Why do rockets work in space?

[jajrussel]

I always thought that escaping gas pushed the rocket along by pushing against atmosphere. Yet rockets work in space. How does that work ?

[swansont]

6 minutes ago, jajrussel said:

I always thought that escaping gas pushed the rocket along by pushing against atmosphere. Yet rockets work in space. How does that work ?

Conservation of momentum, via Newton's third law. The rocket is exerting a force on the gas it expels, so the gas exerts a force on the rocket.

Same idea as throwing a large rock while you are in a frictionless surface (e.g. a sheet of ice)

[jajrussel]

7 minutes ago, swansont said:

Conservation of momentum, via Newton's third law. The rocket is exerting a force on the gas it expels, so the gas exerts a force on the rocket.

Same idea as throwing a large rock while you are in a frictionless surface (e.g. a sheet of ice)

Thanks, does space technically change where this occurs? Is there a drop off in thrust during the transition? If not my understanding is that the rockets Mass stays the same so I would think it would be harder to push unless the escaping gas interaction was completely non dependant on atmosphere?

[Janus]

Another way of looking at it is like this:

 

Consider a completely enclosed sphere like the top diagram below.  Hot gases in side try to expand and push outward against the walls.  But since they push equally in all directions, their is no net movement of the Sphere.

Now cut a hole in one side like in the bottom diagram.  The gases escaping through the hole are not pushing against the wall of the sphere in that direction, but the gas is still pushing against the opposite wall, There is an imbalance in the forces acting on the sphere and you get movement of the sphere to the left.

To answer your second question:

Imagine that the there is an atmosphere outside the sphere and that we have pumped it up to the point where its pressure is equal to the pressure of the hot gases.  Air pushing in at the opening pushes just as hard as the hot gases trying to get out. The result is that the gases are held in the sphere just like is there were no hole.  If the gases can't escape, you can get no movement of the sphere.   If the outside pressure is less, they can escape, but not as much as they could of there was no air outside. 

In other words, the atmosphere outside of the rocket reduces the effect of the exhaust gases and thus reduces the efficiency of the rocket. Rockets perform better in a vacuum than they do inside an atmosphere.

Edited March 22, 2019 by Janus

[Sirjon]

40 minutes ago, jajrussel said:

I always thought that escaping gas pushed the rocket along by pushing against atmosphere. Yet rockets work in space. How does that work ?

AS to my understanding, are you referring that since the 'outerspace', unlike the Earth's atmosphere, has no gases nor air, how is it possible for a rocket to work in an 'empty space'? Am I right? I'm also interested on how it works... so let hear from our friends out there.

[Phi for All]

5 minutes ago, Sirjon said:

I'm also interested on how it works... so let hear from our friends out there.

Do you need more than what swansont and Janus provided? 

[Sirjon]

10 minutes ago, Janus said:

 In other words, the atmosphere outside of the rocket reduces the effect of the exhaust gases and thus reduces the efficiency of the rocket. Rockets perform better in a vacuum than they do inside an atmosphere.

So, the reason why we need oxygen or some kind of fuel to burn? In a vacuum like the outer space, what happen to the gas that been exhausted? Is there a trail like the one we see every time a jet plane pass through the clear sky? 

[DrP]

3 minutes ago, Sirjon said:

I'm also interested on how it works... so let hear from our friends out there.

Did you read the above posts? 3rd law explains it.

[StringJunky]

1 minute ago, Phi for All said:

Do you need more than what swansont and Janus provided? 

My thought too.

[Sirjon]

1 minute ago, Phi for All said:

Do you need more than what swansont and Janus provided? 

Sorry. I have more relevant question.

[DrP]

Quadruple simultaneous post madness!!!

 

[Sirjon]

and it happened the other posts did not show up on time, I apologize

[Janus]

3 minutes ago, Sirjon said:

So, the reason why we need oxygen or some kind of fuel to burn? In a vacuum like the outer space, what happen to the gas that been exhausted? Is there a trail like the one we see every time a jet plane pass through the clear sky? 

Yes, you leave a trail of gases behind; You are trading the gases going in one direction for the rocket going in the other. If you took the mass of the gas trail and rocket together, you would find that the center of mass of the entire system doesn't move.

The difference between the gases escaping from a rocket in a vacuum and the exhaust from a jet is the that the jet exhaust slows down after leaving the jet due to interaction with the surrounding air, while the exhaust gasses for the rocket don't. (unless they encounter something or are acted on by outside forces like gravity. But what happens to them after they leave the rocket has no effect on the rocket.)

[swansont]

57 minutes ago, jajrussel said:

Thanks, does space technically change where this occurs? Is there a drop off in thrust during the transition? If not my understanding is that the rockets Mass stays the same so I would think it would be harder to push unless the escaping gas interaction was completely non dependant on atmosphere?

No, the rocket's mass does not stay the same. That's actually what's going on, if you look at Newton's second law

 

F = dp/dt = d(mv)/dt = m dv/dt + v dm/dt

Usually the second term is zero, and you end up with F = ma. But if your mass changes and you are moving relative to that ejected (or added) mass, there must be a force.  If you are adding a horizontally stationary mass to a moving object, like a dump truck or train, it slows you down, because you have to bring that added mass up to your speed. A rocket is essentially doing the opposite of that.

