ISF Logo   IS Forum
Forum Index Register Members List Events Mark Forums Read Help

Go Back   International Skeptics Forum » General Topics » Science, Mathematics, Medicine, and Technology
 


Welcome to the International Skeptics Forum, where we discuss skepticism, critical thinking, the paranormal and science in a friendly but lively way. You are currently viewing the forum as a guest, which means you are missing out on discussing matters that are of interest to you. Please consider registering so you can gain full use of the forum features and interact with other Members. Registration is simple, fast and free! Click here to register today.
Reply
Old 28th July 2019, 06:51 PM   #1
Horhang
Scholar
 
Horhang's Avatar
 
Join Date: Sep 2017
Posts: 66
Can someone explain this

The people making the video are trying to prove a rocket can’t work in a vacuum. The balloon car they use works, what I can not figure out is why it does
Not work when they put a cone around the exhaust. It should work, but there is something happening that I can’t figure out. Can anyone explain it for me please.
https://m.youtube.com/watch?time_con...&v=Gasaz2-HmBs
Horhang is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 28th July 2019, 07:49 PM   #2
Matthew Ellard
Graduate Poster
 
Join Date: Oct 2009
Posts: 1,873
Originally Posted by Horhang View Post
The people making the video are trying to prove a rocket canít work in a vacuum. The balloon car they use works, what I can not figure out is why it does. Not work when they put a cone around the exhaust. It should work, but there is something happening that I canít figure out. Can anyone explain it for me please.
https://m.youtube.com/watch?time_con...&v=Gasaz2-HmBs
1) At no point do they test the balloon in any form of vacuum, which means they aren't testing their claim.

2) Attaching a heavy cone that drags on the cardboard runway, is increasing the mass and drag of their balloon car, so it doesn't move.

This video is complete nonsense.
Matthew Ellard is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 28th July 2019, 08:05 PM   #3
WhatRoughBeast
Graduate Poster
 
Join Date: Apr 2011
Posts: 1,385
Originally Posted by Matthew Ellard View Post

2) Attaching a heavy cone that drags on the cardboard runway
Sorry, but the cone does not touch the rolling surface.
WhatRoughBeast is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 28th July 2019, 08:21 PM   #4
Norman Alexander
Philosopher
 
Norman Alexander's Avatar
 
Join Date: Nov 2014
Posts: 5,878
What point is that video trying to prove?

The cone adds sufficient weight such that the reactive force cannot exceed the friction of the mass on the wheels. A "balloon" rocket is a piss-weak rocket. It can barely push the balloon itself. It barely got the car rolling without the cone on the back.

And this is completely unrelated to the "rocket in a vacuum" nonsense.
__________________
...our governments are just trying to protect us from terror. In the same way that someone banging a hornetsí nest with a stick is trying to protect us from hornets. Frankie Boyle, Guardian, July 2015
Norman Alexander is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 28th July 2019, 08:22 PM   #5
Norman Alexander
Philosopher
 
Norman Alexander's Avatar
 
Join Date: Nov 2014
Posts: 5,878
DUPE
__________________
...our governments are just trying to protect us from terror. In the same way that someone banging a hornetsí nest with a stick is trying to protect us from hornets. Frankie Boyle, Guardian, July 2015

Last edited by Norman Alexander; 28th July 2019 at 08:23 PM. Reason: Dupe
Norman Alexander is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 28th July 2019, 08:25 PM   #6
Matthew Ellard
Graduate Poster
 
Join Date: Oct 2009
Posts: 1,873
Originally Posted by WhatRoughBeast View Post
Sorry, but the cone does not touch the rolling surface.
What is supporting the cone and how much does it weigh?
Attached Images
File Type: jpg rocket cone.JPG (16.0 KB, 24 views)
Matthew Ellard is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 28th July 2019, 08:28 PM   #7
Matthew Ellard
Graduate Poster
 
Join Date: Oct 2009
Posts: 1,873
Originally Posted by Norman Alexander View Post
And this is completely unrelated to the "rocket in a vacuum" nonsense.
It's the same nonsense and probably the same person, as last time.

