ApolloHoax.net
Apollo Discussions => The Hoax Theory => Topic started by: Apollo 957 on June 07, 2016, 02:34:09 PM
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I've come across a few amateur physicists on YouTube who try to prove that rockets don't work in space (vacuum) because of the 'lack of reactionary force' from the surrounding atmosphere. I won't bore you all with the details, but one countering video to these pointed out that the force of the exhaust product would far outweigh the meagre atmospheric pressure surrounding it.
I decided to put some figures on this, and in my layman's terms, came up with;
"The Saturn V weighed 6.5 million pounds at launch. In order to launch it, the rockets must overcome the weight, and the quoted thrust of the first stage was 7.65 million lb/f. It could not be any less than 6.5 million, or else the machine wouldn't leave the ground.
I have no accurate dimension for the diameter of the engine nozzles, but there are pics online of people standing in front of them and they look to be around 8 feet (96 inches) across. There's 5 of them. Formula for area of a circle - pi * radius squared - 3.14 * 48 * 48 each nozzle, and there's five nozzles, so 3.14 * 48 * 48 * 5 = 36000 square inches. I make that 212 ib/f per square inch.
Atmospheric pressure at sea level - 14.7 pounds per square inch (psi)
The Saturn V, on the face of it, is putting out almost 15 times the ambient atmospheric pressure."
At this point, I thought - "that doesn't seem like a lot". I know it would if you descended to a few hundred fathoms, and experienced it in a diving suit on the seabed, but .....
Is the difference made by the rocket exhaust going at supersonic or hypersonic speed, whilst the atmosphere is just exhibiting Brownian motion?
Can anyone put some numbers on this? Hopefully more impressive ones than "times 14" .....
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You have two kinds of pressure to discuss. There is dynamic pressure along the engine axis, which is what we simplistically consider when we estimate thrust. And there is also static pressure at or about the exit plane, which contributes more or less to thrust but also describes the shape of the exhaust plume with respect to ambient pressure -- in rocketry terms, behavior in a vacuum or during boost upward through an ever-thinning atmosphere.
Dynamic pressure varies across the exit plane, but you can use an average if you want and be within an order of magnitude correctness. Exhaust moves faster at the center of the exit plane. In the case of the Rocketdyne F-1, much faster. The edges of the exit plane are dominated by the film cooling regime in the nozzle extension which, by design, employs a much cooler and much slower flow. Hence if you want to talk of averages, exclude the outer 2-3 inches as they are not part of the ordinary exit-plane shear regime.
The numbers you're looking for in your estimate are 1,522,000 lbf for sea-level thrust (AS-504 and later) and 148 inches diameter (standard fitted nozzle extension). But you really also want to look at such things as mass flow rate (5683 lbm/s) and exhaust velocity (9800 f/s), because these will more accurately describe the dynamics of the exhaust hitting the ambient air. Imagine scouring away dirt from the pavement with your garden sprayer. You feel the reactive thrust against your hand, and that's measurable. But the effect of the fluid hitting the pavement in a directional flow is governed by slightly different physical regimes such as kinetic energy. If you use the numbers above to compute kinetic energy in the exhaust, you'll have a more usable number to describe the effect of it hitting ambient air. (It is no accident that the formulation for aerodynamic drag -- the force of moving fluid against an object -- is related to fluid kinetic energy.) Since the "object" in this case is also a fluid, the formulation becomes enormously more complicated (e.g., Navier-Stokes methods) if you want to work out the exact numbers.
The static pressure component doesn't matter much until you talk about thrust in a vacuum. Canonical atmospheric pressure of 14.7 psia is a measurement of the Brownian effect under those conditions, and ideally you want the exhaust plume to match that level of Brownian motion. If all the motion is linear, leading to ideal thrust in a vacuum, then the ambient atmosphere will act to constrict the plume and change its shape (overexpanded plume). The SSME had overexpanded nozzles because it was tuned for optimal thrust at higher altitude, the result being the Mach cone that forms while the engine is operating at sea level. Nearly all rocket nozzles have a fixed geometry. At altitude the F-1 nozzle underexpands the plume, leading to plume spread at high altitude. The static pressure of the plume is greater than ambient. In a vacuum, static exhaust pressure at the exit plane forms a measurable percentage of thrust, as much as half the thrust produced by the LM DPS.
