Does Rocketry Work beyond Earth's atmosphere?

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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby dblitz on April 2nd, 2018, 12:49 pm

Starfish Prime, you said:

Rather, the exhaust molecules/atoms will disperse in every direction, counteracting each other, and generating almost no thrust.


This is not true. The best you will get is a half sphere of possible directions. So, I have saved 50% of our thrust from free expansion, where does it go?

Or, to put it another way, your idea would work if we had a rocket with a nozzle at both ends, then the freely expanding gases would counteract each other, as you said, and the rocket stays still. Now turn off one of the nozzles and what do you expect will occur?
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby bongostaple on April 2nd, 2018, 3:55 pm

dblitz » April 2nd, 2018, 11:07 am wrote:If I understood your question bongostaple; how about a wheeled chair and a high pressure hose?


Assuming it's water pumping down the hose, I guess so, but I'd prefer self-oxidising fuel that's on fire at the nozzle. Really the main bit about rockets working in the atmosphere (i.e. not in space) that I don't feel fits reality is that the NASA rocket thrust equations seem to ignore the density of whatever matter/air/etc is already sitting there when the burning fuel comes out of the nozzle. Are we expected to believe that a rocket on a launchpad and a rocket already in the air produce exactly the same thrust? That certainly appears to be the gist of the rocket thrust equations - I can't see why there wouldn't be a huge difference. But obviously if we extend that thinking from solid ground - air, and compare air - vacuum, then it starts looking rather unthrusty.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby patrix on April 2nd, 2018, 4:48 pm

bongostaple » April 2nd, 2018, 3:55 pm wrote:
dblitz » April 2nd, 2018, 11:07 am wrote:If I understood your question bongostaple; how about a wheeled chair and a high pressure hose?


Assuming it's water pumping down the hose, I guess so, but I'd prefer self-oxidising fuel that's on fire at the nozzle. Really the main bit about rockets working in the atmosphere (i.e. not in space) that I don't feel fits reality is that the NASA rocket thrust equations seem to ignore the density of whatever matter/air/etc is already sitting there when the burning fuel comes out of the nozzle. Are we expected to believe that a rocket on a launchpad and a rocket already in the air produce exactly the same thrust? That certainly appears to be the gist of the rocket thrust equations - I can't see why there wouldn't be a huge difference. But obviously if we extend that thinking from solid ground - air, and compare air - vacuum, then it starts looking rather unthrusty.


Come on. This has been gone through before in the thread. I suggest you start reading.

Sitting on a wheel chair with a fire hose? Then you are constantly adding mass to the system. See any hoses that reach up to space attached to the "space" rockets?

And water has many hundred times the density of gas.

Why all these weird analogies when it's easy to conclude the non effect of gas expansion in vacuum? Release gas in a space where it can expand freely. No work. Case closed. Doesn't matter whatsoever how fast said expansion occurs.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby Penelope on April 2nd, 2018, 9:43 pm

Star prime, you're missing the crux of my argument. You and hoi polloi say, "As the exhaust exits the nozzle, it will not be directed by an atmosphere into a condensed stream generating thrust in the opposite direction. Rather, the exhaust molecules/atoms will disperse in every direction, counteracting each other, and generating almost no thrust."

Don't be silly. Exhaust plume after exit isn't the source of thrust.

\According to Newton's 3d thrust is a product of the mass x velocity of the superheated gas & particles just before the instant of separation. By definition, mutual separation means the two each move away from the other & no longer affect each other.

As I throw the sandbag away from my wheeled chair the instant which determines the thrust of my chair is the one in which my palms & sandbag push against each other.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby Penelope on April 2nd, 2018, 9:48 pm

Patrix said, "Why all these weird analogies when it's easy to conclude the non effect of gas expansion in vacuum?"

The superheated, compressed gas is NOT within a vacuum when it has its effect; it's still inside the nozzle and its effect-- propulsion of both rocket & gas-- is manifest upon exit.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby Altair on April 2nd, 2018, 9:51 pm

There's one important difference between gas expansion and the 'pushing' of a liquid or solid body. Intermolecular forces between molecules of a gas follow a completely different set of laws, and you cannot apply Newton's Laws to them.
A very simple experiment (that seems nobody has carried on, at least I didn't found anything valid) would be to open the valve of a CO2 fire extinguisher in an appropriately sized vacuum chamber and measure the forces exerted.
It's strange that there seems not to be any scientific literature, nor tests regarding the working of rockets in vacuum, and this would be of paramount importance before sending things up there, as a very small deviation in calculated vs. real thrust would mean that the object would not achieve the desired orbit.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby Penelope on April 2nd, 2018, 10:04 pm

bongostaple says

April 2nd, 2018, 6:55 am NASA rocket thrust equations seem to ignore the density of whatever matter/air/etc is already sitting there when the burning fuel comes out of the nozzle. Are we expected to believe that a rocket on a launchpad and a rocket already in the air produce exactly the same thrust? That certainly appears to be the gist of the rocket thrust equations - I can't see why there wouldn't be a huge difference. But obviously if we extend that thinking from solid ground - air, and compare air - vacuum, then it starts looking rather unthrusty.


