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Old 17th April 2019, 06:10 AM   #1
Gingervytes
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Exclamation Rockets cannot propel in the vacuum of space.

They ASSUMED incorrectly (on purpose), that there is an equal and opposite force from gas movement due to pressure gradient force. Escaping gas needs something to push off of



Mathematical proof that the thrust equation is false

308053F1-9673-4230-ACBD-27616D0428EE.jpg
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Old 17th April 2019, 06:37 AM   #2
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Well it's not that the rockets push against something, it's that the equal reaction pushes the rocket forward as it throws stuff backward.

ETA: Or, in other words, the rocket pushes against the stuff that it shoots out its back.
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Old 17th April 2019, 06:49 AM   #3
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Originally Posted by Gingervytes View Post
They ASSUMED incorrectly (on purpose), that there is an equal and opposite force from gas movement due to pressure gradient force. Escaping gas needs something to push off of



Mathematical proof that the thrust equation is false

Attachment 39978
Your proof appears to assume that the fluid in the pipe is never accelerated.

ETA: If you combine two equations and get a result that contradicts one of them, then you've made a mistake in your math. This is just a fact about math and says nothing about physics.
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Last edited by dasmiller; 17th April 2019 at 07:04 AM.
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Old 17th April 2019, 06:56 AM   #4
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Why do you write the "NASA" rocket equation? You do realize that NASA didn't invent rocketry in a vacuum?
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Old 17th April 2019, 07:03 AM   #5
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Originally Posted by JayUtah View Post
Why do you write the "NASA" rocket equation? You do realize that NASA didn't invent rocketry in a vacuum?


Well of course they didn't! Everyone knows rockets can't work in a vacuum!
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Old 17th April 2019, 07:08 AM   #6
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Dang. I'm almost finished with a class on space mission planning over at edx.org. does this mean I'm wasting my time?

That's a trick question. I already know I'm wasting my time, in a practical sense, but it has been really cool learning about this stuff.

Meanwhile, I applaud dasmiller for taking a look at the submission to point out a flaw. It took me a long time to understand how rockets moved. As a child, I, too, thought they pushed against the atmosphere, until someone pointed out that there was no atmosphere in space. Even after I could do the math related to conservation of momentum, I still knew that something had to push the spacecraft, and it took me a while to figure out what that was.
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Old 17th April 2019, 07:29 AM   #7
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Originally Posted by Belz... View Post
Well it's not that the rockets push against something, it's that the equal reaction pushes the rocket forward as it throws stuff backward.

ETA: Or, in other words, the rocket pushes against the stuff that it shoots out its back.
The equation describes momentum thrust and pressure thrust. For something like the Apollo lunar module DPS, pressure thrust accounts for about 40 percent of the overall thrust. This "proof" conflates the concepts of propellant mass flow rate, which is needed to understand how much momentum is in the exhaust, and the exhaust flow rate. It ignores the most exciting part about rocket engines, which is the massive acceleration imparted to the working fluid by the release of stored chemical energy as heat. The de Laval nozzle converts the results of that energy to velocity, which is where Ve comes from.
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Old 17th April 2019, 07:32 AM   #8
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Wow, this guy is foaming all over the forum trying to discredit NASA. This should be entertaining.
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Old 17th April 2019, 07:34 AM   #9
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Originally Posted by JayUtah View Post
The equation describes momentum thrust and pressure thrust. For something like the Apollo lunar module DPS, pressure thrust accounts for about 40 percent of the overall thrust. This "proof" conflates the concepts of propellant mass flow rate, which is needed to understand how much momentum is in the exhaust, and the exhaust flow rate. It ignores the most exciting part about rocket engines, which is the massive acceleration imparted to the working fluid by the release of stored chemical energy as heat. The de Laval nozzle converts the results of that energy to velocity, which is where Ve comes from.
I really wanted to fully understand that, but some of it went right over my head.

