# If space cannot be compressed

I guess my confusion lies in propulsion. Not fantasy, real propulsion. If space cannot be compressed then what do the expanding gases push against to gain propulsion( forward motion). All systems currently in use as propulsion in space rely on the expansion of gases that are directed in one direction to gain momentum in the opposite direction. If space cannot be compressed then what is the expanding gas pushing against? I understand that there is a negative pressure in space but you must be able to increase that pressure in order to gain any momentum. I am not a physicist. My background is very mechanical. I have ideas for propulsion in space but I need a better understanding of the compressibility of space.

Chestermiller
Mentor
You're sitting on a frozen lake of ice, and to start moving, you take your shoe off and throw it backwards so that you can move forwards. You are accelerating your shoe, and to do this, you have to apply a force to it. The shoe pushes back on you with the same force, and you start moving.

Chet

Yes, I understand that it takes a greater action to create a reaction. That just makes sense since you are notOperating within the atmosphere. It would take a greater action to create the same reaction. But you are in fact compressing the space

sophiecentaur
Gold Member
Hello and welcome to PF. That's a very common question. But the answer is entirely different from your picture of things.
Rocket engines, out in space, do not work by pressing on anything outside the rocket. They are reaction engines and Newton's Third Law describes what happens. Just like, when you are in a boat and throw a heavy mass over the stern, the boat moves forward because of the reaction force from the Mass on your hand. So, it is the expelled gases (which have mass) that produce a reaction force against the pressure in the combustion chamber and that pushes the rocket forward. Consider the Momentum involved and remember that Momentum is always conserved in an isolated system. You have a small amount m of mass being expelled out of the back at very high speed V. This causes the large mass M of the rocket to be propelled forward by a lower speed v. The total change in momentum per second will be zero so
mV + Mv = 0
therefore the change v in velocity of the rocket will have the opposite sign to the change V in velocity of the gases. The total mass of rocket plus propellant on board has to get less and less. The higher the velocity of the 'ejecta', the less mass needs to be ejected per second and the longer the rocket can work for.

It isn't at all like what happens in a gun barrel, where there is a high pressure inside, pushing the bullet forward. In that case, the acceleration is much higher and for a shorter time.

sophiecentaur
Gold Member
But you are in fact compressing the space
No "in fact" you are not and your view of what happens doesn't hold water, I'm afraid. Action and reaction forces are Equal and you cannot have them unequal. You may be thinking in terms of friction forces but that is not the same thing at all.

So you are saying that inside the atmosphere. The expanding gases push the rocket forward but outside of it the loss of mass is pulling the rocket?

That changes my understanding a lot. Thank you

Chestermiller
Mentor
So you are saying that inside the atmosphere. The expanding gases push the rocket forward but outside of it the loss of mass is pulling the rocket?
Nothing is pulling. The rocket is pushing the gases out the back, and the gases are pushing forward on the rocket with the same force. It's the same thing as throwing your shoe on the lake of ice. The shoe represents the gases, and your body represents the rocket.

chet

sophiecentaur
Gold Member
So you are saying that inside the atmosphere. The expanding gases push the rocket forward but outside of it the loss of mass is pulling the rocket?
I did not say that at all. In a vacuum or in a gas, noting can get "pulled" because there are no molecular bonds. You cannot have a negative pressure in any gas. The presence or absence of an atmosphere makes little or any difference to how a rocket engine works - a jet engine uses the Oxygen in the atmosphere to burn the fuel and it expels air at high speed but it is still a reaction engine. When a rocket is on the launch pad and stationary, there is an excess pressure underneath the tail which provides a certain about of lift but, once it is a few metres up, that pressure begins to vanish.
It would be an idea if you tried to use the accepted terms for these effects. You will have great difficulty in understanding if you don't.

I apologize but I do not know the accepted term. As I wrote, I am not a physicist. I am trying to understand how this works. I know how a jet engine works, I worked on many gas turbines in the military. I guess what I don't understand is that if the expanding gases are pushing against the Rocket, then they must be also pushing against or off of something. I like the way you explain things. Is there a way that you can explain that to me?

