Does Rocketry Work beyond Earth's atmosphere?

[kalliste]

Somebody explain this to me, in this answer from Quora

https://www.quora.com/profile/Johnny-Pa ... liams-1067

Peter Williams
Ph.D. in Physics, The University of Texas at Austin (Graduated 2000)

Several ppl have responded that the answer is “zero.”

Actually, it’s a bit more complex than that. First, note that the question is a bit vague: what is the antecedent of “it?”

If by “it” you mean the vacuum, then yes, the answer is zero (neglecting fancy arcane stuff like the Casimir effect).

If by “it” you mean the gas, then the answer is not necessarily zero. True, if we imagine a box with a partition, gas on one side and vacuum on the other, and we remove the partition - poof! - then the work done on (and by) the gas is zero, after we wait around for the gas to come back into thermal equilibrium. For an ideal gas, all we have done is change its entropy, not its energy. Neither heat nor work has been exchanged between the gas and anything else.

If, however, we imagine that on the vacuum side of the partition, the box extends to infinity, then after we remove the partition, the gas expands indefinitely. It cools adiabatically and gains bulk momentum , approaching Mach 1 or so. The gain of momentum comes from the unbalanced pressure force on the walls, and the mechanical energy comes from the thermal energy of the molecules. In accordance with the 2nd law, you can’t get it all out, of course, but you can definitely get a good chunk of it, especially if you build a nice de Laval nozzle.

This is not just an academic exercise. You can now imagine, say, floating out in space, and having a can filled with high-pressure gas, and you open one side of the can. You will experience a force; work is being done on you, and work is being done on the gas that is expanding in the opposite direction (accelerating it), the energy coming from the adiabatic expansion of the gas.

So in that case, no, the answer isn’t “zero.”

There’s no paradox here; it’s just that usually questions like this about the thermodynamics of ideal gases, until you move on to the field of gas dynamics, are framed in very idealistic conditions. The field should really be called “thermostatics,” not “thermodynamics.” One of the idealizations is usually that you consider the bulk state of the gas to be characterized by a very small number of thermodynamic state variables: density, temperature, pressure. That’s it. That’s fine if the gas is in complete thermodynamic equilibrium, but when the gas is in motion, and in particular when the gas is moving different speeds at different points (I don’t mean the gas molecules themselves, I mean the gas viewed as a continuum), then pretty much by definition it’s not in thermodynamic equilibrium. Instead, in that case, ppl adopt the approximate view that in small enough little volumes (a “fluid particle”), the gas is in some kind of local thermodynamic equilibrium (LTE), but not when considered as a bulk. And of course, at some point, even that approximation breaks down, and the entire concept of thermodynamic quantities like “temperature” and “pressure” start to lose their meaning, even in that restricted LTE sense.

Just had to point this all out b/c it can be a bit confusing when you consider the thought experiment of the astronaut with the aerosol can (the example I gave above), and trying to make that jive with basic ideal gas thermodynamics.

And again, in all cases, the work done on the vacuum is always zero.

What kind of magic is this? It's like you're watching sleight of hand knowing it's a magic trick but you still can't see how it's done. In the case of a gas expanding in a vacuum though surely any work done is a temperature change? And then there's that killer last sentence!

[glg]

Somebody explain this to me, in this answer from Quora

https://www.quora.com/profile/Johnny-Pa ... liams-1067

Peter Williams
Ph.D. in Physics, The University of Texas at Austin (Graduated 2000)

Several ppl have responded that the answer is “zero.”

Actually, it’s a bit more complex than that. First, note that the question is a bit vague: what is the antecedent of “it?”

If by “it” you mean the vacuum, then yes, the answer is zero (neglecting fancy arcane stuff like the Casimir effect).

If by “it” you mean the gas, then the answer is not necessarily zero. True, if we imagine a box with a partition, gas on one side and vacuum on the other, and we remove the partition - poof! - then the work done on (and by) the gas is zero, after we wait around for the gas to come back into thermal equilibrium. For an ideal gas, all we have done is change its entropy, not its energy. Neither heat nor work has been exchanged between the gas and anything else.

If, however, we imagine that on the vacuum side of the partition, the box extends to infinity, then after we remove the partition, the gas expands indefinitely. It cools adiabatically and gains bulk momentum , approaching Mach 1 or so. The gain of momentum comes from the unbalanced pressure force on the walls, and the mechanical energy comes from the thermal energy of the molecules. In accordance with the 2nd law, you can’t get it all out, of course, but you can definitely get a good chunk of it, especially if you build a nice de Laval nozzle.

