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I've been meaning to post this for a while.
Radioisotope thermal propulsion is like NTR, except with radioisotopes such as pu-238 or u-232 or Th-228 instead of a full blown nuclear reactor. The ones that were tested have Isp's in the 650-700 range, but if built today, we would see more like 700-800. This isn't the answer to our problems, but it is an improvement. What do you think?
-Josh
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Thrust is too low for a main propulsion system. How much mass is a 1 N radioisotope thruster compared with a chemical one? Radioactives need heavy casings to prevent radiation leaks in an accident.
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If you use alpha emitters, such as the isotopes I mentioned in my other post, there will be no radiation leaks. For th-228, and u-232, it's alpha emission and beta minus decay all the way down to lead. I'm not really sure how much 1 n of thrust is, Is it a little or a lot?
I'm not really sure, there isn't too much info as it doesn't seem to be well known, but I would assume that we're talking economies of scale.
-Josh
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A nuclear fission thermal propulsion system would provide better power to weight ratio, would be controllable and could be made extremely compact. The core of a NTR using plutonium oxide as fuel might not be much bigger than a coffee cup.
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If you use alpha emitters, such as the isotopes I mentioned in my other post, there will be no radiation leaks. For th-228, and u-232, it's alpha emission and beta minus decay all the way down to lead. I'm not really sure how much 1 n of thrust is, Is it a little or a lot?
Thorium-228, Uranium-232 and Plutonium-238 are all hazardous materials, they all need to be robustly encased to prevent leaks in an accident, for example at launch.
1 N of thrust is typical for satellite station keeping in GEO, it's a very small propulsion system. Shuttle thrusters are about 100 N (a Shuttle Main engine has 1.8 million N)
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Oh, so this isn't something that could get us to mars, or even the moon?
If you want to accelerate very slowly it will get you to the Moon - though Mars would need too much propellant.
Radioisotopes are rather nasty to handle in any great quantity - they usually have low critical masses and so can't be made into a high-power nuclear core as a result. That and being red-hot all the time.
Better with a metal-oxide fission reactor using uranium - according to Scott Howe the release of fission products can be all but eliminate making the exhaust stream essentially non-radioactive. Problem is that the old style NERVA design is under-powered and has a narrow thrust range. A better NTR design is the DUMBO system, which you can look up as it has recently been declassified and is available on the Web.
BUT there is another option...
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...which is?...
Use what is abundant and build to last
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Oh, so this isn't something that could get us to mars, or even the moon?
If you want to accelerate very slowly it will get you to the Moon - though Mars would need too much propellant.
Radioisotopes are rather nasty to handle in any great quantity - they usually have low critical masses and so can't be made into a high-power nuclear core as a result. That and being red-hot all the time.
Better with a metal-oxide fission reactor using uranium - according to Scott Howe the release of fission products can be all but eliminate making the exhaust stream essentially non-radioactive. Problem is that the old style NERVA design is under-powered and has a narrow thrust range. A better NTR design is the DUMBO system, which you can look up as it has recently been declassified and is available on the Web.
BUT there is another option...
Wiki as ever, provides a useful description of RTGs which adds context to this discussion: http://en.wikipedia.org/wiki/Plutonium-238
The most popular RTG material is Plutonium-238, which is a powerful Alpha emmitter. It isn't fissile, so critical mass is not an issue. Cost is a serious issue, given that Pu-238 must be prepared by neutron bombardment of Neptunium-237, which is an actinide waste product that must be chemically seperated from spent nuclear fuel. All of the favourite RTG isotopes are nasty and toxic, because of their relatively short half lives and the high biological effectiveness of alpha and beta radiations in human tissue.
Basically RTGs are costly to prepare, low in power and the decay reaction is obviously impossible to control, given that it follows radioactive decay laws with the rate of decay being inversly proportional to half-life of the heat generating material. Not really an ideal choice for a Mars or lunar propulsion system. a solar powered system would provide similar or greater thrust level at lower cost. a nuclear fission system will greatly outperform and RTG.
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The other option was fusion pulse using fusion ignitors using high-energy density materials - like hi-energy nitrogen, which has recently been produced in a lab. Such solid nitrogen might have enough omph to be a totally non-fission fusion trigger for an Orion-style launcher.
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Accelerate Ionized Duetranium out the back at high speed so they fuse and ride the blast?
Use what is abundant and build to last
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