Dale
Mentor
2021 Award
if the expanding gases are pushing against the Rocket, then they must be also pushing against or off of something.
This is simply not true. The expanding gasses push the rocket. By Newtons 3rd law the rocket pushes the expanding gasses with an equal and opposite force. Nothing else is needed, the "something" you mention does not exist in space.

Bandersnatch
Rocket engine is like having some hot gas in a closed box. On average, the same amount of molecules hits every side every second, so the resultant forces cancel out to 0 and the box stays put.
Remove one side, and the gasses pushing on the side opposite to the one removed are no longer balanced - they exert force on that side, but escape through the hole .
If there is some other gas (like air) blocking the exit, the exhaust cannot escape so easily and thrust is reduced.
In vacuum there is no gas blocking the exit, so the engine is the most efficient there.

I think I understand. The gases expand equally in all directions. The Rocket being the only mass around the expansion is pushed by the expanding bubble. That makes sense.

Bandersnatch
You've got it.

My experience as a mechanic teaches me that I can bring a pressure down to a vacuum, and vice versa. Why can I not do that in space

sophiecentaur
Gold Member
The Rocket being the only mass around the expansion is pushed by the expanding bubble. That makes sense
It doesn't make any sense actually because it (still) ignores the concept of reaction forces. As I said before, whilst you use your own private language, you are making it harder and harder to understand.

OK. try this. The expanding gases in the combustion chamber and the tapered nozzle are exerting a pressure on the solid surfaces. The lateral forces will all cancel each other out but the forward components will add together, providing thrust for the rocket. This can only happen if there is a hole, somewhere, for gases to escape (i.e. the back of the engine) and there needs to be enough pressure there to keep up the pressure on the inside of the engine. Hence you need the expanding gases to be ejected fast enough to produce a high pressure over the mouth of the nozzle to maintain the pressure inside. Each molecule of the gas that's pushed out, has to provide a reaction force against the internal molecules - maintaining the pressure. Without the reaction, here, it couldn't work. But that's only introducing a complicated and needless middle stage. All that's needed is to talk about the reaction forces from the ejecta, providing a force to push the rocket forward.

When you throw a heavy rock, you are not 'compressing' anything. You are just feeling a reaction force, whilst you are accelerating the rock. Without the rock, and with the same throwing action and speed, do you actually feel any reaction force? You would feel it, if you were compressing something. 'Pretend throw' as fast as you can and I doubt that you would feel any significant resistance, even then.
If you won't accept the reaction force significance, you will not get this. There is no point, either, in trying to restate what I am writing, using as many of your own word and concepts as you can, in an attempt to be 'not quite wrong'. I know it's a great temptation.

Maybe the words that I use do not exactly fit with your way of thinking, but it is the way I think. I am in complete understanding of how a rocket engine works in space. Thank you for all your input and I do apologize if my lack of technical knowledge offended you, that was definitely not my intention. I am not as well educated on the subject but I am trying to understand. In my last post I commented that I can very easily change a pressure to a vacuum and a vacuum to a pressure. As a mechanic I did that regularly. In fact, the compressor on your automotive air conditioner does that every time you turn it on. You are obviously very well educated on this subject and I wonder if you could explain to me why I cannot do this same thing in space.

Chestermiller
Mentor
You can do it in an enclosed system because the gas is trapped within the system, and by increasing and decreasing its volume, you are decreasing and increasing its pressure; the container gives the gas something to push against. In space, you don't have an enclosed gas, and you have nearly perfect vacuum (zero pressure), so there is nothing to push against. When you release the gas, it can only expand into the infinite volume of space until its pressure drops to zero.

Chet

sophiecentaur
Gold Member
Maybe the words that I use do not exactly fit with your way of thinking

I am not at all offended. :)
It's not 'my way of thinking'. It's the way everyone else looks at it - because it is a very successful approach. I suggest that you are 'in complete understand' on your own assessment of your level of understanding. If you understand how a rocket operates in space, why do you introduce a weird concept of 'compressing space'? A rocket operates the same way everywhere; by reaction. An aeroplane wing works on reaction too, but the fluid it forces downward is the air it flies through. There is no fluid in space so you have to supply it all yourself in the propellant.
It's a pity that you seem to want to feel you understand this topic without actually acknowledging the notion of Reaction. It can't be done. What you are saying about refrigeration compressors is just not relevant to propulsion in space.