This is not just an academic exercise. You can now imagine, say, floating out in space, and having a can filled with high-pressure gas, and you open one side of the can. You will experience a force; work is being done on you, and work is being done on the gas that is expanding in the opposite direction (accelerating it), the energy coming from the adiabatic expansion of the gas.

So in that case, no, the answer isn’t “zero.”

There’s no paradox here; it’s just that usually questions like this about the thermodynamics of ideal gases, until you move on to the field of gas dynamics, are framed in very idealistic conditions. The field should really be called “thermostatics,” not “thermodynamics.” One of the idealizations is usually that you consider the bulk state of the gas to be characterized by a very small number of thermodynamic state variables: density, temperature, pressure. That’s it. That’s fine if the gas is in complete thermodynamic equilibrium, but when the gas is in motion, and in particular when the gas is moving different speeds at different points (I don’t mean the gas molecules themselves, I mean the gas viewed as a continuum), then pretty much by definition it’s not in thermodynamic equilibrium. Instead, in that case, ppl adopt the approximate view that in small enough little volumes (a “fluid particle”), the gas is in some kind of local thermodynamic equilibrium (LTE), but not when considered as a bulk. And of course, at some point, even that approximation breaks down, and the entire concept of thermodynamic quantities like “temperature” and “pressure” start to lose their meaning, even in that restricted LTE sense.

Just had to point this all out b/c it can be a bit confusing when you consider the thought experiment of the astronaut with the aerosol can (the example I gave above), and trying to make that jive with basic ideal gas thermodynamics.

And again, in all cases, the work done on the vacuum is always zero.

What kind of magic is this? It's like you're watching sleight of hand knowing it's a magic trick but you still can't see how it's done. In the case of a gas expanding in a vacuum though surely any work done is a temperature change? And then there's that killer last sentence!

Yes kalliste it is a magic sleight of hand trick but it must not necessarily be the case that the magician is mentally above the trick knowing better.

In this instance I try to imagine myself in a vacuum and by contorting and releasing my muscles in a certain way I try to leap forward in space. It's hard to imagine if it will work or not, but once you try to imagine it working more then once the mental endeavor becomes comical.
But if I imagine myself in a vacuum tied to a rope attached to the moon for instance, then the same bodily contortion and release trying to leap forward will definitely get me swinging forward and back again.
I say this, because further up this thread, there is a video of an exploding can in a vacuum chamber attached to a string(!).
My question is, would this same can leap forward in a vacuum if it was not attached to a string?
Because if it would, then it would not be inconceivable to me, that successive explosions could leap the can ever more forward in a vacuum.

edit: of course the rope or the string must be taut to act as translators and that adds the influence of gravity to my examples, but really the question is, if a closed system can move directionally coherent through space without the aid of translators?

[Mansur]

[ If a gas expands against a vacuum, what is the work done on it? ]

If, however, we imagine that on the vacuum side of the partition, the box extends to infinity… etc.

Perhaps the trick is that - in this second round or attempt - he doesn't make the reader 'imagine' an infinite vacuum, but rather an infinitely long box, so he tries to present it as a closed system as it were.

it is a magic sleight of hand trick but it must not necessarily be the case that the magician is mentally above the trick knowing better.

I've read quite a few of these explanations on the web, written by professionals (mostly teachers); in a sense, they could even be called bona fide. For them, the outcome is a foregone conclusion: then years of routine seem to come to their rescue in the form of a 'lucky idea'.

When a man is cornered in his persuasion, his ingenuity increases rather than decreases. It doesn't work with these people: already in the second round they either 'quit' or resort to undignified means.

My question is, would this same can leap forward in a vacuum if it was not attached to a string?

I think we should see that video.

[kalliste]

Then there's this:

https://www.scientificamerican.com/arti ... n-the-air/

"On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs."

Maybe the joke is on us and rockets do work in a vacuum but NASA and the scientists know they can't really explain it because according to accepted physics it shouldn't happen. The "infinite vacuum" (which obviously can't really exist) has to be conjured up to save them. Maybe the way they work is such that it can't be explained at all if we did but know what really happens.

[glg]

My question is, would this same can leap forward in a vacuum if it was not attached to a string?

