Re: starship-design: Fusion Cone Scoop

[Steve VanDevender <stevev@efn.org>]

bugzapper writes:
 > >  > 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
 > afford.

But that energy still has to come from somewhere, and you have to get
enough energy to make this work.  You can't accelerate your proton beam
to high relativistic speeds without acquiring several times more energy
than the mass of the protons.  If you're pulling in fuel for nuclear
fusion, that means pulling in several hundred times as much interstellar
hydrogen as will go into the proton beam.  Pulling that hydrogen in will
induce drag; if your forward conical beam succeeds in putting a
concentrated stream of hydrogen in front of your ship, then you've just
shifted the location of the drag from the cone to the ship, as now the
ship is encountering that denser concentrated stream of gas.  It's not
even worth thinking about collecting the gas just to react it
chemically, because the payoff is literally millions of times lower.

 > 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.

What makes you think the forward beam can successfully induce fusion at
a high rate?  It's much more likely that the forward beam will just
scatter the gas atoms around.

Also, I'm rather doubtful that your conical beam will be able to collect
even as much gas as it will take to make the beam in the first place;
that combined with the beam producing lots of thrust in the wrong
direction makes this whole thing still sound really untenable to me.

 > >  > 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.
 > 
 > 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.

Run the numbers.  It's already doubtful that you can gather enough
interstellar gas with a ramscoop to make a fusion-powered starship
feasible if you need millions of times as much gas as your starship
weighs; if you need trillions of times as much hydrogen for chemical
fuel as your starship weighs, and your best estimate of interstellar gas
density is only about a hundred times what's likely to exist, you've
already lost.