Expander cycle (rocket)

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Expander rocket engine (closed cycle). Heat from the nozzle and combustion chamber powers the fuel and oxidizer pumps.

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 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. 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 pure expander cycle is limited to engines with up to 300 kN thrust. 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. 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 that easily reach their boiling point.

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 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.

Advantages

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.

Some examples of an expander cycle engine are the Pratt & Whitney RL10 and RL60 and the Vinci engine for the future Ariane 5 ME [1].