The most basic equation is Force = mass x velocity. Velocity at the relevant instant is already net of all affective factors. Those factors occurring after separation of Newton's two paired entities, the rocket and its propulsive gases, are irrelevant to the propulsive force already determined.

The results of that propulsive force will of course differ in that atmosphere exerts drag on the rocket and vacuum doesn't. Inertia of rest vs inertia of motion also is a factor.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby Penelope on April 2nd, 2018, 10:25 pm

by Altair on April 2nd, 2018, 9:51 pm

There's one important difference between gas expansion and the 'pushing' of a liquid or solid body. Intermolecular forces between molecules of a gas follow a completely different set of laws, and you cannot apply Newton's Laws to them.

Newton's laws apply to material bodies, whether solid, liquid or gas as these are the forms of matter.

A very simple experiment (that seems nobody has carried on, at least I didn't found anything valid) would be to open the valve of a CO2 fire extinguisher in an appropriately sized vacuum chamber and measure the forces exerted.

The differential pressure between the tank of CO2 and 0 pressure of vacuum would determine the speed of exit of the CO2. Other factors like size of opening; temperatures of both, which are a factor in the number of molecular collisions, which would slow down exit of the CO2.

It's strange that there seems not to be any scientific literature, nor tests regarding the working of rockets in vacuum, and this would be of paramount importance before sending things up there, as a very small deviation in calculated vs. real thrust would mean that the object would not achieve the desired orbit.


Vacuum is just lack of atmosphere. Of course they're tested. Google "testing rocket nozzles in vacuum" to get 451,000 results which cover more than nozzles. Obviously not everything is truthful, but it's the most readily available data we have. If you want something more technical exert yourself to find it,
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby patrix on April 3rd, 2018, 4:31 am

Penelope » April 2nd, 2018, 9:48 pm wrote:Patrix said, "Why all these weird analogies when it's easy to conclude the non effect of gas expansion in vacuum?"

The superheated, compressed gas is NOT within a vacuum when it has its effect; it's still inside the nozzle and its effect-- propulsion of both rocket & gas-- is manifest upon exit.


So the NASA rockets move by pushing at themselves with "superheated" compressed gas from the inside then. Same old same old. Physics says no... ( https://youtu.be/AJQ3TM-p2QI )
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby Penelope on April 3rd, 2018, 5:24 am

by patrix » April 2nd, 2018, 7:31 pm
So the NASA rockets move by pushing at themselves with "superheated" compressed gas from the inside then. Same old same old. Physics says no


If the movement of the gas inside the rocket causes it to exit, that movement will propel the rocket.
If the movement of the gas inside the rocket is prevented from causing exit, the rocket will not be propelled. (provided the gas has enough mass & velocity, of course)

Try it yourself. Sit in a wheeled chair on a smooth surface with a 40 lb dog on your lap. You, the dog & chair are all considered one object now, one mass (like the gas inside the rocket is included in the rocket's mass). The dog may make all the attempts he likes to jump free, but so long as you hold onto him, you are still considered as one mass. But if you open your arms so that he actually jumps free, your chair will be propelled backward. It doesn't matter whether he jumps into air or vacuum. Only his movements and velocity while he is still on the way to exit matter. When his paws are clear of you he can no longer affect the chair's movement.

If no dog, use sand in a garbage bag.

The idea that the rocket is propelled by its exhaust pushing on the air is a misunderstanding of Newton's 3d, which deals with paired objects and the forces they exert. Rocket and gases until separation are a pair involved in the transactional force which gives both a motion which separates them.

The gases after separation are exhaust. Within atmosphere the air resists the exhaust. The exhaust being initially more energetic pushes it aside, continuing to travel away from the rocket until equlibrium is reached. This going-away exhaust does not affect the rocket, even if it goes into vacuum.



Even though when he made those exact same movements while you prevented his leap your chair did not move.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby Altair on April 3rd, 2018, 8:16 am

Sorry for the retro-quote, but here is my take on it:

Altair » October 31st, 2017, 5:32 pm wrote:I think it has been mentioned in this thread or in some one related, but it seems that free expansion of gases cannot produce any work, in physical sense. There is a pretty good discussion here: https://www.physicsforums.com/threads/w ... um.725702/

So, the idea is that work is done by pressure. In an atmosphere, the gas ejected from the rockets would build a pressure gradient, from a maximum near the exhaust, and gradually decreasing until it levels off with atmospheric pressure. So, it's the pressure buildup what pushes the rocket upwards. A nice way to model this would be to imagine a compressed air cilinder and then opening the valve.

