3 ms·

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 nu

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.

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.