eli5: If space is a vacuum, how can rockets work? What are the thrusters pushing *against* if there is nothing out there?

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I’ve never really understood the physics of this. Obviously it works somehow — I’m not a moonlanding denier or anything — but my (admittedly primitive) brain continues to insist that a rocket thruster needs something to push *against* in order to work.

So what is it pushing against if space is essentially a void?

In: 7157

31 Answers

Anonymous 0 Comments

The exploding fuel pushes against the rocket. That’s all that’s necessary to propel the rocket forward, away from the mass of the ignition

Anonymous 0 Comments

You know how in some video games you shoot like a massive shotgun or bazooka and it flings you way back? It’s just that but bigger.

Anonymous 0 Comments

It’s the “every force has an opposite and equal reaction” concept. One fart could theoretically propel you across the universe given enough time.

Anonymous 0 Comments

Try grabbing a heavy object that you can carry (if you have kettlebell or maybe a sack of rice or flour), sit on an office chair with wheels with said object in your lap, and lift your feet off the ground. Now lift the object and push/throw it away quickly. You’ll notice that you’ll move in the direction opposite to your throw. That’s basically the principle.

In space, while a spaceship burns fuel, the fuel will expand, pushing out against the spaceship, and in reaction, the ship is pushed back.

Anonymous 0 Comments

Imagine that rocket fuel and its oxidizing agent, when combined and ignited, is like trillions of microscopic baseball pitchers all throwing baseballs in every direction.

Now you put these into a shaped nozzle so that all the baseballs exit the nozzle in roughly the same direction.

When a tiny pitcher throws a baseball, the ball moves away at high speed, but due to Newton’s laws of equal and opposite reaction, the pitcher is pushed slightly in the opposite direction.

So trillions of pitchers throwing their baseballs results in a lot of baseballs being ejected at very high speed out of the end of the nozzle, and the equal and opposite force pushes the pitchers the other way, against the nozzle and the rocket it is attached to, pushing the rocket in the opposite direction of the baseballs. The baseballs don’t themselves have to push against anything. It was the force of throwing the baseball out the back of the nozzle that caused the pitcher to be pushed in the opposite direction.

This keeps happening until all the baseballs are used up (solid fuel rocket) or until the rocket control system stops adding pitchers and baseballs into the nozzle (liquid fuel rocket).

Anonymous 0 Comments

There’s a handy analogy that’s perhaps a bit odd, and certainly very American: imagine firing a gun at a range. The gun pushes the bullet, which is relatively light, out at enormous speeds. But it also pushes back on you, sometimes quite hard. If you were to mount yourself on a rolling platform, you could propel yourself backwards with sufficient ammunition and firing rate. Rockets do a similar thing: they throw mass(propellant, or reaction mass) out the back of a spacecraft and take advantage of the “recoil” to accelerate the spacecraft itself.

Anonymous 0 Comments

Fun fact — you’re in no worse company than [The New York Times](https://www.forbes.com/sites/kionasmith/2018/07/19/the-correction-heard-round-the-world-when-the-new-york-times-apologized-to-robert-goddard/), here.

[This was the actual text published](https://www.rfcafe.com/miscellany/factoids/images/ny-times-correction-goddard.jpg) on Apollo 11’s launch day. 😛

(And, to make it even better: Robert Goddard *really* *did* have some fundamentally flawed ideas about how rockets work; just not *that* one. He bought into something called the “pendulum rocket fallacy,” believing a rocket would be more stable if the engine nozzle was at the top of the ship, rather than the bottom, which we now know to be false. So the moral is… rocket science is just un-intuitive sometimes. :P)

Anonymous 0 Comments

From my understanding, it’s still pushing away from something. It’s just that the something is its fuel. Same thing as when you let go of the end of an air filled balloon. It’s newton’s 3rd law, I think. It works better in space because there’s no air resistance to push back.

You can do the same thing by getting in an office chair and throwing a basketball ball or using a foaming fire extinguisher.

Summary: Rockets work because, in space, farting propellants is the same as pushing away from a wall.

Anonymous 0 Comments

“every action has an equal and opposite reaction” is the rule at play here. Take note that it doesn’t say “in the presence of an atmosphere…”

When you’re standing near a person, and you suddenly push them away, what happens to you? If you are standing with your feet together, you’ll fall over as well. When you shove your friend, your body pushes on their body, but their body also pushes on your body, causing you to fall over as well.

This is exactly what a rocket does. The fuel is built so that it can sustain an ignition reaction even without an atmosphere, and that reaction causes it to expand. Normally it would expand outward, but this is constrained by the rocket nozzle to only expand in one direction. The expansion of the fuel is the action, and the reaction is a change in velocity to the rocket, in the opposite direction. The fuel pushes on the rocket, and the rocket pushes on the fuel.

Anonymous 0 Comments

It is pushing against the particular of gas it is shooting out. Your rocket ships is jumping off the mass of the gas.

Imagine this. You are standing still on a skateboard and you throw a basketball in front of yourself. You will move backwards. Why? Because YOU are pushing off the mass of the basketball. Now imagine you basketball is full of water instead of air. Now imagine it is full of concrete or lead. Which makes you go further? Why? Well the heavier objects, because they have more mass and therefore more momentum. Now imagine the basketball is the earth. Why is it that you can jump against the earth? Because it has mass.

Now go back to the rocket ship. Imagine that each molecule of gas it pushes against pushes you a tiny bit in the opposite direction. Then look at how much gas comes out the back of a rocket!

Or consider a gun. When you shoot it, what happens? The gunpowder converts itself from a solid that takes up very little space into a gas which takes up a lot of space and it forces a bullet out the front. What is the bullet pushing against? Well the gas is pushing on the bullet and one the gun and you. You are pushed back a bit (recoil) because you are pushing against the bullet. Now imagine a gun that shoots blanks. You still feel recoils as the expanding gas pushes in all directions and leaves the muzzle. Now imagine a rocket is just a blank firing gun.