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It's hard to do any kind of nuclear reactor in space. You don't have bodies of water or even an atmosphere so radiative cooling it is, and that requires huge ra

It's hard to do any kind of nuclear reactor in space. You don't have bodies of water or even an atmosphere so radiative cooling it is, and that requires huge radiators. They alone weigh a lot. You must pipe coolant through them. What about micrometeoroids? You probably have to have valves to shut off leaking radiators. You don't have gravity so you don't have convection or separation of gas and liquid. Hard to test such systems on Earth. You can't make repairs easily. Solar cells start to look really good really fast. Maybe on Mars surface they could make some sense, or on the Moon where the night lasts for two weeks. Though on the Moon one would preferably settle near the poles without such problems. But even on the surface, just regular batteries look really good on the simplicity front, and fuel cells beyond that.

kbenson 9y ago
> You don't have bodies of water or even an atmosphere so radiative cooling it is, and that requires huge radiators. This is what I thought. To my (admittedly fairly uninformed) knowledge, nuclear power is really nuclear steam engines, at least the ones I know of. As awesome as a space steam engine sounds, it doesn't seem too feasible for the reasons you presented.
- jessriedel 9y ago
  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.)
jessriedel 9y ago
Internal combustion engines once look really complicated compared to horses too -- so many new problems to solve! -- but it was a forgone conclusion that cars would replace horses once the fundamental energy source could be harnesses. That nuclear powered space travel is more economical and efficient at large scales isn't disputed by experts. The barriers reaching that scale are funding and political, not the engineering ones you've highlighted.
- Gravityloss 9y ago
  A more apt comparison might be coal fired steam. It was very unwieldy but replaced horses in some ways. Railroads and steam ships changed the world but still the technology was too heavy for personal mobility. If you can invent any kind of technology at will, sure, it can be great. Then let's have light and reliable space nuclear power plants. At the moment we are very far from that. It might have looked like a good choice forty years ago, but solar cells have improved a lot, as well as batteries. We do have some newish things on the nuclear side that could help, like gas cooled reactors or supercritical power cycles. You need to get the temperature up (to make the radiators small) and the power density too.
  - jessriedel 9y ago
    Nuclear rockets don't look much like the reactors used for power generation. We are much closer to the tech for efficient nuclear rockets than we are to the tech for an (even partially) self-sustaining colony on Mars.

kbenson 9y ago

> You don't have bodies of water or even an atmosphere so radiative cooling it is, and that requires huge radiators. This is what I thought. To my (admittedly fairly uninformed) knowledge, nuclear power is really nuclear steam engines, at least the ones I know of. As awesome as a space steam engine sounds, it doesn't seem too feasible for the reasons you presented.

jessriedel 9y ago
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.)

jessriedel 9y ago

Internal combustion engines once look really complicated compared to horses too -- so many new problems to solve! -- but it was a forgone conclusion that cars would replace horses once the fundamental energy source could be harnesses. That nuclear powered space travel is more economical and efficient at large scales isn't disputed by experts. The barriers reaching that scale are funding and political, not the engineering ones you've highlighted.

Gravityloss 9y ago
A more apt comparison might be coal fired steam. It was very unwieldy but replaced horses in some ways. Railroads and steam ships changed the world but still the technology was too heavy for personal mobility. If you can invent any kind of technology at will, sure, it can be great. Then let's have light and reliable space nuclear power plants. At the moment we are very far from that. It might have looked like a good choice forty years ago, but solar cells have improved a lot, as well as batteries. We do have some newish things on the nuclear side that could help, like gas cooled reactors or supercritical power cycles. You need to get the temperature up (to make the radiators small) and the power density too.
- jessriedel 9y ago
  Nuclear rockets don't look much like the reactors used for power generation. We are much closer to the tech for efficient nuclear rockets than we are to the tech for an (even partially) self-sustaining colony on Mars.