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Basically, Thrust = mass flow * exhaust velocity + exit plane * (exhaust pressure - ambient pressure)
In other words, rockets work better in a vacuum.
That is assuming you the nozzle is designed for proper expansion, which is easier when there is a non-zero ambient pressure so the devil is in the detail.
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Basically, Thrust = mass flow * exhaust velocity + exit plane * (exhaust pressure - ambient pressure)
In other words, rockets work better in a vacuum.
That is assuming you the nozzle is designed for proper expansion, which is easier when there is a non-zero ambient pressure so the devil is in the detail.
Except for the YouTube fringe.
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In other words, rockets work better in a vacuum.
Dramatically, in most cases.
That is assuming you the nozzle is designed for proper expansion, which is easier when there is a non-zero ambient pressure so the devil is in the detail.
Exactly. In practice you can't make a deLaval nozzle that achieves zero static pressure at the exit plane. But since static pressure contributes to thrust in a vacuum, it doesn't matter much.
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In other words, rockets work better in a vacuum.
That is assuming you the nozzle is designed for proper expansion, which is easier when there is a non-zero ambient pressure so the devil is in the detail.
Depends on what you mean by "work better".
Let's say we have two engines that are identical in all ways expect for their expansion ratios. At sea level, the one with the low expansion ratio will produce greater thrust than the one with the high expansion ratio. And in a vacuum, the one with the high expansion ratio will produce greater thrust than the one with the low expansion ratio. But both engines will produce greater thrust in a vacuum then they do at sea level.
So we could say the low expansion ratio engine "works better at sea level" because it is the better of the two engines in that particular environment. But we could also say the low expansion ratio engine "works better in a vacuum" because to produces more thrust in a vacuum than it does at sea level.
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Exactly. In practice you can't make a deLaval nozzle that achieves zero static pressure at the exit plane.
Emphasis added. Can you in theory?
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Yes, if you make the nozzle infinitely long.
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...whereupon a variant of Poiseuille's Law would reduce exhaust momentum to zero.
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How? The longer the nozzle, the greater the momentum thrust and the smaller the pressure thrust. The former increases faster than the latter decreases, so overall thrust increases. But the effect rapidly reaches diminishing returns.
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See that over there, way over in the distance, that's my comfort zone...
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How?
On second thought it wouldn't, because in addition to being infinitely long, the ideal nozzle has an infinite expansion ratio, which disqualifies the Poiseuille regime. I was thinking of infinite length for a given expansion ratio, which would result essentially in a tube of infinite length. Real-world rocket nozzles don't offer much resistance to flow, which was the limiting factor I was imagining.
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In other words, rockets work better in a vacuum.
That is assuming you the nozzle is designed for proper expansion, which is easier when there is a non-zero ambient pressure so the devil is in the detail.
Depends on what you mean by "work better".
Let's say we have two engines that are identical in all ways expect for their expansion ratios. At sea level, the one with the low expansion ratio will produce greater thrust than the one with the high expansion ratio. And in a vacuum, the one with the high expansion ratio will produce greater thrust than the one with the low expansion ratio. But both engines will produce greater thrust in a vacuum then they do at sea level.
So we could say the low expansion ratio engine "works better at sea level" because it is the better of the two engines in that particular environment. But we could also say the low expansion ratio engine "works better in a vacuum" because to produces more thrust in a vacuum than it does at sea level.
So I guess that designers of multi-stage rockets take that into account when designing rocket motors for each stage?
