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Comment by simonh | Hacker News Reader

You do what you would on a terrestrial reactor, and shut down the reactor e.g. by using control rods, or pulling out fuel rods to shut down the chain reaction. These wouldn't be simple radiothermal devices, but proper actively managed reactor cores.

Having said that you absolutely would need active cooling even in a 'shut down' state, probably by circulating a cooling fluid to radiator fins. You might use the same fluid as the propellant or might not. In the former case sure, you wouldn't be able to use all of it as propellant.

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Absolutely not, in space you have the option of jettisoning the core, carrying coolant for a controlled shut down is a waste of weight. You might even go as far as having 4 disposable cores, one for each acceleration stage.

Serious questions...

Would the core just float off into space? Couldn't it make its way back to Earth? Or compromise potential life on Mars?

It's pretty straightforward to put it in a safe, permanent orbit.

https://en.wikipedia.org/wiki/Graveyard_orbit

Brilliant solution man.

Additional nuclear fuel, unlike the popular belief says, will not be taking any much significant weight. A critical mass of plutonium is only 10kg, and you will likely have it reduced with moderation and reflectors.

If you have to keep disposing heat away from a powered down, but still residually active core, you will have to keep a no joke mass of radiators just for that.

Not sure why this is being downvoted jettison the core would likely be the go-to solution you can put into a stable orbit or even set on course with the sun and let it burn off...

There is no much need for a complex control mechanism in fact keeping those core as orbital modules on mars would likely allow you to skip many of the complications of having a complex control system to prevent a run away effect and modulate power output no one really cares if the core would go critical even at low orbit on Mars as there is so much radiation there from natural sources anyhow.

If the core goes critical mid way then either have a fallback core or just jettison it and wait for a rescue mission.

The vessels would likely have enough supplies for such occasions and as far as risk to the crew goes nuclear propulsion in space is much less dangerous than chemical propellants that can actually explode.

With a nuclear core you can safely vent hydrogen and xenon to space not worrying that much about radiation or long term contamination the cores can be more or less exposed with large fin stacks for radiative cooling.

"set on course with the sun and let it burn off" - https://laughingsquid.com/why-it-is-more-difficult-to-crash-...

So you would not build in mechanisms to moderate the heat output of the core, and have it generating at full power constantly? Ok, what state is the core in before you want to fire the engines? Suppose you want to burn to leave Earth, and agin to slow down at Mars, what happens to the engine core on the coasting period of many months in between? If you're carrying several disposable cores, if there's no way to moderate their output how do you store them safely when not in use and switch them on? Also, whole propulsion cores are lighter than the cooling system for a single core? Really?

A core is just nuclear fuel with enough other parts that it can be handled. Of course there needs to be a control mechanism for adjusting the output, you just can't use that for eliminating heat output. Once the core has been turned on there is decaying fission products producing heat, that is why you can't turn off a nuclear power plant in an instant, and why you might want to get rid of the core. Unused cores can be stored with few special precautions as the unspent fuel decay too slowly to be a major hazard.

The ship would carry one small nuclear reactor, and several tons of coolant/propellant. The only way the cooling system is actually going to keep up is by ejecting the hot coolant into space, which is also what creates thrust. You can't recycle the coolant as there is no efficient way to cool the coolant.

In effect, if you don't want to dispose of the core you have to keep the engine on consuming coolant/propellant, depending on a lot of factors that may or may not be viable.

>You can't recycle the coolant as there is no efficient way to cool the coolant.

Alright, this is the crux of the problem. Yes you can, the ISS has huge radiator panels in it's shadow for thermal management. The shuttle also had radiator panels in the bay door structure, they were so crucial the vehicle had to re-enter soon after closing the doors to avoid excessive thermal buildup. This is a known solved problem. And that's on vehicles that aren't carrying any nuclear power systems, and just have to contend with solar radiative heating.

The ISS doesn't have a heat budget anywhere close to that of a reactor core, you'd need to carry a much larger radiator construction, just for this job. Extra cores cost way less weight.

But if you don’t have mechanisms to moderate those cores output, they’ll be blazing away all the time. You can’t get away from the fact you need a way to moderate the reaction, including For ‘spare cores, which you at least need a mechanism to activate, and you need an active cooling system.

And on top of all that, even with reaction mass cooling the core when it’s active, that not going to be enough to capture all the generated heat.

your option of coolant without loss implies radiative cooling, how large a surface would be needed for how large a rocket for radiative cooling in the off shut down state? would the sunny side need to be reflective while the dark side needs to be emissive?

Nuclear reactors can stand quite high temperatures, so it will be much easier than you would assume if you looked at other rocket projects. Remember that radiating power increases with the 4rth power of the temperature.

Also, a shut down reactor quickly gets to ~1% of the energy of an active one, so, again, it's easier than it seems. Generated energy goes down exponentially with time.

Of all the problems of creating a nuclear rocket, I don't think cooling stands up.

but what cools the radiator fins, in the absence of air?

Infrared radiation from the hot fins to outer space should be taking energy out (cooling it) a bit, not sure how much though.

I think he is wondering about the utility of fins geometrically speaking in steradians when it comes to radiative cooling?

It's radiative cooling. Same way the sun heats the earth.

If you look at diagrams of the International Space Station you'll see they have rather large radiators, so it does work in space. They're always positioned in shade to radiate heat most efficiently. It's the two long pieces labeled 11 in this diagram: https://www.nasa.gov/sites/default/files/thumbnails/image/it...

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