Types Of Chemical Rocket Propulsion

SOLID PROPELLANT ROCKET MOTORS

The propellant to be burned is contained within the combustion chamber or case. The solid propellant is sometimes called grain and it contains all the chemical components needed for burning (oxidizer and fuel).

The word propellant usually refers to the mixture of a fuel and an oxidizer. For example, in a car, the propellant would be the gas and the oxygen.
Super-simplified scheme of a solid rocket motor.

Once the propellant is ignited, it burns smoothly at a determined rate on all the internal surfaces. Initial burning take place on the internal surfaces of the hole(s) there is in the grain. As the propellant is burned, the cavity keeps growing.

Cross-sectional area of the grain at ignition.
Cross-sectional area of the grain after some time of burning.
Cross-sectional area of the grain after all the propellant has been consumed.

The resulting hot gases flows through the nozzle to impart thrust. Once the motor is ignited, the propellant will burn till it has been consumed.

These type of engines are considered to be the most simple due to the fact there aren’t feed systems like in hybrid or liquid engines.

An example of this type of rocket engines are the ones that were used on the Space Shuttle boosters or the ones used on the Ariane 5 boosters.

Space Shuttle at lift off with its solid boosters.
Credit: NASA
Ariane 5 at lift off with its solid boosters.
Credit: ESA – CNES Arianespace Optique video du CSG JM Guillon

LIQUID PROPELLANT ROCKET ENGINES

Liquid propellant rocket engines use liquid propellants that are fed at high pressures from tanks into a thrust chamber. Depending on the number of liquids that are used we can distinguish between:

  • Monopropellant engines: They use a single liquid that contains both oxidizing and fuel species. It decomposes into hot gas when properly catalyzed. They are usually used on altitude control systems (ACS).
  • An Altitude Control System (ACS) is used to control the orientation of a rocket, a satellite or a spacecraft with respect to one point.
    Super-simplified scheme of a monopropellant engine.
    Monopropellant engine used on the Apollo Moon lander for altitude control.
    Credit: NASA
  • Bipropellant engines: The liquid propellants consists of a liquid oxidizer (e.g. liquid oxygen) and a liquid fuel (eg. kerosene). Nowadays, they are the most used type of rocekt engines for orbital rockets.
  • Super-simplified scheme of a bipropellant engine.
    Credit: ESA – CNES Arianespace Optique video du CSG JM Guillon
  • Tripropellant rocket engines: It uses three propellants. Tripropellant systems can be designed to have high specific impulse and have been investigated for single stage to orbit designs. While tripropellant motors have been tested by Rocketdyne and Energomash, no tripropellant rocket has been built or flown.
  • Super-simplified scheme of a tripropellant engine.

In the thrust chamber the propellants react to form hot gases, which are accelerated and ejected at high velocities through the nozzle, imparting momentum to the vehicle.

Some of the liquid rocket engines permit repetitive operation and can be started and shut off various times (eg. Merlin (SpaceX)). If the thrust chamber is provided with adequate cooling, it is possible to run liquid rocket engines for long periods.

In this image there is a regenerative-cooling rocket engine. Here fuel is pumped on the rocket nozzle, then it flow till arriving on the injectors when enters the combustion chamber.

The liquid propellant engines are considered to be the most complex due to the fact they require precision valves and a complex feed mechanism which includes propellant pumps, turbines, propellant pressurizing device and a complex thrust chamber.

An example of these type of rockets can be found in many of the active rockets. For example the Merlin engines used on the Falcon 9 or the Rutherford used on the Electron rocket.

Merlin 1D test at SpaceX McGregor, Texas test facility.
Credit: SpaceX
9 Rutherford engines test.
Credit: Rocket Lab
The tank with the name "He" on the schemes is a tank that is usually connected to the fuel/oxidizer tank in order to pressurize the fuel/oxidizer. It is basically a tank with helium (He) at high pressure that pushes the fuel/oxidizer out.

GASEOUS PROPELLANT ROCKET ENGINES

This type of engines uses a stored high-pressure gas such as air, nitrogen or helium as their propellant. Due to the high pressures, this type of engines require heavy tanks. This type of engines have been used and are still used today as altitude control systems (ACS).

Super-simplified scheme of a gaseous propellant rocket engine.

On this type of rocket engines we can heat the gas to improve the performance. This type of engines are the so called warm gas propellant rocket propulsion.

HYBRID PROPELLANT ROCKET PROPULSION SYSTEM

This type of rocket engines use a liquid and a solid propellant. For example, it can be injected a liquid oxidizing agent into a combustion chamber filled with a solid grain. Then the chemical reaction takes place and produce hot gases. This can also be done injecting liquid propellant agent into a combustion chamber filled with a solid oxidizer.

Super-simplified scheme of an hybrid rocket engine.

There are also combination systems that both has solid and liquid propellants. One example is a pressurized liquid propellant engine that uses a solid propellant to generate hot gases for tank pressurization.


Written by David Esteba Fàbrega on 2019-09-30.

TEST YOURSELF WITH THIS SURVEY!

Which type of chemical rocket engine will usually burn until the propellant has been consumed?

  • Solid rocket motors
  • Liquid propellant rocket engines
  • Gaseous propellant rocket engines
  • Hybrid propellant rocket engine

EXPLANATION

Usually solid rocket motors will burn till the propellant has been consumed. The other types of engines can be started and shut off when desired.

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Which type(s) of rocket engines are used on Altitude Control Systems (ACS)?

  • Nuclear and ion rocket engines
  • Liquid and solid rocket engines
  • Monopropellant and gaseous rocket engines
  • Gaseous and solid rocket engines

EXPLANATION

Most of the times the gaseous and monopropellant rocket engines are used on the ACS because they can be started and shut off many times with an excellent precision.

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Which type of chemical rocket engine is considered to be the most complex?

  • Solid rocket motors
  • Liquid propellant rocket engines
  • Hybrid rocket engines
  • Gaseous propellant rocket engines

EXPLANATION

Liquid propellant rocket engines are considered to be the most complex due to the fact they require precision valves and a complex feed mechanism which includes propellant pumps, turbines, propellant pressurizing device and a complex thrust chamber.

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Which type of rocket engine is the most used on orbital rockets?

  • Solid rocket engines
  • Electromagnetic engines
  • Ion engines
  • Liquid propellant rocket engines

EXPLANATION

Liquid propellant engines are the most used on orbital rockets due to the fact they have the best T/W ratio and the best performance.

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