4 ms·

I'm just an armchair space enthusiast, but both of those problems seem like they have possible workarounds or solutions. Using methane instead of hydrogen could

by sq_ 6y ago

I'm just an armchair space enthusiast, but both of those problems seem like they have possible workarounds or solutions. Using methane instead of hydrogen could significantly reduce the required tank volume since CH4 is denser, and you don't lose that much performance with it (based on Project Rho's table linked above). For shielding, if you put the fuel mass between the engine and the humans aboard, you should be able to get a decent amount for "free".

CydeWeys 6y ago
It's not clear that methane would be as workable; that's a large change from hydrogen to that. We at least know that hydrogen is workable, because an engine was built and tested. As for the shielding, you can't use solely propellant as shielding because you still need shielding once the propellant is exhausted (the engine remains highly radioactive after having been used). And it's not clear that hydrogen provides suitable shielding for all the kinds of radioactivity generated here anyway; you might actually just need lead. Plus you still need substantial shielding in the event of break-up/crash on launch.
- evv555 6y ago
  At what distance is radioactivity no longer an issue? The engine could be attached to payload at a longer distance with relatively lightweight structures. It doesn't have to be designed to stand up to the stress of launch/land with a payload. Just to ferry between points in space.
  - valuearb 6y ago
    Spars can’t be that lightweight when NTR engines have a lot of mass, and get very, very hot.
- baybal2 6y ago
  Just throw away a reactor after, and carry a second one for a back trip
valuearb 6y ago
ISP of Methane in an NTR is less than 600. And the engine, shielding, and cooling mass increases the ship mass at least 2x to 3x over chemical.
- sq_ 6y ago
  It's definitely worse than H2's ISP in an NTR, but ~600s is still a solid boost over the ~450s that's about the best a practical LOX/LH2 chemical engine can do. My curiosity is essentially just whether the performance lost between an H2 and CH4 NTR would be gained back through reduced tank mass and boil-off. Shielding/engine mass will certainly be a big mass penalty versus chemical rockets.

by sq_ 6y ago

CydeWeys 6y ago

It's not clear that methane would be as workable; that's a large change from hydrogen to that. We at least know that hydrogen is workable, because an engine was built and tested. As for the shielding, you can't use solely propellant as shielding because you still need shielding once the propellant is exhausted (the engine remains highly radioactive after having been used). And it's not clear that hydrogen provides suitable shielding for all the kinds of radioactivity generated here anyway; you might actually just need lead. Plus you still need substantial shielding in the event of break-up/crash on launch.

evv555 6y ago
At what distance is radioactivity no longer an issue? The engine could be attached to payload at a longer distance with relatively lightweight structures. It doesn't have to be designed to stand up to the stress of launch/land with a payload. Just to ferry between points in space.
- valuearb 6y ago
  Spars can’t be that lightweight when NTR engines have a lot of mass, and get very, very hot.
baybal2 6y ago
Just throw away a reactor after, and carry a second one for a back trip

valuearb 6y ago

ISP of Methane in an NTR is less than 600. And the engine, shielding, and cooling mass increases the ship mass at least 2x to 3x over chemical.

sq_ 6y ago
It's definitely worse than H2's ISP in an NTR, but ~600s is still a solid boost over the ~450s that's about the best a practical LOX/LH2 chemical engine can do. My curiosity is essentially just whether the performance lost between an H2 and CH4 NTR would be gained back through reduced tank mass and boil-off. Shielding/engine mass will certainly be a big mass penalty versus chemical rockets.