5 ms·

Do RTG reach the >2170K necessary for this?

jvanderbot 5y ago
Apparently not, as someone below pointed out. Oh well, I was briefly excited but there's a big enough gap that it probably won't work as is
- paledot 5y ago
  Bear in mind that, according to TFA, this work is noteworthy specifically because of the high temperature - past work has achieved the same results, albeit with lower efficiency, at lower temperatures.
perihelions 5y ago
Apparently not. This 1991 document [0] says the GPHS [1] heat source's iridium cladding has an upper operating temperature limit of 1,300 °C (1,573 K). There's a suggestion this could be safely raised to 1,500 °C. (This is the structural part that contains the Pu-238 fuel; the plutonium itself could be hotter than this, and the thermoelectric junction is much colder). (Note the reason given for this limit is to guarantee that, if this capsule accidentally falls to Earth, the cladding has the mechanical performance to stay intact on impact. If iridium gets too hot, its strength is permanently degraded (so it says). I suspect if you voided this requirement -- if you created a separate of class of spacecraft "no longer capable of impacting earth" -- the limit could be raised much higher). [0, pdf] https://ntrs.nasa.gov/api/citations/19910015359/downloads/19910015359.pdf https://ntrs.nasa.gov/api/citations/19910015359/downloads/19... (2.2 "Temperature Constraints") [1] https://en.wikipedia.org/wiki/GPHS-RTG https://en.wikipedia.org/wiki/GPHS-RTG
- dredmorbius 5y ago
  I also suspect that thermal ranges are limited due to the challenges of radiating away heat. Cooling things in space turns out to be hard. There's no convection or conduction, so all cooling occurs via radiation. I believe the ISS radiators are rated at about 1 watt per m^2, which means that you can calculate the radiator area (and mass) given your power budget. To a first approximation, the ISS radiators are about the same size as its solar panels. This is a factor missing from virtually all sci-fi representations of space ships. I had this realisation a few years back talking about 2001 --- the Discovery should have had a huge set of radiator fins given its nuclear propulsion (ion drive I believe, in the story universe). Wikipedia states that there were radiators in the book, though I don't recall that element. https://en.wikipedia.org/wiki/Discovery_One https://en.wikipedia.org/wiki/Discovery_One
  - perihelions 5y ago
    That's a constraint on thermal power, not on temperature (?). Hotter objects radiate faster (T^4 scaling even), so it's *easier* to cool them in a vacuum, all else being equal. MMRTG [0] dissipates 2,000 W, in a package that only has several m^2 of radiator fins. That's >100x more efficient than the ISS (1 W/m^2). ISS' radiators have to operate below room temperature; MMRTG fins [1] are in the 100 °C - 200 °C range. Higher temperature -> higher heat dissipation per fin area. (By the way, the comment we're replying to asks about nuclear quadcopters on Titan [2], which is in dense atmosphere not a vacuum!) [0] https://en.wikipedia.org/wiki/Multi-mission_radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Multi-mission_radioisotope_the... [1] https://sci-hub.se/10.1063/1.2169255 https://sci-hub.se/10.1063/1.2169255 [2] https://en.wikipedia.org/wiki/Dragonfly_(spacecraft) https://en.wikipedia.org/wiki/Dragonfly_(spacecraft)
    - dredmorbius 5y ago
      That's somewhat the conclusion I was coming to as I was thinking through the question. And yes, Titan / cold atmosphere would make for highly-efficient thermal gradients.

Apparently not, as someone below pointed out. Oh well, I was briefly excited but there's a big enough gap that it probably won't work as is

paledot 5y ago
Bear in mind that, according to TFA, this work is noteworthy specifically because of the high temperature - past work has achieved the same results, albeit with lower efficiency, at lower temperatures.

perihelions 5y ago

Apparently not. This 1991 document [0] says the GPHS [1] heat source's iridium cladding has an upper operating temperature limit of 1,300 °C (1,573 K). There's a suggestion this could be safely raised to 1,500 °C. (This is the structural part that contains the Pu-238 fuel; the plutonium itself could be hotter than this, and the thermoelectric junction is much colder). (Note the reason given for this limit is to guarantee that, if this capsule accidentally falls to Earth, the cladding has the mechanical performance to stay intact on impact. If iridium gets too hot, its strength is permanently degraded (so it says). I suspect if you voided this requirement -- if you created a separate of class of spacecraft "no longer capable of impacting earth" -- the limit could be raised much higher). [0, pdf] https://ntrs.nasa.gov/api/citations/19910015359/downloads/19910015359.pdf https://ntrs.nasa.gov/api/citations/19910015359/downloads/19... (2.2 "Temperature Constraints") [1] https://en.wikipedia.org/wiki/GPHS-RTG https://en.wikipedia.org/wiki/GPHS-RTG

dredmorbius 5y ago
I also suspect that thermal ranges are limited due to the challenges of radiating away heat. Cooling things in space turns out to be hard. There's no convection or conduction, so all cooling occurs via radiation. I believe the ISS radiators are rated at about 1 watt per m^2, which means that you can calculate the radiator area (and mass) given your power budget. To a first approximation, the ISS radiators are about the same size as its solar panels. This is a factor missing from virtually all sci-fi representations of space ships. I had this realisation a few years back talking about 2001 --- the Discovery should have had a huge set of radiator fins given its nuclear propulsion (ion drive I believe, in the story universe). Wikipedia states that there were radiators in the book, though I don't recall that element. https://en.wikipedia.org/wiki/Discovery_One https://en.wikipedia.org/wiki/Discovery_One
- perihelions 5y ago
  That's a constraint on thermal power, not on temperature (?). Hotter objects radiate faster (T^4 scaling even), so it's *easier* to cool them in a vacuum, all else being equal. MMRTG [0] dissipates 2,000 W, in a package that only has several m^2 of radiator fins. That's >100x more efficient than the ISS (1 W/m^2). ISS' radiators have to operate below room temperature; MMRTG fins [1] are in the 100 °C - 200 °C range. Higher temperature -> higher heat dissipation per fin area. (By the way, the comment we're replying to asks about nuclear quadcopters on Titan [2], which is in dense atmosphere not a vacuum!) [0] https://en.wikipedia.org/wiki/Multi-mission_radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Multi-mission_radioisotope_the... [1] https://sci-hub.se/10.1063/1.2169255 https://sci-hub.se/10.1063/1.2169255 [2] https://en.wikipedia.org/wiki/Dragonfly_(spacecraft) https://en.wikipedia.org/wiki/Dragonfly_(spacecraft)
  - dredmorbius 5y ago
    That's somewhat the conclusion I was coming to as I was thinking through the question. And yes, Titan / cold atmosphere would make for highly-efficient thermal gradients.