Why Rocketry Doesn't Work in the Vacuum

[jadebox]

I realize that I started it with my "clueless" remark, but I don't think this is the place to insult people even if they aren't participants in the forum (and even though their understanding of basic physics is lacking).

It might be more interesting to consider how you might explain, for example to a child, how a rocket works in the vacuum of space? Of course, there are many examples of rockets working in vacuums in space and on earth which the participants in that thread dismiss, so just stating the obvious, that rockets do work in space, isn't enough.

-- Roger

 

[GDJ]

The Stupids are more dangerous than The Crazies.

Yeah, you can (kind of) reason with a crazy person. Stupid is just.....well......just tell them to back to the sandbox.

 

[Sully]

I realize that I started it with my "clueless" remark, but I don't think this is the place to insult people even if they aren't participants in the forum (and even though their understanding of basic physics is lacking).

It might be more interesting to consider how you might explain, for example to a child, how a rocket works in the vacuum of space? Of course, there are many examples of rockets working in vacuums in space and on earth which the participants in that thread dismiss, so just stating the obvious, that rockets do work in space, isn't enough.

-- Roger

This is true. The premise they are working with is that all space activity is fictional/hoax, and they cite the rocket-in-a-vacuum argument to support that. Producing more and more evidence of our activity in space only serves to demonstrate (to the paranoid) how extensive the hoax is.

If I am sitting next to someone who says "but how does it work? I remember a little high school physics and it seems like it shouldn't work..." telling them that they're an idiot and showing them a photo of Earth rising over the lunar surface isn't going to be very effective or helpful. (the idiot part may be true, but still not helpful).

There is a nice explanation here:
https://www.straightdope.com/columns/read/1846/how-do-rockets-work-in-the-vacuum-of-space

Uncle Cecil gives the example of throwing a medicine ball while standing on ice. The backward acceleration you experience changes depending on the size/weight of the ball, so the air resistance isn't relevant. He also suggests pushing against the air directly with your hands to see if that works (which it would if "pushing against air" was the reason for acceleration).

It seems obvious to me that the expanding gasses in a rocket have plenty of things to "push against" aside from the vacuum of space. Why this isn't obvious to the doubters I am not sure, but if you want to have a dialog, you have to change the terms to something they will understand better.

Sure, some people don't want to be convinced and aren't worth the time (in my eyes, these are the "crazy" ones). But providing clear explanations limits the influence they have over others who are simply underinformed.

 

[JPVegh]

Launching a rocket in a vacuum is a bad idea, it might ignite the dust in the bag and set the whole house on fire.

 

[shrox]

A vacuum is cold, therefore rockets can't work in a vacuum because it absorbs all the heat. Sounds right...nitrogen gas thrusters are just a hoax.

 

[GDJ]

Besides, this question was answered in Isaac Newton's book Principia Mathematica, published in 1673. That's 340 years ago. A hell of a long time to keep a mass conspiracy going.

 

[Sully]

Launching a rocket in a vacuum is a bad idea, it might ignite the dust in the bag and set the whole house on fire.

Shhh..... Don't give away my oddroc plans for LDRS 34!

 

[gpoehlein]

I realize that I started it with my "clueless" remark, but I don't think this is the place to insult people even if they aren't participants in the forum (and even though their understanding of basic physics is lacking).

It might be more interesting to consider how you might explain, for example to a child, how a rocket works in the vacuum of space? Of course, there are many examples of rockets working in vacuums in space and on earth which the participants in that thread dismiss, so just stating the obvious, that rockets do work in space, isn't enough.

-- Roger

You are correct, Roger, and I apologize to anyone who was offended by my "gene pool" comment - it was snarky and beneath me. I shall endeavor not to do so again.

 

[billspad]

I realize that I started it with my "clueless" remark, but I don't think this is the place to insult people even if they aren't participants in the forum (and even though their understanding of basic physics is lacking).

It might be more interesting to consider how you might explain, for example to a child, how a rocket works in the vacuum of space? Of course, there are many examples of rockets working in vacuums in space and on earth which the participants in that thread dismiss, so just stating the obvious, that rockets do work in space, isn't enough.

-- Roger

Didn't you ever wonder if these people are for real or if they just put this stuff out there just to watch the reaction?

 

[jadebox]

Didn't you ever wonder if these people are for real or if they just put this stuff out there just to watch the reaction?

It's hard to tell the trolls from the true believers on the 'net. I'm sure that some of it is just trolling for attention.

-- Roger

 

[billspad]

It's hard to tell the trolls from the true believers on the 'net. I'm sure that some of it is just trolling for attention.

