Topics Space and the universe

How do rockets work in space?

Rockets work in space by throwing something out the back very fast. The exhaust goes one way, and the rocket goes the other. This is Newton's third law: every action has an equal and opposite reaction. A rocket does not push against air or the ground. It pushes against its own exhaust, which is why it works just as well in a vacuum, and in some ways better, since there is no air to slow it down.

The interesting part is everything around that simple idea. A rocket has to carry both its fuel and the oxygen, or another oxidizer, to burn it, because space has none. That makes most of a rocket's weight into propellant, which is why rockets are built in stages and drop empty parts along the way. Then there is the odd fact that reaching orbit is less about going up and more about going sideways very fast.

An episode would walk through thrust, staging, orbits and why a small burn in the right place can change where a spacecraft ends up. On bre, the hosts are AI, so they can make mistakes, and you can press Talk to ask a question mid-episode, like why rockets are so tall or whether they could run on something else.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. Action, reaction and the empty vacuumWhy a rocket needs nothing to push against, and the common mistake of thinking exhaust pushes on the air.
  2. Carrying your own oxygenWhy rockets bring both fuel and oxidizer, and what kinds of propellant are used, from liquid fuels to solid ones.
  3. Why rockets come in stagesDropping empty tanks and engines means the rest of the rocket has less mass to push, which is the core of the rocket equation's tough tradeoff.
  4. Sideways is the hard partReaching orbit means gaining enough horizontal speed that you keep falling around Earth instead of into it.
  5. Steering and changing orbitsHow small engine burns, thrusters and gravity assists change a spacecraft's path once it is coasting.
  6. Other ways to move in spaceA look at ion engines and solar sails: much gentler pushes that can work for a very long time.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Arlo. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Let's start with the question everyone has at some point. There is no air in space. So what is the rocket pushing against?

  2. ArloAI host

    Itself. Nothing else.

  3. breAI host

    Okay, say more. Picture someone on a skateboard throwing a heavy bag backward. The bag goes one way, they roll the other.

  4. ArloAI host

    Right. And that's the whole trick. The rocket throws exhaust, the exhaust throws the rocket.

  5. breAI host

    So the air was never the point. If anything, air is in the way.

  6. ArloAI host

    Yeah. Air just adds drag. Vacuum is easier to move through.

  7. breAI host

    Then why do rockets need so much fuel? If pushing is that simple, why is a launch so enormous?

  8. ArloAI host

    Because you carry the fuel. Then you burn fuel to carry the fuel. That's the problem.

Questions people also ask

Do rockets need air to work?
No. A rocket pushes itself forward by expelling exhaust backward, so it does not need air to push against. Rockets carry their own oxidizer to burn fuel, since space has no oxygen. In a vacuum they work fine, and without air drag they can be more efficient.
Why do rockets have stages?
Fuel tanks and engines are heavy, and once they are empty they are just dead weight. Dropping them means the remaining engines have less mass to accelerate. Staging lets a rocket reach much higher speeds than a single piece of the same size could.
How do spacecraft turn in space?
They use small thruster engines that fire short bursts of gas, and many also use spinning wheels inside the craft to rotate without using fuel. With no air, a craft can also keep spinning or drifting until something counteracts it.
Why does a rocket have to go sideways to reach orbit?
Going straight up only gets you high, and then you fall back. Orbit means moving sideways fast enough that as you fall, the curve of Earth drops away beneath you at the same rate. That speed is why most of a launch's energy goes into horizontal motion.

Related topics

More: all 300 topics, space and the universe, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.