"svrco pas" who posted this you-tube video is posting videos by other people, just like the previous person Jerry Sprocket, who posted other people's flat earth and anti-rocket nonsense videos.
Matthew Ellard is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 28th July 2019, 08:29 PM   #8
fritznien
New Blood
 
Join Date: Oct 2016
Posts: 23
a nozzle makes a big difference to a rocket engine. so putting a bad nozzle will greatly decrease thrust. the long paper nozzle is aimed up to clear the track, the air might be hitting the side and wasting most of the thrust.
fritznien is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 28th July 2019, 08:48 PM   #9
Ziggurat
Penultimate Amazing
 
Ziggurat's Avatar
 
Join Date: Jun 2003
Posts: 43,794
Originally Posted by fritznien View Post
a nozzle makes a big difference to a rocket engine. so putting a bad nozzle will greatly decrease thrust. the long paper nozzle is aimed up to clear the track, the air might be hitting the side and wasting most of the thrust.
The problem is probably mostly the reduction in thrust because of the nozzle size. The gradual expansion means the air flow will expand with the cone, and will decrease in velocity, leaving the cone at much lower velocity. If itís, say, a factor of 5 bigger diameter, thatís a factor of 25 more in area, so a factor of 25 less in exit velocity and a factor of 25 less in force. At some point, the force wonít overcome friction.
__________________
"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law
Ziggurat is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 28th July 2019, 09:47 PM   #10
abaddon
Penultimate Amazing
 
abaddon's Avatar
 
Join Date: Feb 2011
Posts: 18,868
Originally Posted by Horhang View Post
The people making the video are trying to prove a rocket can’t work in a vacuum. The balloon car they use works, what I can not figure out is why it does
Not work when they put a cone around the exhaust. It should work, but there is something happening that I can’t figure out. Can anyone explain it for me please.
https://m.youtube.com/watch?time_con...&v=Gasaz2-HmBs
There is already a thread that has done this to death.

If rockets cannot work in a vacuum then satellites are impossible. Thus satellite TV does not exist. This will come as somewhat of a surprise to the countless millions of people who use it.
__________________
Who is General Failure? And why is he reading my hard drive?


...love and buttercakes...
abaddon is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 12:27 AM   #11
Matthew Ellard
Graduate Poster
 
Join Date: Oct 2009
Posts: 1,873
Originally Posted by fritznien View Post
a nozzle makes a big difference to a rocket engine. so putting a bad nozzle will greatly decrease thrust. the long paper nozzle is aimed up to clear the track, the air might be hitting the side and wasting most of the thrust.
If you look at the photo I posted the nozzle isn't straight from above. Additionally, if you look at the nozzles opening's radius, it is larger than the trolley is off ground meaning its either scraping on the ground or bent upwards and simply pushing down against the rear axle and acting as a brake.

That's probably why the two idiots, who made the video, shot the video from above.
Matthew Ellard is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 01:50 AM   #12
DevilsAdvocate
Philosopher
 
DevilsAdvocate's Avatar
 
Join Date: Nov 2004
Posts: 5,188
Originally Posted by abaddon View Post
There is already a thread that has done this to death.

If rockets cannot work in a vacuum then satellites are impossible. Thus satellite TV does not exist. This will come as somewhat of a surprise to the countless millions of people who use it.
This thread is not that thread. This thread is about explaining what happens in this video.

I would like an answer and be able to understand that answer. I don't think a sufficient answer has been provided to explain what we see in the video.

Why doesn't the car move? Would the same thing happen in space? I don't know. I would like to hear answers.
__________________
"Nothing scarier than piled up adirondack chairs." - AvE
DevilsAdvocate is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 02:58 AM   #13
GnaGnaMan
Graduate Poster
 
Join Date: Sep 2008
Posts: 1,707
Originally Posted by Horhang View Post
The people making the video are trying to prove a rocket canít work in a vacuum. The balloon car they use works, what I can not figure out is why it does
Not work when they put a cone around the exhaust. It should work, but there is something happening that I canít figure out. Can anyone explain it for me please.
https://m.youtube.com/watch?time_con...&v=Gasaz2-HmBs
The increased friction from the increased mass will be doing something. My guess is that the nozzle redirects the air upwards and/or sidewards, too, thus leaving no net forward impulse.

I don't understand how they interpret this. It's not a vacuum, so shouldn't this still work in their model?
__________________
It makes no difference whatever whether they laugh at us or revile us, whether they represent us as clowns or criminals; the main thing is that they mention us, that they concern themselves with us again and again. -Hitler
GnaGnaMan is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 05:26 AM   #14
Ziggurat
Penultimate Amazing
 
Ziggurat's Avatar
 
Join Date: Jun 2003
Posts: 43,794
Originally Posted by Matthew Ellard View Post
If you look at the photo I posted the nozzle isn't straight from above. Additionally, if you look at the nozzles opening's radius, it is larger than the trolley is off ground meaning its either scraping on the ground or bent upwards and simply pushing down against the rear axle and acting as a brake.

