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Your intuition doesn't even agree with 1970s technology. Radioisotope thermoelectric generators developed then are still more efficient than modern battery po

Your intuition doesn't even agree with 1970s technology. Radioisotope thermoelectric generators developed then are still more efficient than modern battery power sources. They don't use steam and they have no moving parts. https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

Gravityloss 9y ago
Thermoelectric generators are at the moment not useful for primary propulsion use as the amount of energy generated is so small. Though nowadays you could maybe improve them with advanced quantum technology, like long wavelength solar cells. The cell itself still has to be cooler than the source, so you still need radiators... https://en.wikipedia.org/wiki/Thermophotovoltaic https://en.wikipedia.org/wiki/Thermophotovoltaic
kbenson 9y ago
Does that scale to the level required here? Also, while it's not a steam engine, it is an engine based on heat transfer, so it's not entirely different, and a lot of the same problems apply. So, while my assumption doesn't hold, the requirements imposed by it may (and may be worse, RTGs can't vary their heat output in any way). Do we have numbers on how much heat a current fairly standard RTG generates (for what appears to be hundreds of Watts), and how hard that is to radiate in space?
SigmundA 9y ago
They are actually extremely inefficient since they use a thermocouple. They only convert about 5% of the thermal energy to electricity, the other 95% of the heat must be rejected somewhere. A steam turbine nuclear reactor is a Rankine cycle heat engine, their efficiency is above 40%. The reason RTG are used is simplicity and no moving parts, but efficiency greatly suffers. If we could figure out more efficient thermocouple all sorts of opportunities open up, haven't seen much work in this area.
- jessriedel 9y ago
  The efficiency I'm talking about here is power and thrust per weight, not absolute (percentage) efficiency in the thermodynamic sense. (Yes, the latter is important for worrying about heat dissipation, but the former is the key physical limitation for interplanetary travel.)

Gravityloss 9y ago

Thermoelectric generators are at the moment not useful for primary propulsion use as the amount of energy generated is so small. Though nowadays you could maybe improve them with advanced quantum technology, like long wavelength solar cells. The cell itself still has to be cooler than the source, so you still need radiators... https://en.wikipedia.org/wiki/Thermophotovoltaic https://en.wikipedia.org/wiki/Thermophotovoltaic

kbenson 9y ago

Does that scale to the level required here? Also, while it's not a steam engine, it is an engine based on heat transfer, so it's not entirely different, and a lot of the same problems apply. So, while my assumption doesn't hold, the requirements imposed by it may (and may be worse, RTGs can't vary their heat output in any way). Do we have numbers on how much heat a current fairly standard RTG generates (for what appears to be hundreds of Watts), and how hard that is to radiate in space?

SigmundA 9y ago

They are actually extremely inefficient since they use a thermocouple. They only convert about 5% of the thermal energy to electricity, the other 95% of the heat must be rejected somewhere. A steam turbine nuclear reactor is a Rankine cycle heat engine, their efficiency is above 40%. The reason RTG are used is simplicity and no moving parts, but efficiency greatly suffers. If we could figure out more efficient thermocouple all sorts of opportunities open up, haven't seen much work in this area.

jessriedel 9y ago
The efficiency I'm talking about here is power and thrust per weight, not absolute (percentage) efficiency in the thermodynamic sense. (Yes, the latter is important for worrying about heat dissipation, but the former is the key physical limitation for interplanetary travel.)