Bart: Go, Dad, go!
Lisa: How doth the hero, strong and brave, a celestial path to the heavens paved!
(The family stares at her.)
Lisa (dejected): Go, Dad, go. -The Simpsons
Last week, I got a question from one of my online friends, cmgraves. His question was straightforward:
How do thrusters work in space? On Earth, when we want to speed up, slow down, change our direction, or to change our motion in general, we always have something to push against. This is true whether you’re a runner and have the ground to push against,
or a turbofan engine with the air to push against.
But in the vacuum of space, your attempts to fly a plane would fail just as surely as your attempts to run, since there’s nothing to push against! How, then, is it possible to change your motion in space?
In other words, how do thrusters work? Take a look at the image below.
This is what happens when you explode a little bit of rocket fuel inside that rocket. Note the big difference between what happens here, inside of a rocket, and what happens when you just blow up something up on its own.
If I just let something explode, the explosion is going to move spherically outwards. For every little bit that moves to the left, there’s a little corresponding chunk that moves to the right. For every gram that moves up with a certain velocity, there’s a corresponding amount that moves down with a certain velocity. This is what we call the conservation of momentum.
But if I have a controlled explosion, I can spew all of that combusted fuel out — at a very high speed — in one direction.
But now, all of that high-velocity, combusted fuel gets blown out in a controlled direction! And if I want to accelerate, all I have to do is point my thruster in the opposite direction I want to move, burn a little fuel, and I’m done!
This is the same reason why, when you fire a rifle, there’s a large kickback that throws you backwards. There’s an explosion that goes on inside of the gun, part of the energy propels the bullet forwards (out of the barrel) at a certain velocity, and part of the energy propels the gun and everything it’s attached to (including you) backwards at a different, smaller velocity.
(Any bet as to how far we’ll get in the comments before the inevitable joke about farts in space?)