Science is hard, but you don't see me making stuff up to compensate, like some other people.
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Old 17th April 2019, 07:36 AM   #10
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Quote:
This states that mass flow rate exiting a pipe is force.
Velocity is no longer needed to calculate force.
Obviously flow rate by itself is not force.
Is he/she saying that you might as well pour the rocket fuel directly out of the exhaust pipes without igniting it to create thrust because the mass is all that matters???
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Old 17th April 2019, 08:09 AM   #11
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Originally Posted by JayUtah View Post
Why do you write the "NASA" rocket equation? You do realize that NASA didn't invent rocketry in a vacuum?
Lets see, maybe a Russian named Tsiolkovsky in 1903?

https://en.wikipedia.org/wiki/Tsiolk...ocket_equation
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Old 17th April 2019, 08:12 AM   #12
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Originally Posted by dann View Post
Is he/she saying that you might as well pour the rocket fuel directly out of the exhaust pipes without igniting it to create thrust because the mass is all that matters???
Sort of. From the equations, I think he/she is saying that if you take a pipe, squirt fluid into it at one end, have that fluid come out of the other end at the same speed, then there won't be any thrust on the pipe.

Which is true, but it's not a good description of how rockets work.
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Last edited by dasmiller; 17th April 2019 at 08:15 AM. Reason: a bit of clarification
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Old 17th April 2019, 08:13 AM   #13
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Originally Posted by bknight View Post
Lets see, maybe a Russian named Tsiolkovsky in 1903?

https://en.wikipedia.org/wiki/Tsiolk...ocket_equation
That was my first thought too, but nothing in the picture looked anything like Tsiolkovsky's rocket equation.
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Old 17th April 2019, 08:29 AM   #14
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Originally Posted by dasmiller View Post
That was my first thought too, but nothing in the picture looked anything like Tsiolkovsky's rocket equation.
I agree but that is the equation dealing with rocket thrust that he should have been using.
Jay differentiated in post 7.
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Old 17th April 2019, 08:34 AM   #15
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Originally Posted by Gingervytes View Post
They ASSUMED incorrectly (on purpose), that there is an equal and opposite force from gas movement due to pressure gradient force. Escaping gas needs something to push off of



Mathematical proof that the thrust equation is false

Attachment 39978
Nope, rockets send material in one direction, if they are in space they still send that material in one direction.
Newton's law for every action there is an equal and opposite reaction then says that as the gases in the combustion vessel are moved in one direction the vessel is moved in the other.

No atmosphere needed
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Old 17th April 2019, 08:41 AM   #16
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Originally Posted by Belz... View Post
I really wanted to fully understand that, but some of it went right over my head.
It's relatively easy to break down.

Rocket thrust in a vacuum comes from two sources: the momentum of the exhaust and the static pressure of the exhaust. The first term in the equation looks at momentum, which has been written extensively about since Newton first discovered it. The momentum of the exhaust leaving in one direction matches the momentum of the rocket moving in the opposite direction, per Newton's third law of motion. The mass part of the momentum formula is the mass of the propellants, here described as the mass applied per unit time, because the exhaust leaves the system and must be replaced over time by new propellant.

The key here is that the mass enters the thrust chamber as a liquid -- often a dense cryogenic liquid. But it leaves the engine as a gas of much greater volume and much less density than before. To be sure, the mass flow rate into the thrust chamber is the same as the mass flow rate out of the thrust chamber, but the exhaust mass is in a different form that must move much faster in order to sustain that flow rate.

The propellant is first converted to gas in the top of the thrust chamber. It's sprayed together in such a way that it mixes thoroughly, and then the radiant heat from the reaction downstream vaporizes it. Those thoroughly mixed gases are then ignited, creating vast amounts of thermal energy in the working fluid. Every gas responds to an increase in thermal energy by trying to increase its volume and/or pressure. The urge to do so in a rocket engine is extremely powerful. A wonderful Victorian-era gadget called a de Laval nozzle -- a convergent-divergent nozzle -- lets the gas escape from the only remaining hole in the thrust chamber in a way that collimates the flow. All the gas molecules are going in the same direction, maximizing the momentum. Otherwise, pressurized gas escaping from a plain hole in a pressure vessel will expand in a cone-shaped pattern.

The conversion of chemical energy thermodynamically to gas pressure, and from there to gas velocity, is what the poster's proof is missing.

