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Jairo
2010-Oct-29, 01:37 PM
Do you know any educative video showing that a rocket produces thrust in vacuum?

I'm searching for examples with baloons inside vacuum chambers, but I haven't found one until now.

grapes
2010-Oct-29, 01:46 PM
I'm searching for examples with baloons inside vacuum chambers, but I haven't found one until now.I doubt you'd find balloons in a vacuum chamber--the balloon would either burst or fill the chamber, probably.

What about the video of the liftoff from the moon surface?

AndreasJ
2010-Oct-29, 01:48 PM
If they don't believe elementary school physics, why'd they believe a video? Vaccum isn't visible.

George
2010-Oct-29, 02:29 PM
Do you know any educative video showing that a rocket produces thrust in vacuum?

Here (http://www.youtube.com/watch?v=cOdzhQS_MMw&NR=1&feature=fvwp) is the Apollo 17 LM (Ascent Stage) lift-off from the Moon.

slang
2010-Oct-29, 02:41 PM
Here (http://www.youtube.com/watch?v=cOdzhQS_MMw&NR=1&feature=fvwp) is the Apollo 17 LM (Ascent Stage) lift-off from the Moon.

Wanna bet that the person causing Jairo to ask this will claim that video is fake? :)

George
2010-Oct-29, 02:44 PM
Wanna bet that the person causing Jairo to ask this will claim that video is fake? :) Yeah, I probably used the worst example possible. :)

shadmere
2010-Oct-29, 03:48 PM
Would a basic illustration help at all?

From Wikipedia. (http://en.wikipedia.org/wiki/File:R%C3%BCckstoss1600.png)

Just explain how the burned propellant is pushing in every direction, but the only direction that isn't canceled out is "up." In an atmosphere, there is air that the escaping propellant has to push through, so it seems a rocket would actually work better in a vacuum. (Nothing to impede its escape.)

Isn't the definition of a rocket basically, "A jet engine that could potentially work in a vacuum"? Maybe explain that the fact that it works in a vacuum is pretty much the only reason that it's considered a rocket instead of a jet.

Strange
2010-Oct-29, 06:07 PM
I find it hard to understand why anyone would imagine it wouldn't work in a vacuum. If anything, I would expect it to be more efficient because there is less resistance to the exhaust gas (don't know if that is true or not). Presumably they assume it won't work because they don't want it to.

grapes
2010-Oct-29, 06:34 PM
I find it hard to understand why anyone would imagine it wouldn't work in a vacuum.Probably assume it won't burn. 'Course, that's why rocket fuels have their own oxidant.

shadmere
2010-Oct-29, 06:38 PM
This link here (http://www.straightdope.com/columns/read/1846/how-do-rockets-work-in-the-vacuum-of-space) (The Straight Dope) explains why rockets work in a vacuum, and also explains why someone might think otherwise.

They may believe that rockets work by "pushing" against something. Intuitively, it makes a bit of sense. You see a rocket taking off from a launchpad, you assume that the flame of the engine is pushing against the ground, and thus pushing the rocket into the air.

But that's not how rockets work. As shown by the Wiki image I linked to above, the thrust is created because of unequal forces inside the engine itself. There doesn't need to be anything there to "push" against.

Edit: I thought that was how rockets worked myself, when I was a kid. If you never researched it, and didn't pay much attention in science class, I guess there's no reason to realize otherwise.

NEOWatcher
2010-Oct-29, 06:59 PM
Edit: I thought that was how rockets worked myself, when I was a kid. If you never researched it, and didn't pay much attention in science class, I guess there's no reason to realize otherwise.
Same here, but I think it might be an issue of how it's taught.
We are all (hopefully) taught Newton's laws, but when I look back, all the demostrations had to do with someone pushing something against something else.
If it was the balloon example, then your mind just connected pushing the air against air while the rest of the balloon the air is just "stopped" against the balloon.

I'm not sure how I would stress the idea of shedding weight, but that's probably because I was never formally taught that.

Jairo
2010-Oct-29, 07:25 PM
I doubt you'd find balloons in a vacuum chamber--the balloon would either burst or fill the chamber, probably.

Actually, I found many videos showing ballons expanding in vacuum chambers. Whether it bursts or not, depends on how much air it had for starters. But that's all. Nobody releases them to move.


What about the video of the liftoff from the moon surface?

And, yeah, people said that just as an argument against the Moon landings. And they even say it's "because of the third Newton's Law". Should I post this question at the conspiracy board?

Strange
2010-Oct-29, 07:43 PM
This link here (http://www.straightdope.com/columns/read/1846/how-do-rockets-work-in-the-vacuum-of-space) (The Straight Dope) explains why rockets work in a vacuum, and also explains why someone might think otherwise.

