4 ms·

There's one more: the boring idea of combining a closed, electric-generating nuclear fission system with an (electric-powered) ion engine. Both components being

There's one more: the boring idea of combining a closed, electric-generating nuclear fission system with an (electric-powered) ion engine. Both components being flight tested and not crazy.

jerf 4y ago
That doesn't sound like something that could get you off of the Earth. It's probably a great choice for once you're already in space, where being able to apply even very small thrusts nearly-continuously really adds up.
- walrus01 4y ago
  The Soviets literally launched reactors into space in the late 70s for very high RF power radar satellites (intended to track US navy, among other things). One deorbited over northern Canada. https://www.google.com/search?client=firefox-b-d&q=soviet+rorsat https://www.google.com/search?client=firefox-b-d&q=soviet+ro... Of course anything related to hall effect/ion thrusters isn't going to get you off the earth even with some of the theoretical/bench-tested-only 100kW (electrical load) thrusters under development. These are things with very low thrust at any instantaneous moment in time but very high specific impulse from 3800 to 6000 ISP or more. Meant to be used in vacuum only.
  - Schroedingersat 4y ago
    Space based fission electric isn't very good though. Hence why they use PV now. NASA are developing a system that aims to be 6.67We/kg https://en.wikipedia.org/wiki/Kilopower https://en.wikipedia.org/wiki/Kilopower Compare to 50We/kg for cheap consumer grade flexible PV designed to be kicked around for a decade including an inverter. And that's with an atmosphere to use as a heatsink instead of radiators.
Schroedingersat 4y ago
What is the specific power of a nuclear reactor + heat engine + crew shielding + radiators? Naval reactors are on the order of a few watts to tens of watts per kg and they have the benefit of unlimited cooling. Seems an order of magnitude short of PV.
- perihelions 4y ago
  Possibly north of 200 W(elec.)/kg [0]. (I can't assess this stuff personally, but it's some MIT person's PhD thesis, so I assume some baseline credibility). Key distinction: you're looking at a conservative kilowatt-scale proposal, one which isn't intended for propulsion. But propulsion-optimized designs are on the megawatt scale: and the mass/power scaling is sub-linear. [0] https://dspace.mit.edu/handle/1721.1/67559 https://dspace.mit.edu/handle/1721.1/67559
  - Schroedingersat 4y ago
    Radiators scale at best linearly, so once they are dominant you don't get any improvement with power. A mylar mirror is extremely cheap and light (kW or tens of kW of heat can be trivially concentrated per kg), so the fact that there are no even semi-serious proposals for space based CSP should be fairly telling as to the viabity. At 200W/kg, a hypothetical fission generator is strictly worse than a current generation solar array in the inner planets for space based acceleration. Seems useful outside mars though.

There's one more: the boring idea of combining a closed, electric-generating nuclear fission system with an (electric-powered) ion engine. Both components being flight tested and not crazy.

jerf 4y ago

That doesn't sound like something that could get you off of the Earth. It's probably a great choice for once you're already in space, where being able to apply even very small thrusts nearly-continuously really adds up.

walrus01 4y ago
The Soviets literally launched reactors into space in the late 70s for very high RF power radar satellites (intended to track US navy, among other things). One deorbited over northern Canada. https://www.google.com/search?client=firefox-b-d&q=soviet+rorsat https://www.google.com/search?client=firefox-b-d&q=soviet+ro... Of course anything related to hall effect/ion thrusters isn't going to get you off the earth even with some of the theoretical/bench-tested-only 100kW (electrical load) thrusters under development. These are things with very low thrust at any instantaneous moment in time but very high specific impulse from 3800 to 6000 ISP or more. Meant to be used in vacuum only.
- Schroedingersat 4y ago
  Space based fission electric isn't very good though. Hence why they use PV now. NASA are developing a system that aims to be 6.67We/kg https://en.wikipedia.org/wiki/Kilopower https://en.wikipedia.org/wiki/Kilopower Compare to 50We/kg for cheap consumer grade flexible PV designed to be kicked around for a decade including an inverter. And that's with an atmosphere to use as a heatsink instead of radiators.

Schroedingersat 4y ago

What is the specific power of a nuclear reactor + heat engine + crew shielding + radiators? Naval reactors are on the order of a few watts to tens of watts per kg and they have the benefit of unlimited cooling. Seems an order of magnitude short of PV.

perihelions 4y ago
Possibly north of 200 W(elec.)/kg [0]. (I can't assess this stuff personally, but it's some MIT person's PhD thesis, so I assume some baseline credibility). Key distinction: you're looking at a conservative kilowatt-scale proposal, one which isn't intended for propulsion. But propulsion-optimized designs are on the megawatt scale: and the mass/power scaling is sub-linear. [0] https://dspace.mit.edu/handle/1721.1/67559 https://dspace.mit.edu/handle/1721.1/67559
- Schroedingersat 4y ago
  Radiators scale at best linearly, so once they are dominant you don't get any improvement with power. A mylar mirror is extremely cheap and light (kW or tens of kW of heat can be trivially concentrated per kg), so the fact that there are no even semi-serious proposals for space based CSP should be fairly telling as to the viabity. At 200W/kg, a hypothetical fission generator is strictly worse than a current generation solar array in the inner planets for space based acceleration. Seems useful outside mars though.