That's probably why the two idiots, who made the video, shot the video from above.
Itís bent up a little bit. But the extra pressure it puts on the axle wonít matter, that will be far less than itís weight, so it wonít be much of a brake. The problem is simply the thrust is just reduced too much by widening the nozzle so much.
__________________
"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law
Ziggurat is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 05:29 AM   #15
Ziggurat
Penultimate Amazing
 
Ziggurat's Avatar
 
Join Date: Jun 2003
Posts: 43,794
Originally Posted by DevilsAdvocate View Post
This thread is not that thread. This thread is about explaining what happens in this video.

I would like an answer and be able to understand that answer. I don't think a sufficient answer has been provided to explain what we see in the video.

Why doesn't the car move? Would the same thing happen in space? I don't know. I would like to hear answers.
Look at how fast the car accelerates without the nozzle: itís pretty slow. The force is small to begin with. Cut that force by a factor of, say, 25, and you wonít even be able to overcome friction.
__________________
"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law
Ziggurat is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 05:36 AM   #16
smartcooky
Penultimate Amazing
 
smartcooky's Avatar
 
Join Date: Oct 2012
Location: Nelson, New Zealand
Posts: 12,222
What Zig said, and IIUC, its got a name...“Krushnic Effect"

Krushnic Effect happens when the motor is recessed into the body tube by more than one tube diameter. The cylindrical volume below the motor forms a secondary expansion chamber which allows the exhaust gasses to expand below atmospheric pressure before leaving the rocket. Surrounding air aspirated into the exhaust stream causes turbulence which negates much of the thrust. The result is that the rocket makes a tremendous amount of noise and smoke but doesn’t go anywhere.

This is named after Richard Krushnic, a rocket engineer who first characterized the effect in the late ’60s.
__________________
ďGive me your tired, your poor, your huddled masses yearning to breathe free, the wretched refuse of your teeming shore - if they're white!"
If you don't like my posts, my opinions, or my directness then put me on your ignore list.
This will be of benefit to both of us; you won't have to take umbrage at my posts, and I won't have to waste my time talking to you... simples! !

Last edited by smartcooky; 29th July 2019 at 05:38 AM.
smartcooky is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 07:08 AM   #17
Ziggurat
Penultimate Amazing
 
Ziggurat's Avatar
 
Join Date: Jun 2003
Posts: 43,794
Originally Posted by smartcooky View Post
What Zig said, and IIUC, its got a name...ďKrushnic Effect"

Krushnic Effect happens when the motor is recessed into the body tube by more than one tube diameter. The cylindrical volume below the motor forms a secondary expansion chamber which allows the exhaust gasses to expand below atmospheric pressure before leaving the rocket. Surrounding air aspirated into the exhaust stream causes turbulence which negates much of the thrust. The result is that the rocket makes a tremendous amount of noise and smoke but doesnít go anywhere.

This is named after Richard Krushnic, a rocket engineer who first characterized the effect in the late í60s.
I'm not sure this is the Krushnic effect. Even if the flow remains laminar, the fact that the nozzle expands so much will still drop the thrust by a huge factor. Of course, I'm 100% positive that the flow will be laminar in this case, but it sounds like either way the nozzle is screwing things up.
__________________
"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law
Ziggurat is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 08:14 AM   #18
Delvo
الشيطان الأبيض
 
Delvo's Avatar
 
Join Date: Jul 2008
Location: Harrisburg, PA
Posts: 8,019
We don't even need an explanation for why the added "nozzle" would yield no thrust. We're talking about people who are out to prove that rockets don't work. If they built something showing that it did still work, they'd rig it to fail anyway, like by putting a piece of tape on the bottom when they pick it up to attach the "nozzle".
Delvo is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 09:36 AM   #19
GnaGnaMan
Graduate Poster
 
Join Date: Sep 2008
Posts: 1,707
Originally Posted by smartcooky View Post
Krushnic Effect happens when the motor is recessed into the body tube by more than one tube diameter. The cylindrical volume below the motor forms a secondary expansion chamber which allows the exhaust gasses to expand below atmospheric pressure before leaving the rocket. Surrounding air aspirated into the exhaust stream causes turbulence which negates much of the thrust. The result is that the rocket makes a tremendous amount of noise and smoke but doesnít go anywhere.

