Does Rocketry Work beyond Earth's atmosphere?

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Discumbobulate
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by Discumbobulate »

The scientific theory of whether a rocket engine would produce thrust in a vacuum was answered by James Joules 1843 experiment as pointed out correctly by several posters.

I'd like to present the engineering principles , which tie into this experiment , which I haven't come across whist reading through the thread.

The rocket engine is the most inefficient of the various types of internal combustion engines - surprised me when I first learnt of that at tech college many years ago.

Internal combustion engines take the thermal energy produced by chemical reaction (fuel burn) and through mechanical means convert thermal energy to kinetic energy. In the case of automotive engines the combustion takes place in a closed chamber or valve and the expansion of the hot gases push against a piston which in turn drive a camshaft to produce kinetic energy.

In the case of a rocket engine the fuel burn must encounter a resistance at the exit nozzle to enable the thermal energy to create the reactive force of thrust. The atmosphere, or the launchpad, is the equivalent of the piston.

So rocket thrust is entirely dependant on the hot mass flow encountering a resistance to produce kinetic energy . No resistance = no thrust = no kinetic motion.

All this ties in with Newtons laws of motion and the laws of thermodynamics.

The amount of thrust produced by a rocket in the atmosphere is dependant on the outer atmospheric pressure and the area of exit nozzle. Directly proportional acting according to the inverse square law , if I recall correctly, an atmospheric pressure decrease of 1/2 result in a decrease of thrust to 1/4 unless the nozzle area changes.

Sorry if this has already been posted by others , it may have and I just haven't come across it yet.

Nasa rocket equation is maths trickery.
kalliste
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by kalliste »

I think the rocket engineers have a bit of explaining to do entirely based on what they say: the whole thing about the nozzle having to change depending on the atmospheric pressure. If a rocket engine really is a "reaction engine" and it's all about the velocity and mass of the stuff it's throwing out the back then what does the nozzle really matter. There's no logic to worrying about the nozzle. It's all about the speed you can deliver the combustible mixture to the combustion chamber and set fire to it and eject the hot gasses out the hole or in the case of nuclear thermal how hot you can get the gas coming out of the reactor. The size of the hole you're throwing the stuff out of might be an issue but what's a nozzle got to do with it. Worrying about the nozzle only matters if it's standing in for a piston in an internal combustion engine. They're busted by their tomfoolery with rocket nozzles.
kalliste
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by kalliste »

Now I've found somewhere claiming jet engines are "reaction engines" as well as rocket engines and that an afterburner creates "extra thrust" by heating up the exhaust of the primary part of the engine:
https://science.howstuffworks.com/trans ... ion374.htm
But then Wikipedia have to go and spoil it all with their exposition of how a turbofan engine works:
https://en.wikipedia.org/wiki/Turbofan
...which manages to produce a method of working that directly contradicts itself in the text, claiming that the high velocity of the exhaust represents wasted energy whilst at the same time saying it's the exhaust that creates the thrust.

It seems they've had to rewrite how the jet engine works to protect their precious rocket engines. My contention is that jet engines are essentially high speed fans that pull the aircraft through the air by reaction against the blades. Increasing the velocity of the exhaust will let it pull more air through the fans. Turbofans work more fuel efficiently because they're pushing against a greater mass of air.

As for the stuff about water jets and hoses, when someone is wrestling with a hose it's about the hydraulic pressure inside the pipe working on the walls of the tube not about what's squirting out the end. The guy standing on a jet of water from the hose is another misdirection - the water pressure in the hose is making the hose rigid: you could make an arrangement where the hoses were squirting out the top and he was hanging off the side of the hose(s). Think about those advertising thingies that blow air through a tube of fabric to understand what I mean or find a high pressure hose and mess with it. Water jet boats have impellers that are pushing against the water same a jet turbine fans and aircraft propellers, the stuff coming out the end isn't doing the pushing.

Then we have ramjets in case you think I've forgotten. They're like rocket engines, an internal combustion engine with the nozzle end acting in lieu of a piston but they use atmospheric oxygen as the fuel oxidizer. Actually when you get into ramjets and scramjets there are shock waves inside the engine to consider in the combustion cycle so I expect they are a somewhat more complicated thing to get working in spite of the apparent simplicity of construction. Somebody should probably ask the rocket scientists why, if force equals mass times velocity with some foo factors due to the mass of the rocket going down due to the expulsion of fuel, a ramjet and a scramjet need any fuel at all once they get up to speed.
Discumbobulate
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by Discumbobulate »

Hi kalliste, the point I was making about the rocket nozzle is that it is just basically an exhaust pipe - its open to the atmosphere or vacuum. The only way to convert thermal energy of the fuel burn is to provide a means for conversion to kinetic energy. Vacuum conditions provide no means for converting thermal to kinetic energy.

It doesn't matter how hot the gas is , it's just thermal energy and will remains so until a resistance to it's flow out of the nozzle encounters a mechanism for conversion to kinetic energy - atmosphere or a solid surface. All it does in the supposed vacuum of space is expand without providing any thrust , maybe raise the temperature by a miniscule amount.

The jet engine also works by converting thermal energy to kinetic. Not really looked into those since they don't pretend to use those in outside the atmosphere.

It seems to me that the term "internal combustion" engine has been quietly shoved aside by the controlling buffoons , introducing instead the term "reaction" engines . Muddy the waters so to speak, however the thermal energy always has to convert to kinetic to provide thrust . Thermal energy cannot thrust upon itself although it may warm things up a bit.

That is the basic scientific or engineering principle that prevents the rocket engine working in a vacuum.
kalliste
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by kalliste »

kalliste wrote: Fri Sep 23, 2022 12:46 am Somebody should probably ask the rocket scientists why, if force equals mass times velocity with some foo factors due to the mass of the rocket going down due to the expulsion of fuel, a ramjet and a scramjet need any fuel at all once they get up to speed.
By which I meant to say the ramjet working by expulsion of high velocity gas surely by this argument a tube with one end constricted and the larger end traveling forwards would continue on forever in perpetual motion because the high velocity air out the narrow end would push it as per rocket equation. In fact this isn't the case because a ramjet/scramjet is an internal combustion engine not a "reaction engine."
kalliste
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by 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
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by glg »

kalliste wrote: Sat Sep 24, 2022 8:06 am 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
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by 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.
glg wrote: Sat Sep 24, 2022 3:07 pmit 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
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by 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
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by glg »

Mansur wrote: Sat Sep 24, 2022 8:52 pm
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
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by 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
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by glg »

kalliste wrote: Sun Sep 25, 2022 5:59 am 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
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by kalliste »

glg wrote: Sun Sep 25, 2022 9:03 am 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
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by 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
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread post by kalliste »

glg wrote: Sun Sep 25, 2022 10:49 am
(..) 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.
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