• Post category:Rocket Science
  • Reading time:3 mins read

In this short article, we will understand Rocket Engines, their working principle, and different types.

Myth: Rocket works by applying force on the ground, and the ground pushes the rocket up.

Revisiting Newton’s 3rd Law

Truth: Rocket works by applying force on the propellants (and throwing it out). The propellant mass applies opposite force on the rocket. This is also apparent by the fact that the rocket keeps accelerating even when away from the ground(!).

Rocket-Engines-Working-Newtons-third-law

Working of Rocket is commonly explained by using Newton’s third law (please refer the above figure). As per Newton’s third law, action and reaction forces act on two different surface/objects. In the case of rockets, the two objects being – “rocket” itself and the “exiting propellant”. It is very similar to the case of a person standing on a boat (placed in water). When the he/she throws a heavy mass away, this action causes a reaction making the boat (and the person) to move in the opposite direction.

Principle of Operation

Rocket engines produce thrust by the expulsion of an exhaust fluid that has been accelerated to high speed through a propelling nozzle. The fluid is usually a gas created by high-pressure combustion of solid or liquid propellants, consisting of fuel and oxidizer components, within a combustion chamber. As the gases expand through the nozzle, they are accelerated to very high (supersonic) speed, and the reaction to this pushes the engine in the opposite direction. Combustion is most frequently used for practical rockets, as high temperatures and pressures are desirable for the best performance.

Rocket-Engines-sectional-view-l
Different sections [1-6] of rocket in context of rocket engine and propellant

The above image (Courtesy: Pbroks13) shows a simplified diagram of a liquid-fuel rocket. 1. Liquid rocket fuel. 2. Oxidizer. 3. Pumps carry the fuel and oxidizer. 4. The combustion chamber mixes and burns the two liquids. 5. The hot exhaust is choked at the throat, which, among other things, dictates the amount of thrust produced. 6. Exhaust exits the rocket.

Types of Rocket Engines: –

1. Physically powered: This includes water rockets and cold gas thrusters, which uses pressurized water and inert gases respectively for getting the desired thrust.

2. Chemically powered: In this, thrust is obtained by an exothermic chemical reaction among the propellants. Various subtypes include

  • Solid rockets: This contains a single chamber of solid fuel and oxidizer mixture.
  • Hybrid rockets: Solid fuel, liquid oxidizer. Separate tanks.
  • Monopropellant: A single propellant flows over a catalyst and decomposes exothermically.
  • Bipropellant: Two-fluid propellants are brought together. (Same as Falcon 9 and the rocket principle explained above).
  • Dual Mode: Rocket takes off as a bipropellant rocket, then turns to use just one propellant.

3.    Electrically Powered

  • Resistojet Rocket: A monopropellant is imparted energy by heating a resistive element.
  • Arcjet rocket: Similar to above, except the heating element is replaced by an electric arc.
  • Pulsed plasma: Plasma is used to ignite solid propellants.

4.    Thermal Rockets: Hot water is stored in tanks at high temperature and pressure, converted into steam and exited via a nozzle.

Other than above, nuclear rockets, nuclear thermal and solar-thermal rockets are also currently being developed and experiments are being carried out in controlled environments.

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