[ExI] Fusion Rocket

[John Clark]

I sent this to the list back in 2002.
=========================

The efficiency of a rocket depends on its exhaust velocity, the faster the
better. The space shuttle's oxygen hydrogen engine has a exhaust
velocity of about 4500 meters per second and that's pretty good for
a chemical rocket, the nuclear heated rocket called NERVA tested
in the 1960's had a exhaust velocity of 8000 meters per second, and
ion engines are about 80,000. Is there any way to do better, much
better, say around 200,000,000 meters per second? Perhaps.

The primary products of a fission reaction are about that fast, but if you
use Uranium 235 or Plutonium 239 the large bulk of the material will
absorb the primary fission products and just heat up the material,
that slows things way down. However the critical mass for the little
used element Americium-242 (half life about a century) is less than 1%
that of Plutonium. This would be great stuff to make a nuclear bomb
you could put in your pocket, but it may have other uses.

In the January 2000 issue of Nuclear Instruments and Methods Physics Research A
Yigal Ronen and Eugene Shwagerous calculate that a metallic film of
Americium 242 less than a thousandth of a millimeter thick would
undergo fission. This is so thin that rather than heat the bulk material the
energy of the process would go almost entirely into the speed of the primary
fission products, they would go free. They figure a Americium-242 rocket
could get to Mars in two weeks not two years as with a chemical rocket.

There are problems of course, engineering the rocket would be tricky and I'm
not sure I'd want to be on the same continent as a Americium 242 production
facility, but it's an interesting idea.

 John K Clark



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