5 ms·

An other quite interesting concept (also a little less "out there" and actually, successfully tested) is the NERVA engine, also linked at the bottom of the arti

An other quite interesting concept (also a little less "out there" and actually, successfully tested) is the NERVA engine, also linked at the bottom of the article[1]. It uses a nuclear reactor to heat up a single propellant (liquid hydrogen) which then rapidly expands as it goes out the nozzle at the end, resulting in quite high specific impulse and is more efficient than a chemical rocket. If we want to go really far out in terms of crazy rocket designs, there is also the concept of the nuclear salt-water rocket[2][3]. This one uses enriched plutonium or uranium salts, dissolved in water, inside a neutron-absorbent fuel tank design. The water is pushed out through a pipe, where an uncontrolled chain reaction starts. If it goes fast enough, it explodes right inside the engine bell, pushing the rocket. The proposed design in the original paper is supposed to generate hundreds of gigawatts of thermal power. This is comparable to the Chernobyl reactor #4 when it exploded, but unlike Chernobyl (which only exploded once) or the pulsed Orion engine, it could run continuously. The stuff that flies out of the nozzle easily reaches escape velocity of the solar system. [1] https://en.wikipedia.org/wiki/NERVA?useskin=vector https://en.wikipedia.org/wiki/NERVA?useskin=vector [2] https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket?useskin=vector https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket?uses... [3] Obligatory Scott Manley video: https://www.youtube.com/watch?v=cvZjhWE-3zM https://www.youtube.com/watch?v=cvZjhWE-3zM

perihelions 4y ago
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.
atoav 4y ago
The nuclear salt reactor one is the answer to the question: "How can we make rocket engine testing more dangerous."
- soco 4y ago
  We don't have to build/test them on Earth.
AtlasBarfed 4y ago
I don't think it is "out there" really. It's pretty simple. Bombs, plate, boom. It went through a pretty thorough development cycle at General Atomics, and they didn't have modern simulators. I think it would be great if we get a moon base going that could build it from regolith. Then you ship up the bombs in a few runs of Starship and you have a badass solar system exploration rig. From the docs, they could get to Saturn in something crazy like 1 month.
panick21_ 4y ago
And DoD and NASA are right now funding this research again. Edit: NERVA style, not Salt water thing.

perihelions 4y ago

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.

atoav 4y ago

The nuclear salt reactor one is the answer to the question: "How can we make rocket engine testing more dangerous."

soco 4y ago
We don't have to build/test them on Earth.

AtlasBarfed 4y ago

I don't think it is "out there" really. It's pretty simple. Bombs, plate, boom. It went through a pretty thorough development cycle at General Atomics, and they didn't have modern simulators. I think it would be great if we get a moon base going that could build it from regolith. Then you ship up the bombs in a few runs of Starship and you have a badass solar system exploration rig. From the docs, they could get to Saturn in something crazy like 1 month.

panick21_ 4y ago

And DoD and NASA are right now funding this research again. Edit: NERVA style, not Salt water thing.