9 ms·

I don't think heat engines would work in space generally due to the low mass of stuff around you that can absorb heat.

by foota 8y ago

Baeocystin 8y ago
They work great as long as you have a source of heat and a good radiator. You don't need mass as a heat sink.
- foota 8y ago
  Interesting, I wasn't thinking about that. Thanks
  - theothermkn 8y ago
    At a practical level, your initial instinct was right. In space, nobody can feel you convect. Essentially all of the waste heat has to be disposed of via radiation, and this is orders of magnitude less effective than convection at the temperatures that any known material can withstand. Worse, you don't have to radiate "just" the waste heat. Eventually, all of your electric energy becomes heat, too. In other words, all of the energy you extract from your decay source will end up as heat that you have to get rid of via radiation. (Well, you could wind a giant spring, I suppose. But, a spring that large would be a better radiator than energy storage device.)
    - woodandsteel 8y ago
      >In space, nobody can feel you convect. I know I'm on HN, as opposed to one of the political sites I read, when I run across that sort of remark.
    - Baeocystin 8y ago
      All that being said (and true!), you also have a very cold sink to reject waste heat to, whether you want to or not. The end result is that heating is as much an issue as cooling. Thus spacecraft usually wind up with blankets and radiators, both. :) Here's a detailed look at the thermal control system for the ISS, for the curious: https://www.nasa.gov/pdf/473486main_iss_atcs_overview.pdf https://www.nasa.gov/pdf/473486main_iss_atcs_overview.pdf
JackFr 8y ago
https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...
- acqq 8y ago
  From there: “RTGs use thermoelectric generators to convert heat from the radioactive material into electricity. Thermoelectric modules, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3–7%” Also interesting, the price of RTGs: "Plutonium 238, which is used to power Radioactive ThermoGenerators such as the ones that power the Voyagers, Galileo, and Cassini probes has not been produced in the US for 25 years, but NASA and DOE have budgeted $50 million to restart production of about 2 Kg per year for 5 years…an average cost of about $5 million per Kg. That is expensive stuff!" One SNAP-27 unit, the type used in Apollo missions, providing only around 70 W, had around 4 Kg, which means its Pu price alone would be at least around 20 million, if it would be produced now.
namibj 8y ago
So, a kind of thermal solar panel that could possibly scale down/cheap better than a solar panel?

I don't think heat engines would work in space generally due to the low mass of stuff around you that can absorb heat.

by foota 8y ago

I don't think heat engines would work in space generally due to the low mass of stuff around you that can absorb heat.

Baeocystin 8y ago

They work great as long as you have a source of heat and a good radiator. You don't need mass as a heat sink.

foota 8y ago
Interesting, I wasn't thinking about that. Thanks
- theothermkn 8y ago
  At a practical level, your initial instinct was right. In space, nobody can feel you convect. Essentially all of the waste heat has to be disposed of via radiation, and this is orders of magnitude less effective than convection at the temperatures that any known material can withstand. Worse, you don't have to radiate "just" the waste heat. Eventually, all of your electric energy becomes heat, too. In other words, all of the energy you extract from your decay source will end up as heat that you have to get rid of via radiation. (Well, you could wind a giant spring, I suppose. But, a spring that large would be a better radiator than energy storage device.)
  - woodandsteel 8y ago
    >In space, nobody can feel you convect. I know I'm on HN, as opposed to one of the political sites I read, when I run across that sort of remark.
  - Baeocystin 8y ago
    All that being said (and true!), you also have a very cold sink to reject waste heat to, whether you want to or not. The end result is that heating is as much an issue as cooling. Thus spacecraft usually wind up with blankets and radiators, both. :) Here's a detailed look at the thermal control system for the ISS, for the curious: https://www.nasa.gov/pdf/473486main_iss_atcs_overview.pdf https://www.nasa.gov/pdf/473486main_iss_atcs_overview.pdf

JackFr 8y ago

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

acqq 8y ago
From there: “RTGs use thermoelectric generators to convert heat from the radioactive material into electricity. Thermoelectric modules, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3–7%” Also interesting, the price of RTGs: "Plutonium 238, which is used to power Radioactive ThermoGenerators such as the ones that power the Voyagers, Galileo, and Cassini probes has not been produced in the US for 25 years, but NASA and DOE have budgeted $50 million to restart production of about 2 Kg per year for 5 years…an average cost of about $5 million per Kg. That is expensive stuff!" One SNAP-27 unit, the type used in Apollo missions, providing only around 70 W, had around 4 Kg, which means its Pu price alone would be at least around 20 million, if it would be produced now.

namibj 8y ago

So, a kind of thermal solar panel that could possibly scale down/cheap better than a solar panel?