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I've heard that rockets accelerate fastest when travelling horizontally to the ground, not downwards or upwards. Is that true, and why?

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All else being equal, the rocket accelerates faster downward, because gravity helps. So you heard wrong. This is obvious, no matter what anybody says. I think this question is too trivial--- it is only nontrivial because somebody said something stupid (namely that horizontal is faster than downward), and a moment's thinking shows this is wrong. Don't listen to people. –  Ron Maimon Jun 3 '12 at 18:46
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It was a question on the BBC panel show 'QI' hosted by Stephen Fry. Someone said "downwards" but the correct answer was horizontal. It's just a TV quiz show, maybe they made a mistake. But I don't think it's fair to say "a moment's thinking shows it's wrong". E.g. maybe gravity doesn't apply if you're already going faster than terminal velocity by your own propulsion, so it doesn't increase your rate of acceleration, and maybe the air pressure behind the rocket when falling doesn't provide as much push. I'm just speculating; I don't know. But my point is, don't dismiss it as a stupid question. –  callum Jun 3 '12 at 19:28
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Television is stupid, and QI got it wrong. The correct answer is downwards. They made a mistake. A moment's thinking shows that it is wrong, and the person who said "downward" did this moment's thinking. The stuff you say after your first sentences is nonsense, and it is a stupid question. The answer is "the TV people got it wrong", and that's that. There's no confounding factor. –  Ron Maimon Jun 3 '12 at 19:47
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@RonMaimon Your attitude is a bit harsh. I was just speculating to demonstrate that it's not necessarily obvious to everyone that it would be downwards. I said I didn't know. It's OK that you think the answer is very obviously downwards, and to tell me that. But there's no need say my speculation is "nonsense" and my question is "stupid". Relax. –  callum Jun 3 '12 at 20:35
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@callum: TV makes mistakes, it's not internet. I get annoyed when somebody says something right, and then expert say "no, counterintuitively, the obvious thing you said is not right!" This is how Earth-centered cosmology survived for thousands of years--- thousands of students saying "I think the Earth rotates" and the expert saying "Although it looks like it, it's not so!" When it is so. This is the great enemy of science--- people saying "it's more complicated than that". Just because some expert says something doesn't make it so. In this case it takes less than a minute to see the answer. –  Ron Maimon Jun 4 '12 at 4:22
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3 Answers

Pretty straightforward, really:

  • Accelerating upwards, the rocket can accelerate at T-g, where T is thrust, and g is the acceleration due to Earth's gravity.
  • Accelerating downwards, the rocket can accelerate at T+g
  • Accelerating horizontally, the acceleration will be T

So you can see that it will accelerate fastest downwards.

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Only until it hits terminal velocity. And then its acceleration becomes 0. –  Ignacio Vazquez-Abrams Jun 3 '12 at 19:52
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But you will also hit terminal velocity in the other directions as well. It will just take longer as acceleration is lower. –  Rory Alsop Jun 3 '12 at 19:54
    
Sure. But in the meantime, it's accelerating faster in the other direction. –  Ignacio Vazquez-Abrams Jun 3 '12 at 19:55
    
But surely you need to take the most obvious case - which is from zero to x seconds. If you take too long, the rocket will have hit the ground (as per your answer) and as that will happen relatively rapidly, terminal velocity is not really going to become a major factor here. –  Rory Alsop Jun 3 '12 at 19:57
    
Which is why I worded my answer the way I did. And somehow I still managed to get a downvote. –  Ignacio Vazquez-Abrams Jun 3 '12 at 19:59
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Less Air Resistance, and there is gravity which prevents not only prevents the rocket from moving quickly but also slows it at down (at about 9.8 meters per second squared).

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Rory Alsop explained why the idea is wrong, but it may originated from the following reasoning.

When a space rocket takes of, it does so vertically. At that time it is fully loaded with fuel and hence its acceleration is slow. When you watch a video of a space rocket take-off, it seems to crawl along the launch tower.

However, in order to achieve orbit, the rocket has to travel 7 km/sec horizontally. To achieve that, after a while the rocket's path starts to curve towards the horizontal. At that point the first stage may already have dropped off and a large amount of fuel has been burned, so the rocket is a lot lighter. Because the acceleration is inversely proportional to the mass the rocket will be accelerating significantly faster at that point. At the same time, because the rocket is now fairly high up, the air pressure has dropped significantly, and the reduced drag also increases acceleration.

Hence, the rocket accelerates faster when it is going horizontally. Somebody could then take that as meaning "faster than upwards as well as downwards"

Edit

Another issue is the is the "dynamic pressure" which is created by the speed and air-drag. Because of this, the engines may not be run at full power until past the "max-q" point. In the case of the Shuttle, the main engines ran at 65% for the first minute or so of the flight. Only then was it throttled up to 100%, increasing acceleration. See http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml

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