Spaceship speed #2
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.
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Update: June 2012