I think we should see that video.

full link: http://www.youtube.com/watch?v=T8MOoUuLnug

Please, now that I have resupplied that video, remember again what I wrote because it seems neither the uploader nor the good people in the comments section pay heed to the string the can is attached to acting perhaps as a crucial translator of directional motion.

glg wrote::
In this instance I try to imagine myself in a vacuum and by contorting and releasing my muscles in a certain way I try to leap forward in space. It's hard to imagine if it will work or not, but once you try to imagine it working more then once the mental endeavor becomes comical.
But if I imagine myself in a vacuum tied to a rope attached to the moon for instance, then the same bodily contortion and release trying to leap forward will definitely get me swinging forward and back again.
I say this, because further up this thread, there is a video of an exploding can in a vacuum chamber attached to a string(!).
My question is, would this same can leap forward in a vacuum if it was not attached to a string?
Because if it would, then it would not be inconceivable to me, that successive explosions could leap the can ever more forward in a vacuum.

edit: of course the rope or the string must be taut to act as translators and that adds the influence of gravity to my examples, but really the question is, if a closed system can move directionally coherent through space without the aid of translators?

[kalliste]

I left a comment on the video thus:

There are multiple problems with this. There's something called "ground effect" that is very poorly understood, so the walls of the chamber are too close.
The other is the whole top of the can blew off. Nobody is disputing Newton's 3rd Law so blowing the top of the can off moves the can the other way.
The problem is we're not convinced that the rocket equation is real because in a vacuum we don't believe it applies to gases in free expansion. The contention is that a rocket is in actual fact a (very inefficient) internal combustion engine where the nozzle/opening and external pressure act in lieu of the piston in a more conventional internal combustion engine. Thus in atmosphere the thermal energy of the gas is converted to kinetic energy because it does work but with combustion In a vacuum the free expansion of gas means it no longer works as an engine and you go nowhere. Rocket engineers have to fiddle with the nozzle to get better results at a different atmospheric pressure precisely because of the rocket engine in actual fact being an internal combustion engine. Calling them "vacuum pressure nozzles" is misdirection, they're really thin atmosphere nozzles.
Actually there are some people that have issues with Newton's 3rd but we don't need to go there.

[glg]

I left a comment on the video thus:

There are multiple problems with this. There's something called "ground effect" that is very poorly understood, so the walls of the chamber are too close.
The other is the whole top of the can blew off. Nobody is disputing Newton's 3rd Law so blowing the top of the can off moves the can the other way.
The problem is we're not convinced that the rocket equation is real because in a vacuum we don't believe it applies to gases in free expansion. The contention is that a rocket is in actual fact a (very inefficient) internal combustion engine where the nozzle/opening and external pressure act in lieu of the piston in a more conventional internal combustion engine. Thus in atmosphere the thermal energy of the gas is converted to kinetic energy because it does work but with combustion In a vacuum the free expansion of gas means it no longer works as an engine and you go nowhere. Rocket engineers have to fiddle with the nozzle to get better results at a different atmospheric pressure precisely because of the rocket engine in actual fact being an internal combustion engine. Calling them "vacuum pressure nozzles" is misdirection, they're really thin atmosphere nozzles.
Actually there are some people that have issues with Newton's 3rd but we don't need to go there.

These are all interesting and valid points you make kalliste and I believe other members of this forum have critiqued this experiment in similar terms some ways back in this thread.
I even think the experimenter himself was aware of some of the problems you mention which is why he focuses on one very crucial instant of the experiment only namely where the gas has not yet reached the wall and where the can has not yet blown its lid off but where nonetheless the can already pushes forward.
So if that instant where conserved and the can somehow managed to keep the rest of its pressure inside to repeat that instant as long as there is pressure left to waste, would then not that can be somewhat of a forward moving rocket through the vacuum of space?

Which is why I mentioned and pointed out the string.

[kalliste]

These are all interesting and valid points you make kalliste and I believe other members of this forum have critiqued this experiment in similar terms some ways back in this thread.
I even think the experimenter himself was aware of some of the problems you mention which is why he focuses on one very crucial instant of the experiment only namely where the gas has not yet reached the wall and where the can has not yet blown its lid off but where nonetheless the can already pushes forward.
So if that instant where conserved and the can somehow managed to keep the rest of its pressure inside to repeat that instant as long as there is pressure left to waste, would then not that can be somewhat of a forward moving rocket through the vacuum of space?

Which is why I mentioned and pointed out the string.

As I said, ground effect is not well understood so this is meaningless without a very much larger vacuum chamber.

The gas is the CO2 dissolved in the soda not the liquid you can see. So to claim you can see the invisible CO2 as what you're seeing is quite likely misdirection... or the the guy is an idiot. I can argue the CO2 gas has to do work against the liquid it's confined in to become a free gas so isn't behaving as an ideal gas. This doesn't help a rocket engine because a rocket engine doesn't remotely work in this way. We don't know at what point the top separated and became a separate mass either.

