Expander cycle (rocket)
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. 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.)
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. However, in some cases, such as the Japanese LE-5A/B, the thrust performance gains can take precedence over absolute efficiency.
The expander cycle has a number of advantages over other designs:
- Low temperature. The advantage is that after they have turned gaseous, the fuels are usually near room temperature, and do very little or no damage to the turbine, allowing the engine to be reusable. In contrast Gas-generator or Staged combustion engines operate their turbines at high temperature.
- Tolerance. During the development of the RL10 engineers were worried that insulation foam mounted on the inside of the tank might break off and damage the engine. They tested this by putting loose foam in a fuel tank and running it through the engine. The RL10 chewed it up without problems or noticeable degradation in performance. Conventional gas-generators are in practice miniature rocket engines, with all the complexity that implies. Blocking even a small part of a gas generator can lead to a hot spot, which can cause violent loss of the engine. Using the engine bell as a 'gas generator' also makes it very tolerant of fuel contamination because of the wider fuel flow channels used.
- Inherent safety. Because a bell-type expander-cycle engine is thrust limited, it can easily be designed to withstand its maximum thrust conditions. In other engine types, a stuck fuel valve or similar problem can lead to engine thrust spiraling out of control due to unintended feedback systems. Other engine types require complex mechanical or electronic controllers to ensure this does not happen. Expander cycles are by design incapable of malfunctioning that way.
Expander cycle engines include the following:
Expander Bleed cycle engines include the following:
Expander cycle engines have been used in:
- Centaur upper stage
- DC-X planned to be the first Single Stage to Orbit Technology demonstration rocket. completed 12 test flights before destruction of the only flight vehicle in an accident.
- Ariane 5 upper stage
- Saturn I
Expander Bleed cycle engines have been used in:
See also 
- "Pratt & Whitney Space Propulsion – RL60 fact sheet". Retrieved 2008-12-28.