-- Roger

I find the trolls who are playing us sort of amusing. The others scare me.

 

[shrox]

I find the trolls who are playing us sort of amusing. The others scare me.

Nothing scares me, except The Blob!

 

[GDJ]

You owe me a scream Shrox.

 

[cerving]

I think the whole thing is a troll... kinda like the Flat Earth Society guys that get together every year for a big party. Ditto for the "apocalyse of the moment" guys. (Remember the Mayan end of the world?) Looks like they found plenty here willing to take the bait...

It's hard to tell the trolls from the true believers on the 'net. I'm sure that some of it is just trolling for attention.

-- Roger

 

[shrox]

You owe me a scream Shrox.

No, The Blob!

 

[GDJ]

Oh, THAT blob. Sorry.

 

[BABAR]

I think the author is confusing the difference between a vacuum and just plain sucking.

“Some people are like a Slinky. They’re not really good for much; but they do bring a smile to your face when you push them down a flight of stairs”.

Nobody is completely useless. They can always serve as a bad example.

 

[powderburner]

.....I don't think this is the place to insult people even if they aren't participants in the forum (and even though their understanding of basic physics is lacking).

I know it is not "nice" to call people names----even if they really are that stupid. And I have had many conversations along the same lines as the original post, when people find out that I am an aerospace engineer, and they start asking questions. It is amazing and frightening at the widespread lack of understanding of science and physics. It's like they never ever went to school, never heard of Newton. Sine? Cosine? Oooh that hurts my head. Just forget about asking them to solve for x.

My own father-in-law is a brilliant Bible scholar, knows every word of that book, and can intelligently discuss any aspect of any verse. Yet, I was once forced to just about yell at him when trying to convince him that he should not follow an invitation to invest in a business that was trying to sell perpetual motion machines.

I still think you can almost make a case for name-calling for the ones who refuse to learn, who stubbornly cling to their witchcraft-based beliefs, who seem to enjoy pulling others down with them.

 

[Mushtang]

Many years ago my job took me and a co-worker to a Naval Air Station where we were able to look at a display of a jet engine that had been cut away showing all the interior parts and surfaces. I told him I was amazed that the engine bell was able to withstand the force that it takes to push the plane with that much acceleration.

He said it doesn't, the acceleration comes from the blast out the back of the jet.

I tried to explain that while that was mostly true, in order for the blast to leave the engine backwards it first had to push on the back of the engine (moving the engine forward), and once the gas leaves the engine it does nothing to help the engine to forward. Sort of like the guy sitting on the ice that throws a rock forward so that he can slide backwards - the rock pushes the guy on the palm of his hand to get him to move. Once the rock leaves his hand the rock no longer helps the guy slide.

But my attempts at communicating that idea were apparently done in a bad way, because he never did buy into the fact that the expanding gas pushes the engine forward. He left there saying that the engine moves because of the blast of fire behind the engine, everyone knows that, and I must be an idiot not to realize it.

 

[powderburner]

He left there saying that the engine moves because of the blast of fire behind the engine, everyone knows that, and I must be an idiot not to realize it.

I have tried to explain conservation of momentum (how much more simple can you get than MxV = MxV ?) and all I ever get is glazed-over-eyeballs.

 

[kjmccarx]

Indeed, folks: we are living on the bottom of a huge ocean of air

According to him a rocket will fly farther if we launch it from inside a black hole.

 

[rbelknap]

I'm resurrecting this old thread, because I found this over on the discovery science website: https://www.discovery.com/tv-shows/mythbusters/videos/its-rocket-science/ and it made me think of this old discussion

 

[Rex R]

somebody is bound to point out that, their starting vacuum isn't as good as space and or that once the motor fired it was no longer a vacuum.
Rex

 

[boomtube-mk2]

I don’t know why you all keep referring to the “Vacuum of Space”.

Obviously space is not a vacuum or it would suck all of the atmosphere off of the Earth.

 

[Mushtang]

I'm resurrecting this old thread, because I found this over on the discovery science website: https://www.discovery.com/tv-shows/mythbusters/videos/its-rocket-science/ and it made me think of this old discussion

I LOVE the show Mythbusters!! It's very entertaining. But they are so often very far from doing actual science that it surprises me. Some of their jumps in logic are ridiculous and they often come to conclusions that they've proved something when they've actually done no such thing. This is one of those times.

 

[jmattingly13]

Oi! The original article hurts to read...