Good explanation, with a practical experiment (although something makes me thing some people will claim they have been tricked, even if they do it themselves!). I liked this aside:

(If you think this is because the medicine ball pushed on the air, then try the experiment without the medicine ball--just push on the air with your hands, see how far you lurch backwards.)

George
2010-Oct-29, 08:00 PM
And, yeah, people said that just as an argument against the Moon landings. And they even say it's "because of the third Newton's Law". Well, if they accept Newton's 3rd law and they want something real basic and believable, perhaps this ice skate girl throwing bowling ball (http://www.youtube.com/watch?v=vPkkCOlGND4&feature=&p=AA60EF53A97F5082&index=0&playnext=1).

JohnD
2010-Oct-29, 09:11 PM
Jairo,

See another Wiki article, on the Manned Maneuvering Unit: http://en.wikipedia.org/wiki/Manned_Maneuvering_Unit
Used untethered, in orbit so difficult to fake in the studio, the MMU is a set of multidirectional rockets, that use pressurised nitrogen raher that a chemical reaction for thrust.
What a great demonstration of Newton's 3rd Law!

And here's Bruce McAndless 'flying' it: http://www.youtube.com/watch?v=jDqRLXg_tTc&feature=related
If rockets don';t work in a vacuum, how did he get out 300 feet from the Shuttle and back again?

JOhn

kleindoofy
2010-Oct-29, 10:16 PM
I can't remember exactly in which grade at school it was that I learned Newton's law "for every action there is an equal and opposite reaction," but I'm pretty sure it was a single digit grade. Perhaps I'm just over-estimating humanity, but I would think that anybody capable of typing a sentence on the internet would have heard of that law at some point in their life.

So why on Earth would anybody doubt the effectiveness of a rocket anywhere, vacuum or none?

Or do they think that the rocket exhaust has to push against something?

Strange
2010-Oct-29, 10:18 PM
Or do they think that the rocket exhaust has to push against something?

Apparently. I don't remember ever thinking that. But then again, I don't remember much!

pzkpfw
2010-Oct-29, 10:21 PM
And, yeah, people said that just as an argument against the Moon landings. And they even say it's "because of the third Newton's Law". Should I post this question at the conspiracy board?

Do these same people discount:
GPS, weather and communication satellites?
Probes to Mars and the other planets?
Missions to the Moon by other (i.e. non U.S., non Russian) nations recently?

....not just the getting of these things to where we want them, but the manouvering of them (i.e. how they turn).

Van Rijn
2010-Oct-29, 10:25 PM
And, yeah, people said that just as an argument against the Moon landings.


Are they also arguing against all satellites, all space missions? If not, how do they think spacecraft maneuver?

ETA: Heh. Cross posted with pzkpfw, I see.

kleindoofy
2010-Oct-29, 10:26 PM
Do these same people discount: ... Probes to Mars and the other planets? ...
I would be funny if some woo who believes that the "face" on Mars was made by ET's were to deny that any man-made probe ever went to Mars.

No probe, no picture of the "face." :think:

slang
2010-Oct-30, 12:18 AM
Should I post this question at the conspiracy board?

Nah, why? It is a good question, and a common misconception that rockets push, not just among HB's.

Van Rijn
2010-Oct-30, 12:33 AM
Nah, why? It is a good question, and a common misconception that rockets push, not just among HB's.

And there's the famous case of the New York Times claim based on that misconception, and their later retraction, discussed here:

http://www.popsci.com/military-aviation-amp-space/article/2009-07/new-york-times-nasa-youre-right-rockets-do-work-space


JULY 17, 1969: On Jan. 13, 1920, Topics of The Times, an editorial-page feature of The New York Times, dismissed the notion that a rocket could function in a vacuum and commented on the ideas of Robert H. Goddard, the rocket pioneer, as follows.

"That Professor Goddard, with his 'chair' in Clark College and the countenancing of the Smithsonian Institution, does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react -- to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools."

Further investigation and experimentation have confirmed the findings of Isaac Newton in the 17th century and it is now definitely established that a rocket can function in a vacuum as well as in an atmosphere. The Times regrets the error."

Heh.

speedfreek
2010-Oct-30, 09:18 AM
I blame the original "Thunderbirds" TV series for inadvertently introducing the misconception that a rocket needs something "solid" to push against. As a child, I was convinced that when Thunderbird 2 took off, its rockets were pushing against that concrete flap that lifted up behind it, rather than it being a device to make sure no tell-tale burn marks were left on the concrete.

http://farm4.static.flickr.com/3391/3328459394_f68850d925.jpg