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Solid propellant is actually used to propel many rockets. Many of the US ICBMs (intercontinental ballistic missiles) used solid rocket fuel to power them into space. The solid booster rockets on the Space Shuttle used a combination of solid and liquid fuel. The advantage of solid fuel is that rockets that use it are usually safer and simpler. However liquid fuel rockets are able to control the fuel flow which can give you varying thrust. Solid fuel rockets cannot stop burning once ignited.