 

[Sensei]

1 hour ago, Sirjon said:

So, the reason why we need oxygen or some kind of fuel to burn?

Oxygen is oxidizer. Fuel is e.g. Hydrogen, Ethanol, Methanol, Benzene etc. etc. There are other alternative oxidizers possible instead of liquid Oxygen.

1 hour ago, Sirjon said:

In a vacuum like the outer space, what happen to the gas that been exhausted? Is there a trail like the one we see every time a jet plane pass through the clear sky? 

Spaceship ejecting gas from engines could give similar effect like comet (it's ejecting Hydrogen, Oxygen and Water molecules.. comet is boiling in the close presence of the Sun).

1 hour ago, Sirjon said:

Is there a trail like the one we see every time a jet plane pass through the clear sky? 

Jet plane trail is completely something else.. it is not direct result of work of engines.. engineless plane at high altitude, flying at high enough speed, like Space Shuttle will leave similar trail as ordinary jet plane.

[MigL]

Most air breathing ( atmospheric oxygen is the oxidizer ) are virtually smokeless these days.
The vapor trails you see are mostly condensation due to pressure difference above and below wings and other lifting surfaces.

An oxygen/hydrogen rocket will also leave a water vapor trail.

And as others have stated, a mass of reacted gases accelerated backwards, will have an equal and opposite reaction on the ejecting rocket.

[studiot]

2 hours ago, jajrussel said:

I always thought that escaping gas pushed the rocket along by pushing against atmosphere. Yet rockets work in space. How does that work ?

Rocket propulsion depends on mass.

A rocket is a device which separated itsself (it's mass) into two parts by ejecting one part backwards or leaving it standing.

The ejected part would be the exhaust.
This may be ions, gaseous matter, beta rays.

The part moving 'forward' we normally think of as 'the rocket', but of course the exhaust started off as part of the rocket.

So a quiescent rocket beofre it blasts off is an isolated system that has mass.

Because it has mass it has a centre of mass.

Since the system is isolated, not forces act on it.

Therefore the system cannot change its centre of mass, which must remain in the same place as the business end progresses forward.

Three things assist here.

Firstly the 'rocket' can leave behind further parts than the exhaust (spent stages).

Anything left behind reduces the payload mass.

Secondly once the payload is beyond the reach of restraining gravity it needs no more propulsion and moves forward at constant speed in space.

Thirdly the attracting gravity of another body may influence the 'rocket' further on in its flight.

So in a sense the rocket does 'push against' its exhaust.

 

 

[jajrussel]

Thanks to all of you this should keep me busy for a while...

[koti]

Quote

I always thought that escaping gas pushed the rocket along by pushing against atmosphere. Yet rockets work in space. How does that work ?

Crude answer is rockets work in space for the same reason as a ball thrown in space will hit you and you will feel it exactly the same as you would feel it on earth. 
A punch in space would feel exactly the same as a punch on Earth.

Edited March 22, 2019 by koti

[mohammed sameer]

newton's third law is the basic principle here,but we can imagine a pseudo force acting on the rocket to conserve linear momentum.

[Ken Fabian]

Maybe it's easier to comprehend rocket thrust as coming from pushing those gases away (like pushing against rocks as you throw them) rather than (incorrectly) the stream of gases pushing against something external (which is like the rocks hitting something after you throw them).

Edited April 11, 2019 by Ken Fabian

[swansont]

2 hours ago, Ken Fabian said:

Maybe it's easier to comprehend rocket thrust as coming from pushing those gases away (like pushing against rocks as you throw them) rather than (incorrectly) the stream of gases pushing against something external (which is like the rocks hitting something after you throw them).

Yes. A rifle does't wait for the bullet to hit before it recoils. It recoils because the momentum of the bullet must be balanced by the momentum of the rifle + person, as there is no net external force on the system.

[theHassan]

On 3/22/2019 at 5:36 PM, Sirjon said:

So, the reason why we need oxygen or some kind of fuel to burn? In a vacuum like the outer space, what happen to the gas that been exhausted? Is there a trail like the one we see every time a jet plane pass through the clear sky? 

Rockets come in many different varieties.

A bipropellant chemical rocket burns a fuel (liquid hidrogen, kerosene, ethanol, hydrazine...) with an oxidizer (liquid oxigen, fluorine, nitric acid...) to produce hot gases to eject. There are also solid fuels, like black gunpowder and various plastic-metal mixes. These are the biggest rocket motors, which carry (carried) the Soyuz, the Apollo, the Shuttle and many more space vehicles up to there.

There are monopropellant chemical rockets, much smaller things, where hydrazine is reacted in a catalytic chamber to generate the gases. Here nothing is burned, the chemistry is different. 

Even smaller rockets use pressurized gas (nitrogen usually), and just release the amount of gas as needed.

These smaller rocket motors are used to orient the spacecraft, that is, to turn it around to look wherever it needs to look to do its job.

Ion engines are also rockets, but they use electric fields to accelerate ionized gas, which is ejected from the engine.

However you produce the mass flow out from the rocket motor, it always comes down to the principles already described. 

[donnieporko]

interesting questions, thanks for you all detail reply, I learned a lot