Space Nuclear Propulsion

24 points by neverminder 2 years ago | 20 comments

This is the only viable future for space travel. In orbit assembled space ships with nuclear thermal propulsion. Travel time to Mars with conventional chemical propulsion takes just way too long.

Apofis 2 years ago | |

I can really see this happening with Malten Salt Reactors finally getting traction. China already has built a demonstrator and is now building a full-scale version of an MSR Reactor and now finally the US is building a demonstrator as well.

https://www.thecooldown.com/green-business/us-nuclear-test-r...

credit_guy 2 years ago | | |

No, molten salt reactors are not the right technology for nuclear propulsion. The idea of a nuclear thermal rocket engine is to heat up very light molecules to very high temperatures, and so to achieve higher exhaust velocities than chemical rockets. If you plug a higher exhaust velocity in the rocket equation, you end up needing less fuel mass for the same cargo mass. In practice, the best nuclear thermal rockets achieve a lower temperature than chemical rockets, but they can dedicate it to heat only hydrogen (H2), rather than the combustion products in chemical rockets (such as H2O or CO2), so overall the exhaust velocity can be approximately twice as high.

Still, temperature is quite important, you want the core of the reactor to run as hot as possible. You are limited by the fact that you don't want the core to disintegrate. The NERVA project [1] achieved temperatures in excess of 2200 K.

Molten salt reactors are designed to reach about 1000 K. That gives up most of the benefit of using a nuclear reactor. You would still beat chemical rockets, but only by 25%, not by a factor of 2. Why would you do that? If you build on the NERVA project and use TRISO fuel (which was not available at the time) you can end up with a specific impulse of more than 1000 s, which is 2.2 times higher than what the best chemical rockets can deliver, and 2.85 times higher than SpaceX Starship.

[1] https://en.wikipedia.org/wiki/NERVA#Reactor_and_engine_test_...

pfdietz 2 years ago | |

There are non-nuclear alternatives, particularly inward of the asteroid belt.

PV in space can be made very thin. The absorption length for photons in CdTe, for example, is just 0.1 microns. Without having to be mechanically robust against wind and rain, great gossamer PV arrays could have very high power/mass ratios. These could drive plasma engines with high Isp.

thegrim33 2 years ago | | |

None of that has anything to do with reducing travel times to Mars unless your entire payload is on the order of a couple of pounds.

That's like replying to someone saying it takes too long to drive from New York to Seattle, by saying that we could build an efficient 1000 mile per gallon car, that travels at .01 miles per hour. How efficient the vehicle is isn't the slightest bit useful to solve their complaint.

A high thrust to weight ratio when the weight is a couple of pounds isn't useful. What's useful is having a huge amount of thrust that's large enough to shove multiple tons of mass at high accelerations.

Qem 2 years ago | | |

For crewed missions, wouldn't the low trust lead to very large transit times anyway, in a Mars mission scenario?

whaleofatw2022 2 years ago | | |

> Without having to be mechanically robust against wind and rain,

What about micro meteorids?

dgoodell 2 years ago | |

I think nuclear electric propulsion is probably a viable option as well.

neverminder 2 years ago | | |

Indeed. Probably a combination of both nuclear-thermal and nuclear-electric or ion drive I think it's called otherwise. The nuclear thermal would provide initial boost, then ion drive can do continuous acceleration half way and then deceleration the other half way. That would get a ship to Mars in a fraction of time.