starship-design: Lots O Stuff...

[Steve VanDevender <stevev@efn.org>]

Ken Wharton writes:
 > What IÕm thinking of here

I've noticed, Ken, that whatever editor you're using doesn't insert
normal ASCII apostrophes (') but instead some funny character that
doesn't show up as an apostrophe on other displays.  You might want to
check your editor settings.

 > Adding this structural matter to 
 > the equivalent amount of regular hydrogen matter that we need for fuel, 
 > and you start to see why I was talking about a ŌminimumĶ 
 > matter/antimatter ratio.

Calculations that Timothy and I did some time ago indicated that to
reach high relativistic speeds (above 0.8 c) with a ship that carries
its own fuel, you need to convert over four times as much mass to energy
as you have payload mass, and the 4:1 fuel:payload ratio is only if you
use matter/antimatter for fuel.

 > On the flywheel subject, Timothy writes:
 > 
 > >I've thought of a flywheel before, but never mentioned it because it
 > >seemed too unpractical:
 > 
 > >- Its estimated weight was too high.
 > >- Containing/guiding a huge fly wheel that rotates with super high
 > >  velocities seemed almost infeasable.
 > 
 > >>So already, for a 200 meter radius flywheel,
 > >>we're talking about an energy storage of 10^10 J/Kg.  We'd probably
 > >>need at least 10^14 J/Kg to make a decent spaceship.
 > 
 > >Actually I think that 1E16 J/kg is the absolute minimum if you want to
 > >get into the direction of relativistic velocities.

Since 1 kg of matter converts to 9E16 J, you're still not getting much
energy density out of that relativistic flywheel.  The flywheel could
barely accelerate itself to relativistic speeds, so getting a good speed
for the ship that carries it is even more unlikely.

 > I suppose a 20-kilometer flywheel is out of the question.  Anyone know 
 > how strong perfect diamond is, anyway? Or what about a two-stage engine; 
 > a flywheel to get the ship moving fast enough where a ramjet-type drive 
 > could take over.  I know you guys have talked about ramjets, but IÕm not 
 > sure what the current consensus is...

Conventional ramjets have the problem that it doesn't take long before
the drag of collecting interstellar hydrogen balances the thrust from
fusing it; the figure tossed around is that this is likely to happen at
about 0.1 c.

There has been discussion about ramjet-type drives that don't capture
and fuse interstellar gas, but instead accelerate it backwards to a
higher ship-relative velocity, so that it would be possible to get
higher speeds.  The physics involved is quite exotic, though.  At high
relativistic speeds you also don't have much time to work on a quantity
of gas.