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Yes, definitely. Take a look at the two different models of the Merlin RP-1/LOX engine on the Falcon 9. I don't know of a picture that shows them together, but the vacuum model has a much longer nozzle.
An even better illustration: a Delta II with 9 SRBs. Six are lit on the ground and the other three in the air at about T+60 sec, and the air-lit (vacuum-lit, actually) solids have noticeably longer nozzles.
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Indeed, nozzle design is an art. Which is to say, a very enriched science. Also, solids tend to use conical nozzles. This is not because of math but because of heat rejection. Most are built of and/or lined with ablative cooling materials; the conical shape is easier to deal with.
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I found a picture of a 9-SRB Delta II that shows the different nozzle lengths pretty well:
http://spaceflightnow.com/delta/d341/mst/01.jpg
Six have short nozzles and are lit at liftoff, the remaining three have long nozzles and are lit at T+60 sec, about when the first set burn out. In theory, lighting all nine at liftoff would be more fuel-efficient but it would unacceptably increase stresses.
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When I come across someone making the argument that rockets would not work in a vacuum, I ask them to make a prediction about the following scenario and provide a video which I made as an example of an experiment they can perform.
Take one of those kitchen spray nozzles and wrap a rubber band around it so that it is always on. Turn the water on and of course the nozzle will be thrust backwards due to the water exiting the nozzle at some velocity. Then balance the nozzle so that the thrust equals gravity, and the nozzle is suspended in the air. I then ask what they predict would happen if you placed some solid object (in my case a paper plate) directly in front of that nozzle where the water is spraying directly on a solid "wall" an inch away. Those that believe that rockets need air to push off of no doubt believe that this would increase the thrust of the nozzle. I then link them to the following video. I have never had anyone come back with a counter. I don't know if they just give up and run away or if they perform the experiment and realize they were wrong, but it seems to work to at least enlighten a few, and that's all I care about. Here is a link to the video :
I am not in any way trying to promote this video or my channel (which only consists of this single video I believe!). I legitimately think it is a good experiment to have those that don't understand how rockets work perform. These people don't understand that rockets don't need something to "push off of". Their education is limited, and getting into expansion rates and rocket equations will do no good for these people (in my opinion). They don't understand all that stuff. They can do this experiment and see first hand that they are wrong. Explaining to them WHY they are wrong does no good for these people until they accept that they ARE wrong. This experiment does that in my experience. THEN, will they be able to accept an explanation as to WHY they were wrong.
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Brilliant. Loved the last bit, where it was obvious, the nozzle wasn't supported by anything other than water spray. And the symbolic "fall" of hoax ideas.
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An excellent test clearly showing that something behind the exhaust of a rocket has no influence on the thrust.
Here is another one done by Mythbusters. I know I have posted this before, but this is a good place to have it for the benefit of lurkers...
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I've tried all these arguments, including the Mythbusters video. Nothing works, because these people are simply delusional.
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My answer is an experiment you can do sitting on a chair with wheels. If you get, a medicine ball, or other heavy object and a balloon of the same size. Sit on the chair and throw the medicine ball. Then repeat with the balloon.
1:- If the amount you move is due to pushing against the air you will move the same distance.
2:- As this is not the case, you will move further with the medicine ball, because the "thrust" is proportional to the mass of the object you throw.
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There's a larger scale version of Willoughby's video that was done for a TV show with a guy on a platform bouyed up by high pressure fire hoses. he went from hovering above a land based surface to hovering over a water surface some feet below him with absolutely no loss in height.
However, as ka9q says, these people are delusional and the only thing they represent is the waste of taxes spent on educating them.
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Great pair of videos.
After watching the Mythbusters one I saw a link to one titled "No Rockets go near or enter Space, so simple to see it hurts". Interestingly, that video contained a link to our old mate Anders Bjoerkman of Heiwa fame...
