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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?

callum
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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 03 '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 03 '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 03 '12 at 19:47
  • QI does get things wrong - sometimes they admit their mistakes on later shows, other times they don't. :-) – Rory Alsop Jun 03 '12 at 19:50
  • The other possibility is that you just are not reporting the question correctly to us. – Raskolnikov Jun 03 '12 at 20:06
  • Have you considered the situation that acceleration and velocity are not in the same direction? And that direction of the rocket's nose is not in the direction of the velocity? There are plenty of various situations and in one of them maybe it is possible to get surprising results. I'd say that the question is a bit ambiguous. – Pygmalion Jun 03 '12 at 20:14
  • @RoryAlsop I'm dissapointed at BBC. Our small national television produced dozens of different quizzes, some even original ones and I can't recall they made more than two mistakes in last two decades. – Pygmalion Jun 03 '12 at 20:16
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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 03 '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 04 '12 at 04:22
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    I gave you a comment with the answer, and you could think about it yourself to see that it is right, but instead, you brought all sorts of obviously irrelevant confounding factors you just thought of, and instead of seeing they don't make a difference, you make it sound all complicated. This is the anti-scientific attitude that must be fought. You have to think "If gravity and rocket are pushing together, how can it go slower?" The answer is "it can't" and that's the end of it. Hence the (slight) harshness. You brought up the "stupid question" business, BTW. – Ron Maimon Jun 04 '12 at 04:26
  • For a little more information on what was in the minds of the TV producers see this discussion Apparently "flying downwards" was excluded because "rockets don't burn their engines in this situation". – twistor59 Jun 04 '12 at 07:56
  • @twistor59 Well... the discussion there is quite appalling. But yes, there are rockets that are normally fired downwards. – mmc Jun 04 '12 at 12:59
  • @RonMaimon: "In this case it takes less than a minute to see the answer." – good for you. But not everyone is so smart; some people have to ask others to explain things. I heard something on TV that confused me, so I signed up to a physics Q&A website and asked about it. You took the time to make me feel unwelcome and a bit stupid, which doesn't encourage enquiry. Do you participate here because you like to help people understand physics, or is there another reason? That's not a rhetorical question; please answer. – callum Jun 05 '12 at 13:43
  • @callum: Not good for me. Shame on you. Everyone is that smart. Any child can see the answer to this question in one second, and nobody needs someone else to explain this. The only reason that you are asking is because someone with authority told you something stupid, and instead of saying "look, another moron in power!" You said "Oh, this person must have some secret point." This shows a deference to authority which is appaling. There is no way that you thought about it independently and failed to see the answer. You just had dissonance, because "how could bigwigs be so stupid?" They can. – Ron Maimon Jun 05 '12 at 15:07
  • @RonMaimon: Wow. See my first comment: "It's just a TV quiz show, maybe they made a mistake." – callum Jun 05 '12 at 15:39
  • @callum: You need to think it through, not trust experts. Even if it were published with the most famous of expert authors, you sometimes have such an elementary error. For example, if you look among Einstein's 1905 papers you find such an error. The problem is no independent thinking and reliance on other people. One cannot rely on social mechanisms for weeding out nonsense, it doesn't work, because the social mechanisms are permanently broken. Why? Because in every human society in history, the comments I just made to you above would be socially unacceptable, and make me a pariah. – Ron Maimon Jun 05 '12 at 15:42
  • @RonMaimon If you are in a circular orbit thrusting upwards or downwards is thrusting at 90 degrees angle to your velocity. Cosine of 90 degrees is 0, so you are not changing the rockets energy and the energy of your fuel is being "wasted". Depending on what the wording of the question was I would say there is a good possibility that QI got it right. Edit: Also, sorry about the necromancy – Taemyr Mar 03 '15 at 14:49
  • @Taemyr: That was not the question, it's your personal nonsense stretched interpretation to make sense of the answer. One must never be charitable toward idiots. The assumption that the orbit is circular was never made here, and I am sure it was never made in the program. It is possible that your correct statement is what the idiots at the TV station misinterpreted to make their stupid question, but there is next to no chance that QI got it right, because, from experience, TV never gets anything scientific right, because they don't have this kind of open review, and TV is written by stoners. – Ron Maimon Mar 03 '15 at 21:49
  • @RonMaimon It is correct that the question is not "did QI get it right?". The correct answer to the question is "rockets accelerate fastest when the acceleration is paralell to the velocity". In several important instances this is horizontally. Thrusting at periapsis or apoapsis are perhaps better examples than a circular. – Taemyr Mar 04 '15 at 08:10
  • @Taemyr: No, no, no! This is both completely intuitively obvious, and not what they were talking about. They were asking about acceleration relative to the gravity pull, which way is best to accelerate, and the idiotic intuition they were exploiting is that the Earth's rotation is helping you in some way. Pay attention to the actual question, don't make up nonsense to make authority look less stupid than it always is. – Ron Maimon Mar 04 '15 at 11:37

4 Answers4

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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

hdhondt
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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.

Rory Alsop
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  • Only until it hits terminal velocity. And then its acceleration becomes 0. – Ignacio Vazquez-Abrams Jun 03 '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 03 '12 at 19:54
  • Sure. But in the meantime, it's accelerating faster in the other direction. – Ignacio Vazquez-Abrams Jun 03 '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 03 '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 03 '12 at 19:59
  • +1. I remember a number 11g is about the limit of what a pilot can survive. So 1g + or - is not a major part of acceleration for rockets, not talking about unpiloted rockets –  Jun 03 '12 at 21:56
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To move from one orbit to an orbit that is further out you want to add energy to your rocket. This is most efficiently done by adding thrust in the direction you are traveling.

Furthermore, in a Hohmann transfer you are making two burns. - When increasing your orbital radius this is one burn to raise your apoapsis, and one burn to raise your periapsis. The most efficient way to raise your apoapsis is to burn at periapsis, and the most efficient way to raise your periapsis is to burn at apoapsis. On both periapsis and apoapsis your trajectory is parallel to the ground, and so you get the highest efficiency when accelerating horizontally.

Taemyr
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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).

Coder404
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