The air compressor in my garage forces atmospheric air into a sealed chamber until enough air is in the chamber to produce the required pressure. Why can you not pull air/space from outside the vehicle and force it into a chamber until pressure is achieved. I understand that with conventional compressors. It would require far too much energy to complete the process but maybe it is the compressor that needs to be rethought.

I am very ill and at this time very tired. I hope I have given you something to think about. I will rest and check back later.

Don't let the way things are. Stop you from seeing the way things can be

Chestermiller
Mentor
Are you saying that a vacuum cannot be compressed?
Yes. That's what we're saying. A perfect vacuum cannot be compressed, because there's nothing there to compress.

Chet

Bystander
Homework Helper
Gold Member
Why can you not pull air/space from outside the vehicle and force it into a chamber until pressure is achieved.
How long are you prepared to wait for a stray gas molecule to wander into your compressor intake?

Is space a vacuum for just the lack of pressure?

sophiecentaur
Gold Member
Space is just - well, empty - until you find some particles in it. In the deepest inter galactic space, it's estimated there's about one hydrogen atom for every metre cube. That's the deepest vacuum you will get. Your compressor would be working for a good few years to get to atmospheric pressure if you ran it out there, waiting for molecules to turn up and be stuffed into your gas bottle.

russ_watters
Mentor
Is space a vacuum for just the lack of pressure?
Yes -- because it is empty.

I don't mean to imply that we could use my shop compressor in the zero pressure environment of space. I am just trying to get a conversation started in the direction. An external combustion engine is a compressor of sorts. I don't really like using the word vacuum when talking about the environment outside our atmosphere. A vacuum implies that there is a negative pressure. The environment outside our atmosphere is not a negative pressure. It is to my knowledge, an area of no pressure at all. Absolute zero. You cannot compress a vacuum but since that area is really not a vacuum. It should be compressible. By that I mean it is possible create a high pressure from a low pressure. I find it hard to believe that the area known as space is completely empty and I am sure that in the future we will figure out a way to use the incredibly small particles there but for now, we may need to introduce an element to create the desired reaction. Gas hydrate( frozen methane). Like many other compounds are only stable under intense pressure. When exposed to a lower pressure It will revert very quickly to a gas, changing the pressure in any container. It will compress by expansion.Frozen carbon dioxide will do the same thing without the dangers Of methane gas.

At sea level, atmospheric pressure( pressure created by The weight of our atmosphere) Is fairly high. As you reach the upper levels of our atmosphere, the pressure drops too low for us without a pressure suit but is not a vacuum. When we leave our atmosphere. There is no pressure. Zero, not a vacuum.

Dale
Mentor
2021 Award
but for now, we may need to introduce an element to create the desired reaction. Gas hydrate( frozen methane). Like many other compounds are only stable under intense pressure. When exposed to a lower pressure It will revert very quickly to a gas, changing the pressure in any container.
Which is what a rocket does.

When we leave our atmosphere. There is no pressure. Zero, not a vacuum.
Look, you are admittedly not an expert in physics, so rather than trying to tell us what you think the word "vacuum" implies why don't you ask what it actually implies.

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Nugatory
Mentor
A vacuum implies that there is a negative pressure.

This is simply not true.

Chestermiller
Mentor
This is simply not true.
Yes. As nugatory says, vacuum does not imply negative pressure. There are two pressure quantities that scientists use: absolute pressure and gauge pressure. Absolute pressure is self-explanatory. Gage pressure is equal to absolute pressure minus 14.7 psi (average atmospheric pressure at the surface of the earth). Perfect vacuum implies zero absolute pressure, but negative gauge pressure (-14.7 psi). Negative gauge pressure means that, if you open your container at the surface of the earth, air will rush in.

Chet

Dale