Why space-bound rockets have multiple stages

16:00, Jan 28 2013
Apollo XII
SPACE FLIGHT: The Saturn V rocket, which carried Neil Armstrong to the moon, had jettisoned during the flight into space.

For early science fiction enthusiasts, getting to the moon was a simple, if daring, affair. You jump in your rocket on Earth, blast off into space and then land the whole thing - precariously - blunt end down on the moon.

But it turns out that rocketry isn't that simple, and even just getting to orbit requires a rocket with multiple stages that are jettisoned during flight. Why is that?

There are a few reasons why multistage rockets are the way to go but the most important one is simple elimination of weight, because when you're shooting something into space, every kilogram counts. There's a kind of snowball effect: to loft an extra kilogram of payload to orbit you need to carry more fuel. But then you need to take even more fuel to lift that extra fuel, and so on. A rocket has to lift not just itself but all of its fuel and adding a little to the final payload can add a lot to the initial fuel and that in turn adds to the size and weight of the rocket's structure.

But it goes both ways. If you can remove some weight during the flight then you don't have to carry fuel to lift it from that point on and the snowball continues in reverse resulting in considerable savings in weight. And what ballast can we throw overboard? Well, why not a piece of the rocket itself?

That's why rockets are divided into multiple stages, each a rocket in its own right. When one is exhausted you just let it drop away while the rest continues merrily on its way. The savings are significant. The Saturn V moon rocket, complete with Apollo spacecraft and astronauts, weighed about 250 tons not counting fuel. But the rocket was divided into three stages so that by the time it reached Earth orbit all but about 65 tons had been shed as stages one and two detached. Shortly after leaving for the moon the third and final stage was abandoned, too, leaving about fifty tons of actual spacecraft to head to the moon.

The space shuttle used a different kind of staging. Its main engines fired right from lift-off to space but it also carried a pair of solid rocket boosters that helped out early in the flight before dropping off, thus acting as a first stage. Even SpaceShipOne, the Scaled Composites rocket-plane that carried the first privately launched astronaut into space, used a conventional aircraft as its first stage.

Multistage launchers have other advantages, too, but none of them come free. It would certainly be easier and less wasteful if we could return to the simpler situation imagined last century where you just hop in your spaceship and take a jaunt into space without dropping bits of it off into the ocean along the way. With current propulsion and construction techniques it just isn't possible to build a rocket that can propel itself directly into space. In the meantime if you want to get into space you just have to do it in stages.


The Press