That's a more nuts-and-bolts explanation of where the momentum thrust comes from. Pressure thrust comes from the static pressure of the exhaust gas. The gas streaming in linear fashion out of the de Laval nozzle has momentum. But it's still a gas with measurable static pressure. It doesn't have zero density. As such, it pushes against the walls of the nozzle just like the contained air in a balloon pushes against the balloon walls, even though the balloon air isn't hot and isn't moving. It's ordinary gas pressure. The term for this effect is "adiabatic," and it's the same principle by which steam locomotives conserved water by opening the steam valve only a little bit at the beginning of the power stroke.

If the static pressure of the exhaust is greater than the ambient into which it is exhausted, it will continue to expand in static fashion irrespective of its velocity. That urge to expand into a relatively unpressurized space is the ability to do what engineers call "pressure and volume work," in this case, to continue pushing in all directions. "All directions" in this case includes the direction of the rocket nozzle, which results in thrust. That's the second term of the equation -- the static pressure of the exhaust per unit area, minus the static pressure of the ambient (i.e., the pressure difference) times the area of the exit plane of the nozzle -- sort of like the area of the piston face in a steam cylinder.

Ironically, the poster here thinks he has cleverly discarded momentum thrust. But he hasn't dealt with the notion that in a vacuum, the ambient pressure is zero so the pressure component of the rocket equation actually contributes more. The notion of "having something to push against" is actually the opposite of what makes rockets more efficient as they climb.

Quote:
Science is hard, but you don't see me making stuff up to compensate, like some other people.
That's just it. Science is hard, but if overturning well-established, well-used principles were as easy as scribbling some poorly-remembered physics onto a sheet of notebook paper, it wouldn't come across as so inaccessible.

NASA didn't invent rocketry. NASA isn't the only state-funded space program. NASA isn't even the biggest consumer of rocketry. NASA relies on private industry to supply its rockets, the same private industry that sells access to space to other private industries. Spacefaring is a multibillion-dollar industry. It doesn't give a rat's patootie about some ideological spat.
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Old 17th April 2019, 08:47 AM   #17
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Originally Posted by JayUtah View Post
It's relatively easy to break down.

Rocket thrust in a vacuum comes from two sources: the momentum of the exhaust and the static pressure of the exhaust. The first term in the equation looks at momentum, which has been written extensively about since Newton first discovered it. The momentum of the exhaust leaving in one direction matches the momentum of the rocket moving in the opposite direction, per Newton's third law of motion. The mass part of the momentum formula is the mass of the propellants, here described as the mass applied per unit time, because the exhaust leaves the system and must be replaced over time by new propellant.

The key here is that the mass enters the thrust chamber as a liquid -- often a dense cryogenic liquid. But it leaves the engine as a gas of much greater volume and much less density than before. To be sure, the mass flow rate into the thrust chamber is the same as the mass flow rate out of the thrust chamber, but the exhaust mass is in a different form that must move much faster in order to sustain that flow rate.

The propellant is first converted to gas in the top of the thrust chamber. It's sprayed together in such a way that it mixes thoroughly, and then the radiant heat from the reaction downstream vaporizes it. Those thoroughly mixed gases are then ignited, creating vast amounts of thermal energy in the working fluid. Every gas responds to an increase in thermal energy by trying to increase its volume and/or pressure. The urge to do so in a rocket engine is extremely powerful. A wonderful Victorian-era gadget called a de Laval nozzle -- a convergent-divergent nozzle -- lets the gas escape from the only remaining hole in the thrust chamber in a way that collimates the flow. All the gas molecules are going in the same direction, maximizing the momentum. Otherwise, pressurized gas escaping from a plain hole in a pressure vessel will expand in a cone-shaped pattern.

The conversion of chemical energy thermodynamically to gas pressure, and from there to gas velocity, is what the poster's proof is missing.

That's a more nuts-and-bolts explanation of where the momentum thrust comes from. Pressure thrust comes from the static pressure of the exhaust gas. The gas streaming in linear fashion out of the de Laval nozzle has momentum. But it's still a gas with measurable static pressure. It doesn't have zero density. As such, it pushes against the walls of the nozzle just like the contained air in a balloon pushes against the balloon walls, even though the balloon air isn't hot and isn't moving. It's ordinary gas pressure. The term for this effect is "adiabatic," and it's the same principle by which steam locomotives conserved water by opening the steam valve only a little bit at the beginning of the power stroke.

