5 ms·

what happens at the end of such a trip, when the propellant is exhausted, and can no longer evaporatively cool the core? it has to contain enough propellant to

what happens at the end of such a trip, when the propellant is exhausted, and can no longer evaporatively cool the core? it has to contain enough propellant to come back to Earth orbit to refuel the propellant coolant? even if the core is throttled to its lowest levels, it will still produce heat in the vacuum of space? or will radiative cooling balance the remaining production of heat?

simonh 7y ago
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.
- om3n 7y ago
  but what cools the radiator fins, in the absence of air?
  - tandr 7y ago
    Infrared radiation from the hot fins to outer space should be taking energy out (cooling it) a bit, not sure how much though.
    - DoctorOetker 7y ago
      I think he is wondering about the utility of fins geometrically speaking in steradians when it comes to radiative cooling?
  - peterwoerner 7y ago
    It's radiative cooling. Same way the sun heats the earth.
  - exhilaration 7y ago
    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/its_2.jpg https://www.nasa.gov/sites/default/files/thumbnails/image/it...
- DoctorOetker 7y ago
  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?
  - marcosdumay 7y ago
    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.
- NohatCoder 7y ago
  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.
  - dvdhnt 7y ago
    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?
    - ceejayoz 7y ago
      It's pretty straightforward to put it in a safe, permanent orbit. https://en.wikipedia.org/wiki/Graveyard_orbit https://en.wikipedia.org/wiki/Graveyard_orbit
  - baybal2 7y ago
    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.
  - dogma1138 7y ago
    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.
dclowd9901 7y ago
Why do you believe they haven't/aren't already thought/thinking about these things and solving them? Or are you just genuinely curious?
Symmetry 7y ago
With a nuclear thermal rocket generally you do 90% of your burn all at once to maximize the Oberth effect then you shut down the reactor. But you've still got secondary decays going on for a while so you slowly use the remaining 10% of the propellant to cool the engine while getting a bit more thrust.
- DoctorOetker 7y ago
  this is getting close to the answer, but still not there, the secondary decays decay for a long long time, what do we do with the rocket or core after use? how much propellant is needed in theory given radiative cooling? is 10% enough?
  - Symmetry 7y ago
    10% is enough for the fast decaying parts that give off most of the energy. I'm sure you still need a small radiator for the longer half-lived parts but any spacecraft is going to need radiators anyways.
dogma1138 7y ago
Most likely it would be a very simple core without power throttling or control at all and will be used as a disposable booster and jettisoned once the acceleration phase is complete. Another core can then be used for deceleration and disposed off as well before entering earth orbit where for the final orbital insertion a much smaller chemical rocket can be used. Alternatively you don’t get back to earth at all but rather back to the moon where you could jettison the core even on the surface during final approach without giving much thought to radiation. Then use moon to earth transit via chemical rockets.

simonh 7y ago

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.

om3n 7y ago
but what cools the radiator fins, in the absence of air?
- tandr 7y ago
  Infrared radiation from the hot fins to outer space should be taking energy out (cooling it) a bit, not sure how much though.
  - DoctorOetker 7y ago
    I think he is wondering about the utility of fins geometrically speaking in steradians when it comes to radiative cooling?
- peterwoerner 7y ago
  It's radiative cooling. Same way the sun heats the earth.
- exhilaration 7y ago
  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/its_2.jpg https://www.nasa.gov/sites/default/files/thumbnails/image/it...
DoctorOetker 7y ago
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?
- marcosdumay 7y ago
  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.
NohatCoder 7y ago
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.
- dvdhnt 7y ago
  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?
  - ceejayoz 7y ago
    It's pretty straightforward to put it in a safe, permanent orbit. https://en.wikipedia.org/wiki/Graveyard_orbit https://en.wikipedia.org/wiki/Graveyard_orbit
- baybal2 7y ago
  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.
- dogma1138 7y ago
  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.

dclowd9901 7y ago

Why do you believe they haven't/aren't already thought/thinking about these things and solving them? Or are you just genuinely curious?

Symmetry 7y ago

With a nuclear thermal rocket generally you do 90% of your burn all at once to maximize the Oberth effect then you shut down the reactor. But you've still got secondary decays going on for a while so you slowly use the remaining 10% of the propellant to cool the engine while getting a bit more thrust.

DoctorOetker 7y ago
this is getting close to the answer, but still not there, the secondary decays decay for a long long time, what do we do with the rocket or core after use? how much propellant is needed in theory given radiative cooling? is 10% enough?
- Symmetry 7y ago
  10% is enough for the fast decaying parts that give off most of the energy. I'm sure you still need a small radiator for the longer half-lived parts but any spacecraft is going to need radiators anyways.

dogma1138 7y ago

Most likely it would be a very simple core without power throttling or control at all and will be used as a disposable booster and jettisoned once the acceleration phase is complete. Another core can then be used for deceleration and disposed off as well before entering earth orbit where for the final orbital insertion a much smaller chemical rocket can be used. Alternatively you don’t get back to earth at all but rather back to the moon where you could jettison the core even on the surface during final approach without giving much thought to radiation. Then use moon to earth transit via chemical rockets.