starship-design: Fusion Cone Scoop

[Steve VanDevender <stevev@efn.org>]

bugzapper writes:
 > If we could spread out our beam of accelerated particles, in a surface
 > of conical shape, each particle trajectory deflected by the same angle, we
 > could build a cone of accelerated particles.

Which will generate a substantial amount of thrust -- backwards.

 > The cone of fusion traversing forward establishes a shock front, which
 > pushes progressively inward on both neutral gas and plasma, to serve as an
 > effective ram scoop. Though drag forces may occur on the ship, in its own
 > interaction with the medium, this drag is proportional to the ship's cross
 > sectional area, straight hydrodynamics. No back pressure is felt by the
 > accelerator, from remote fusion events many kilometers down its beam. So no
 > drag forces are due to the compression event, but all retarding forces are
 > from direct interactions of the ship with the onrushing column of gas.

Generating the conical particle beam is retarding force enough.

 > Producing the cone by sending out light ions forward gives a decelerating
 > force, but (hopefully) this may be negligible compared to the chemical
 > energy released.

You're trying to reach relativistic speeds with _chemical_ reactions?
Pardon me, but ha ha ha ha ha ha ha.  It's difficult enough, in terms of
fuel-to-payload ratio, to use fusion to reach even low relativistic
speeds; with fusion fuel you're looking at very roughly 10^6:1
fuel:payload.  You're talking about reactions that are several orders of
magnitude less efficient at mass-energy conversion, which means more
orders of magnitude increase in the fuel-to-payload ratio.  Even your
wildly optimistic estimates of interstellar gas density won't get you
enough hydrogen to boost a ship by chemically reacting the hydrogen, nor
will you get enough forward thrust to overcome the backwards thrust
you're generating with your conical particle beam.