If the static pressure of the exhaust is greater than the ambient into which it is exhausted, it will continue to expand in static fashion irrespective of its velocity. That urge to expand into a relatively unpressurized space is the ability to do what engineers call "pressure and volume work," in this case, to continue pushing in all directions. "All directions" in this case includes the direction of the rocket nozzle, which results in thrust. That's the second term of the equation -- the static pressure of the exhaust per unit area, minus the static pressure of the ambient (i.e., the pressure difference) times the area of the exit plane of the nozzle -- sort of like the area of the piston face in a steam cylinder.

Ironically, the poster here thinks he has cleverly discarded momentum thrust. But he hasn't dealt with the notion that in a vacuum, the ambient pressure is zero so the pressure component of the rocket equation actually contributes more. The notion of "having something to push against" is actually the opposite of what makes rockets more efficient as they climb.
Jay, let me be the first to say: please never, ever die. You are a fountain of knowledge.

It's just too bad that the OP will ignore you.
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Old 17th April 2019, 08:54 AM   #18
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Originally Posted by Belz... View Post
Jay, let me be the first to say: please never, ever die. You are a fountain of knowledge.

It's just too bad that the OP will ignore you.
Jay adds a lot of sensibility to the forum and to http//www.apollohoax.net/
Especially when dealing with nut burgers who post nonsense like this.

I do hope he lives for some more time, but I don't think never is in the cards.
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Old 17th April 2019, 08:55 AM   #19
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The rocket doesnít push the gas out. The gas moves out due to pressure gradient force. Thatís the false ASSUMPTION made. No one here can demonstrate that there is an equal and opposite force from gas movement due to pressure gradient force.
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Old 17th April 2019, 09:04 AM   #20
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Originally Posted by Gingervytes View Post
They ASSUMED incorrectly (on purpose), that there is an equal and opposite force from gas movement due to pressure gradient force. Escaping gas needs something to push off of

Mathematical proof that the thrust equation is false

Attachment 39978
Hi, Gingervytes. Welcome to the forum.

Thank you for your clearly-written work. Jay has already addressed the errors in it, but the larger issue is that rockets are observed to work in a vacuum - we operate them all the time. That should have tipped you off that your work was incorrect.
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Old 17th April 2019, 09:06 AM   #21
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Originally Posted by Gingervytes View Post
They ASSUMED incorrectly (on purpose), that there is an equal and opposite force from gas movement due to pressure gradient force. Escaping gas needs something to push off of



Mathematical proof that the thrust equation is false

Attachment 39978
Sounds like he graduated from the Anders Bjorkman space academy.
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Old 17th April 2019, 09:08 AM   #22
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Originally Posted by Gingervytes View Post
The rocket doesnít push the gas out. The gas moves out due to pressure gradient force. Thatís the false ASSUMPTION made. No one here can demonstrate that there is an equal and opposite force from gas movement due to pressure gradient force.
Translation: I've decided I don't want to believe this, and if reality disagrees then reality can just shut up.

The only rational response is attributed to Daniel Patrick Moynihan: "Everyone is entitled to his own opinion, but not his own facts."

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Old 17th April 2019, 09:10 AM   #23
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Originally Posted by Gingervytes View Post
The rocket doesnít push the gas out. The gas moves out due to pressure gradient force. Thatís the false ASSUMPTION made.
No, it is not assumed -- it is observed and measured. The de Laval nozzle dates back to Victorian times, used in steam turbine engines. The pressure "gradient" is converted to velocity, which is expressed in your equation as Ve.

Quote:
No one here can demonstrate that there is an equal and opposite force from gas movement due to pressure gradient force.
I already pointed out that your derivation of the "pressure gradient force" was in error because you wrongly conflated two concepts in the derivation of the rocket equation. You did not address that. Before you claim that no one has refuted you, you must address the posts in which they do just that.
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Old 17th April 2019, 09:11 AM   #24
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Originally Posted by sts60 View Post
Hi, Gingervytes. Welcome to the forum.

