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Expander_cycle_(rocket)". A list of authors is available in Wikipedia.|