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fredTOG's avatar

How does rocket thrust work in the vacuum of space?

Asked by fredTOG (526points) April 6th, 2012

If space is basically a vacuum and void of atmosphere, how do rockets alter the direction and speed of space craft? In other words, how do they “push off” against nothing? does the space program give a false representation of how they work?

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22 Answers

gondwanalon's avatar

Lets say that you are floating out in space while holding a bowling ball. If you push the ball away from you then you and the ball will be put into motion in opposite directions.

The same principle occurs with the rocket engine. When the engine fires up, it pushed the blast in one direction as the rocket is pushed in the opposite direction.

King_Pariah's avatar

m1 x a1 = m2 x a2, action force = reaction force, Newton’s 3rd Law,

The exhaust persay is exerting a force on the rocket (pushing the rocket in desired direction) which in turn exerts a force back onto the exhaust (pushing itself off towards desired direction)

fredTOG's avatar

push against what the nothingness of space?

King_Pariah's avatar

The exhaust pushes against the rocket, the rocket pushes off the exhaust, I don’t know (at the moment) how to state it any more simply than that. :/ (just about midnight and I am exhausted from a days worth of putting up fences)

fredTOG's avatar

so if you kick your self in the ass you can move in space?

ragingloli's avatar

imagine you are jumping, and every time you jump up, a stone slab falls out of your arse rear, on which you land. then you jump again.

gasman's avatar

Imagine shooting a gun (which works in a vacuum) and experiencing the backward recoil in a direction opposite the bullet’s path. Shoot again and get another backward impulse. Keep firing bullets and the gun will keep recoiling backward, each shot slightly increasing your backward speed. This arises from Newton’s laws: conservation of momentum.

The rocket nozzle is shooting high-velocity gas molecules in one direction (analogous to bullets), accelerating the rocket in the opposite direction (analogous to gun recoil). The presence or absence of atmosphere is irrelevant. The presence or absence of gravity is irrelevant. It is purely action & reaction as described by Newton 300 years ago.

Rock2's avatar

Sit in a chair with wheels on it. Be on a tile floor. Keep your feet off of the floor. Hold a basketball. Throw the baskball horizontally straight out. You will probably need to make a two handed throw from your chest out.. Do you move? In which direction relative to the ball’s direction?

LuckyGuy's avatar

The rocket is not pushing off anything. It is pushing away exhaust ( Mass x velocity) and that equal and opposite reaction pushes the rocket in the opposite direction.
If you kick yourself in the ass you are still hanging on to your leg so you can’t go anywhere. If you ripped your leg off and threw it, you would propel yourself forward.

fredTOG's avatar

I know what your saying, oh by the way where did you all get your PHD from ? what I’m saying is a complete vacuum is impossible as it’s forbidden by quantum mechanics. Fluctuations in energy must occur, thereby violating the definition of a complete vacuum.

King_Pariah's avatar

Sorry for not reading between the lines to get to what you were really saying. ~

fredTOG's avatar

I just wanted to ask a question.

LuckyGuy's avatar

Ahhh, but @fredTOG, you said the “vacuum of space” and “basically a vacuum” in your question and details. You did not say “complete vacuum” or ‘total vacuum”.
Once the rocket expels exhaust in the area, it is no longer in a total vacuum.
Welcome to Fluther by the way.

Rock2's avatar

@LuckyGuy
The mass is the mass of the rocket fuel being expelled. The force on the rocket fuel is f=ma. The force moving the rocket is the opposite and equal reaction to that.

flutherother's avatar

For every action there is an equal and opposite reaction. So if rocket exhaust gases are blasted out towards the Earth the rocket will accelerate in the opposite direction away from the Earth. I should know I have PhD from the University of the Moon.

mazingerz88's avatar

I knew it. @flutherother is no other than Gingrinch himself!

Rock2's avatar

@flutherother
I tried to transfer to the University of the Moon but was denied.

jerv's avatar

Once exhaust comes out, space is no longer a vacuum, at least not in a small area for a short time. More importantly, there is no physical connection between the exhaust and the rocket like there is between your foot and your ass. If you kick yourself in the ass, your foot will basically be pushing off of itself, try to drag itself along, and Conservation of Momentum will leave you stationary.

With a rocket, a small mass goes one way at high speed which causes a large mass to go in the opposite direction at relatively low speed. Both masses push off from each other. And this continues as long as the rocket keeps shoving mass out.

Some science fiction speculates “reactionless” drives that operate without ejecting reaction mass, but those are purely fictional as they violate the laws of physics as we currently know them. Somewhere in the middle are drives that eject less mass at higher velocity, allowing for comparable thrust with less fuel required and thus a smaller, lighter spacecraft, but many of those rely on things we currently lack, like fusion reactors, or antimatter

fredTOG's avatar

you can change direction a little but your not making any U turns.

King_Pariah's avatar

@fredTOG it’s possible
example: exhaust vents on front right/left and rear left/right of vehicle or gyroscopes inside vehicle

jerv's avatar

@fredTOG That is where bow thrusters and maneuver jets come in. Even many boats have them since the main engine is set up solely for straight-line thrust and the rudders (which are useless in space) can only do so much. Of course, if your engines are set up far enough apart, you can vary the thrust on each side and get some turning that way, just as a boat can slow/reverse props on one side and impart uneven thrust.

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