Thank you for your clearly-written work. Jay has already addressed the errors in it, but the larger issue is that rockets are observed to work in a vacuum - we operate them all the time. That should have tipped you off that your work was incorrect.
It is a shame that non scientific types don't want to understand that you people work in the aerospace field everyday. And you get to see your work propelled into space.
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Old 17th April 2019, 09:17 AM   #25
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Originally Posted by sts60 View Post
[R]ockets are observed to work in a vacuum - we operate them all the time.
Since he has expressed elsewhere on the forum a warning that belief in NASA is a religion and that NASA and space travel are a great political lie promulgated by the United States, I gather he will express himself further as one of those who delusionally claim that there is only NASA-related space flight and that the only observations of space flight come via NASA.

Quote:
That should have tipped you off that your work was incorrect.
As I write above, he's probably already convinced himself that the only observations that could dispute his findings come from the organization whose validity he is challenging and can therefore be set aside.

Another reason to question one's own work is the ease with which it comes about. If it were that easy to refute the principle by which rocket engines work, why would one think himself to be the first person to have done it? Or the only person who knows about it?
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Old 17th April 2019, 09:23 AM   #26
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"Further investigation and experimentation have confirmed the findings of Isaac Newton in the 17th Century and it is now definitely established that a rocket can function in a vacuum as well as in an atmosphere. The Times regrets the error."

- Editorial, The New York Times, July 17th 1969.

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Old 17th April 2019, 09:23 AM   #27
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Originally Posted by JayUtah View Post
No, it is not assumed -- it is observed and measured. The de Laval nozzle dates back to Victorian times, used in steam turbine engines. The pressure "gradient" is converted to velocity, which is expressed in your equation as Ve.



I already pointed out that your derivation of the "pressure gradient force" was in error because you wrongly conflated two concepts in the derivation of the rocket equation. You did not address that. Before you claim that no one has refuted you, you must address the posts in which they do just that.
How is it observed? Why is there no unedited video of a rocket in space? When wind blows on the back of your head, why donít you feel the air in front of you push off your face as it moves away from you?
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Old 17th April 2019, 09:23 AM   #28
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Originally Posted by Belz... View Post
Jay, let me be the first to say: please never, ever die. You are a fountain of knowledge.
Thank you, but I'm more a fountain of available time and patience. These are not new concepts, or little-known concepts. Not to make light of your understandable confusion, but these are well-known topics. Sometimes it just takes someone who knows the right way to explain something to dispel that confusion. That person may not have vast knowledge of truth, but his skill lies in having sympathy for and experience with confusion. Having suffered myself as an early student, I recall the particular misconceptions that I had to overcome.

Newton's third law has stood upon a solid foundation for some three hundred years. It tells us how rockets must work, but more importantly it tells us about how so much of the observable universe works. If it suddenly didn't work in this case, that would be very strange indeed.

Quote:
It's just too bad that the OP will ignore you.
...and continue to insist that he's right and that no one can refute him.
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Old 17th April 2019, 09:33 AM   #29
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Well, the SpaceX launch I watched last week from just outside the Vehicle Assembly Building was a commercial endeavor - they had a customer who paid them to put their vehicle into orbit. So NASA provided launch infrastructure, but had nothing to do with the rocketry in space. That’s just the latest example of decades of commercial space flight. Not to mention Soviet/Russian, European, Chinese, Indian, Japanese, and other countries’ national and commercial space programs. The idea that the theory, let alone the practice, of rocketry somehow belongs to NASA is manifestly false.

On a more specific note, I’ve personally commanded a spacecraft to “fire” its thruster to move away from the Shuttle - a cold gas N2 rocket with a whopping few ounces of thrust. It worked just fine, as we observed the results directly - including tracking the vehicle with our own (not just NASA’s) S-band antenna. So, yes, I have direct personal experience that rockets work in a vacuum.
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Old 17th April 2019, 09:33 AM   #30
JayUtah
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Originally Posted by Gingervytes View Post
How is it observed?
The velocity of gas exiting a de Laval nozzle for a given pressure is observed many times in the laboratory. The de Laval nozzle is not used only in rocketry.

