B What does a rocket push against?

[homerwho]

TL;DR Summary

Propulsion from thrusters is pushing against something.

I feel ok that in our atmosphere there is pressure that is a force to push against. But in a "vacuum" there is very little element available to push off of. I had some experience with near space high vacuum here on earth. The machinery was able to get down to analysis of element composition of elements in a hi vac chamber. I am recalling that at 10 ^ -15 torr it was close to space pressure. But with classical physics what is there to push against at that void?

 

[phinds]

Summary: Prepulsion from thrusters is pushing against something.

I feel ok that in our atmosphere there is pressure that is a force to push against. But in a "vacuum" there is very little element available to push off of. I had some experience with near space high vacuum here on earth. The machinery was able to get down to analysis of element composition of elements in a hi vac chamber. I am recalling that at 10 ^ -15 torr it was close to space pressure. But with classical physics what is there to push against at that void?

This mistaken belief that a rocket exhaust needs something to push against was debunked very publicly about 100 years ago when some "science writer" in a major magazine was roundly castigated for saying that it did.

Newton, of course, was aware of what was happening, long before that science writer was born.

 

[symbolipoint]

Some basic Physics teaches about this; the only vocabulary I can remember being "Momentum". Rocket does not push against anything. Engine makes hot material come out from one end, and at a very very high speed. In order to keep in balance, rocket needs to move in the other direction. The knowledgeable (many many of them) Physicists and engineers reading your post can answer more thoroughly.

 

[Thejas15101998]

Hi, the rocket in space qualifies as an isolated system; most of the systems on Earth which are of interest to us like cars don't qualify to be isolated as they interact with the surrounding environment to serve their assigned purpose. Momentum Conservation principle (MCP) can only be applied to isolated systems (ISBN 978-0805390469, chapter 10). MCP can't be applied to latter events on Earth as friction plays role in helping cars to move ("pushed against")- motion is expounded along this line of thinking.
in the former situations (rockets), MCP can be applied and combusted gas is ejected out of the rockets with a certain momentum which in turn push the rocket with certain momentum, in chime with MCP.

 

[Dale]

If you like you can say that the rocket pushes against the exhaust.

 

[Delta2]

If you like you can say that the rocket pushes against the exhaust.

More precisely the rocket pushes back the gas that is leaving from the exhaust, and the gas is pushing rocket in the opposite direction(forward) due to Newton's 3rd Law.

 

[dsaun777]

It would be like if you were in space and started firing a gun in one direction while holding the gun firmly in place, the recoil would send you in the opposite direction. As long as aimed normal to your center of mass.

 

[homerwho]

If you like you can say that the rocket pushes against the exhaust.

This is what I was thinking.

 

[cjl]

As long as aimed normal to your center of mass.

It doesn't even need the force vector to be through your center of mass. If it's offset, it'll both give you a lateral and a rotational velocity, while if it's straight through your center of mass, you'll just get a lateral velocity.

 

[dsaun777]

It doesn't even need the force vector to be through your center of mass. If it's offset, it'll both give you a lateral and a rotational velocity, while if it's straight through your center of mass, you'll just get a lateral velocity.

Yes, but using normal vector would give you the most efficiency for linear travel. Unless you want to tumble through a space...

 

[Chestermiller]

Suppose you are sitting in the middle of a lake covered with ice, and the ice is frictionless. You need to get to the shore. What do you do? You take off your shoe and throw it. You are exerting a force on the shoe to accelerate it, and the shoe pushes back on you with the same force. So you start to move. The same thing happens in a rocket. The unburned fuel is traveling at the same speed that you are. But, when it is burned, its volume increases greatly, and it gets pushed out the back of the rocket. So the rocket is exerting a force to accelerate the exhaust gas. The gas pushes back on the rocket with the same force.

 

[Filip Larsen]

I feel ok that in our atmosphere there is pressure that is a force to push against.

In addition to what else has been said, also note that the rocket exhaust actually has to work against any ambient atmospheric pressure thus giving less net thrust compare to when running in vacuum (everything else being equal). See description at https://en.wikipedia.org/wiki/Rocket_engine#Net_thrust

Further more, to avoid under and over expansion, first stage nozzles are typically shorter than vacuum rated nozzles. See description at https://en.wikipedia.org/wiki/Rocket_engine#Nozzle