What is the difference between fuel and propellant?Robert Frost:
A fuel is a substance that can burn in the presence of oxygen. An oxidizer is a source of oxygen. A propellant is the combination of fuel and oxidizer. It is the chemical mixture that is burned to provide propulsion.
The three most common rocket propellants are liquid (hypergolic), liquid (non-hypergolic), and solid rocket.
For hypergolic fuels, these two substances are so reactive that just coming into contact with each other causes them to ignite. For non-hypergolic fuels, an ignition source is provided.
Pumps inject fuel and oxidizer into a combustion chamber where they burn, producing hot, rapidly expanding gases that are looking for a way out.
Solid rockets have a single substance that is pre-mixed to contain fuel and oxidizer. That substance is cut or molded into shapes and thicknesses appropriate for the types of burns the rocket needs to perform. That material is put inside the rocket fuselage. Sometimes it is bonded to the sides, sometimes it isn't. And then ignition is provided in the center of the fuselage so that the fuel burns from the inside out.
With the liquid propellants, thrust can be easily manipulated using the turbo pumps that feed the fuel and oxidizer into the combustion chamber. The engine can be stopped and restarted. Solid rockets can usually only be controlled by their shape. Once they are burning, it really isn't practical to stop them from burning. Some rockets work around this by using multiple sections of solid fuel and then burning them separately.
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