<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><span class="Apple-style-span" style="font-size: small; ">I sent this to the list back in 2002.</span></div><div><span class="Apple-style-span" style="font-size: small; ">=========================</span></div><div><span class="Apple-style-span" style="font-size: small;"><br></span></div><div><span class="Apple-style-span" style="font-size: small; ">The efficiency of a rocket depends on its exhaust velocity, the faster the</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">better. The space shuttle's oxygen hydrogen engine has a exhaust</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">velocity of about 4500 meters per second and that's pretty good for</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">a chemical rocket, the nuclear heated rocket called NERVA tested</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">in the 1960's had a exhaust velocity of 8000 meters per second, and</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">ion engines are about 80,000. Is there any way to do better, much</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">better, say around 200,000,000 meters per second? Perhaps.</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">The primary products of a fission reaction are about that fast, but if you</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">use Uranium 235 or Plutonium 239 the large bulk of the material will</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">absorb the primary fission products and just heat up the material,</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">that slows things way down. However the critical mass for the little</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">used element Americium-242 (half life about a century) is less than 1%</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">that of Plutonium. This would be great stuff to make a nuclear bomb</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">you could put in your pocket, but it may have other uses.</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">In the January 2000 issue of Nuclear Instruments and Methods Physics Research A</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">Yigal Ronen and Eugene Shwagerous calculate that a metallic film of</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">Americium 242 less than a thousandth of a millimeter thick would</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">undergo fission. This is so thin that rather than heat the bulk material the</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">energy of the process would go almost entirely into the speed of the primary</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">fission products, they would go free. They figure a Americium-242 rocket</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">could get to Mars in two weeks not two years as with a chemical rocket.</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">There are problems of course, engineering the rocket would be tricky and I'm</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">not sure I'd want to be on the same continent as a Americium 242 production</span><span class="Apple-style-span" style="font-size: small; "><br></span><span class="Apple-style-span" style="font-size: small; ">facility, but it's an interesting idea.</span></div><div><span class="Apple-style-span" style="font-size: small; "><br></span></div><div><span class="Apple-style-span" style="font-size: small; "> John K Clark</span></div><div><br></div><div><br></div><br></body></html>