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Re: starship-design: LINAC efficiency

Johnny Thunderbird writes:
 > Having a hungry linac may be fun and exciting, but we will need to figure
 > out what to feed it. I favor the ram scoop method pioneered by Bussard.

There's a problem with ramscoops.  Yes, you can sweep up interstellar
hydrogen and fuse it for thrust.  But once your ship is at speed,
sweeping up that hydrogen induces drag.  If you're trying to put that
hydrogen into a fusion reactor, you have to bring it up to the speed of
the ship to get it in there; eventually, the thrust you get from the
hydrogen only matches the drag of the ramscoop.

In the case where you have to bring the hydrogen fully up to the speed
of the ship before you fuse and expel it, it's expected that the ship
will top out at about 0.1 c.  The incoming stream of hydrogen moving at
0.1 c relative to your ship is exactly balanced by an equal mass of
outgoing fusion products moving at 0.1 c.  At that point you may as well
shut down the ramscoop and coast.

It's not physically impossible that you might be able to use a ramscoop
to gather hydrogen and "pinch" it into a concentrated stream without
slowing it down appreciably, and then induce it to fuse, _and_ then be
able to get the fusion exhaust to interact with the ship such that it
actually induces thrust.  That might be able to squeeze a couple more
tenths of a c more out of a ramscoop drive, but the problem of confining
and inducing fusion in a relativistic stream of hydrogen is non-trivial.

I can see how Lee would be tempted by the idea of "reactionless" drives,
but conservation of momentum and energy seem to be rather fundamental to
physics as we know it, and probably even as we don't yet know it.  Most
likely a "reactionless" drive would just produce some exotic reaction
product; Larry Niven's fictional reactionless drive that turns energy
into a coherent stream of neutrinos, or other hypothetical drives that
produced photons or gravity waves (which also carry momentum) would
really still be reaction drives.

Also, no matter what drive you use, in order to get to high relativistic
speeds you need some extremely copious source of energy, whether it be a
huge mass of hydrogen or a smaller, but still quite impressive, quantity
of matter and antimatter.  Getting a ship up to 0.7 c or so requires
about as much energy as would be obtained by converting an amount of
mass equal to the ship's own mass entirely to energy.  That's why
interstellar travel at high relativistic speeds is currently considered
to be a big problem.  No matter what your drive system, you need quite a
bit more fuel than ship -- and if your power source is as relatively
inefficient (compared to the ideal of total mass-energy conversion) as
fusion, you need a _lot_ more fuel and reaction mass than final payload
mass, on the order of a million-to-one ratio.