5 ms·

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 requ

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
        Yes, you can. Hydrogen, for the same temperature, has a far higher speed of sound (which is close to the average speed of the gas molecules) than air or water vapor. This is why your voice is higher pitched when you breathe in helium (also a light gas like hydrogen). Basic kinetic gas theory stuff.
      - perihelions 4y ago
        Huge difference between the molecular weight of H₂ (2) and its lightest combustion product OH (17).
  - Robotbeat 4y ago
    It is true. Basic gas theory stuff, the average molecular speed (close to the speed of sound) at a given gas temperature is, to first order, higher for a lower molecular mass. Otherwise, why bother with such a difficult to store propellant which you’re not even extracting energy from (as the energy comes from the reactor, not the propellant as in chemical rockets)? Chemical rockets reach over 3500 Kelvin, but Nerva only got to around 2300 Kelvin.

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
      Yes, you can. Hydrogen, for the same temperature, has a far higher speed of sound (which is close to the average speed of the gas molecules) than air or water vapor. This is why your voice is higher pitched when you breathe in helium (also a light gas like hydrogen). Basic kinetic gas theory stuff.
    - perihelions 4y ago
      Huge difference between the molecular weight of H₂ (2) and its lightest combustion product OH (17).
- Robotbeat 4y ago
  It is true. Basic gas theory stuff, the average molecular speed (close to the speed of sound) at a given gas temperature is, to first order, higher for a lower molecular mass. Otherwise, why bother with such a difficult to store propellant which you’re not even extracting energy from (as the energy comes from the reactor, not the propellant as in chemical rockets)? Chemical rockets reach over 3500 Kelvin, but Nerva only got to around 2300 Kelvin.