What does a rocket push against in space?

originally posted by: buddha
Has any one calculated how much fuel you would
need to accelerate then decelerate to get to mars?

my point is a rocket in earth atmosphere
has some thing to push against.
But in the vacuum of space you have Nothing to push against.
Only the Gas the rocket pushes out the back!

think about that. when it starts it has nothing at All to push against.
and then as the gas builds up behind it. it will push against that.
So all the weight of the rocket is pushing against a Tiny mass of gas.

Please comment, I would like to know!

Youre thinking of rocket thrust incorrectly.

When a rocket is standing vertical on earth before it launches, the thrusters arent actually touching the ground. Very little if any of the pressure created from a reflection the downward thrust to the ground forces the rocket up.

You have to look at the combustion of the fuel as a standalone object.

If you are in a vacuum and you place 2 feathers next to an explosive on opposite ends, or even just 1 feather on one end.. and set the explosive off the feather or feather will go flying off in the direction opposite of the explosion. whether there is a medium or not.

The "thrust" is essentially a directed explosion.

The rocket is not being forced forward by the thrust pushing off its own exhaust behind it.. its being thrusted forward by the explosive gas pushing the back of the rocket forward

What's happening is both the rocket and the exhaust are being pushed but in opposite directions. Whenever anything pushes something else, it is itself pushed by that thing with equal force, but in the opposite direction. You might say there is not very much exhaust to push against... first, I would ask you, is that true? But secondly, the exhaust is being pushed to a very great speed.

originally posted by: roadgravel
a reply to: dragonridr

Lower gravity means less mass to move.

The mass is the same - it doesn't decrease.

No gravity has an effect in relativistic mass. So indeed it does change and the less gravity the less energy needed to move the mass. However our space craft doesn't change so your right in that regard.

a reply to: Tearman

Lots of engineering in focusing that expansion linearly. Like, how do you turn a bullet explosion into a projectile, with no chamber to focus the energy in one direction? That's a lot of energy to waste, 360 degrees of expansion for one direction of travel. So it's all in engineering the exhaust.. Has to be.

Anyways it might as well be a heat source behind the ship. If you can direct the heat to dissipate linearly, you can use it for propulsion. Call it the linear entropy heat wave engine. Just a giant hair dryer in a funnel, yeaah

A rocket moves because NOT because it pushes against air (or atoms in space or whatever, BUT because of Newton's Third Law, which tells us that that there will be a force inside the rocket (specifically on the side opposite the nozzle) that will be equal to the force of the gasses leaving through the nozzle.

Here's a simplified graphic that helps describe this. Imagine that this is the inside of the rocket combustion chamber. This particular combustion chamber is totally sealed, so any forces due to combustion (the blue arrows) will pushes equally in all directions. Newton's Third Law -- equal forces in opposite direction.


Now, image that there is a nozzle out the back, like rockets have. Now there be a force of gasses flowing out through the nozzle. However, let's not forget Newton's Third Law. That powerful force shooting out the back of rocket through the nozzle will have a force in the opposite direction that would be equal to the force through the engine nozzle.

That force out through the nozzle and the equal and opposite force are shown in red in the graphic below. The force out the back of the rocket has an equal force pushing "forward" (the opposite direction of the nozzle):

a reply to: buddha

You are discounting that in space there is no resistance..like gravity or air..etc.

originally posted by: dragonridr

originally posted by: roadgravel

Lower gravity means less mass to move.

Sorry, but gravity has no effect on mass. 5 kg mass on earth will have 5 kg mass in space. Gravity affects weight, not mass.

a reply to: dragonridr

The force of gravity between masses decreases with distance. As it move from Earth, gravity has less effect.

Most of the thrust comes from the pressure difference between the inside of the combustion chamber and where the gas is expelled. There is a high pressure side in the combustion chamber and a low pressure side where the gas is expelled.

