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Stages

Rockets and landing · level 3, lesson 5 of 6

A question to start

You are going on a long walk, carrying ten bottles of water in a big backpack. As you drink them, you could keep the empty bottles in the pack, or throw them in a bin as you go. Which is easier?

The idea

Throwing the empties away, of course: you stop carrying weight you do not need. Rockets do the same thing, with stages.

A staged rocket is a stack of rockets, one on top of the other.

  1. At lift-off, the big first stage fires. It has huge tanks and powerful engines.
  2. When its tanks are empty, the first stage drops off: tanks, engines and all. That is tonnes and tonnes of metal the rocket no longer has to push.
  3. The smaller second stage fires. It only has to push itself and the payload, so the same amount of fuel speeds it up far more.
  4. Some rockets have a third stage, or more.
1. first stage fires2. it drops off3. second stage fires

The first stage of a big rocket usually burns for two or three minutes and falls back. Some now land again to be used once more: SpaceX's Falcon 9 first stages fly back and land upright on a pad or a ship at sea.

A trip to Mars has more steps than a trip to orbit, so it needs more "stages" in a different sense: a rocket to orbit, a burn to leave Earth for Mars, a heat shield and engines to land, and a separate rocket to come home.

Worked examples

1. Saturn V. Three stages. The first burned about 2.5 minutes and lifted the rocket to about 68 km. The second burned about 6 minutes. The third put the astronauts into orbit and then, later, fired again to send them to the Moon.

2. Why not one huge stage? A single-stage rocket would carry its huge empty tanks and engines all the way to orbit. With today's materials and fuels that is so hard that no rocket has yet reached orbit with one stage.

Try it

  1. What is a rocket stage?
  2. Why does a rocket drop its stages when they are empty?
  3. Is the second stage of a rocket usually bigger or smaller than the first?
  4. What can happen to a Falcon 9 first stage after it drops off?
Answers
  1. One of the rockets in the stack, with its own tanks and engines.
  2. So the rest of the rocket does not have to carry their weight.
  3. Smaller.
  4. It can fly back and land, to be used again.

Common mistake

"Stages are there in case an engine breaks." They are there to get rid of weight. Each stage is dropped as soon as it is empty, so the rest of the fuel only has to push what is still needed.

Where it leads