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 fuel in the gaseous state expands through a turbine using the pressure differential from the supply pressure to the ambient exhaust pressure to initiate turbopump rotation. This can provide a bootstrap starting capability as is used on the Pratt & Whitney RL10 engine. This bootstrap power is used to drive turbines that drive the fuel and oxidizer pumps increasing the propellant pressures and flows to the rocket engine thrust chamber. After leaving the turbine(s), the fuel is then injected with the oxidizer into the combustion chamber and burned to produce thrust for the vehicle.
Because of the necessary phase change, the expander cycle is thrust limited by the square-cube rule. As the size of a bell-shaped nozzle increases with increasing thrust, the nozzle surface area (from which heat can be extracted to expand the fuel) increases as the square of the radius. However, the volume of fuel that must be heated increases as the cube of the radius. Thus there exists a maximum engine size of approximately 300 kN of thrust beyond which there is no longer enough nozzle area to heat enough fuel to drive the turbines and hence the fuel pumps. Higher thrust levels can be achieved using a bypass expander cycle where a portion of the fuel bypasses the turbine and or thrust chamber cooling passages and goes directly to the main chamber injector. Aerospike engines do not suffer from the same limitations because the linear shape of the engine is not subject to the square-cube law. As the width of the engine increases, both the volume of fuel to be heated and the available thermal energy increase linearly, allowing arbitrarily wide engines to be constructed. All expander cycle engines need to use a cryogenic fuel such as hydrogen, methane, or propane that easily reach their boiling points.
Some expander cycle engine may use a gas generator of some kind to start the turbine and run the engine until the heat input from the thrust chamber and nozzle skirt increases as the chamber pressure builds up.
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 a decrease in propellant efficiency (lower engine specific impulse). A closed cycle expander engine sends the turbine exhaust to the combustion chamber (see image at right.)
This operational cycle is a modification of the traditional expander cycle. In the bleed (or open) cycle, instead of routing heated propellant through the turbine and sending it back to be combusted, only a small portion of the propellant is used to drive the turbine and is then bled off, being vented overboard without going through the combustion chamber. Bleeding off the turbine exhaust allows for a higher turbopump output by maximizing the pressure drop through the turbine. This leads to higher engine thrust at the sacrifice of some efficiency loss due to essentially wasting the bled propellant. However, in some cases, such as the Japanese LE-5A/B, the performance gains can take precedence over absolute efficiency.
The expander cycle has a number of advantages over other designs:
Expander cycle engines include the following:
Expander cycle engines have been used in: