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Spaceship speed
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Question:
Aside from the theory of Special Relativity, is there a theoretical
limit on velocity for spaceships powered by ion engines. I think there is but
I do not understand why because unbalanced forces should always create
accelerations.
Replies:
They do. The limiting problem is that you run out of fuel, unless
you are talking about the famous (and hypothetical) Bussard ramrocket.
The limits you may be thinking about are the basic practical limits to
your final rocket velocity, beyond your more mundane engineering
limits relating to overcoming internal friction in your motor,
powering life support, inefficiencies in your control of the fuel
flow, and faulty O-ring gaskets... These limits are related to the
exhaust velocity of the rocket and the ratio of the fueled to empty
rocket weight. You can get to higher speeds in principle by (a) using
a higher velocity exhaust (this shoves more momentum out back for
every gram of fuel you use up), which is the principal advantage of
ion rockets (which use electric fields to accelerate heavy ions out
back) over chemical rockets; or (b) getting a higher fueled/empty
ratio by either building real thin rockets or multiple stage rockets,
so you throw away non-fuel mass when you are done with it. I think
Robert Goddard was one of the first to really exploit the latter idea,
and of course it dominates modern rocketry.
If you use a solar sail and power your vehicle by light pressure,
there is no upper limit (besides c) on your velocity, but you will run
out of the solar system before you get going too fast.
christopher grayce
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Update: June 2012
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