Gas generator

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For generators driven by an internal combustion engine from gasoline or a gas, see electrical generator.

A gas generator usually refers to a device, often similar to a solid rocket or a liquid rocket that burns to produce large volumes of relatively cool gas, instead of maximizing the temperature and specific impulse. The low temperature allows the gas to be put to use more easily in many applications, particularly to drive turbines. Gas generators are used to power turbopumps in rocket motors,[1] to deploy airbags, and in other cases where large volumes of gas are needed, and storing it as a pressurized gas is undesirable or impractical.

A gas-generator cycle can also specifically refer to a way of designing a turbopump-fed liquid rocket engine, where some of the propellant is burned to drive the turbopump, and the exhaust is dumped overboard (usually through a nozzle) instead of being fed into the main combustion chamber. Many liquid rockets are designed this way, for example the Saturn V F-1[1] and SpaceX Merlin engines. Usually, the propellants are burned in a highly fuel-rich mix to keep flame temperatures low — O:F ratios below 0.5 are common.

Another good example is the V-2 rocket, which used hydrogen peroxide decomposed by a liquid sodium permanganate catalyst solution as a gas generator. This was used to drive the main turbopump to pressurize the LOX-ethanol propellants.

A common use for gas generators is in automobile air bags. A small pyrotechnic charge is usually used to open a valve, which begins the gas generation process. A typical charge might be a pound of sodium azide.

Another common gas generator is the chemical oxygen generator.

Gas generators have also been used to power torpedoes. Hydrogen peroxide is frequently used in such applications.

Gas generators can be also used for powering of auxiliary power units and emergency power units. Hydrazine is often used as a fuel.

Common fuels and reactions used[edit]

Hydrogen peroxide[edit]

Hydrogen peroxide decomposes to oxygen and water. As a high test peroxide it is used to power torpedoes and in some rocket engines.[2]

2 H2O2 → 2 H2O + O2

Hydrazine[edit]

Hydrazine decomposes to nitrogen and hydrogen. The reaction is strongly exothermic and produces high volume of hot gas from small volume of liquid. It is used in some small rocket engines, and some auxiliary power units,[2] e.g. in the Space Shuttle, and emergency power units, e.g. in the F-16 aircraft.

  1. 3 N2H4 → 4 NH3 + N2
  2. N2H4 → N2 + 2 H2
  3. 4 NH3 + N2H4 → 3 N2 + 8 H2

Chlorates and perchlorates[edit]

Sodium, potassium, and lithium chlorates and perchlorates are used for production of oxygen in chemical oxygen generators.

Sodium azide[edit]

Sodium azide decomposes exothermically to sodium and nitrogen. The reaction is employed in some early designs of air bags.

2 NaN3 -> 2 Na + 3 N2

The resulting sodium is hazardous, so other materials are added, e.g. potassium nitrate and silica, to convert it to a silicate glass.

Solid rocket propellants[edit]

Many solid rocket propellant compositions can be used as gas generators.[3]

Production of fuel gas[edit]

Main article: Producer gas

A gas generator fuelled by coke, or other cabonaceous material, may be used to produce fuel gas for industrial use. Portable gas generators of this type were used during World War II to power vehicles as a way of alleviating petrol shortages.[4]

See also[edit]

References[edit]

  1. ^ a b Sutton, George P. (1992). Rocket Propulsion Elements (6th ed.). Wiley. pp. 212–213. ISBN 0-471-52938-9. 
  2. ^ a b Sutton 1992, pp. 344
  3. ^ Sutton 1992, pp. 441–443
  4. ^ Lord Barnby (1941-07-16). "PRODUCER GAS FOR TRANSPORT. (Hansard, 16 July 1941)". Hansard.millbanksystems.com. Retrieved 2014-05-26.