Quote:
Originally Posted by mort
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So what are you measuring the velocity of exhaust flow by? How fast the exhaust is flowing compared to a planet hundreds of thousands of kilometers away, or how fast it is flowing compared to the nozzle it is coming out of?
Because that exhaust will always be moving thousands of miles per hour relative to
something. And it won't matter. What matters is how fast it is moving relative to the spacecraft. Period.
Let's make this simple. You're inside a bullet train moving at 200 mph. Does it take more effort to run towards the front of the train or the back? Because that's what you're doing firing a rocket while moving through space. There's no wind resistance, no drag, no nothing to make you use more force to accelerate along any vector.
Or, if you want a long-winded explanation:
HowStuffWorks "What would happen if you fired a gun on a train moving as fast as a bullet?"