Working principle of Rocket Engines (and associated myths)
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.
Re-visiting 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(!).
More explanation: 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.
Rocket Engines Working
Different sections [1–6] of rocket in context of rocket engine and propellant
Above figure shows the different sections of rocket in context of rocket engine and propellant. Rockets work by the producing high-speed exhaust (6). To produce this, fuel (1) is mixed with an oxidizer (2) with the help of pumps (3) in a combustion chamber (4) and ignited. The resulting hot exhaust is choked at the nozzle (5), ultimately producing the desired high-speed hot exhaust.
Note: The word “propellants” include both fuel and oxidizer.
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.
