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About 50 years ago, the New York Times retracted an unsigned editorial from 1920, in which it was argued that rockets cannot work in vacuum. See this blog, or wikipedia. (alas, I was not able to access the full 1920 article)

Naturally, people who argued thus were wrong, rockets do work in vacuum.

What incorrect model of rocket operation did these people have in their heads, that lead them to this incorrect prediction? Did they think conservation of momentum does not hold in vacuum? Did they think the propellant explosion would impart downward momentum on the surrounding air, without carrying downward momentum itself?

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    $\begingroup$ From the wikipedia article you reference "The basis of that criticism was the then-common belief that thrust was produced by the rocket exhaust pushing against the atmosphere; Goddard realized that Newton's third law (reaction) was the actual principle. Unbeknownst to the Times, thrust is possible in a vacuum, as the writer would have discovered had he read Goddard's paper" $\endgroup$ – JCRM Aug 9 at 11:05
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    $\begingroup$ These people still exist and show up on this site occasionally. They think that rocket plumes have to push on something to work. $\endgroup$ – Organic Marble Aug 9 at 11:09
  • $\begingroup$ "Pushing" is a momentum exchange, from the exhaust to the air. However much downward momentum the air gains, the exhaust lost that much. So wikipedia's "explanation" is really just "they didn't understand conservation of momentum". $\endgroup$ – Wouter Aug 9 at 11:11
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    $\begingroup$ From the source linked in the Wikipedia article "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. " so yes, they didn't understand the conservation of momentum. $\endgroup$ – JCRM Aug 9 at 13:44
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People think that rockets push on the air, and that without air there is nothing to push on.

This is a very common misconception I think: people with physics or other science backgrounds (like me) have spent a long time more-or-less unconsciously training ourselves not to think like this intuitively, to the extent where we may end up thinking that people who do think like this are somehow stupid. But that's wrong: the way things actually do work is not at all intuitively evident to humans I think, and it requires a lot of training to develop the right intuitions.

As an example consider Aristotle's physics: he thought, among other things that among the fundamendal laws of physics were things such as:

  • heavier objects fall faster;
  • the natural state of things is rest (everything will slow down to rest);
  • things don't move unless you push them.

Well, these are just hopelessly wrong as basic laws of physics, of course and someone with a modern physics background is going to laugh at them. But Aristotle was not stupid, clearly: he just hadn't developed the right intuition, because the experiments he could do were limited by being able to do them only in very specific circumstances (and he may have had funny-by-modern-standards attitudes to experimental evidence as well: I'm not sure about that). Aristotle's intuitions about how physics works were, simply, wrong. But it's also wrong to sneer at him for this, because it turns out it takes a lot of time to develop intuitions which either are not wrong or are at least less wrong (can anyone really develop intuitions about how QM works for instance: I don't know, but famous physicists have claimed they can't). And once you have developed the less-wrong intuitions it's then hard to remember that these were things you had to learn, simply because they are now intuitions.

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    $\begingroup$ Aristotle wasn't simply "not stupid", he make accurate and correct observations about the way the world really works: ● Drop a feather and a rock; which falls faster? ● Look at anything on Earth that's moving; does it continue even though nothing is pushing it? ● Look at anything that's still; does it ever move without being pushed? Newton derived laws of motion that required complicated additions to explain why the real world didn't follow those laws (e.g. friction). To the common man, the non-scientist or non-engineer, Aristotle's laws describe the world far more correctly than Newton's. $\endgroup$ – Ray Butterworth Aug 9 at 15:53
  • $\begingroup$ A modern physicist would not laugh, because Aristotle’s laws correctly describe what happens in presence of friction and drag, which is ubiquitous in Earth surface conditions. $\endgroup$ – Neith Aug 9 at 16:01
  • $\begingroup$ @Neith: I've clarified that a modern physicist would deride these as proposed laws of physics because they're so far from correct. But more generally my whole point was that the intuitions we develop are actually very counterintuitive: I was not trying to be rude about Aristotle. $\endgroup$ – tfb Aug 9 at 16:06
  • $\begingroup$ @RayButterworth: but, drop a rock and a heavier rock, and keep doing that with heavier rocks, and you find something interesting (and your name is Galileo perhaps). Galileo wasn't necessarily smarter than Aristotle, but he did do experiments Aristotle could perfectly well have done and found out Aristotle was wrong. $\endgroup$ – tfb Aug 9 at 16:20
  • $\begingroup$ For clarity I would suggest going with the angle that the rules aren't outright wrong, but rather his rules were woefully incomplete and not clearly covering all the factors involved. [All the stated rules are true, they're just missing an * with (conditions apply) to go with them.] $\endgroup$ – TheLuckless Aug 9 at 16:35

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