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What distinguishes fuel from propellant?Robert Frost:
A fuel is any substance capable of combustion when exposed to oxygen, while an oxidizer supplies oxygen. A propellant is the mixture of fuel and oxidizer that burns to create propulsion.
The three main types of rocket propellants are liquid (hypergolic), liquid (non-hypergolic), and solid.
In the case of hypergolic fuels, these substances are so reactive that they ignite instantly upon contact. For non-hypergolic fuels, an external ignition source is necessary.
Fuel and oxidizer are pumped into a combustion chamber, where they ignite, generating hot gases that rapidly expand and seek an escape route.
Solid rockets use a single pre-mixed substance containing both fuel and oxidizer. This mixture is shaped or molded to suit the required burn characteristics and placed inside the rocket's fuselage. The material may be attached to the sides or left unattached, and ignition starts at the fuselage's core, burning the fuel from the inside out.
Liquid propellants allow easy control of thrust through turbo pumps that feed fuel and oxidizer into the combustion chamber, with the engine able to be started and stopped. Solid rockets, however, are generally controlled only by their shape, and once ignited, they cannot be practically halted. Some rockets address this by using multiple sections of solid fuel, burning them separately.
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