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There's a larger scale version of Willoughby's video that was done for a TV show with a guy on a platform bouyed up by high pressure fire hoses. he went from hovering above a land based surface to hovering over a water surface some feet below him with absolutely no loss in height.
However, as ka9q says, these people are delusional and the only thing they represent is the waste of taxes spent on educating them.
But, but, but they are critical thinkers ::)
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There's a larger scale version of Willoughby's video that was done for a TV show with a guy on a platform bouyed up by high pressure fire hoses. he went from hovering above a land based surface to hovering over a water surface some feet below him with absolutely no loss in height.
However, as ka9q says, these people are delusional and the only thing they represent is the waste of taxes spent on educating them.
I dont think you are wasting any taxes on them since they are probably homeschooled or dropped out.
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However, as ka9q says, these people are delusional and the only thing they represent is the waste of taxes spent on educating them.
I dont think you are wasting any taxes on them since they are probably homeschooled or dropped out.
I don't think I'm wasting any taxes on Team Hoax because most all of them appear to be in the USA, and I'm not.
The latest whackjob is one who's put together a camera/lens/teleconverter combo to photograph and film Sirius, the brightest star in our sky. He's ended up with something that looks like a starfish, and thinks he's found the 'true shape' of it, because the 'Government' hadn't accounted for the combination of equipment he's using when they falsified the Sirius imagery in all the other (digital) equipment.......
He admits his equipment won't focus on any other stars, and is useless for photographing terrestrial objects, but seems to think this makes it ideal for showing the 'true shape' of this one star.....
Wow. Just.... Wow.
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However, as ka9q says, these people are delusional and the only thing they represent is the waste of taxes spent on educating them.
I dont think you are wasting any taxes on them since they are probably homeschooled or dropped out.
I don't think I'm wasting any taxes on Team Hoax because most all of them appear to be in the USA, and I'm not.
The latest whackjob is one who's put together a camera/lens/teleconverter combo to photograph and film Sirius, the brightest star in our sky. He's ended up with something that looks like a starfish, and thinks he's found the 'true shape' of it, because the 'Government' hadn't accounted for the combination of equipment he's using when they falsified the Sirius imagery in all the other (digital) equipment.......
He admits his equipment won't focus on any other stars, and is useless for photographing terrestrial objects, but seems to think this makes it ideal for showing the 'true shape' of this one star.....
Wow. Just.... Wow.
What.
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However, as ka9q says, these people are delusional and the only thing they represent is the waste of taxes spent on educating them.
I dont think you are wasting any taxes on them since they are probably homeschooled or dropped out.
I don't think I'm wasting any taxes on Team Hoax because most all of them appear to be in the USA, and I'm not.
The latest whackjob is one who's put together a camera/lens/teleconverter combo to photograph and film Sirius, the brightest star in our sky. He's ended up with something that looks like a starfish, and thinks he's found the 'true shape' of it, because the 'Government' hadn't accounted for the combination of equipment he's using when they falsified the Sirius imagery in all the other (digital) equipment.......
He admits his equipment won't focus on any other stars, and is useless for photographing terrestrial objects, but seems to think this makes it ideal for showing the 'true shape' of this one star.....
Wow. Just.... Wow.
What? All he needs to do is Google images of Sirius and he'll find the same starfish shape. There's a complete lack of fundamental physics going on with these hoaxers.
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I have had some success in at least getting the hoax folks to shut up by humoring them slightly, but trying to show where THEIR favorite theory fails. My argument goes something like this:
Your "push off of air" fantasy isn't credible to ANYONE with scientific experience. Let us examine HOW that would work at the molecular level, shall we? Combustion forces the exhaust out of the nozzle - you agree with that. You say that said combustion does not impart force to the rocket (fundamentally false, but I'll indulge you for this one time), so that means the exhaust molecules have to bounce off the air molecules, return to the rocket, bounce off of it, and thereby produce lift. Do you realize how inefficient that would be? How many exhaust molecules are going to hit air molecules (which are normally randomly moving, or moving in the general direction of local air currents) at PRECISELY the right angle to be reflected back in a manner to strike the rocket? Very, very few, if you know ANY math/geometry whatsoever. If that was how rockets worked (it isn't) their efficiency would be greatly improved by giving them a bell-shaped bottom so more molecules could reflect off it and thereby achieve more thrust. However, the opposite is true in reality, because your visualizations were not thought through to realize such a fundamental failure. Maybe you need to add some air fairies to your hypothesis to produce the needed lift?
