Liquid Propulsion
Liquid propulsion systems
For hundreds of years, man lived with the problems associated with solid-fuel rockets. However, in 1926, after 17 years of theoretical and experimental work, Robert Goddard created the first rocket engine that used liquid fuels-- gasoline and liquid oxygen (LOX). Since these engines can be throttled, they have been preferred for interplanetary propulsion systems.
The largest liquid-fueled propulsion system created to date was the Saturn V, shown here, that was used to propel U.S. astronauts to the moon and back. Every engine and thruster in this enormous spacecraft used liquid fuels. The basic idea is simple. In most liquid propellant rocket engines, a fuel and an oxidizer (for example, liquid hydrogen and oxygen) are pumped into a combustion chamber. There they burn to create a high-pressure and high-velocity stream of hot gases. These gases flow through a nozzle that accelerates them further, and then leave the engine. It is normal for either the fuel of the oxidizer to be a cold liquefied gas like liquid hydrogen or liquid oxygen. One of the big problems in a liquid propellant rocket engine is cooling the combustion chamber and nozzle, so the cryogenic liquids are first circulated around the super-heated parts to cool them. The pumps have to generate extremely high pressures in order to overcome the pressure that the burning fuel creates in the combustion chamber.
The main engines in the Space Shuttle actually use two pumping stages and burn fuel to drive the second stage pumps. Here is a summary of the fuels used in traditional liquid-fueled rockets: Gasoline and LOX (Goddard's early rockets) Alcohol and LOX (German V2 rockets) Kerosene and LOX (first stage of the Saturn V) Liquid hydrogen (LH2) and LOX (Space Shuttle main engines) Nitrogen tetroxide (NTO)/monomethyl hydrazine (MMH) (Cassini's engines) Liquid-fuel rocket engines have two important advantages over solid-fuel engines: The thrust can be easily controlled They can be restarted multiple times...
They also have disadvantages:
- They are very complex and require significant maintenance
- They are much more expensive
- The liquefied gasses must be kept at cryogenic temperatures, which requires insulation and/or refrigeration
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