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Many rockets use both liquid and solid propellant rocket engines to reach space. Solid propellant is cheaper than liquid propellant but it cannot be controlled as well. When a rocket launches from Earth it uses rocket boosters with solid propellant to gain enough thrust to leave the atmosphere. Liquid propellant involves a mixture of liquid fuel and liquid oxidizer. It is different than solid propellant in that it can be controlled, or throttled. But it's also more complicated and requires extra weight on the spacecraft (it needs more "stuff" like piping and pumps).