You're also kind of right that the string is doing something because it physically connects to the wall the can is a pendulum weight so liquid motion inside it against the walls of the can will certainly do something to move the can (sort of Newton's Cradle effect). The moment it ruptures and the liquid is disturbed the can will move.

This video tells us nothing about the rocket equation or rocket engines. Which is unsurprising given it's a can of soda.

While we're on the subject of liquids, if you look at the accounts of water injection in jet engines (as done for instance by the B52 bomber) to increase thrust the debate seems to switch between claiming cooling the engine allows it to operate at higher RPM, otherwise claiming the extra mass of the exhaust gas increases thrust. Neither fit properly with the assumptions of the rocket equation which will be why the explanations are all over the place. Actually it looks like they have no real idea how water, although it's water/methanol so the water can't freeze, helps a turbine engine. Just the usual hand waving.

[glg]

(..) meaningless without a very much larger vacuum chamber.

no matter how large the chamber anything able to release energy attached to a string or taut rope tied to the wall of a no matter how huge vacuum chamber will move (swing).

all other points you make about this ridiculous experiment have value of course but just seeing the string invalidates the experiment instantly - that's all I'm saying.

[kalliste]

(..) meaningless without a very much larger vacuum chamber.

no matter how large the chamber anything able to release energy attached to a string or taut rope tied to the wall of a no matter how huge vacuum chamber will move (swing).

all other points you make about this ridiculous experiment have value of course but just seeing the string invalidates the experiment instantly - that's all I'm saying.

Are the people doing these videos idiots rather than paid misdirection, that's all that the paranoid in me really needs to know.

[Mansur]

I think the main figure of the video is definitely the (space) rocket model, which the guy is waving in front of the camera for a considerable part of the clip.

The 'work' is done, I think it is obvious, by the liquid. If the guy was really interested in the behaviour of the gas, he would have had to use a pressurised bottle with only gas in it; but now he gives the impression, perhaps deliberately, that the soft drink is the liquid fuel... The 'experiment' is complete nonsense, with or without string.

With his HD camera, perhaps he should have been watching the atmospheric pressure sensor to document minutely the change in air pressure - of course, that would probably require an 'HD sensor' too, I don't know if such a thing exists. Anyway, if you want to know something, you have to do hundreds and thousands of experiments, not just one.

Videos of this type are run under the prestigious heading of 'experiment', but should be called a demonstration, not in the regular meaning (demonstrating a known fact with an 'experiment'), but in a purely propaganda sense. On the basis of 'a picture is worth a thousand words'.

Are the people doing these videos idiots rather than paid misdirection, that's all that the paranoid in me really needs to know.

Among other videos of the author gentleman, one shows a balloon and another a mini drone, also in a vacuum chamber. The behaviour of both objects as shown in the two videos respectively, under zero atmospheric pressure, is utter nonsense, so the two psychological categories above could perhaps be brought together under a common heading: a liar. For the 'idiot', unless it is a medical case, I can offer no excuse - he himself made an idiot of himself. (I would rather withdraw my previous indulgence of them.)

As for the 'infinite vacuum': many people raise the idea that if there were a vacuum, the atmosphere could not exist. So when the air runs out, something has to remain. The question is what that is. I guess there is no empirical answer to that. Not just 'not yet', but not at all.

To return: trying to produce experimental conditions, simulations, of which we know nothing about the original seems to be an interesting effort.

[Discumbobulate]

I'm young enough to remember the old Saturn V rocket and the Apollo moon shots. The operation of the rockets explained in those days by physicists of the day as I recall it.

1st stage rocket engine , 5 large area nozzles needed to lift the huge load of the launch pad . These quickly lose efficiency as the rocket clears the launch pad and takes a ballistic flight path to prevent a stall . Rocket accelerates and when the first stage fuel is spent the second stage fuel ignites ( still within the atmosphere) as the first stage falls away .

2nd stage rocket engine has smaller nozzle area giving increased efficiency in reduced atmospheric pressure. Also kicks against the momentum of the first stage giving more acceleration.

3rd stage as above.

These 3 stages were said to produce enough velocity to escape the clutches of earths gravity - along with the supposed slingshot effect produced by orbiting earth at high speed.

Very interesting to me as a youngster. The rocket equations of that showed that thrust depended totally on outer atmospheric pressure or launch pad / used stage.

Having been made redundant in the 1990s , and still having the belief that scientists were truthful souls , I thought I'd go to Uni , hopefully to study for a degree in rocket science.

