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RE: starship-design: Bussard drive
Timothy et. al.,
There seems to be two separate issues here, one of which may be valid and
one is not.
First, Kyle is correct that when drag equals thrust, acceleration will stop.
Note that I said acceleration, you will still have to maintain thrust in
order to maintain velocity or drag will slow you down. Of, course this
actually applies to the scoop area and assumes that scoop area is a
constant, so I suppose you could "fold" the scoop and reduce drag. For that
matter, you could reduce the scoop area constantly to maintain a constant
acceleration irregardless of drag...remember, drag is a function of area and
so is scoop efficiency. If the scoop area decreases for a given amount of
power, then its efficiency will increase as a function of field density
while drag is decreasing at a proportional rate. The trick is to balance
scoop area against the interstellar hydrogen density to maintain a more or
less constant thrust. If engine ISP is also increased then the area of the
scoop can be increased accordingly. There will always be some "magic point"
where scoop area, hydrogen density, and engine ISP will maximize thrust
WITHOUT an upper limit on velocity.
Second, the upper limit on velocity IS NOT the exhaust velocity of the
engine, go back to your freshman physics book if you don't believe me, and I
don't care who you've been reading. With enough reaction mass, you can get
to light speed by THROWING tennis balls off the back of the Santa Maria!
The only real problems (if one forgets the low local density) here are scoop
design and efficiency, engine efficiency (ISP) and design lifetime. An
antimatter drive fed by onboard hydrogen initially and later by interstellar
hydrogen could approach ANY fraction of the speed of light and do it fairly
quickly. By sacrificing a bit of thrust, it can even replenish its onboard
fuel enroute for the deceleration run.