Assistance in answering the CluesForum "why rocketry doesn't work in a vacuum" debate

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BABAR

Builds Rockets for NASA
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First, I (as I suspect just about everyone here) understand that rocketry DOES indeed work in a vacuum. But I have some free time on vacation and saw this post bumped
https://www.rocketryforum.com/showt...y-Doesn-t-Work-in-the-Vacuum&highlight=Vacuum
And thought what the heck, maybe it will be fun . I know....I need to get out more.
Anyway, these guys take a certain amount of science and layman's thinking and try to contradict the science of Rocketry in general and NASA in particular.
In any case they raise some legitimate questions and for the most part are polite. In reviewing the posts I find a tremendous misunderstanding if what a vacuum is (and more importantly IS NOT!). Some of the concepts I think I may need a bit of help to answer. I will not claim to represent THE ROCKETRY FORUM, so if I make mistakes it will not reflect on our Forum. I do think some of these questions may also be educational for newbees (and some oldsters on our forum, hence posting here.)
question 1
While I do no doubt the fact at action/reaction is sufficient and certainly the dominant force in rocket propulsion, DOES the presence of air (mass) behind the rocket have any effect, positive or negative, in the degree of force generated (does is provide any additional "push"). One of the examples used in ClueForum was a jet ski. The suggestion was that when a jet ski jet is not in the water it produces no force. While I understand that this is indeed wrong (and furthermore is a liquid, not a gas), the acceleration of the water pumped in jet form out the back does cause some action reaction, it is I believe much less than the same jet pumped into a water medium.

My expectation is that the presence of gas behind the rocket (particularly at lift off when velocity of rocket is slow) may get compressed by the rapidly exiting gas plume and provide a slight (likely negligible) push. Not sure what happens when rocket gets up to speed.

But I would like a on answer on the jet ski question as well. Likely I am comparing apples to asparagus.

Thanks
 
The jet ski doesn't work in the air because it has no way to suck up water to use to squirt out the back. If you were to hook up a firehose to the intake so that it could get water, while the jetski sat on ice (or on wheels, or some other way to reduce friction) it would easily push the water out the back with enough force to move.
 
Rocket engines burn fuel which is a chemical reaction that causes the exhaust to move out in all directions with a LOT of speed. The exhaust that hits the rocket engine bell loses some of it's speed and delivers it to the rocket, much like a cue ball can cause a billiard ball to move away with a lot of speed, or a baseball can leave a bat very fast.

A billiard ball doesn't move away because it's pushing on the table - and a rocket doesn't push on the air.
A baseball doesn't move away from a bat because it's pushing on the air - and a rocket doesn't push on air either.
 
If I understand correctly, the slipstream of air moving around the rocket (or being blown out of the path of the exhaust at launch) does exert pressure to the side of the exhaust stream and helps shapes it - almost like a nozzle extension. I think that's why you see engines rated 'in a vacuum' - the air pressure has a small influence on the performance.

I think the simplest analysis is that the chamber pressure would be 1 bar higher at sea level than it would be in a vacuum - all other things being equal. so 14 psi out of 3000 in the case of the space shuttle main engine. or 0.5%
 
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To heck with this discussion...
I wanna know why there's an attractive girl dressed only in a pink halter top for Road Kill T-shirts in the middle of several of these posts! The shirt reads, "With an amazing top like this, who needs pants..." At least I think that's what it says.... I have trouble concentrating when I try to read it.... LOL!
 
I've looked at that thread. Amusing and SAD.

Here is the best advice you are ever going to get, when you are dealing with people like THAT.

Don't waste your time.

It's like trying to convince the Moon Hoaxers or 9/11 Truthers that they are wrong, they always find ways to ignore or dispute the most basic facts. You'll never "win", or convince them of anything. Just waste your time.

Although I found Buzz Aldrin's solution to a harassing Hoaxer's "in your face" style, with his own "in your face" literal response, to be quite satisfying. Still didn't convince that loon though.