 

[luke strawwalker]

I LOVE the show Mythbusters!! It's very entertaining. But they are so often very far from doing actual science that it surprises me. Some of their jumps in logic are ridiculous and they often come to conclusions that they've proved something when they've actually done no such thing. This is one of those times.

That's why I don't watch it much. Sometimes it's neat, then they do something incredibly stupid like the Chinese flying rocket chair. Instead of just using a cluster of "off the shelf" motors or a single large commercially available rocket motor, they go through some convoluted crap-shoot custom motor design that just LOOKING at it you know it won't work... then when the thing fails UTTERLY due to their cobbled-together rocket motor burning through and basically catoing, they declare the myth 'busted', because it didn't work... nevermind that it failed because of their crummy rocket motor and failed setup of the experiment.

Oh well... least common denominator. Blow $h!t up= better ratings... and ratings is what its all about anyway...

Later! OL JR

 

[maskedwarrior]

Hi Rocketry Forum.

Please don't think I'm trying to be at all divisive here... I'm not a flat-earther, nor do I sport a tinfoil hat. I'm just a normal guy who's getting bamboozled looking for a simple, straightforward answer to what is, doubtless, a fundamentally very easy thing to put me on the straight and narrow about!

I've seen the thread that you're all referring to. And yes, there's a lot of cod science from a mixture of people with, shall we say, 'varying degrees of scientific understanding'. However a couple of points made are bugging me, because I can't find much material online to put them in any context!! For a layman such as I, it would be really great to get your feedback on this. To help me understand these concepts better, if nothing else.

One of the main points made by the original poster on Clues Forum relates to the principle of Free Expansion of gas and how this appears to disagree with Newton's Third Law - the usual explanation given for rocket-powered propulsion in space. Apparently, if I understand correctly, the unique properties of a gas in a vacuum cause it to disperse almost instantly, with no transfer of energy. And the assertion made, in the thread, is that this invalidates the 'bowling ball chucked off a skateboard', Newtonian explanation ie. because gas could not have a force greater than 0 in a vacuum due to Free Expansion (as it could not hope to be accelerated, rather snatched away far more quickly than it could possibly be propelled by the rocket) Therefore, states the original thread, since there can certainly be no 'pushing' or other external force involved in propelling a rocket in the vacuum of space, the equation F1 = -F2 would have to read 0 = -0. Hence, a stationary rocket.

The following independent links were cited on the original thread, including a link relating to the speed with which gas travels in a vacuum. Could you please take a quick look and fill me in on where the original poster's reasoning is faulty and/or confused? This would really help me. The web really needs a simple, straightforward breakdown of this problem to avoid anymore wasted time. Thanks a lot.

https://www.etomica.org/app/modules/sites/JouleThomson/Background2.html

https://www.physicsforums.com/threads/expansion-of-ideal-gas-into-a-vacuum.151250/

https://pubs.acs.org/doi/abs/10.1021/j150043a002?journalCode=jpchax.2

https://prettygoodphysics.wikispaces.com/file/view/expansionIntoVacuum.pdf

https://adsabs.harvard.edu/full/1953ZA.....33..251K

And also, one small question about a post on this very thread, that may indeed not have been intended seriously as no one challenged it (or maybe I'm more ignorant than I thought!):

I don’t know why you all keep referring to the “Vacuum of Space”.

Obviously space is not a vacuum or it would suck all of the atmosphere off of the Earth.

I just wondered whether this is technically true in some way I don't appreciate?

Thanks so much for having me on your forum.
Tony

 

[jadebox]

Rocket motors have a nozzle which forces the gas to accelerate. So there is no free expansion of the gas.

The thing about the vacuum of space sucking away the earth's atmosphere is based on a misunderstanding. A vacuum doesn't "suck." Using a vacuum cleaner as an example, a lower pressure is created in the hose and the outside air pressure pushes stuff into the hose. Earth's atmosphere is held to the earth by gravity. There's nothing trying to suck it away.

-- Roger

 

[Mushtang]

Firing a gun on earth results in a recoil in the opposite direction.
Firing a gun in space would do the same thing.
The bullet and expanding gas behind it don't leave the gun (and therefore are not subjected to Free Expansion) until well after putting pressure on the gun in the opposite direction (the recoil). As soon as the bullet leaves the shell inside the gun there is recoil and the bullet leaves the barrel a short time later.

In a rocket, the expanding gas produces recoil on the rocket inside the combustion chamber and the exhaust gas leaves through the nozzle. The recoil happens inside the chamber as well as in the engine bell as it's leaving.

The nuts that don't believe rockets work in space will believe zero of this, so telling them is wasting your time.