Now, I may have missed something in my interpretation, but I have never gotten any counter explanation.
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Another real-life example to give the hoax folks is to tell them to blow on their hand as hard and quick as they can. A healthy person can feel the force on their hand, but no matter how close they get to their mouths, they will feel no significant force returned to their face.
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The hoaxers have no clue as to physics and reactions from forces. They rank right up there with the Flat Earth beliefs.
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Welcome to ApolloHoax, MBDK. It looks like you might have been lurking for a few weeks before posting, which is what I did too, and some hoax-believers don't have the sense to do (i.e. size up the opposition first).
As you can see, it's pretty quiet here now and there's not many of them about compared with 10-12 years ago, but it's still a great board for learning the fascinating minutiae of Apollo and other space-related things.
...Now, I may have missed something in my interpretation, but I have never gotten any counter explanation.
It is a good interpretation and quite simple, which makes it very good. And you're in the right place to be put right if you have missed something. There's some real expertise here.
Regarding your next post, I made myself hell-of-a dizzy trying, and you were right. Most of what hit my face with hand flat, bounced off the ball of my thumb, but it was only a tiny amount of the pressure hitting my hand. Cupping my hand spread it over a bigger area of my face, but it was still weak.
I've sometimes directed hoax-believers (HBs) to my two favourite sayings below, but it doesn't seem to help.
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Welcome to ApolloHoax, MBDK. It looks like you might have been lurking for a few weeks before posting, which is what I did too, and some hoax-believers don't have the sense to do (i.e. size up the opposition first).
It is a good interpretation and quite simple, which makes it very good. And you're in the right place to be put right if you have missed something. There's some real expertise here.
Thank you kindly for the welcome, Kiwi. Yes, I was a lurker prior to registering, but not as long as I lurked at the Cosmoquest forum (probably a couple of years). There I use the Monkeyboysdontknow moniker, and MBDK is just a shorthand version. I came to both sites for the intellectual conversations and hoax-busting explanations, and this hoax section seems to get much more traffic these days. I argue under my longer pseudonym on You Tube against many CTers, and have found a lot of useful information here and on Cosmoquest. I know many think that tactic is an exercise in futility, but knowing that YouTube's viewers/posters are younger overall, I feel that if I can reach even a few, I may be able to help them develop some useful critical thinking skills to help them now, and later in life.
Hopefully I'll get around to giving my avatar an image and a quote or two. The posters here seem to have gobbled up the best ones, so I'll exit with this quote (directed at the hoax proponents) from the Buckaroo Bonzai movie (where part of my posting name came from):
"Laugh while you can monkey boy!" - Dr. Emilio Lizardo
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Aha, you've been where many of us have before. Some of the regulars here started at the Bad Astronomy Bulletin Board (Phil Plait's forum, which was very good) which became BAUT (Bad Astronomy and Universe Today) and bigger, and then CosmoQuest and bigger still.
Some of us got a bit tired of the heavy-handed moderating there. Owner of this board, LunarOrbit, has it just about dead right, in my opinion, but it's obviously much easier on a smaller board. At least HBs are given plenty of opportunity here to have their say. Brief bouts of hot heads are tolerated as long as they settle down again*, and banning usually only occurs after severe rule-breaking and a few warnings.
