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Chapter: Mechanical - Gas Dynamics and Jet Propulsion - Space Propulsion

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Types of Rocket Engines

Rocket or rocket vehicle is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine.

Types of Rocket Engines


Rocket or rocket vehicle is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction. Rocket engines push rockets forwards simply by throwing their exhaust backwards extremely fast.


Rockets for military and recreational uses date back to the 13th century. Significant scientific, interplanetary and industrial use did not occur until the 20th century, when rocketry was the enabling technology of the Space Age, including setting foot on the moon.


Rockets are used for fireworks, weaponry, ejection seats, launch vehicles for artificial satellites, human spaceflight and exploration of other planets. While comparatively inefficient for low speed use, they are very lightweight and powerful, capable of generating large accelerations and of attaining extremely high speeds with reasonable efficiency.


Chemical rockets are the most common type of rocket and they typically create their exhaust by the combustion of rocket propellant. Chemical rockets store a large amount of energy in an easily released form, and can be very dangerous. However, careful design, testing, construction and use minimize risks.


Rocket vehicles are often constructed in the archetypal tall thin "rocket" shape that takes off vertically, but there are actually many different types of rockets including, tiny models such as balloon rockets, water rockets, skyrockets or small solid rockets that can be purchased at a hobby store missiles space rockets such as the enormous Saturn V used for the Apollo program rocket cars rocket bike, rocket powered aircraft (including rocket assisted takeoff of conventional aircraft- JATO), rocket sleds rocket trains rocket torpedos, rocket powered jet packs, rapid escape systems such as ejection seats and launch escape systems space probes




A propellant is a material that is used to move ("propel") an object. The material is usually expelled by gas pressure through a nozzle. The pressure may be from a compressed gas, or a gas produced by a chemical reaction. The exhaust material may be a gas, liquid, plasma, or, before the chemical reaction, a solid, liquid or gelled. Common chemical propellants consist of a fuel; like gasoline, jet fuel, rocket fuel, and an oxidizer. Propellant used for propulsion Technically, the word propellant is the general name for chemicals used to create thrust.


For vehicles, the term propellant refers only to chemicals that are stored within the vehicle prior to use, and excludes atmospheric gas or other material that may be collected in operation.

Amongst the English-speaking laymen, used to having fuels propel vehicles on Earth, the word fuel is inappropriately used. In Germany, the word Treibstoff—literally "drive-stuff"—is used; in France, the word ergols is used; it has the same Greek roots as hypergolic, a term used in English for propellants which combine spontaneously and do not have to be set ablaze by auxiliary ignition system.


In rockets, the most common combinations are bipropellants, which use two chemicals, a fuel and an oxidiser. There is the possibility of a tripropellant combination, which takes advantage of the ability of substances with smaller atoms to attain a greater exhaust velocity, and hence propulsive efficiency, at a given temperature. Although not used in practice, the most developed tripropellant systems involves adding a third propellant tank containing liquid hydrogen to do this.

Solid propellant


In ballistics and pyrotechnics, a propellant is a generic name for chemicals used for propelling projectiles from guns and other firearms. Propellants are usually made from low explosive materials, but may include high explosive chemical ingredients that are diluted and burned in a controlled way (deflagration) rather than detonation. The controlled burning of the propellant composition usually produces thrust by gas pressure and can accelerate a projectile, rocket, or other vehicle. In this sense, common or well known propellants include, for firearms, artillery and solid propellant rockets: Gun propellants, such as:


Ø     Gunpowder (black powder)


Ø     Nitrocellulose-based powders


Ø     Cordite


Ø     Ballistite


Ø     Smokeless powders


Composite propellants made from a solid oxidizer such as ammonium perchlorate or ammonium nitrate, a rubber such as HTPB, or PBAN (may be replaced by energetic polymers such as polyglycidyl nitrate or polyvinyl nitrate for extra energy) , optional high explosive fuels (again, for extra energy) such as RDX or nitroglycerin, and usually a powdered metal fuel such as aluminum.


Some amateur propellants use potassium nitrate, combined with sugar, epoxy, or other fuels / binder compounds.


Potassium perchlorate has been used as an oxidizer, paired with asphalt, epoxy, and other binders.




Propellants are used in forms called grains. A grain is any individual particle of propellant regardless of the size or shape. The shape and size of a propellant grain determines the burn time, amount of gas and rate produced from the burning propellant and consequently thrust vs time profile.


There are three types of burns that can be achieved with different grains.


Ø     Progressive Burn:


Usually a grain with multiple perforations or a star cut in the center providing a lot of surface area.


Ø     Digressive Burn


Usually a solid grain in the shape of a cylinder or sphere.


Ø     Neutral Burn


Usually a single perforation; as outside surface decreases the inside surface increases at the same rate.




There are four different types of solid propellant compositions:


Single Based Propellant:


A single based propellant has nitrocellulose as its chief explosives ingredient. Stabilizers and other additives are used to control the chemical stability and enhance the propellant’s properties.


Double Based Propellant:


Double based propellants consist of nitrocellulose with nitroglycerin or other liquid organic nitrate explosives added. Stabilizers and other additives are used also. Nitroglycerin reduces smoke and increases the energy output. Double based propellants are used in small arms, cannons, mortars and rockets.


Triple Based Propellant


Triple based propellants consist of nitrocellulose, nitroquanidine, nitroglycerin or other liquid organic nitrate explosives. Triple based propellants are used in cannons.




Composites contain no nitrocellulose, nitroglycerin, nitroquanidine or any other organic nitrate. Composites usually consist of a fuel such as metallic aluminum, a binder such as synthetic rubber, and an oxidizer such as ammonium perchlorate. Composite propellants are used in large rocket motors.

Liquid propellant


Common propellant combinations used for liquid propellant rockets include:


Ø     Red fuming nitric acid (RFNA) and kerosene or RP-1


Ø     RFNA and Unsymmetrical dimethyl hydrazine (UDMH) Dinitrogen tetroxide and UDMH, MMH and/or hydrazine Liquid oxygen and kerosene or RP-1


Ø     Liquid oxygen and liquid hydrogen


Ø     Liquid oxygen and ethanol


Ø     Hydrogen peroxide and alcohol or RP-1


Ø     Chlorine pentafluoride and hydrazine


Common monopropellant used for liquid rocket engines include:


Ø     Hydrogen peroxide


Ø     Hydrazine


Ø     Red fuming nitric acid (RFNA)


Introducing propellant into a combustion chamber

Rocket propellant is mass that is stored, usually in some form of propellant tank, prior to being ejected from a rocket engine in the form of a fluid jet to produce thrust.


Chemical rocket propellants are most commonly used, which undergo exothermic chemical reactions which produce hot gas which is used by a rocket for propulsive purposes. Alternatively, a chemically inert reaction mass can be heated using a high- energy power source via a heat exchanger, and then no combustion chamber is used.


A solid rocket motor:


Solid rocket propellants are prepared as a mixture of fuel and oxidizing components called 'grain' and the propellant storage casing effectively becomes the combustion chamber. Liquid-fueled rockets typically pump separate fuel and oxidiser components into the combustion chamber, where they mix and burn. Hybrid rocket engines use a combination of solid and liquid or gaseous propellants. Both liquid and hybrid rockets use injectors to introduce the propellant into the chamber. These are often an array of simple jets- holes through which the propellant escapes under pressure; but sometimes may be more complex spray nozzles. When two or more propellants are injected the jets usually deliberately collide the propellants as this breaks up the flow into smaller droplets that burn more easily.


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