Getting something to orbit Earth means accelerating it to about 17,500 mph. That takes a huge amount of fuel - but fuel is heavy, and carrying heavy stuff means you need even more fuel to lift it. The math quickly spirals into impossibility. A single-stage rocket would need almost all of its mass to be fuel just to reach orbit, leaving nothing for the spacecraft.
The trick, worked out by Russian theorist Konstantin Tsiolkovsky in 1903, is to throw away parts you don’t need anymore. A multi-stage rocket stacks two or three smaller rockets on top of each other. The biggest one fires first to lift the whole stack off the pad, then drops away when its fuel is gone. The next stage takes over with a lighter load to push.
Almost every orbital rocket today uses two or three stages. The Saturn V used three. The Falcon 9 uses two. By the time the final stage fires, it’s only pushing the spacecraft itself, so its engines can boost it the rest of the way into orbit. The dropped stages either burn up in the atmosphere or, for newer rockets, fly back down to be reused.
