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Re: starship-design: Fusion Cone Scoop
Steve laughs at:
> > a cone of accelerated particles.
> Which will generate a substantial amount of thrust -- backwards.
Yeah, sure will.
> > direct interactions of the ship with the onrushing column of gas.
> Generating the conical particle beam is retarding force enough.
It might be, but it isn't sure to be. My LINAC beam, exiting straight out
the back and intended to provide thrust, is a beam made of protons that we
have stepped on hard. We want to give them the largest relativistic mass
multiplication factor we can, so they will have great inertia, so we can
have the momentum transfer benefits of throwing out a lot heavier jet of
exhaust than that much hydrogen would seem to weigh, in a balloon at 1g.
This beam meant for propulsion, is discharged from a machine we have tuned
to give them the maximum possible acceleration, for the energy we can
But nuclear fusion does not require such a hard impact. With the ramscoop
beam, we are not trying to make a proton weigh a pound. We need a high
current, true, to assure the density of our cone wall close to the ship. We
don't have to send these particles out so darn fast. We don't want to affect
an unlimited region of space. Our cone boundary is limited by dissipation,
as the final errant particles meet their space resident proton and flare in
fusion. Relative to "cold" space gas, the immediate velocity of the ship
which projects the particles is added to the speed at which they're shot.
The particles of the cone beam do not need relativistic speeds at all. Just
throwing them over the side, at the ship's speed, is fast enough to generate
nuclear fusion. Our need to give them speed, is so they will travel a long
way to gain us lots of gas, but that doesn't mean they need to get near the
speed of light. If it can be adequately collimated, a single stage of
electrostatic acceleration might be adequate to project this cone.
> > Producing the cone by sending out light ions forward gives a
> > 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.
Look closer: how much of this fuel am I carrying? None, right? What does
that make the ratio? It would be fine to keep a 1:1, or even a 10:1 ice
ratio aboard, for shielding, but it isn't needed for fuel. Hydrogen can be
harvested any time from the rushing stream about.
> 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.
You didn't name the real problem this time, but you get credit anyway,
because I've seen you mention it before. The real problem is exhaust
velocity. Even the best chemical reaction, which is this one, gives a lousy
exhaust velocity. Fusion does a lot better, by stepping out of the thermal
mode for excitation of its exhaust particles, but not even fusion ash
particles approach lightspeed. Antimatter is too costly. What you're left
with is an accelerator or an electromagnetic (light) beam. I'll take matter,
because it's heavier, not because it's darker.
Gulping umpteen cubic kilometers of nearly nothing, seems like it might get
you something. You're harvesting the stuff real fast. I offered a scoop
proposal which isn't drag-limited, and it doesn't have any real constraints
on the area (volume) harvested. If there's anything out there, this will get