Re: starship-design: LINAC efficiency

[Steve VanDevender <stevev@efn.org>]

bugzapper writes:
 > All right, Steve, I'll get to work on the numbers for a 1 g scenario, but I
 > don't feel limited to using those technologies (ramscoops and fusion) you've
 > indicated you don't like. Anything that works is fair. I already agreed
 > ramscoop drag, as you pointed out, is a major factor restricting its use,
 > particularly approaching relativistic velocity. I did mention this drag
 > factor is a positive contribution during deceleration phase. Like a fusion
 > generator, I think I can show that a ram scoop is better to have, than to
 > lack.

My point is not so much that ramscoops are bad as that ramscoops
probably cannot do the things you want them to do, in particular sustain
1 g acceleration to high relativistic speeds.

And I agree that using a ramscoop field to brake may actually be very
useful, particularly if combined with an external source of thrust in
the origin system to help boost the ship up to speed.

 > As I pointed out in my last post, none of us know the local interstellar gas
 > density. To recognize that, is better than to think we know it, when we
 > don't. I shared all the most recent publications I noticed, which favor a
 > higher density figure, than the ones which have been taken as rendering
 > ramscoops unfeasible.

My guess is that the higher density estimates you're citing are probably
too optimistic.  Unfortunately my copy of _Astrophysical Formulae_
doesn't clearly indicate the observational bounds on the density of
interstellar hydrogen (although it has lots of nice complicated formulae
on how to compute column density of interstellar gas from observed
emission and absorption spectra).  If I get a chance I'll see if I can
chase down some of the references it has to other publications that
might have more definite statements.  Whatever the local interstellar
medium density is, it can be confirmed by observational measurements.

 > Kelly, the silicate dust excited Dr. Landgraf, specifically because it marks
 > gas clouds elsewhere. Seeing it here, probably means we're in a gas cloud,
 > just like the others seen previously. Being in a gas cloud means ram scoops
 > deserve a closer look.

Given the other limits of ramscoops, being in a higher-density region of
the galaxy probably would only mean we can get the ship up to the fairly
low velocity limit of the ramscoop sooner.  Interestingly, in some of
the more credible hard science fiction novels I've read that have
slower-than-light universes, the authors have had ramscoop ships that
reach about .3 to .4 c at their limits of effectiveness.  I'd be curious
what research they did to choose that range of limit velocities.