OK so this has to be the third "physics" question on ATS. All related to high school physics. So now my alarm bell is ringing loudly. WTF is going on ? Have schools become so devoid of basic education that high school physics is now a mystery?

There's a (topical political) joke over here in the UK that the DUP's website went down and they all thought it was because the steam engine broke. Is the Trump anti science agenda dumbing people down in the US? It seems that way.

Archimedes , Galileo , Copernicus, Newton, Einstein all shaking their heads in despair in their graves at how dumb 21st century man is trying to become........

a reply to: buddha

What if its actually pushing on the luminiferous aether?

a reply to: buddha

Who says they need forward "propulsion"...when they could just as well be "pulled" magnetically?

Think out of the box a little....I dont believe they need "thrusting rocket engines" throughout deep space...thats a pretty juvenile way (and prob the only way we can think of with only limited experience)...but I believe its something else altogether....

a reply to: buddha

Were you not taught Newton's Laws of Motion?

Newton's Third Law- Every action has an equal an opposite reaction.

When the exhaust leaves the back of the rocket going in one direction, the Rocket is moved in the opposite direction.

Simple.

a reply to: buddha

If the concept still bothers you, remember that the rocket is expelling gas into space. Therefore the incoming rocket exhaust is pushing on its own exhaust gas. But the previous posts are correct about Newton's laws.

a reply to: [post=22369798]buddha.

But but but air particles....

Your theory stands up to Warner Bros law of motion, if you move your legs fast enough you can use air particles to climb up through the air but only if you have big shoes. Don't go too high though because unless you can move your arms really fast and grab hold of the air particles coming down you might twist your ankle.

In space you just run on the spot until a 5 point shooting star stabs you.

Ruthless.

My God, is this thread for real, even after the first 2 pages of explanation, the OP still does not get it.

In Public School here in Australia, we learned the laws of motion in 5th and 7th grade, it takes a very limited mind to not be able to wrap your mind around an answer even when the said answers are in picture form!

Newton's 3rd law has been experimented over and over for over 200 years.....it's a fact.

College and High School students do experiments that show how the 3rd law works....and work it does.

Take a balloon, fill it with air, then let go and watch it fly...because the compressed gas is escaping out at high velocity (we call this Thrust and Impulse).

But it's pushing on air! I can hear that being claimed now.

No problem: place balloon in a vacuum chamber and do the same thing. Watch it fly around....no air to push on needed.

Place a small model rocket in that same chamber and light off the engine: watch it fly...again, no air or anything else needed to push it..

But my rocket in space has a mass of 10 tons!

So?

In space, other than gravity, you normally do not have any other force acting upon the rocket. If the rocket is just sitting there, not moving, the only thing the rocket engine needs to over come is the rocket's inertia.

The impulse from a rocket engine just has to over come the inertia of the rocket....and it will move. If the rocket is already moving, and you want to change direction, then that inertia will be a LOT more than if it's just sitting there. If the rocket fires and adds it's force in the same direction that the rocket is already moving, that inertia will be a lot less.

Ion Drives are a perfect example of Newton's 3rd law. All that is coming out of the rocket's exhaust port is charged particles......but because they are leaving the rocket at a high rate......the space craft will move in the opposite direction. Slowly at first....ion drives have a very, very small impulse....but the craft will move.

So what would be a rocket's top speed then?

Easy: it's top speed will be that of the exhaust velocity....but only if you have enough fuel for the rocket to reach that speed. Once a rocket's forward velocity equals that of the exhaust velocity, it can go no faster (assuming you're in a vacuum with no opposing force to that motion).

a reply to: buddha

"Every action has an equal and opposite reaction."

Nice job, correct answer, and nothing else need be said.

a reply to: buddha

You must understand now that energy is different from mass. And all a mass needs inorder to move or accelerate is a force. Its not about being pushed by particles of liquid we call gas. Its about the reaction it has, when exhausted.

You should also ask yourself "What would happen after the rocket runs out of fuel?" It would drift on forever, or until it hit something, or got caught in an orbit.