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?
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Rockets don’t work by pushing off anything. They work by throwing stuff out the back.
It is a law of physics that every force has an equal and opposite reaction (Newton’s third law of motion). This basically means if you push on something, it also pushes back. If you were sitting in a rolling chair and threw a bowling ball, you’d roll in the opposite direction you threw the ball. This works with anything that has mass.
The hot gasses you get from burning fuel have mass. Throwing them out the back of the rocket puts a forward force on the rocket. The faster you thrown the gasses out the back, the greater the force on the rocket and the faster it goes.
Imagine a scuba tank. You have a top end, a tube, and a bottom end. Inside is gas compressed to 200 times normal air pressure. It’s pushing extremely hard on the inside of the tank.
It’s pushing with several tons of force on the top end, and also several tons of force on the bottom end.
Now, imagine the bottom end stops existing. Poofs away. Now the pressure inside is just pushing against the top end. The tank starts rising rapidly because the pressure isn’t counteracting itself by pushing on both ends.
A rocket’s combustion chamber is a scuba tank that’s missing the bottom. The huge pressure pushes the rocket in the direction you want to go.
In very simple terms, the thrust is pushing against the rocket body.
It doesn’t need to push against the air or lack there of, unlike a fixed or rotary engined vehicle that generates lift via that interaction.
Stand on a skateboard and throw something. You move backwards a little bit in the opposite direction. The faster you throw and the heavier the thing you throw the more you will move.
Rockets are like the skateboard. Rocket fuel is the thing being thrown. The explosion in the back is throwing a lot of fuel out the back of the rocket very fast, which means the rocket moves very fast in the opposite direction.
Well rockets on earth also don’t push off of air. It’s like a controlled explosion that sends nearby objects flying away in all directions. Rocket’s design is so that this direction is forward.
You throw a ball away from you in space, you’ll move the other way.
The propellant is the ball. You’re throwing propellant away from you.
To push is to give something a shove into another object. With helicopters it is air being shoved into the ground. The helicopter would generate equally as much lift if it was flipped upside down in our atmosphere
Now with rockets you are throwing a shit ton of hot gas out the back. In space this hot gas will not slow down. who knows if it will eventually hit a planet, asteroid or just get slowed down by colliding with particles in space.
Either way you threw mass away from you and that’s what makes you go in the opposite direction.
Rockets don’t work by pushing against something. They work by throwing something out in one direction really fast (rocket exhaust) and the reaction is that they move in the opposition direction.
Physically, it’s the same as if you were sitting in a rolling chair and threw a heavy bowling ball or something, it’ll push you back. The rocket is just throwing lots of little bowling balls (particles) really really really fast (and in physics fast particles = hot).
Get on a skateboard with a big rock or bag of sand or something heavy. Stand on the skateboard and Throw the heavy thing as hard as you can off the back. You’ll find that you will move in the other direction.
Try it again with a bucket full of stones and throw them out one by one. Same thing happens, just instead of one big push, you get a bunch of small ones.
Same thing happens with rockets, but instead of one heavy thing, it’s ‘thowing’/pushing out lots of expanding gas out the back. It all adds up and the rocket goes!
Newton’s 3rd law of motion is the result.
Like you are five?
Sit on a swing with a 10kg rock in your lap.
Now toss the rock as hard and fast as you can, and you will swing backwards. Not because you are pushing against air, but because you are pushing against the rock.
In a spacecraft, the push is because the exhaust has mass and is thrown out back really really fast.
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