This is named after Richard Krushnic, a rocket engineer who first characterized the effect in the late í60s.
Google lead me to this diagram.
https://www.rocketryforum.com/attach...69-jpg.360280/
I think you may be right on it.
__________________
It makes no difference whatever whether they laugh at us or revile us, whether they represent us as clowns or criminals; the main thing is that they mention us, that they concern themselves with us again and again. -Hitler
GnaGnaMan is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 09:48 AM   #20
GnaGnaMan
Graduate Poster
 
Join Date: Sep 2008
Posts: 1,707
Originally Posted by Ziggurat View Post
The problem is probably mostly the reduction in thrust because of the nozzle size. The gradual expansion means the air flow will expand with the cone, and will decrease in velocity, leaving the cone at much lower velocity. If itís, say, a factor of 5 bigger diameter, thatís a factor of 25 more in area, so a factor of 25 less in exit velocity and a factor of 25 less in force. At some point, the force wonít overcome friction.
What's the physics behind this?
__________________
It makes no difference whatever whether they laugh at us or revile us, whether they represent us as clowns or criminals; the main thing is that they mention us, that they concern themselves with us again and again. -Hitler
GnaGnaMan is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 12:01 PM   #21
Elagabalus
Illuminator
 
Join Date: Dec 2013
Posts: 4,630
Originally Posted by DevilsAdvocate View Post
This thread is not that thread. This thread is about explaining what happens in this video.

I would like an answer and be able to understand that answer. I don't think a sufficient answer has been provided to explain what we see in the video.

Why doesn't the car move? Would the same thing happen in space? I don't know. I would like to hear answers.
Because svrco pas aka Jerry Sprockets is a disingenuous prat.
Elagabalus is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 12:17 PM   #22
Ziggurat
Penultimate Amazing
 
Ziggurat's Avatar
 
Join Date: Jun 2003
Posts: 43,794
Originally Posted by GnaGnaMan View Post
What's the physics behind this?
Simple momentum transfer.

Force is the change in momentum over time. The forward momentum you add to the rocket is equal to the backward momentum you give to the exhaust. And momentum is mass times velocity. The mass flowing out the back is controlled by the balloon, that's the same in either case. What changes is the exhaust velocity. If the nozzle is small, the exhaust velocity is large. If the nozzle is big, that same amount of air can pass out the back in the same amount of time at much lower velocity. Hence, lower momentum and less force transmitted to the rocket.

Now, you might naively say that since the air enters the cone at a high velocity, it should still exert a large (relatively speaking) force on the rocket cart. And if you chop the system up into parts and look at just that part, then yeah, the force should still be large. But you still can't ignore the cone. In order for the air to enter the cone at high velocity but leave the cone at low velocity, it has to slow down. So a net force has to act on the air. The cone has to, in effect, "suck" at the air. In more detail, the air is at lower pressure inside the cone than outside the cone, with a pressure gradient that slows down the air (Bernoulli's principle). Because of the cone's shape, lower pressure inside than outside means there's a net force pushing backwards on the cone, so that the high force at the entrance to the cone is largely cancelled by the forces acting on the cone itself.

The net effect is the same as if you don't chop the system into parts: low exhaust velocity means small net force, and for the purposes of propulsion we don't care about any internal stresses we're creating within the rocket itself (though on a real rocket, you'd need to make sure any internal forces don't break anything).
__________________
"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law
Ziggurat is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 12:50 PM   #23
Elagabalus
Illuminator
 
Join Date: Dec 2013
Posts: 4,630
svrco pas shoves what was the nozzle way back into the plastic(main body) of the car and then attaches the cone nozzle . Given the amount of offset to the cone nozzle(so it doesn't touch the runway surface) I wouldn't be surprised if some restrictions developed or perhaps some significant leaks.


I'm still not sure why he thinks this this "test" is proof the rockets don't work in a vacuum.
Elagabalus is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 12:52 PM   #24
Ziggurat
Penultimate Amazing
 
Ziggurat's Avatar
 
Join Date: Jun 2003
Posts: 43,794
Originally Posted by Elagabalus View Post
I'm still not sure why he thinks this this "test" is proof the rockets don't work in a vacuum.
Really? That seems like the most obvious part of this whole thing to me.
__________________
"As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law
Ziggurat is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 12:56 PM   #25
abaddon
Penultimate Amazing
 
abaddon's Avatar
 
Join Date: Feb 2011
Posts: 18,868
Originally Posted by DevilsAdvocate View Post
This thread is not that thread. This thread is about explaining what happens in this video.

I would like an answer and be able to understand that answer. I don't think a sufficient answer has been provided to explain what we see in the video.

