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 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 jonkc@att.net
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