Rockets get into space by burning fuel that pushes hot gas out the back. There are two main types of rocket fuel, and they behave very differently. Solid rockets use a hard, rubbery propellant packed tightly into a tube - basically a giant, scientifically engineered firework. Once it’s lit, it burns through every last bit and there is no off switch.
Liquid rockets are the opposite. They store fuel and oxidizer in separate tanks, mix them in a combustion chamber, and burn them as needed. That means engineers can throttle them up and down, restart them in flight, or shut them down early if something goes wrong. Almost every rocket that lands itself back on Earth uses liquid engines.
Most big launches use both. Solid boosters give a huge thump of power at takeoff to lift everything off the pad, then drop away after a couple of minutes. Liquid main engines, more controllable and efficient, push the rest of the way to orbit. The Space Shuttle and many modern rockets used exactly this combo.
