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I want to understand more about the upper limit of spaceshipip speed. Should not unbalanced forces as given by ion engines be independent of exhaust velocities? Also, what is theoretical limit of ion engines, of Bussard ramrockets?

The force produced by a rocket engine is CAUSED by the exhaust velocity - the center of mass of the rocket + exhaust must continue moving at constant velocity if no outside influences are involved (Newton's first law) and there is various formulas (look up the "rocket equation" in any mechanics text book) for how they relate. The result is that to get a rocket speed faster than the exhaust speed requires an exponential relation between the mass of the rocket and the mass of the exhaust, and becomes prohibitively expensive for faster final velocities. The ramrockets get around this by grabbing new fuel along the way, instead of carrying it with them from launch. If you can power your rocket with hydrogen fusion, there is plenty of hydrogen in interstellar space to keep you going.

Arthur Smith

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