A gas generator is a device for generating gas. A gas generator may create gas by a chemical reaction or from a solid or liquid source, when storing a pressurized gas is undesirable or impractical. The term often refers to a device that uses a

rocket propellant Rocket propellant is used as reaction mass ejected from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines. Overvi ...

to generate large quantities of gas. The gas is typically used to drive a turbine rather than to provide

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...

as in a

rocket engine A rocket engine is a reaction engine, producing thrust in accordance with Newton's third law by ejecting reaction mass rearward, usually a high-speed Jet (fluid), jet of high-temperature gas produced by the combustion of rocket propellants stor ...

. Gas generators of this type are used to power turbopumps in rocket engines, in a

gas-generator cycle The gas-generator cycle, also referred to as the GG cycle or colloquially as an open cycle, is one of the most commonly used power cycles in bipropellant liquid rocket engines. Propellant is burned in a gas generator (analogous to, but distinct ...

. It is also used by some

auxiliary power unit An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. Aircraft APUs generally produce 115&n ...

s to power

electric generator In electricity generation, a generator, also called an ''electric generator'', ''electrical generator'', and ''electromagnetic generator'' is an electromechanical device that converts mechanical energy to electrical energy for use in an externa ...

s and

hydraulic pump A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy ( hydrostatic energy i.e. flow, pressure). Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. They gen ...

s. Another common use of the term is in the

industrial gas Industrial gases are the gaseous materials that are Manufacturing, manufactured for use in Industrial sector, industry. The principal gases provided are nitrogen, oxygen, carbon dioxide, argon, hydrogen, helium and acetylene, although many other ...

es industry, where gas generators are used to produce gaseous chemicals for sale. For example, the

chemical oxygen generator A chemical oxygen generator is a device that releases oxygen via a chemical reaction. The oxygen source is usually an inorganic superoxide, chlorate, or perchlorate. Ozonides are a promising group of oxygen sources, as well. The generators are usu ...

, which delivers breathable oxygen at a controlled rate over a prolonged period. During

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

, portable gas generators that converted coke to

producer gas Producer gas is a fuel gas manufactured by blowing air and steam simultaneously through a coke or coal fire. It mainly consists of carbon monoxide (CO), hydrogen (H2), as well as substantial amounts of nitrogen (N2). The caloric value of the pro ...

were used to power vehicles as a way of alleviating

petrol Gasoline (North American English) or petrol ( Commonwealth English) is a petrochemical product characterized as a transparent, yellowish, and flammable liquid normally used as a fuel for spark-ignited internal combustion engines. When formul ...

shortages. Other types include the gas generator in an automobile airbag, which is designed to rapidly produce a specific quantity of inert gas.

Common applications

As a power source

The

V-2 rocket The V2 (), with the technical name ''Aggregat (rocket family), Aggregat-4'' (A4), was the world's first long-range missile guidance, guided ballistic missile. The missile, powered by a liquid-propellant rocket engine, was developed during the S ...

used

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...

decomposed by a liquid

sodium permanganate Sodium permanganate is the inorganic compound with the formula sodium, NaPermanganate, MnO4. It is closely related to the more commonly encountered potassium permanganate, but it is generally less desirable, because it is more expensive to produce ...

catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

solution as a gas generator. This was used to drive a turbopump to pressurize the main LOX-

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

propellants. In the

Saturn V The Saturn V is a retired American super heavy-lift launch vehicle developed by NASA under the Apollo program for human exploration of the Moon. The rocket was human-rated, had multistage rocket, three stages, and was powered by liquid-propel ...

F-1 and

Space Shuttle The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...

main engine, some of the main propellant was burned to drive the turbopump (see

and

staged combustion cycle The staged combustion cycle (sometimes known as topping cycle, preburner cycle, or closed cycle) is a power cycle of a bipropellant rocket engine. In the staged combustion cycle, propellant flows through multiple combustion chambers, and is th ...

). The gas generator in these designs uses a highly fuel-rich mix to keep flame temperatures relatively low. The Space Shuttle

and the

F-16 The General Dynamics F-16 Fighting Falcon is an American single-engine supersonic multirole fighter aircraft originally developed by General Dynamics for the United States Air Force (USAF). Designed as an air superiority day fighter, it e ...

emergency power unit An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. Aircraft APUs generally produce 115&n ...

(EPU) use

hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydraz ...

as a fuel. The gas drives a turbine which drives

s. In the F-16 EPU it also drives an

. Gas generators have also been used to power

torpedo A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, such ...

es. For example, the US Navy

Mark 16 torpedo The Mark 16 torpedo was a redesign of the United States Navy's standard Mark 14 torpedo in use during World War II. It incorporated war-tested improvements into a weapon designed to be used in unmodified United States fleet submarines. Due to hi ...

was powered by

. A concentrated solution of

is known as

high-test peroxide High-test peroxide (HTP) is a highly concentrated (85 to 98%) solution of hydrogen peroxide, with the remainder consisting predominantly of water. In contact with a catalyst, it decomposes into a high-temperature mixture of steam and oxygen, with n ...

and decomposes to produce oxygen and water (steam). : 2 H2O2 -> 2 H2O + O2

Hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydraz ...

decomposes to mixtures of nitrogen, hydrogen and ammonia. The reaction is strongly exothermic and produces high volume of hot gas from small volume of liquid. # 3 N2H4 -> 4 NH3 + N2 # N2H4 -> N2 + 2 H2 # 4 NH3 + N2H4 -> 3 N2 + 8 H2 Many solid chemical rocket propellant compositions can be used as gas generators.

Inflation and fire suppression

Many automobile airbags use

sodium azide Sodium azide is an inorganic compound with the formula . This colorless salt is the gas-forming component in some car airbag systems. It is used for the preparation of other azide compounds. It is highly soluble in water and is acutely poisonou ...

for inflation (). A small pyrotechnic charge triggers its decomposition, producing

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

gas, which inflates the airbag in around 30 milliseconds. A typical airbag in the US might contain 130 grams of sodium azide. Similar gas generators are used for fire suppression.

Sodium azide Sodium azide is an inorganic compound with the formula . This colorless salt is the gas-forming component in some car airbag systems. It is used for the preparation of other azide compounds. It is highly soluble in water and is acutely poisonou ...

decomposes exothermically to sodium and nitrogen. : 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.

Oxygen generation

delivers breathable oxygen at a controlled rate over a prolonged period. Sodium, potassium, and lithium

chlorate Chlorate is the common name of the anion, whose chlorine atom is in the +5 oxidation state. The term can also refer to chemical compounds containing this anion, with Chlorate#Compounds (salts), chlorates being the salt (chemistry), salts of chlo ...

s and

perchlorate A perchlorate is a chemical compound containing the perchlorate ion, , the conjugate base of perchloric acid (ionic perchlorate). As counterions, there can be metal cations, quaternary ammonium cations or other ions, for example, nitronium cat ...

s are used.

Generation of fuel gas

A device that converts coke or other carbonaceous material into

may be used as a source of fuel gas for industrial use. Portable gas generators of this type were used during

to power vehicles as a way of alleviating

shortages.

References