Why doesn't the car move? Would the same thing happen in space? I don't know. I would like to hear answers.
Fine then, I will hand you the relevant equations, but just be aware that while you might learn about nozzle design, it will tell you nothing about the balloon car video because we have none of the relevant numbers.

Enjoy

https://www.grc.nasa.gov/WWW/K-12/rocket/nozzle.html

ETA: Note the important bit though, for subsonic gas velocity (which this car definitely has) Increasing nozzle area decreases gas velocity. I suspect we are seeing that very effect. But absent measurements, nobody can prove that.
__________________
Who is General Failure? And why is he reading my hard drive?


...love and buttercakes...

Last edited by abaddon; 29th July 2019 at 01:08 PM.
abaddon is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 01:11 PM   #26
Elagabalus
Illuminator
 
Join Date: Dec 2013
Posts: 4,630
Apropos of nothing, here's a long winded YT about V-2 Turbopumps:

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


I rather like this channel. Very soothing, calming even. Try having it in the background while you're gardening!



Originally Posted by abaddon View Post
...I suspect we are seeing that very effect. But absent measurements, nobody can prove that.

Hah! Gotcha'!!

Last edited by Elagabalus; 29th July 2019 at 01:15 PM.
Elagabalus is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 01:24 PM   #27
abaddon
Penultimate Amazing
 
abaddon's Avatar
 
Join Date: Feb 2011
Posts: 18,868
Originally Posted by Elagabalus View Post
Apropos of nothing, here's a long winded YT about V-2 Turbopumps:

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


I rather like this channel. Very soothing, calming even. Try having it in the background while you're gardening!






Hah! Gotcha'!!
You basket! LOL.
__________________
Who is General Failure? And why is he reading my hard drive?


...love and buttercakes...
abaddon is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 01:29 PM   #28
GnaGnaMan
Graduate Poster
 
Join Date: Sep 2008
Posts: 1,707
Originally Posted by Ziggurat View Post
Simple momentum transfer.

Force is the change in momentum over time. The forward momentum you add to the rocket is equal to the backward momentum you give to the exhaust. And momentum is mass times velocity. The mass flowing out the back is controlled by the balloon, that's the same in either case. What changes is the exhaust velocity. If the nozzle is small, the exhaust velocity is large. If the nozzle is big, that same amount of air can pass out the back in the same amount of time at much lower velocity. Hence, lower momentum and less force transmitted to the rocket.

Now, you might naively say that since the air enters the cone at a high velocity, it should still exert a large (relatively speaking) force on the rocket cart. And if you chop the system up into parts and look at just that part, then yeah, the force should still be large. But you still can't ignore the cone. In order for the air to enter the cone at high velocity but leave the cone at low velocity, it has to slow down. So a net force has to act on the air. The cone has to, in effect, "suck" at the air. In more detail, the air is at lower pressure inside the cone than outside the cone, with a pressure gradient that slows down the air (Bernoulli's principle). Because of the cone's shape, lower pressure inside than outside means there's a net force pushing backwards on the cone, so that the high force at the entrance to the cone is largely cancelled by the forces acting on the cone itself.

The net effect is the same as if you don't chop the system into parts: low exhaust velocity means small net force, and for the purposes of propulsion we don't care about any internal stresses we're creating within the rocket itself (though on a real rocket, you'd need to make sure any internal forces don't break anything).
I see now. Thanks.
__________________
It makes no difference whatever whether they laugh at us or revile us, whether they represent us as clowns or criminals; the main thing is that they mention us, that they concern themselves with us again and again. -Hitler
GnaGnaMan is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 06:24 PM   #29
Robin
Philosopher
 
Join Date: Apr 2004
Posts: 9,942
Originally Posted by Horhang View Post
The people making the video are trying to prove a rocket can’t work in a vacuum. The balloon car they use works, what I can not figure out is why it does
Not work when they put a cone around the exhaust. It should work, but there is something happening that I can’t figure out. Can anyone explain it for me please.
https://m.youtube.com/watch?time_con...&v=Gasaz2-HmBs
If I understand what they are doing, they start with a balloon car on a flat surface and show that it moves.

Then they attach a conical nozzle to the exhaust and also put long straws into the nozzle.

After this the car doesn't move.

I am not sure if I missed something there.

But imagine the balloon is left uninflated and a string is attached to it and strung over a pulley at the end of the table and a certain weight put on the end.

The balloon car would move.

But if you replaced the weight with a much lighter one, the balloon car would not move.