No such course. That's when I lost my faith.

Rocketry is pure engineering. Nasa's rocket equations are laughable .Sealed bombs or cans of soda pop will explode in vacuums but rocket engines cannot
produce kinetic energy in a vacuum - it would be a violation of the known laws of physics and thermodynamics.

Of course maybe we dont have an outer space or even a vacuum. Nasa lies whichever way

[glg]

(..)The 'experiment' is complete nonsense, with or without string.(..)

I disagree: The experiment is complete nonsense precisely and foremost because of the string.
I mentioned the string because it is the primary receiver and translator of the slightest disturbance in inertia of the can.

For any experiment which wants to test secondary or tertiary effects devaluating the experiment like gas hitting the chamber wall or ¨ground effect¨ or such, must first discard the primary obstacle to those hypothesis and that is the string.
Of course, once the string is gone, this experiment ceases to exist. People like the string though because it fools them into believing that the can is floating in a vacuum which allows them to study things like gas hitting the chamber walls asf. but those things actually cannot be studied correctly because here the string is the primary receiver and translator of the slightest disturbance in inertia of the can.

I don't want to make an issue out of it other then that I think it matters to see where the primary faults in an experiment lie, because those are the first that need to go to see which other design faults might be next in line.

[kalliste]

Rocketry is pure engineering. Nasa's rocket equations are laughable .Sealed bombs or cans of soda pop will explode in vacuums but rocket engines cannot
produce kinetic energy in a vacuum - it would be a violation of the known laws of physics and thermodynamics.

Of course maybe we dont have an outer space or even a vacuum. Nasa lies whichever way

That's my theory in the ISS thread. The mystery of why they set the ISS in a low orbit that means it continually has to be resupplied with fuel to boost itself back into orbit is no mystery at all if that's as high as a rocket can get. I agree with what I interpret to be your thinking, rockets clearly work up to a point and they've milked it to the limit by engineering tinkering after decades of empirical measurement and good old trial and error. They could get Apollo into Earth orbit of sorts but they had to fake the rest, hence the faked picture of approaching the Moon that was clearly taken in Earth orbit that Bart Sibrel has exposed. The very first space suits were little more than slighty modified high altitude aircraft pilot pressurised flight suits and after that space suits become increasingly ridiculous and impractical as space flight had to be faked. I watched a video made in the sixties about the Apollo Moon suit backpack and it was hilarious. How the guy explaining how it worked kept a straight face was the most impressive part.
Like Elon Musk says, you know it's real because it's fake...


full link: http://www.youtube.com/watch?v=-40rmID-U6o

[kalliste]

I've come up with a hypothesis on how rocket engines can work in the vacuum of space if they indeed they can be shown to work in the vacuum of space. They're pushing against their own exhaust gases just as NASA and the bullshit rocket equation pretends. The rocket equation is nonsense though and nothing to do with what is happening except in so far as you want the highest velocity exhaust gas you can get. The trick is that so long as the exhaust is moving fast enough and can generate shock waves in the exhaust flow while in a vacuum the rocket will work. The shock waves are the mechanism to convert the heat energy of the gas to kinetic energy that can do work on the rocket. The shock waves in the exhaust hold up the gas coming out of the rocket nozzle hence no more free expansion. The nozzle design would be a critical factor according to this hypothesis. The engine will work so long as the exhaust stream can be made to generate shock waves, I imagine near the rocket, in a vacuum. In fact it's still working as an internal combustion engine but what's happening to the rocket is that the centre of mass of the system is moving around and generating momentum (think of Newton's Cradle but there's nothing keeping the pendulums in the Cradle). I'm totally guessing but maybe you can even move in the same direction as the engine nozzle is pointing if you do it right.

What gave me this idea is watching a video about the SR71, which I've seen in a museum. The speed of the SR71 was limited by the formation of shock waves in the inlet of the jet engines which would choke the flow of air into the engine. It had a special input nozzle arrangement to control the airflow but above a certain speed it ran out of adjustment and the engines flamed out because shock waves formed and stopped the flow of air. This actually happened to an SR71 when pilots ignored protocol and wanted to see how fast they could go. They nearly died because they had trouble relighting the engines.

I'd be interested to hear whether forum members think this will fly. Another thought occurs to me: if you were outside the rocket but inside the gas cloud generated by the rocket exhaust you should be able to hear the shock waves, so according to my hypothesis as long as you're close enough you can hear a rocket flying in space. All those SF movies were correct after all!