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NASA Wants to Send Nuclear Rockets to the Moon and Mars

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Re: NASA Wants to Send Nuclear Rockets to the Moon and Mars

#41

Post by Symmetry »

greglindahl wrote: post #36
Symmetry wrote: post #30

Earlier quoted context omitted.

Well, I'm no nuclear engineer but I'd hope that if you could make one of those that was light enough NASA would use that instead.

It's called an RTG, and they've been using them for decades.

I'm not sure what you're talking about? RTGs don't have short decay paths, they have to have long decay paths to last through a mission. P238 is what we use for most probes and has a half life of 88 years. It decays to U234 which has a half life of 200,000 years, short enough to be dangerous but long enough to almost never go away. RTGs tend to produce on the order of 100 watts of electricity from 500 watts of heat. A good nuclear engine will want to use 100+ megawatts when in use.

And more importantly RTGs don't put out nearly enough heat to make a usable nuclear thermal rocket. The important thing is being able to turn them on when you're doing a burn but then turn them off when you're coasting to your destination then turn them on again to stop there. RTGs can't do that.

Re: NASA Wants to Send Nuclear Rockets to the Moon and Mars

#42

Post by DoctorOetker »

Symmetry wrote: post #39
DoctorOetker wrote: post #26

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?

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.

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?

Re: NASA Wants to Send Nuclear Rockets to the Moon and Mars

#43

Post by om3n »

simonh wrote: post #37
DoctorOetker wrote: post #26

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?

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 fl…

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

Re: NASA Wants to Send Nuclear Rockets to the Moon and Mars

#44

Post by DoctorOetker »

simonh wrote: post #37
DoctorOetker wrote: post #26

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?

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 fl…

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?

Re: NASA Wants to Send Nuclear Rockets to the Moon and Mars

#45

Post by tandr »

om3n wrote: post #43
simonh wrote: post #37

Earlier quoted context omitted.

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 fl…

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.

Re: NASA Wants to Send Nuclear Rockets to the Moon and Mars

#46

Post by Symmetry »

DoctorOetker wrote: post #42
Symmetry wrote: post #39

Earlier quoted context omitted.

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.

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?

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.

Re: NASA Wants to Send Nuclear Rockets to the Moon and Mars

#47

Post by DoctorOetker »

tandr wrote: post #45
om3n wrote: post #43

Earlier quoted context omitted.

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?

Re: NASA Wants to Send Nuclear Rockets to the Moon and Mars

#48

Post by peterwoerner »

om3n wrote: post #43
simonh wrote: post #37

Earlier quoted context omitted.

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 fl…

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

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

Re: NASA Wants to Send Nuclear Rockets to the Moon and Mars

#49

Post by greglindahl »

Symmetry wrote: post #41
greglindahl wrote: post #36

Earlier quoted context omitted.

It's called an RTG, and they've been using them for decades.

I'm not sure what you're talking about? RTGs don't have short decay paths, they have to have long decay paths to last through a mission. P238 is what we use for most probes and has a half life of 88 years. It decays to U234 which has a half life of 200,000 years, short enough to be dangerous but long enough to almost never go away. RTGs tend to produce on the order of 100 watts of electricity from 500 watts of heat.…

What was asked for sounded like an RTG to me. I wasn't saying it was a nuclear thermal rocket or that it could be used as one. As you point out, it's a constant power source.
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