5 ms·

Whats the relevance of SpaceX here?

robonerd 4y ago
It's tangentially related insofar as SpaceX says they're planning to go to Mars, and this NTR engine is also for going to Mars. But according to the DARPA announcement, DARPA determined that Falcon 9 doesn't presently support this sort of liquid hydrogen payload. They suggest Vulcan Centaur could do it with fairing modifications. (Vulcan Centaur hasn't flown yet. Where are the engines, Jeff??)
- mlindner 4y ago
  It's not really related as SpaceX has no plans to use them and isn't exactly interested in doing so as they don't see them as needed. Also NTR are kind of a tossup on efficiency as while you get somewhat better fuel efficiency, their mass is huge because you're lugging an entire nuclear reactor core along with with you. The thrust to weight ratio isn't great.
  - leereeves 4y ago
    Can you mount the nuclear reactor far away from the crew module and reduce the mass of shielding, as in old sci-fi?
    - mlindner 4y ago
      It's not just the shielding. An NTR engine needs a lot of the same plumbing that other engines need, including turbopumps, and they have additional cooling requirements because of the much hotter fuel. Add on to that the already very heavy Uranium and control rods. Add on to that I'm not quite sure how you prevent the engine's nuclear reactor from going into meltdown once it shuts off. The residual heat from the decay products in the seconds to minutes after shutdown will be substantial and that heat needs to go somewhere or it'll cause a reactor meltdown the instant you shut off the engine. So you need all the hardware to dump heat somewhere (presumably radiators and a cooling system that pumps hydrogen through the reactor while it's shut off) so that's even more mass. The only way NTR really makes sense to me is if your spacecraft is truly massive, but literally no one has anything like that even in planning stages.
      - Robotbeat 4y ago
        The fuel is actually cooler for NTR than chemical. With chemical, the peak heat can occur in the gaseous state away from anything solid, but for conventional nuclear thermal, the peak heat is generated in solid material and needs to conduct through to fluids, which are therefore at lower temperatures. And the way they handle shut down is they continue a small flow of propellant through the engine until the core cools off and the hottest, shortest lived stuff decays away. NTRs usually run for a few hours at most, not years, so the decay heat a few minutes after shutdown isn’t that bad.
        mlindner 4y ago
        > The fuel is actually cooler for NTR than chemical. With chemical, the peak heat can occur in the gaseous state away from anything solid, but for conventional nuclear thermal, the peak heat is generated in solid material and needs to conduct through to fluids, which are therefore at lower temperatures. Pretty sure this can't be true. In order to have a higher exhaust velocity the fuel temperature needs to be higher than chemical propulsion.
        leereeves 4y ago
        > In order to have a higher exhaust velocity the fuel temperature needs to be higher than chemical propulsion. Are you accounting for the fact that the NTR exhaust (hydrogen) is lighter than chemical rocket exhaust? At the same temperature, both propellants have the same average kinetic energy per molecule, so the hydrogen must be moving faster.
        mlindner 4y ago
        I wasn't accounting for it, but I assumed it wouldn't be significant. The efficiencies claimed are over 2x better than chemical rockets. You don't get that much just from changing gasses.
        Robotbeat 4y ago
mlindner 4y ago
There isn't one really other than SpaceX COO (or was it Musk?) making a single passing reference to them in response to a question at a conference keynote a few years ago.
ClumsyPilot 4y ago
As much as i like them, SpaceX approach of move fast with explosion is best kept away from nuclear :) Also they don't seem to have relevant experience

It's tangentially related insofar as SpaceX says they're planning to go to Mars, and this NTR engine is also for going to Mars. But according to the DARPA announcement, DARPA determined that Falcon 9 doesn't presently support this sort of liquid hydrogen payload. They suggest Vulcan Centaur could do it with fairing modifications. (Vulcan Centaur hasn't flown yet. Where are the engines, Jeff??)

mlindner 4y ago
It's not really related as SpaceX has no plans to use them and isn't exactly interested in doing so as they don't see them as needed. Also NTR are kind of a tossup on efficiency as while you get somewhat better fuel efficiency, their mass is huge because you're lugging an entire nuclear reactor core along with with you. The thrust to weight ratio isn't great.
- leereeves 4y ago
  Can you mount the nuclear reactor far away from the crew module and reduce the mass of shielding, as in old sci-fi?
  - mlindner 4y ago
    It's not just the shielding. An NTR engine needs a lot of the same plumbing that other engines need, including turbopumps, and they have additional cooling requirements because of the much hotter fuel. Add on to that the already very heavy Uranium and control rods. Add on to that I'm not quite sure how you prevent the engine's nuclear reactor from going into meltdown once it shuts off. The residual heat from the decay products in the seconds to minutes after shutdown will be substantial and that heat needs to go somewhere or it'll cause a reactor meltdown the instant you shut off the engine. So you need all the hardware to dump heat somewhere (presumably radiators and a cooling system that pumps hydrogen through the reactor while it's shut off) so that's even more mass. The only way NTR really makes sense to me is if your spacecraft is truly massive, but literally no one has anything like that even in planning stages.
    - Robotbeat 4y ago
      The fuel is actually cooler for NTR than chemical. With chemical, the peak heat can occur in the gaseous state away from anything solid, but for conventional nuclear thermal, the peak heat is generated in solid material and needs to conduct through to fluids, which are therefore at lower temperatures. And the way they handle shut down is they continue a small flow of propellant through the engine until the core cools off and the hottest, shortest lived stuff decays away. NTRs usually run for a few hours at most, not years, so the decay heat a few minutes after shutdown isn’t that bad.
      - mlindner 4y ago
        > The fuel is actually cooler for NTR than chemical. With chemical, the peak heat can occur in the gaseous state away from anything solid, but for conventional nuclear thermal, the peak heat is generated in solid material and needs to conduct through to fluids, which are therefore at lower temperatures. Pretty sure this can't be true. In order to have a higher exhaust velocity the fuel temperature needs to be higher than chemical propulsion.
        leereeves 4y ago
        > In order to have a higher exhaust velocity the fuel temperature needs to be higher than chemical propulsion. Are you accounting for the fact that the NTR exhaust (hydrogen) is lighter than chemical rocket exhaust? At the same temperature, both propellants have the same average kinetic energy per molecule, so the hydrogen must be moving faster.
        mlindner 4y ago
        I wasn't accounting for it, but I assumed it wouldn't be significant. The efficiencies claimed are over 2x better than chemical rockets. You don't get that much just from changing gasses.
        Robotbeat 4y ago

mlindner 4y ago

There isn't one really other than SpaceX COO (or was it Musk?) making a single passing reference to them in response to a question at a conference keynote a few years ago.

ClumsyPilot 4y ago

As much as i like them, SpaceX approach of move fast with explosion is best kept away from nuclear :) Also they don't seem to have relevant experience