Quote:
Why is there no unedited video of a rocket in space?
Assumes facts not in evidence. It is also unclear what evidence you think would be supplied by "video of rocket in space," edited or not. You need to supply a line of reasoning here.

Quote:
When wind blows on the back of your head, why donít you feel the air in front of you push off your face as it moves away from you?
That has nothing to do with how rockets work and nothing to do with fluid dynamics in general. Your expectations are misinformed.
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Old 17th April 2019, 09:38 AM   #31
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I may as well be the one to post this. It usually comes up sooner or later.

YouTube Video This video is not hosted by the ISF. The ISF can not be held responsible for the suitability or legality of this material. By clicking the link below you agree to view content from an external website.
I AGREE
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Old 17th April 2019, 09:39 AM   #32
Gingervytes
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Originally Posted by JayUtah View Post
The velocity of gas exiting a de Laval nozzle for a given pressure is observed many times in the laboratory. The de Laval nozzle is not used only in rocketry.



Assumes facts not in evidence. It is also unclear what evidence you think would be supplied by "video of rocket in space," edited or not. You need to supply a line of reasoning here.



That has nothing to do with how rockets work and nothing to do with fluid dynamics in general. Your expectations are misinformed.
So you are saying that rockets donít work due to pressure build up in the rocket chamber and the eventual release of pressure to low pressure?
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Old 17th April 2019, 09:39 AM   #33
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Originally Posted by sts60 View Post
On a more specific note, Iíve personally commanded a spacecraft to ďfireĒ its thruster...
I have similar personal experience.

Quote:
It worked just fine, as we observed the results directly - including tracking the vehicle with our own (not just NASAís) S-band antenna.
I have similar experience.

Quote:
So, yes, I have direct personal experience that rockets work in a vacuum.
As do I, with rockets operating many thousands of kilometers from Earth, whose nominal thrust was reckoned using the standard equations we're debating, and whose resulting behavior matched exactly what Newtonian dynamics predicted should happen. This was for a commercial customer, where I was on the hook to achieve a contractually agreed-upon result. My observations have nothing to do with preserving some deep, dark NASA secret. They have everything to do with achieving a result that, had it failed, would have put hundreds of millions of dollars of insurance payouts at stake and invited scrutiny from the people eager not to write that check.
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Old 17th April 2019, 09:40 AM   #34
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Originally Posted by Gingervytes View Post
So you are saying that rockets donít work due to pressure build up in the rocket chamber and the eventual release of pressure to low pressure?
I have explained at length how rockets work. I have twice explained what's wrong with your proof. Instead of trying to put additional words in my mouth, please reconcile your proof with the explanations already given.
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Old 17th April 2019, 09:41 AM   #35
sts60
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Originally Posted by JayUtah View Post
...Assumes facts not in evidence. It is also unclear what evidence you think would be supplied by "video of rocket in space," edited or not. You need to supply a line of reasoning here...
Indeed, why would video be the observational standard of evidence that rockets work in space? That makes little sense to me, but then again I work in this field.
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Old 17th April 2019, 09:42 AM   #36
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Originally Posted by sts60 View Post
Indeed, why would video be the observational standard of evidence that rockets work in space? That makes little sense to me, but then again I work in this field.
I assume he is edging toward the "All space flight is fake" claim.
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Old 17th April 2019, 09:42 AM   #37
Gingervytes
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Originally Posted by sts60 View Post
Indeed, why would video be the observational standard of evidence that rockets work in space? That makes little sense to me, but then again I work in this field.
I wonder if they performed the movie Gravity in space as well
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Old 17th April 2019, 09:44 AM   #38
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What is it with conspiracists and their obsession with video?
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Old 17th April 2019, 09:44 AM   #39
Gingervytes
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People need to realize that gas expands freely into a vacuum

https://www.youtube.com/watch?v=AubIFUsq7Ss
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Old 17th April 2019, 09:46 AM   #40
dasmiller
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Originally Posted by Gingervytes View Post
People need to realize that gas expands freely into a vacuum

https://www.youtube.com/watch?v=AubIFUsq7Ss
I think many of us understand that just fine. Can you elaborate on the significance for rocket propulsion?
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