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

The basic principle and working of a liquid fuel rocket is pretty simple. The fuel and oxidizer (together called propellants) are stored in two separate pressurized tanks. These are then pumped into a combustion chamber by use of “pumps”. Finally they exhaust formed because of combustion is made to pass through a de Laval nozzle or a converging-diverging nozzle that accelerates the exhaust gases to very high velocity, resulting in the required thrust.

Heinkel He 176 was a German rocket-powered aircraft. It was the world's first aircraft to be propelled solely by a liquid-fueled rocket
Heinkel He 176 was a German rocket-powered aircraft. It was the world’s first aircraft to be propelled solely by a liquid-fueled rocket. A post-war artist impression.

Advantages and types

Some designs of liquid propellant rockets can be throttled as well as turned off/on. This is unlike solid propellant rockets which cannot be turned off, once ignited. Liquid rockets have been built as monopropellant rockets using a single type of propellant, bipropellant rockets using two types of propellant, or more exotic tripropellant rockets using three types of propellant. Bipropellant liquid rockets generally use a liquid fuel, such as liquid hydrogen or a hydrocarbon fuel such as RP-1, and a liquid oxidizer, such as liquid oxygen.

Some interesting points related to Liquid propellants rockets

  • It is commonly used by rockets requiring high specific impulse at a given thrust.
  • More than 170 different types of liquid propellants has been tested and no new propellant has been discovered in last 30 years.
  • The use of liquid propellants was first proposed by Konstantin Tsiolkovsky and the first liquid fuel rocket was launched by Dr. Robert H. Goddard in 1926
  • German V-2 rocket used alcohol and liquid oxygen (LOX) as the propellant.
Robert Goddard, bundled against the cold weather of March 16, 1926, holds the launching frame of his most notable invention — the first liquid-fueled rocket.
Robert Goddard, bundled against the cold weather of March 16, 1926, holds the launching frame of his most notable invention — the first liquid-fueled rocket. Photo credits: NASA

Problems and Constraints

Bipropellant liquid rockets are simple in concept but due to high temperatures and high speed moving parts, very complex in practice. It makes the rocketry part complex and expensive, with higher chances of failure. The usage of “liquids” leads to phenomena like “sloshing of propellants” that can lead to loss of control of vehicle. Also, most often liquid propellants have to be stored in cryogenic form which again adds to complexity and cost. The materials used in a typical liquid fuel rocket has to handle large variations in temperature — extreme cryogenic temperatures of propellants to extreme high temperature in combustion chamber and nozzle. A lot of performance constraints on such rockets are due to limitations of materials and cooling technologies.

A technician performs maintenance on a Space Shuttle Main Engine, (SSME)
A technician performs maintenance on a Space Shuttle Main Engine, (SSME) Photo Credits: NASA

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