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The week before last I went to the American Physical Society Plasma
Physics conference in Pittsburgh. I've been to the conference a few
times, but this was the first year they actually had a special section
on plasma thrusters for spacecraft. Sure, it was thrown into a tiny
room on the far end of the convention center, and there were only ten
short talks, but it was something.
Most of the talks, though, weren't really usable for interstellar
spacecraft. People are building small plasma thrusters for use on
satellites, and some people talked about bulding large versions to get
to Mars in 3 months instead of 6. (Although one guy actually proposed
building an enormous magnetic mirror spacecraft that, if everything
worked perfectly, would still take 170 days to get to Mars!) A large
magnetoplasma thruster prototype has been built and people are doing
However, there were a few talks that went into interstellar drives, and
one in particular (from Penn State) sounded very interesting. (That is,
interesting in the sense that it might be possible to use the idea in
the next 50 years) The idea (which may have been discussed here
already?) is to use small quantities of antiprotons to catalyze a hybrid
The idea, which has some experimental support (Phys Rev C, v 45, p2332),
is based on the observation that antiprotons can catalyze large fission
and neutron yields in uranium pellets. If you can get 10^11 antiprotons
in a small area, this could heat a small target to many keV and possibly
create conditions that could ignite a fusion reaction.
Their suggested design was to use a series of small (42ng) DHe3 (or DT)
droplets, each doped with 2% U238. These are injected, one at a time,
into an electromagentic trap that contains 10^11 antiprotons. 0.5% of
the antiprotons annihilate, catalyze fission of the U238, which heats
the DHe3. Then they raise the voltage on the electromagnetic trap,
which seperates out the negatively charged particles (the pellet
electrons and unused antiprotons) from the positive ions (the hot D and
He3). The positive ions then fuse at a temperature of 100KeV, giving
off 15kJ of net energy. The used 5 10^8 antiprotons are then
replenished, the antiprotons are put back in the original state, and the
cycle repeats every 20ms with a net average power of 0.75MW. (Or 133MW
for DT targets).
The radiation and fusion products occur inside a silicon carbide shell.
The resulting plasma (Si and C ions) is channeled out of a hole to
They are currently designing a small, unmanned spacecraft based on the
assumption that they could get this to work if they had enough
antiprotons. The plan is not to make it to a star, but rather something
more modest that will travel 10,000 A.U. in a 50 year flight. Their
initial design parameters have a 100kg dry mass and 400kg of fuel. Of
the fuel, most of it is the SiC shell, and only 5.7 micrograms are
antiprotons, which they expect will be about a years supply from
Fermilab 10 years from now (also they predict it wil be 0.14
micrograms/year by 2000; don't know what it is now). All the fuel is
burned in the first 4.4 years; the rest of the time the ship coasts.
The alternate design, using DT pellets, burns all the fuel in only
0.1years and still makes it out to the same distance in the same time,
but it needs 26 micrograms of antiprotons. (Not sure why it needs
more... they didn't go into the DT simulations in too much detail, so
I'm not sure which of the numbers I've given apply for DT).
They also passed out a paper that was presented at a Propulsion
Symposium in October, which had more details but was a different
concept; it used fewer anitprotons (30ng), a 3-day burn, designed for
outer solar system missions (Jupiter in 7.5 months). It also relied on
ion beams to compress much larger fuel pellets, instead of the
electromagnetic trap design they presented at the conference, so I'm not
sure it's comparable at all. In the paper, though, they looked into
engine radiation, and most of the dangerous stuff was neutrons; they put
in 2.2 meters of LiH shielding to protect a crew. Less shielding is
needed to protect the antiprotons themselves. They also had a scheme to
vent an extra 60MW of heat; much more than in the DHe3 interstellar
Anyway, I thought it was interesting. If anyone wants more info, that's
really all I know, so I'm not the one to ask. The people at Penn State
to hunt down are G.A. Smith, R.A. Lewis (Penn State Physics Dept.), B.
Dundore, J. Fulmer (Penn State Aerospace Engineering Dept) and S.
Chakrabarti (Penn State Mech. Eng. Dept).
I assume that with enough antiprotons, this scheme could be scaled up
for an interstellar spaceship. But the key problem is that most of the
fuel is the SiC shell that provides the thrust; less than 0.1% of the
fuel is the DHe3 pellets. So the ratio of the mass of the ejected fuel
to the extractable kinetic energy from the fuel is huge; 250,000 rather
than the the theoretical 250 that a pure DHe3 engine could provide. If
there was a better way to couple the fusion reaction into fewer, faster
thrust particles, there would be a lot of room for improvement.