- George Gassaway


[video=youtube;wptn5RE2I-k]https://www.youtube.com/watch?v=wptn5RE2I-k[/video]
 
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The jet ski doesn't work in the air because it has no way to suck up water to use to squirt out the back. If you were to hook up a firehose to the intake so that it could get water, while the jetski sat on ice (or on wheels, or some other way to reduce friction) it would easily push the water out the back with enough force to move.

[YOUTUBE]aiE58Ri5axQ[/YOUTUBE]

'nuf said... OL JR :)
 
The jet ski doesn't work in the air because it has no way to suck up water to use to squirt out the back. e.

Thanks for the reply. I didn't phrase my question correctly. Assuming adequate intake, is the nozzle output more effective with the NOZZLE above or below the waterline?

I have never ridden a jet ski, so I honestly don't know this. On a jet ski traveling at high speed, is the nozzle above or below the waterline?
 
To heck with this discussion...
I wanna know why there's an attractive girl dressed only in a pink halter top for Road Kill T-shirts in the middle of several of these posts! The shirt reads, "With an amazing top like this, who needs pants..." At least I think that's what it says.... I have trouble concentrating when I try to read it.... LOL!

I want to know why you see that and I don't!
 
OK, against my better advice, here's a page that will help with the jet ski analogy. Jet Skis have more thrust in a less dense medium (Air) than in a denser medium (water). Carried out further, they would thus work better in vacuum than in air.

https://en.allexperts.com/q/Physics-1358/f/Workings-jet-powered-personal.htm

But the nuts in that forum are not going to believe it. They "know" that to move, it has to "push" against something else, and that the Earth is flat, and the Sun, moon and planets revolve around the Flat Earth. The references the Jet Ski guy makes in the above link, mentioning NASA, they will discount every one of those as heresy, as though the words of the devil had been invoked.

I guess the only reason to try it though is to see what "hilarity will ensue" when the bigger nuts come up with their excuses as to what that's wrong and they are right.

Now, in the bigger scheme of things, I know the right thing is to try to educate people. This assumes though that they are open to being educated. But some people are so cultishly deep into their delusions of reality or anti-science (or "alternate science", to coin an Oxy-Moran, and yes I spelled it the way I intended to) that at some point its more cruel to mess with their little world views , sort of like kicking a hornet's nest, than to leave them alone

- George Gassaway
 
Thanks for the reply. I didn't phrase my question correctly. Assuming adequate intake, is the nozzle output more effective with the NOZZLE above or below the waterline?

A rocket engine performs best when the exit pressure is equal to the ambient pressure.

So, assuming that Jet Ski nozzles are designed to maximize performance in the water, not air, the nozzle would be more effective below water.

-- Roger
 
To heck with this discussion...
I wanna know why there's an attractive girl dressed only in a pink halter top for Road Kill T-shirts in the middle of several of these posts! The shirt reads, "With an amazing top like this, who needs pants..." At least I think that's what it says.... I have trouble concentrating when I try to read it.... LOL!

The important thing to remember about this is that you should click on that ad now and then to keep it coming back. I've taken to clicking on Road Kill T-shirts, Snorg Tees and another one that I can't remember which also features a hottie in a T-shirt. I'm not in the market for T-shirts, but I figure the ads are a nice decoration to the site --- better than Chinese characters or mortgage refi ads. I haven't seen the girl in the pink halter top, but I'll keep an eye out.
 
The important thing to remember about this is that you should click on that ad now and then to keep it coming back. I've taken to clicking on Road Kill T-shirts, Snorg Tees and another one that I can't remember which also features a hottie in a T-shirt. I'm not in the market for T-shirts, but I figure the ads are a nice decoration to the site --- better than Chinese characters or mortgage refi ads. I haven't seen the girl in the pink halter top, but I'll keep an eye out.

Darn it. All AdSense thinks I'm interested in are Dremel rotary tools and accessories.

