Space­men need stages

Hay­den Walles re­veals why space-bound rock­ets have mul­ti­ple stages.

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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.

Photo: REUTERS

Space flight: The Saturn V rocket, which car­ried Neil Arm­strong to the moon, had mul­ti­ple stages that were jet­ti­soned dur­ing the flight into space.

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