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starship-design: Fusion Cone Scoop
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.