-- Roger
 
Not that it's going to convince any of the clueless, but the argument that there's nothing for the rocket to push against works against the idea of a rocket motor not working in a vacuum. Imagine bouncing a ball against a wall. The ball bounces back and doesn't (appear to) move because it is anchored to the ground. Float the wall in a vacuum in space then bounce the ball against it. It will move and keep moving because it is not attached to the ground and there is no air for it to push against to slow it down.

A rocket motor works on the same principle. The ball, a particle of exhaust, is accelerated and hits a wall of the motor's chamber. If the chamber was closed, no motion would result, but we leave an opening on one end, the nozzle. Now the forces are not balanced and the balls, err particles, hitting the front of the chamber accelerate the rocket forward with no particles hitting in the opposite direction to counter the acceleration. Therefore, the rocket moves forward.

And that's just the half of it. The particles are accelerated through the nozzle and the same kind of "equal and opposite" force inside the nozzle adds to the acceleration of the rocket.

-- Roger
 
Darn it. All AdSense thinks I'm interested in are Dremel rotary tools and accessories.

-- Roger

Maybe you should try Googling Snorg Tees a few times. Go visit the site. Then maybe it will start showing up in your ads. Unfortunately, I think I may have overplayed this, and now I am getting ads with only the t-shirt graphic and no hottie inside the shirt. I'm not interested in a shirt...
 
The other thing is, I already have three rotary tools .....

-- Roger
 
The idiots on the other forum have based all their arguments on a complete misunderstanding of a few laws of physics.

For one they claim that the exhaust from a rocket gets sucked away into the vacuum of space so quickly there's no way that it could stay around long enough to propel the rocket.

Another misunderstanding is that the rocket has to push against the air to move much like a swimmer has to push against the water or a car had to push against the road.

When people try and point out the problems and correct these misunderstandings they refuse to listen and keep repeating false claims - over and over - and then act like they've clearly won the debate. There's no way to teach someone that truly has no interest in learning.
 
Thanks for the reply. I didn't phrase my question correctly. Assuming adequate intake, is the nozzle output more effective with the NOZZLE above or below the waterline?

I have never ridden a jet ski, so I honestly don't know this. On a jet ski traveling at high speed, is the nozzle above or below the waterline?


The correct answer is when on a plane, jet boats and jet skies nozzles are above the waterline. They develop more thrust in the air than they do in the water. My uncle had a jet boat,always scared the hell out of me with his driving!!
 
The correct answer is when on a plane, jet boats and jet skies nozzles are above the waterline. They develop more thrust in the air than they do in the water. My uncle had a jet boat,always scared the hell out of me with his driving!!

Interesting. Initially this is counterintuitive, to me it just "seems" like it should operate better in water with a medium to push against (and a careful review of replies to this post indicates I am not the only educated person that first thought this.). But the mass velocity product will indeed be greater with the nozzle forcing out the water "mass" against the lower pressure of air than water (same mass but will go faster in air than water.)
So I have already learned something.
George, I have no illusions that I am going to change anyone's minds, I am just in this for the entertainment value, and because it does force me to think some things through, and sometimes it doesn't hurt to think through concepts rather than just take it because it is what you were taught.
An experiment I saw on line is to take a water hose and turn it on so it is a jet acting like a force. Apparently when you put this jet under water the force decreases. By every argument of the "you have to have a force to push against" people, the opposite should be true. .
 
An experiment I saw on line is to take a water hose and turn it on so it is a jet acting like a force. Apparently when you put this jet under water the force decreases.

I was going to bring that up. It is something I learned as a kid, playing around with a water hose jet nozzle and a bucket of water. It had a LOT more force out of the water, than in it.

I wonder if any of those people actually experienced that in real life, if they would refuse to believe what they just saw or felt, or else try to come up with an alternative "scientific" explanation. Sadly, almost 99% sure they would find some way not to believe it or come up with some excuse.

Or else they might just drop that line and say well, that's water, in air, not rocket exhaust, in a vacuum, when jets of water was their own "proof" to begin with. That's what many Moon Hoaxers and conspiracy nuts do when presented with incontrovertible evidence to disprove their claims.

