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17th April 2019, 09:47 AM | #41 |
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If you want to discuss how motion pictures are made, there is probably a suitable place at this forum to do so. And I'm sure there are plenty of people with plenty of experience in practical special effects and cinematic visual effects to answer any questions you may have.
You have presented a physics proof you say establishes that rockets cannot provide momentum thrust in a vacuum. You have failed to address the several people who have shown the simple error you've made. It is unclear what relevance cinematography has to that proof. You have lately asserted that no "unedited video of rockets in space" exists. You have failed to show how that is relevant to your proof. You have even failed to show that your assertion is true. It would help us understand your argument if you would, in addition to blurting out these claims, show how they tie into some overall conclusion. |
17th April 2019, 09:49 AM | #42 |
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That was discussed when I explained pressure thrust. That's the second term in the equation you proposed to disprove. But you need to understand that a rocket employs a nozzle that converts chamber pressure to linear velocity. The pressure in the chamber is produced by thermodynamic means. It is this velocity that resides in the momentum term of your equation. You derived the wrong value for that velocity in your proof by conflating it with mass flow rate. The static pressure that remains after this expansion is what produces vacuum thrust.
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17th April 2019, 09:51 AM | #43 |
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/dann "Stupidity renders itself invisible by assuming very large proportions. Completely unreasonable claims are irrefutable. Ni-en-leh pointed out that a philosopher might get into trouble by claiming that two times two makes five, but he does not risk much by claiming that two times two makes shoe polish." B. Brecht "The abolition of religion as the illusory happiness of the people is required for their real happiness. The demand to give up the illusion about its condition is the demand to give up a condition which needs illusions." K. Marx |
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17th April 2019, 09:53 AM | #44 |
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17th April 2019, 09:53 AM | #45 |
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/dann "Stupidity renders itself invisible by assuming very large proportions. Completely unreasonable claims are irrefutable. Ni-en-leh pointed out that a philosopher might get into trouble by claiming that two times two makes five, but he does not risk much by claiming that two times two makes shoe polish." B. Brecht "The abolition of religion as the illusory happiness of the people is required for their real happiness. The demand to give up the illusion about its condition is the demand to give up a condition which needs illusions." K. Marx |
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17th April 2019, 09:55 AM | #46 |
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17th April 2019, 09:57 AM | #47 |
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17th April 2019, 09:59 AM | #48 |
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The momentum of the rocket exhaust doesn't derive from a "pressure gradient force" in the way your proof describes. The propellants enter the chamber at a certain mass flow rate. The pressure of those propellants is increased dramatically in the chamber by vigorous combustion. The de Laval nozzle converts that pressure into linear flow. The velocity of that linear flow is Ve. It is not the velocity you estimated from "pressure gradient force." The mass flow rate at the exit plane is the same as at the engine injector, but since the volume of the propellants is now vastly increased, it must leave at a faster velocity.
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17th April 2019, 09:59 AM | #49 |
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17th April 2019, 10:00 AM | #50 |
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17th April 2019, 10:01 AM | #51 |
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17th April 2019, 10:01 AM | #52 |
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17th April 2019, 10:03 AM | #53 |
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Although Jay and most regulars are well aware, it's also worth pointing out explicitly that you don’t even need a proper nozzle to generate thrust in space (or on Earth). You just need a hole for the propulsive mass to get out. It’s a lousy way to do it - very inefficient - but it will generate thrust in the opposite direction. The numerous SF scenarios of a stranded spacefarer cracking open an oxygen tank, or opening a small hole in his suit, or for that matter firing a gun in the direction opposite to that desired, all more or less work - it’s controlling one’s thrust vector that’s the problem, not actually generating thrust.
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17th April 2019, 10:04 AM | #54 |
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17th April 2019, 10:06 AM | #55 |
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I suppose you're going to quibble about if this counts as "in space": https://www.youtube.com/watch?v=9HQfauGJaTs |
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17th April 2019, 10:08 AM | #56 |
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Yes, and that was covered under the discussion of pressure thrust. The static pressure of the exhaust as it expands freely acts preferentially on the spacecraft, because the spacecraft is preferentially on one side of the expanding gas volume.
However, how you release the pressure matters a great deal in how much momentum there is in it. And that's where the momentum term in your equation comes from. Specifically, if you release the pressure in a way that creates a unidirectional uniform flow, that flow has net momentum in one direction, whereas the net momentum in an expanding volume of gas is zero. You don't get to ignore the effect of the de Laval nozzle just because it is inconvenient to your proof or your understanding of how it was refuted. Pressure released through such a nozzle creates momentum in one direction. Newton's law says there must be equivalent but opposite momentum, and that is what results in thrust. |
17th April 2019, 10:10 AM | #57 |
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17th April 2019, 10:12 AM | #58 |
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Correct. In fact, solid-fueled motors generally use conical nozzles as a compromise between ideal expansion and the methods used to construct the nozzles out of materials that can take the heat. I don't mean to conflate the optimization of the nozzle geometry with the purer physics of gas expanding in a container with an opening in it.