Because when using force to move a mass against a resistance sometimes the resistance is greater than the force. Also you need a little extra force just to get it started against resistance (there is a name for this, I don't recall it).

So I am guessing that sticking the straw up the outlet of the balloon car restricted the exhaust velocity to the point that it could no longer act against the resistance or couldn't give it that extra little kick to get it started.

I have no idea, incidentally, why they think that this exercise proves that rockets don't work in a vacuum. Where was the vacuum?

(Also, I have no idea what the conical nozzle is supposed to do, especially as they seem to bypass it completely)
__________________
The non-theoretical character of metaphysics would not be in itself a defect; all arts have this non-theoretical character without thereby losing their high value for personal as well as for social life. The danger lies in the deceptive character of metaphysics; it gives the illusion of knowledge without actually giving any knowledge. This is the reason why we reject it. - Rudolf Carnap "Philosophy and Logical Syntax"
Robin is online now   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 06:38 PM   #30
Robin
Philosopher
 
Join Date: Apr 2004
Posts: 9,942
Originally Posted by DevilsAdvocate View Post
Why doesn't the car move? Would the same thing happen in space? I don't know. I would like to hear answers.
I think the first question has been answered.

In the second case, it wouldn't happen in space because there is no (or negligible) friction there.
__________________
The non-theoretical character of metaphysics would not be in itself a defect; all arts have this non-theoretical character without thereby losing their high value for personal as well as for social life. The danger lies in the deceptive character of metaphysics; it gives the illusion of knowledge without actually giving any knowledge. This is the reason why we reject it. - Rudolf Carnap "Philosophy and Logical Syntax"
Robin is online now   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 06:41 PM   #31
Matthew Ellard
Graduate Poster
 
Join Date: Oct 2009
Posts: 1,873
Originally Posted by DevilsAdvocate View Post
I don't think a sufficient answer has been provided to explain what we see in the video. Why doesn't the car move? Would the same thing happen in space? I don't know. I would like to hear answers.
1) At no point have these two idiots tested anything in a vacuum.
2) The cone is not straight from above, thus pushing the cart to the side against the direction of the wheels.
3) As the radius of the wide end of the cone is larger than the height of the wheels it drags on the runway. Alternatively if it is bent upwards it simply pushes down on the rear axle causing even more friction.


Blow up a balloon and place it on a sheet of ice. It moves. Now do the same thing with a small nozzle. (A straw) It will also move. Therefore it isn't the nozzle, it is simply a poorly constructed experiment .
Attached Images
File Type: jpg rocket cone 2.jpg (38.2 KB, 7 views)
Matthew Ellard is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 29th July 2019, 08:50 PM   #32
smartcooky
Penultimate Amazing
 
smartcooky's Avatar
 
Join Date: Oct 2012
Location: Nelson, New Zealand
Posts: 12,222
Originally Posted by Matthew Ellard View Post
1) At no point have these two idiots tested anything in a vacuum.
2) The cone is not straight from above, thus pushing the cart to the side against the direction of the wheels.
3) As the radius of the wide end of the cone is larger than the height of the wheels it drags on the runway. Alternatively if it is bent upwards it simply pushes down on the rear axle causing even more friction.


Blow up a balloon and place it on a sheet of ice. It moves. Now do the same thing with a small nozzle. (A straw) It will also move. Therefore it isn't the nozzle, it is simply a poorly constructed experiment .

Even worse, its a poorly constructed experiment that, even if constructed properly, still would not address the hypothesis it is supposed to be testing.
__________________
ďGive me your tired, your poor, your huddled masses yearning to breathe free, the wretched refuse of your teeming shore - if they're white!"
If you don't like my posts, my opinions, or my directness then put me on your ignore list.
This will be of benefit to both of us; you won't have to take umbrage at my posts, and I won't have to waste my time talking to you... simples! !
smartcooky is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 30th July 2019, 01:05 AM   #33
HansMustermann
Penultimate Amazing
 
Join Date: Mar 2009
Posts: 15,590
Well, here's the saddest thing, though: we know that rockets work in air, at least when reasonably well designed. It's something that everyone who can walk without getting bruised knuckles can find plenty of non-government videos of on youtube, or even test for themselves. You only need a plastic bottle, water and a bicycle pump to see that it works. Or just buy some fireworks and shoot them on Xmas or the 4th of July.

The claim being made by idiot CT-ers is that it works in the air because it pushes against it, while in space it wouldn't because it has nothing to push against. Sure, that too is only proof that someone slept in physics class, but bear with me. That's the claim they're making: that it ONLY works in air.