Also, BTW, in the 1970's, I did some dumb stuff with rockets which included a plastic toy dragster (Top Fuel type, long and skinny), using rocket power, being "launched" across a creek, like Evel Knievel over the Snake River Canyon. Anyway, it went unstable and landed in the water. It used an E5 engine that burned for nearly 5 seconds. After it hit the water, the car floated upside down, and the rocket thrust pushed it VERY slowly for several feet. Clearly the rocket thrust was horribly degraded by the water impeding the exhaust, it was not simply the water drag on the car itself.

- George Gassaway
 
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I was going to bring that up. It is something I learned as a kid, playing around with a water hose jet nozzle and a bucket of water. It had a LOT more force out of the water, than in it.



There you go! Excellent way to let one experience the difference. I experience that every time I wash my vehicles. Hold the hose about a foot from the nozzle fill the bucket a bit and then push the nozzle into the water. As soon as the nozzle hits the water it feels like the hose is actually being pulled in to the bucket do to the change.
 
While I do no doubt the fact at action/reaction is sufficient and certainly the dominant force in rocket propulsion, DOES the presence of air (mass) behind the rocket have any effect, positive or negative, in the degree of force generated (does is provide any additional "push"). One of the examples used in ClueForum was a jet ski. The suggestion was that when a jet ski jet is not in the water it produces no force.
To which the answer is "Gotcha!" :lol:

A jet ski doesn't work out of water because it sucks in water and then throws it out of the back. Likewise a jet engine won't work in vacuum because it sucks in air and then throws it out of the back. (Yes, I know, that's horribly simple. So are the people you're trying to convince. :D)

A rocket will work in a vacuum because it brings its own air with it. Likewise, a water rocket will work out of water because it brings its own water with it. Don't bother showing videos of jet skis, water rockets or anything else - your opponents may claim that they're videos and are obviously fake. Instead, point them at this water rocket or pretty well anything else which shows up if you go onto Amazon and search for "water rocket", and suggest that if they want to play science then they actually do some science, i.e. get one of these things and do an experiment to see for themselves whether a water rocket will work when it's not in the water.
 
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This reminds me of the endless string of people who couldn't fathom the Mythbusters conclusion that an airplane would still take off on a runway that was a giant conveyor belt spinning in the opposite direction as the plane tried to travel, matching the speed exactly.
 
I don't understand why you would bother with those morons.

Rockets don't work in a vacuum. Yeah. And the satellites that relay phone calls all around the world every day were thrown up there by Madison Bumgarner. The guy's got a Hell of an arm.
 
I don't understand why you would bother with those morons.

I think we all know that we're not going to change the minds of any of that forum's die-hards.

But, as I mentioned in the other thread, figuring out ways to explain why they are wrong is an interesting challenge and may help us to better understand the physics.

-- Roger
 
... Or else they might just drop that line and say well, that's water, in air, not rocket exhaust, in a vacuum, when jets of water was their own "proof" to begin with. That's what many Moon Hoaxers and conspiracy nuts do when presented with incontrovertible evidence to disprove their claims.
...
Too late. Several times in that thread the idiots say that the difference in density from water to air is (whatever number they use) but that the density difference from air to the near vacuum of space is (some much bigger number), and therefore any comparison to air and water is invalid when comparing air to near vacuum. And then they'll repeat the "proof" that expanding gas in a vacuum does no work according to some law of physics they've misunderstood and therefore a rocket can't work in space.

They've got all the wrong answers and will throw them at any logic that comes their way.
 
It's impossible to change a person's mind using logical/scientific argument when their starting position is not based on logic or science.
 
It's impossible to change a person's mind using logical/scientific argument when their starting position is not based on logic or science.

Maybe challenge them. Whittle a rifle stock butt down to a bullet-like point, hold it to their shoulder and fire that little particle out the other end. The rocket that is the rifle only has to work against one itty bitty particle o lead. No. This won't convince them of anything but they will run when they see you coming their way, and should be entertaining for the rest of us.
 
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