starship-design: Antiprotons

[wharton@physics.ucla.edu (Ken Wharton)]

Hey all...

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 
actual research.

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 
fission/fusion pellet.  

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 
provide thrust.

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 
proposal.

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.  

Ken