The expander cycle is a power cycle of a bipropellant rocket engine. In this cycle, the fuel is used to cool the engine's combustion chamber, picking up heat and changing phase. The heated gaseous fuel then powers the engine's pumps and turbine before being injected into the combustion chamber and burned.
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. Non-toroidal 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 engines 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.)
Expander bleed cycle (open cycle)
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 heated and 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 decreasing backpressure and maximizing the pressure drop through the turbine. Compared with a standard expander cycle, this leads to higher engine thrust at the cost of sacrificing some efficiency due to essentially wasting the bled propellant by not combusting it.
Expander cycle engines include the following:
Expander Bleed cycle engines have been used in:
- "Pratt & Whitney Space Propulsion – RL60 fact sheet" (PDF). Retrieved 2008-12-28.
- MC-ARIANE5 1811 retrieved 4 June 2014