I repeat: that it works in air.

So if their experiment doesn't even work in air, when they can trivially find designs that do and even the CT says they should, all it proves is that their design sucks. Well, technically, it blows

You don't even need to know anything about nozzles or Newtonian mechanics to figure out that at that point all they've proven isn't "it doesn't work without air" (i.e., "ONLY works with air",) but "doesn't work, period."
__________________
Which part of "Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn" don't you understand?
HansMustermann is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 30th July 2019, 05:07 PM   #34
mgidm86
Philosopher
 
Join Date: Jan 2003
Posts: 5,530
Originally Posted by HansMustermann View Post
Well, here's the saddest thing, though: we know that rockets work in air, at least when reasonably well designed. It's something that everyone who can walk without getting bruised knuckles can find plenty of non-government videos of on youtube, or even test for themselves. You only need a plastic bottle, water and a bicycle pump to see that it works. Or just buy some fireworks and shoot them on Xmas or the 4th of July.

The claim being made by idiot CT-ers is that it works in the air because it pushes against it, while in space it wouldn't because it has nothing to push against. Sure, that too is only proof that someone slept in physics class, but bear with me. That's the claim they're making: that it ONLY works in air.

I repeat: that it works in air.

So if their experiment doesn't even work in air, when they can trivially find designs that do and even the CT says they should, all it proves is that their design sucks. Well, technically, it blows

You don't even need to know anything about nozzles or Newtonian mechanics to figure out that at that point all they've proven isn't "it doesn't work without air" (i.e., "ONLY works with air",) but "doesn't work, period."

__________________
Franklin understands certain kickbacks you obtain unfairly are legal liabilities; however, a risky deed's almost never detrimental despite extra external pressures.
mgidm86 is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 30th July 2019, 06:16 PM   #35
Matthew Ellard
Graduate Poster
 
Join Date: Oct 2009
Posts: 1,873
Originally Posted by smartcooky View Post
Even worse, its a poorly constructed experiment that, even if constructed properly, still would not address the hypothesis it is supposed to be testing.
I agree for two reasons. Firstly it wasn't tested in a vacuum. Secondly the pressure increase from burning fuel expanding, occurs in a rocket's nozzle whereas the balloon model simply has air under pressure pumped into a nozzle and thus is unrelated to rocket nozzles. All the idiots did was add mass and friction to the trolley, which stopped it from moving.

It would be as silly as me making a model of a nuclear reactor using Lego and claiming reactors don't work.
Attached Images
File Type: jpg rocket cone 3.jpg (53.0 KB, 4 views)
Matthew Ellard is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 30th July 2019, 07:05 PM   #36
grmcdorman
Graduate Poster
 
Join Date: Mar 2007
Posts: 1,260
The other thing is that rocket nozzles aren't simple cones. They have a very specific shape: a combustion chamber (usually roughly spherical, I believe), a narrow throat and a bell (not a cone, generally) after that. The performance of the engine depends on the shape of the bell and the exterior pressure; a bell optimized for lower altitudes (i.e. higher ambient pressure) will not perform as well at high altitudes (lower pressure).

The balloon in this "experiment" (if it can be called that) will probably behave differently than a rocket engine. EDIT: Their cone is also, from my limited knowledge, a ridiculous approximation of a rocket engine's bell; it's very long and narrow. I don't think I've ever seen a rocket engine bell even close to that.

However, as JayUtah pointed out in the other thread, there's a very simple consequent to these claims: performance of any engine of this sort (including propellers and jets) should fall off rapidly with altitude. So run your test at sea level, and then again at a significantly higher altitude. For some reason none of these folks even acknowledge this; [sarcasm]wonder why?[/sarcasm]

Discliamer: IANARS (I Am Not A Rocket Scientist). JayUtah may be able to elucidate better; he is a rocket scientist, or more generally an aerospace engineer (with the emphasis very much on space).
__________________
"Hello. My name is Inigo Skywalker. You are my father. Prepare to die."

Last edited by grmcdorman; 30th July 2019 at 07:07 PM.
grmcdorman is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 30th July 2019, 07:29 PM   #37
Matthew Ellard
Graduate Poster
 
Join Date: Oct 2009
Posts: 1,873
The opening post's, ridiculous, You-Tube video, "balloon trolley proves rocket nozzles don't work in a vacuum" was produced by two idiots called "Pete & Peter". These two idiots have not limited themselves to rockets. I link, below, their You-tube page of their other you-tube videos.