But in a vacuum, the gradient 'endpoint' would be 0, so the gas molecules would have no opposing force when moving away from the rocket.

In fact, it's strange that I've found no literature about such a simple experiment as would be opening the valve of a compressed air cilinder in a vacuum chamber and measuring the resulting forces. And then comparing it with the same experiment in the atmosphere. My guess is that the exerted force would be almost negligible, but maybe non-zero.


As for testing in vacuum, yes, there were some results. Maybe the most reasonable one at first glance (obviously, I didn't check all of them) is this one: https://www.reddit.com/r/spacex/comment ... ve_a_large

But still this seems to me too much of a stretch: "So an engine designed to function optimally at sea level will have a diverging section that reduces the gas pressure to 1 atm at the exit; an engine that is optimal for vacuum will try to get a diverging section that will reduce the pressure to 0 atm as the exit (this is impossible, it would require an infinite nozzle, but we can get pretty close with a large enough shape). Since lower pressure at exit = larger nozzle, vacuum-optimized engines have larger nozzles than atmospheric engines."

To put it simply, we can give a hoot about the exit speed of the gas, because what "pushes" the rocket upwards is good old plain pressure against the rocket's structure. If you think that just accelerating the gas will produce such pressure against the nozzle walls, so let it be. But in fact, as the cited article states, accelerating a gas just means that you're trading pressure by speed.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby Altair on April 3rd, 2018, 10:02 am

To put another comparison: think about a hovercraft. It's suspended some inches above the ground/sea because the pressure inside the cushion pushes it upwards. But lift it just a bit more, and it will fall again, because the air exits too quickly and pressure falls.
So in that sense, a rocket's nozzles are just like the hovercraft cushion: you must create enough gas pressure in the inside in order to generate a force that lifts the rocket.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby Penelope on April 3rd, 2018, 11:58 am

Altair at 8:16 AM said
To put it simply, we can give a hoot about the exit speed of the gas, because what "pushes" the rocket upwards is good old plain pressure against the rocket's structure. If you think that just accelerating the gas will produce such pressure against the nozzle walls, so let it be. But in fact, as the cited article states, accelerating a gas just means that you're trading pressure by speed.


Sorry, the de Laval nozzle is far too technical to get into here. The reddit comment you referred to was speaking of extreme cases-- sea level vs vacuum. But the rocket would be operating at a variety of altitudes. Also, he simplified considerably.

Since you suggest looking at a compressed gas cylinder, here is probably my last effort to explain Newton's 3d.

We have lying on a smooth surface a compressed gas cylinder, modified by the addition of a nozzle which is mostly internal to the tank. The gas is under 50 lb/sq in pressure, which means it exerts a force of 50 lbs on each inch of the tank & nozzle. The nozzle's interior is a total of 20 sq inches. So the total force on the nozzle by the gas is 1,000 lbs, plus the nozzle is resisting this pressure with a matching 1,000 lbs. So we have two objects pushing on each other with no means of separation because the exit is closed.

We know that when we open the exit, cylinder and gas will fling away from each other. Just as my palms and the sandbag pressed together until a hairsbreadth of space opened between them. That separation permitted the two to fling themselves in opposite directions by availing themselves of the force built up while they were pressing together.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby patrix on April 3rd, 2018, 2:04 pm

Very interesting Penelope. So with the special "de Laval nozzle" (too complex for us mere mortals to understand) and really really high pressure gas we will achieve the "real" Newtons 3d as you referred to it before, that apart from the old boring one is not dependant on Newtons 1st.

May I suggest another name as to avoid confusion with old plain Newtons 3d. Let's call it the Munchausen effect, since that old Baron was known to be able to lift himself by pulling his own hair.

I understand and sympathize with you Penelope that these new exciting discoveries in physics are hard to explain.

May I therefore suggest that you instead describe an experiment we can perform that will demonstrate this new Munchausen effect. And I take it you understand I prefer to use gas and not wheelchairs or water hoses since gas expansion is the subject matter.
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Re: Does Rocketry Work beyond Earth's atmosphere?

Postby Flabbergasted on April 3rd, 2018, 4:01 pm

Penelope, I may have missed something during my absence, but did you answer Simon´s two questions from his post on 28 March 2018, 9:39 pm?
viewtopic.php?p=2405496#p2405496
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