Of course, some HBs prefer to be banned because it's a badge of honour to them, and it can allow them to claim they were censored and that we couldn't stand their "damning truths." Or whatever. :)
One thing that's occurred to me is, don't argue with a hoax-believer, address the gallery instead -- the fence-sitters and lurkers -- because they are the ones who can most do with good, reliable information. Keep a cool head and point out to them where you believe the HB was wrong, state why, and totally ignore any insults from the HB or, for that matter, from anyone.
In some cases the HB might get annoyed about you not conversing directly with him (for once, it's safe to say that because there are very few or no hers in the field), because some of them are quite egotistical, and might well be narcissists.
It was fascinating here a while back when one HB claimed the lurkers would be on his side, so they were asked to de-lurk and say whether they were or not. A few did, and the HB soundly lost. By God, that was fun!
* For some odd reason, most of the hot heads seem to come from an area that's somewhere between Canada and Mexico. :)
Edited to add:
Last night I followed a link in another thread here to an HB on YouTube and found it really hard (read "painful") to listen to the guy, whose video was almost pointless. With a little more thought it could have been audio only. Well, at least the part I saw.
Then I made the mistake of clicking on the video of another guy I'd never heard of. He was talking about inconsistencies in the tyres on the rover in various videos, and I think in one case he might have been confusing the mesh lunar tyres with pneumatic ones in simulations on earth, but he certainly couldn't figure that a dark background could make the mesh tyre look solid. (Drat! Where's the raised eyebrows or the facepalm emoticon?)
His video was also so tedious and painful that I thought few viewers would bother closely following the guy. Poor presentation certainly doesn't help and nor does pointing out non-existing "anomalies" in photos or videos to people who know better. Short, sweet and to-the-point is much better.
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* For some odd reason, most of the hot heads seem to come from an area that's somewhere between Canada and Mexico. :)
...We can hear you...
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* For some odd reason, most of the hot heads seem to come from an area that's somewhere between Canada and Mexico. :)
I resemble that remark.
Edited to add:
Last night I followed a link in another thread here to an HB on YouTube and found it really hard (read "painful") to listen to the guy, whose video was almost pointless. With a little more thought it could have been audio only. Well, at least the part I saw.
Then I made the mistake of clicking on the video of another guy I'd never heard of. He was talking about inconsistencies in the tyres on the rover in various videos, and I think in one case he might have been confusing the mesh lunar tyres with pneumatic ones in simulations on earth, but he certainly couldn't figure that a dark background could make the mesh tyre look solid. (Drat! Where's the raised eyebrows or the facepalm emoticon?)
His video was also so tedious and painful that I thought few viewers would bother closely following the guy. Poor presentation certainly doesn't help and nor does pointing out non-existing "anomalies" in photos or videos to people who know better. Short, sweet and to-the-point is much better.
Got any YT links? I don't recognize the second video other than expattaffy1 put a video together concerning the rover tires. He was clueless along with myopic and delusional. Joan Evans had a nice rebuttal if that is the one.
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See new thread, YouTube Hoax Believer - Lift the Veil
http://www.apollohoax.net/forum/index.php?topic=1176.msg39149#msg39149
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For some odd reason, most of the hot heads seem to come from an area that's somewhere between Canada and Mexico.
.. and unfortunately, many of those from that part of the world seem to think their country owns the internet, and only their countrymen use YouTube, etc. etc. ....
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For some odd reason, most of the hot heads seem to come from an area that's somewhere between Canada and Mexico.
.. and unfortunately, many of those from that part of the world seem to think their country owns the internet, and only their countrymen use YouTube, etc. etc. ....
Please don't form an opinion of Americans based on YouTube comments and the like.
They are vocal, not representative.
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For some odd reason, most of the hot heads seem to come from an area that's somewhere between Canada and Mexico.
.. and unfortunately, many of those from that part of the world seem to think their country owns the internet, and only their countrymen use YouTube, etc. etc. ....
Please don't form an opinion of Americans based on YouTube comments and the like.
They are vocal, not representative.
Well, they elected one of their own.