4 ms·

When it comes to an RTG lower power density is the same as more stable output and longer life since that's the other side of the half-life coin. So it's not "st

When it comes to an RTG lower power density is the same as more stable output and longer life since that's the other side of the half-life coin. So it's not "strictly worse", it actually has attractive features.

perihelions 2y ago
Yes, but not really, because the crossover point is at around 247 years.
- Dylan16807 2y ago
  The crossover point depends on how the weight is distributed between the radioactive material, the shielding, and the radiators. The first two favor plutonium, the last one favors americium.
  - perihelions 2y ago
    That's an very good point about the radiators—you need some overcapacity with a fast-decaying radioisotope, so you will need more heat dissipation. It's still not a close call, as far as I can tell (I'm not a domain expert). If you plan for even 20 years operating life, then your initial heat dissipation (for Pu-238) would be 1.17 of the nominal power at t+20 years, due it its radioactive decay. The largest RTG, GPHS-RTG, has 13.0 kg of "housing and fins" [0]; if you scale that linearly by 1.17 that would add +2.2 kg. On the flip side, the isotope mass that emits 4,500 Watts of heat goes from 8.3 kg of Pu-238, to 39.5 kg of Am-241—that's +31.2 kg. This is relative to a total mass (GPHS-RTG) of 56.0 kg. (To preempt anyone complaining that this are small quibbles: the total dry mass of New Horzions [1], which contained one of these RTG's, is 401 kg). [0, pdf] https://ntrs.nasa.gov/api/citations/20080003866/downloads/20080003866.pdf https://ntrs.nasa.gov/api/citations/20080003866/downloads/20... (table 1 on page 3, "State-of-the-art RPS. Comparing existing GPHS-RTG with near-term MMRTG and SRG110") [1] https://en.wikipedia.org/wiki/New_Horizons https://en.wikipedia.org/wiki/New_Horizons
  - angiosperm 2y ago
    Strontium-90 has advantages. At density 2.6 vs 12, you can afford to loft more of it to compensate for its 28y half-life, and still come out ahead provided you can dispose of the extra heat at first. Few missions need to run decades.
- foobarian 2y ago
  Any details on this number? I wonder if it accounts for the contributions from the decay products of Pu-238. Would the decay products even matter much?
  - philipkglass 2y ago
    Plutonium-238 decays to uranium-234: https://www.planetary.org/space-images/decay-of-pu-238-to-u-234 https://www.planetary.org/space-images/decay-of-pu-238-to-u-... U-234 has a 246,000 year half life and can be safely ignored when calculating the remaining heat output.
    - foobarian 2y ago
      Heh. I got confused by the article: > ESA’s heater units will not only be a first for Europe, but the first anywhere to use americium-241, a by-product of plutonium decay that packs less power per gram than its predecessor ... except that decay occurs from Pu-241, a different isotope which is not what seems to be commonly used for RTGs, and has a half-life of only about 14 years.

perihelions 2y ago

Yes, but not really, because the crossover point is at around 247 years.

Dylan16807 2y ago
The crossover point depends on how the weight is distributed between the radioactive material, the shielding, and the radiators. The first two favor plutonium, the last one favors americium.
- perihelions 2y ago
  That's an very good point about the radiators—you need some overcapacity with a fast-decaying radioisotope, so you will need more heat dissipation. It's still not a close call, as far as I can tell (I'm not a domain expert). If you plan for even 20 years operating life, then your initial heat dissipation (for Pu-238) would be 1.17 of the nominal power at t+20 years, due it its radioactive decay. The largest RTG, GPHS-RTG, has 13.0 kg of "housing and fins" [0]; if you scale that linearly by 1.17 that would add +2.2 kg. On the flip side, the isotope mass that emits 4,500 Watts of heat goes from 8.3 kg of Pu-238, to 39.5 kg of Am-241—that's +31.2 kg. This is relative to a total mass (GPHS-RTG) of 56.0 kg. (To preempt anyone complaining that this are small quibbles: the total dry mass of New Horzions [1], which contained one of these RTG's, is 401 kg). [0, pdf] https://ntrs.nasa.gov/api/citations/20080003866/downloads/20080003866.pdf https://ntrs.nasa.gov/api/citations/20080003866/downloads/20... (table 1 on page 3, "State-of-the-art RPS. Comparing existing GPHS-RTG with near-term MMRTG and SRG110") [1] https://en.wikipedia.org/wiki/New_Horizons https://en.wikipedia.org/wiki/New_Horizons
- angiosperm 2y ago
  Strontium-90 has advantages. At density 2.6 vs 12, you can afford to loft more of it to compensate for its 28y half-life, and still come out ahead provided you can dispose of the extra heat at first. Few missions need to run decades.
foobarian 2y ago
Any details on this number? I wonder if it accounts for the contributions from the decay products of Pu-238. Would the decay products even matter much?
- philipkglass 2y ago
  Plutonium-238 decays to uranium-234: https://www.planetary.org/space-images/decay-of-pu-238-to-u-234 https://www.planetary.org/space-images/decay-of-pu-238-to-u-... U-234 has a 246,000 year half life and can be safely ignored when calculating the remaining heat output.
  - foobarian 2y ago
    Heh. I got confused by the article: > ESA’s heater units will not only be a first for Europe, but the first anywhere to use americium-241, a by-product of plutonium decay that packs less power per gram than its predecessor ... except that decay occurs from Pu-241, a different isotope which is not what seems to be commonly used for RTGs, and has a half-life of only about 14 years.