Isotope heat sources don't undergo fission. It's just radioactive decay heat.And the achievable temperature is very low - substantially lower, most likely, than that of a fission NTR, but certainly no higher. You'd be restricted to hydrogen if you wanted an Isp above 500. Hydrogen is not easy to store.In contrast, a electric thruster running xenon can pull 3000 seconds easily.
And a bank of electric thrusters powered by a fission reactor can pull high impulse and generate a respectable acceleration for interplanetary travel.
No, in terms of propellant use, an electric thruster would be much more efficient than a radioisotope thermal thruster. In addition to having much higher specific impulse, an electric thruster is much easier to control than an isotope heat source which cannot be turned off.Electric thrusters allowed and Hayabusa and Dawn to successfully complete their missions. Do you know of any radioisotope thruster being currently designed for a spacecraft?There's still no case for manufacturing radioisotopes in space.
[quote author=douglas100 link=topic=27264.msg829231#msg829231 We are talking about too different types of efficient. I was talking about power efficiency. Electric propulsion has better ISP, but much lower power due to conversion loses.Which one is better depends on your mission.
Quote from: 93143 on 11/15/2011 02:20 amIsotope heat sources don't undergo fission. It's just radioactive decay heat.And the achievable temperature is very low - substantially lower, most likely, than that of a fission NTR, but certainly no higher. You'd be restricted to hydrogen if you wanted an Isp above 500. Hydrogen is not easy to store.In contrast, a electric thruster running xenon can pull 3000 seconds easily.The decay of plutonium 238 takes the form of alpha radiation. The alpha radiation gets absorbed within the material thus generating thermal energy. The temperature of the isotope is directly related to the thermal energy contain in it. Like any heat source its temperature will rise until it loses as much heat as it generates. If an thermal insulator is used to prevent heat from escaping than the temperature will rise unabated. Therefore the achievable temperature of plutonium is as high as the system you build can handle.
Quote from: DarkenedOne on 11/15/2011 07:59 pmQuote from: 93143 on 11/15/2011 02:20 amIsotope heat sources don't undergo fission. It's just radioactive decay heat.And the achievable temperature is very low - substantially lower, most likely, than that of a fission NTR, but certainly no higher. You'd be restricted to hydrogen if you wanted an Isp above 500. Hydrogen is not easy to store.In contrast, a electric thruster running xenon can pull 3000 seconds easily.The decay of plutonium 238 takes the form of alpha radiation. The alpha radiation gets absorbed within the material thus generating thermal energy. The temperature of the isotope is directly related to the thermal energy contain in it. Like any heat source its temperature will rise until it loses as much heat as it generates. If an thermal insulator is used to prevent heat from escaping than the temperature will rise unabated. Therefore the achievable temperature of plutonium is as high as the system you build can handle. You're talking to someone with two degrees in mechanical engineering, and most of a third in aerospace. In other words, I know all that. (Except the "unabated" part, which is an oversimplification - there's no such thing as a perfect thermal insulator.)I'm not willing to allow that a radioisotope thermal rocket would definitely be able to hit NTR temperatures, partly becaue you can't turn it off (which creates practical issues), and partly because the power density is so low - radiative cooling, for instance, might limit the achievable peak temperature. I haven't done any detailed analysis, but I believe my remark was more or less fair considering the time I had available to put into it. Perhaps you read it wrong...?
Remember Heinlein's "The man who sold the moon" ? The radioactive rocket fuel was made in orbit instead of being made on earth.Can Pu 238 be made in space, can we make a sort of fuel depot for very deep space missions?
Guys read this article.http://en.wikipedia.org/wiki/Radioisotope_rocketAs you can see they have tested versions that operate at temps similar to NTR.
http://en.wikipedia.org/wiki/Radioisotope_rocketAs you can see they have tested versions that operate at temps similar to NTR.