A hole in one end of the thrust chamber creates non-uniform expansion and a net force. Constraining the expansion to result in a linear flow optimizes and maximizes that effect. |
17th April 2019, 10:13 AM | #59 |
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17th April 2019, 10:14 AM | #60 |
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Your garbled interpretation of Newtonian mechanics aside, why would video be better than tracking data, or radar, or onboard inertial telemetry, the primary ways we (people who launch and operate spacecraft for a living) actually measure the response of spacecraft to rocket operation in space?
Bonus question: you do understand that NASA did not invent the equations of motion, nor the equations of rocketry, and that spacecraft are routinely operated by civil, military, and commercial organizations from many nations? Right? |
17th April 2019, 10:16 AM | #61 |
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17th April 2019, 10:18 AM | #62 |
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17th April 2019, 10:24 AM | #63 |
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17th April 2019, 10:24 AM | #64 |
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17th April 2019, 10:25 AM | #65 |
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C'mon all you science guys! There is a video! This trumps all of your sciencey mumbo jumbo. Rockets don't work and never have. You just have to believe hard enough.
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17th April 2019, 10:29 AM | #66 |
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Yeah, that's kind of my point. One of the first things he did here was jump over to the religion section and, in a thread about atheism (now happily cleansed of the distraction), shout out of the blue that belief in NASA and space was a religion. Someone that intent on a crusade is probably not going to get the physical science right, or have any interest in being corrected on it.
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17th April 2019, 10:31 AM | #67 |
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why Is there no opposite force when smoke rises up. Shouldn’t the smoke from a campfire push off the ground?
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17th April 2019, 10:33 AM | #68 |
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17th April 2019, 10:34 AM | #69 |
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Smoke is an aerosol entrained in the ambient. Its motion has nothing to do with rockets. And if your contention is that smoke rises and should be analogous to rocket exhaust, why would an equal an opposite reaction in the Newtonian sense want to make the campfire also rise? You really are stumbling over basic ideas in Newtonian dynamics.
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17th April 2019, 10:36 AM | #70 |
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17th April 2019, 10:37 AM | #71 |
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17th April 2019, 10:38 AM | #72 |
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Yet another case of not understanding basic principles. I don't blame him though. People learn 'action equals reaction', but the implication of the fact are hard to grasp. And not taught well most of the time.
One of the implications for example is, that every closed system maintains its center of gravity. That also applies to rocket in space. You start with rocket full of fuel, standing still in vacuum of space ('standing still' in some frame of reference). You mark position of the enter of gravity of the rocket, including fuel. Then you fire the rocket. The fuel burns, shoots out of the rocket, and the rocket moves in the opposite direction. Now, at any time, if you take the rocket (moving in one direction), and the byproducts of the burning (moving in opposite direction), the center of gravity of this system stays still in the same spot in the original frame of reference. Things like that are IMHO really counter-intuitive. And some could even argue, that rocket indeed does not work, because the system did not move. Just parts of the system did move in opposite direction, compared to some others. The rocket engine is thus not device to move stuff. It's device to expand stuff ;-) |
17th April 2019, 10:38 AM | #73 |
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This is actually a more complicated question than you think. The smoke we see above a campfire isn't in contact with the ground, so it can't push off the ground. When the smoke is first created within the flame, there's a lot of expanding gas so I'd expect that there is some net pressure on the ground.
Do you think there isn't any net pressure on the ground from the expanding gasses in the flame, and if so, why do you think that? ETA: But the smoke is rising due to buoyancy. Any initial acceleration at the flame would have almost immediately been lost to air drag. |
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17th April 2019, 10:45 AM | #74 |
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You can’t prove anything
All you can pick at is that I used nasa. Well I got the equation from NASA’s website |
17th April 2019, 10:47 AM | #75 |
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And, if you want to get picky, some rocket propellant formulations can produce solid species in the exhaust, which are entrained and become part of the exhaust flow.
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17th April 2019, 10:49 AM | #76 |
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And gas moving out of the chamber is due to pressure gradient force
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17th April 2019, 10:50 AM | #77 |
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We don't have to. Newton proved it centuries ago. You're literally trying to undermine one of the most well-established principles in elementary physics.
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17th April 2019, 10:51 AM | #78 |
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17th April 2019, 10:51 AM | #79 |
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Do you believe that rockets can generate thrust in an atmosphere? If so, why is it possible with an atmosphere but impossible without?
Now for my balloon & steel plate: Imagine you have a big vacuum with a balloon sitting just to the left of a big vertical steel plate. Now, pop the balloon. Will the expanding gas exert any net force on the steel plate? |
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17th April 2019, 10:51 AM | #80 |
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Well, actually, your proof was explicitly found to be incorrect. And numerous counter examples to your claim were provided.
Which you described as essentially belonging to NASA, although as has been repeatedly pointed out the theory was developed long before there was such a thing as NASA. Please don’t blame us for your failure to understand this. You still haven’t answered my questions and counterexamples, by the way. Will you? |
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