Gas generator

A gas generator usually refers to a propellant mixture, often similar to a solid rocket propellant, that burns to produce large volumes of gas. It is similar to a rocket propellant, but is usually designed to produce large volumes of cool gas, instead of maximizing the energy available. The low temperature allows the gas to be put to use more easily in many applications. Gas generators are used to power turbopumps in rocket motors, 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 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 the car air bags. A small pyrotechnic charge is usually used there.

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

Hydrogen peroxide

Hydrogen peroxide decomposes to oxygen and water. As a high test peroxide it is used to power torpedoes and in some rocket engines.: 2 H₂O₂ → 2 H₂O + O₂

Hydrazine

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, e.g. in the Space Shuttle, and emergency power units, e.g. in the F-16 aircraft.
#3 N₂H₄ → 4 NH₃ + N₂
#N₂H₄ → N₂ + 2 H₂
#4 NH₃ + N₂H₄ → 3 N₂ + 8 H₂

Chlorates and perchlorates

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

odium azide

Sodium azide decomposes exothermically to sodium and nitrogen. The reaction is employed in some early designs of air bags.: 2 NaN₃ -> 2 Na + 3 N₂The resulting sodium is hazardous, so other materials are added, e.g. potassium nitrate and silica, to convert it to e.g. a silicate glass.

olid rocket propellants

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