"and you thought plants release oxygen?"
"Water is water and not hydrogen and oxygen"
"End game to Apollo moon landings hoax"
"The ISS is a 100% fake"
"The sun is not 93,000,000 miles away"
"water is the 119th element on the periodic table"


https://www.youtube.com/playlist?lis...lIhEsh5VlQcR12

https://www.youtube.com/channel/UCRf...i_Yk3PWqNETzyQ

They are, as you may have already guessed, flat-earthers.

"Hi there, welcome to Peter & Pete's channel revealing through free thought, that most things in man's world are a pure deception.........Since April 2015 and realising the Earth is indeed FLAT, the channel has gone through significant changes and incorporates a great deal of info regarding the rubbish Globe Earth and the real reason why people lie so much - to conceal the true reality of human life."
https://www.youtube.com/channel/UCRf...WqNETzyQ/about
Matthew Ellard is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 30th July 2019, 08:23 PM   #38
8enotto
Muse
 
Join Date: Dec 2018
Location: Mexico
Posts: 950
The air escaping the balloon is not a proper expanding gasses reaction. It is a pressure release from the balloon but at a set diminishing rate. A rocket engine turns fuel into pressures much higher than the fuel in the tank.

Even a scientific illiterate like me can see the fail in that.
8enotto is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 30th July 2019, 08:52 PM   #39
DevilsAdvocate
Philosopher
 
DevilsAdvocate's Avatar
 
Join Date: Nov 2004
Posts: 5,188
Originally Posted by abaddon View Post
Fine then, I will hand you the relevant equations, but just be aware that while you might learn about nozzle design, it will tell you nothing about the balloon car video because we have none of the relevant numbers.

Enjoy

https://www.grc.nasa.gov/WWW/K-12/rocket/nozzle.html

ETA: Note the important bit though, for subsonic gas velocity (which this car definitely has) Increasing nozzle area decreases gas velocity. I suspect we are seeing that very effect. But absent measurements, nobody can prove that.
Thanks. the bits I can understand are interesting. It's a bit more mathematical than I can translate into something I can understand.
__________________
"Nothing scarier than piled up adirondack chairs." - AvE
DevilsAdvocate is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 30th July 2019, 09:09 PM   #40
smartcooky
Penultimate Amazing
 
smartcooky's Avatar
 
Join Date: Oct 2012
Location: Nelson, New Zealand
Posts: 12,222
Originally Posted by Matthew Ellard View Post
The opening post's, ridiculous, You-Tube video, "balloon trolley proves rocket nozzles don't work in a vacuum" was produced by two idiots called "Pete & Peter". These two idiots have not limited themselves to rockets. I link, below, their You-tube page of their other you-tube videos.

"and you thought plants release oxygen?"
"Water is water and not hydrogen and oxygen"
"End game to Apollo moon landings hoax"
"The ISS is a 100% fake"
"The sun is not 93,000,000 miles away"

"water is the 119th element on the periodic table"


https://www.youtube.com/playlist?lis...lIhEsh5VlQcR12

https://www.youtube.com/channel/UCRf...i_Yk3PWqNETzyQ

They are, as you may have already guessed, flat-earthers.

"Hi there, welcome to Peter & Pete's channel revealing through free thought, that most things in man's world are a pure deception.........Since April 2015 and realising the Earth is indeed FLAT, the channel has gone through significant changes and incorporates a great deal of info regarding the rubbish Globe Earth and the real reason why people lie so much - to conceal the true reality of human life."
https://www.youtube.com/channel/UCRf...WqNETzyQ/about
The highlighted is a dead giveaway of them being Flat Earthers, and is probably even money that they are YEC's as well.
__________________
ďGive me your tired, your poor, your huddled masses yearning to breathe free, the wretched refuse of your teeming shore - if they're white!"
If you don't like my posts, my opinions, or my directness then put me on your ignore list.
This will be of benefit to both of us; you won't have to take umbrage at my posts, and I won't have to waste my time talking to you... simples! !

Last edited by smartcooky; 30th July 2019 at 09:51 PM.
smartcooky is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Reply

International Skeptics Forum » General Topics » Science, Mathematics, Medicine, and Technology

Bookmarks

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -7. The time now is 11:41 PM.
Powered by vBulletin. Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.

This forum began as part of the James Randi Education Foundation (JREF). However, the forum now exists as
an independent entity with no affiliation with or endorsement by the JREF, including the section in reference to "JREF" topics.

Disclaimer: Messages posted in the Forum are solely the opinion of their authors.