[Physics] Why do rockets accelerate fastest horizontally

rocket-science

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

Best Answer

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