6 ms·

This isn't taking into effect the added mass of the engine itself, though. You'd need heavy radiation shielding, heavy shielding around the reactor itself so t

This isn't taking into effect the added mass of the engine itself, though. You'd need heavy radiation shielding, heavy shielding around the reactor itself so that it wouldn't be blown to bits in the event of a launch explosion, etc. Plus, hydrogen is the least dense propellant so your fuel tank needs to be much bigger than e.g. the RP-1/LOX fuel tanks used by the Falcon 9, Saturn V first stage, etc. You end up in a much worst place than even hydrolox, because at least in hydrolox the liquid oxygen fuel tank at least can be relatively small. With NERVA it's only hydrogen. So, once you take into account these factors (rather than looking at the raw efficiency of just the engine alone), it ends up not being as much of an obvious win.

arethuza 6y ago
The reactor isn't going to be running during launch - so it's not going to be highly radioactive. Also I'd expect the reactor to be fairly small, dense and robust - I doubt if it would be "blown to bit" in the event of a problem with the rest of the launcher. e.g. During the 1980 Damascus Titan missile accident the missile exploded underground in a bunker and the warhead was thrown a fair distance but was recovered relatively intact: https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_explosion https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_ex... And that was something that was intended to explode!
- CydeWeys 6y ago
  It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point. It needs to be able to survive any kind of catastrophic detonation/break-up/crash landing you can think of, because the alternative is spreading a large amount of radioactive material directly into the atmosphere/onto the Earth's surface. For example, the Challenger orbiter itself was fine, but when the SRB blew up it took up the orbiter with it. So if there'd been any nuclear materials in the orbiter, even if they weren't used at launch, they still would've needed serious shielding. And it doesn't matter if the consequences of said radioactive material release aren't actually that bad in the grand scheme of things (like Fukushima looks not to have been) -- what matters is that the public reaction to such an event would preclude the possibility of ever launching it again.
  - arethuza 6y ago
    I suspect the public should be worried more about the thousands of other rockets sitting around with nuclear materials on top! Edit: Regarding the Challenger accident - the crew compartment survived the explosion and the crew were still alive for a while after :-( https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster#Cause_and_time_of_death https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disas...
    - CydeWeys 6y ago
      It didn't survive the crash though, which is what you'd need to prevent a radioactive materials release.
      - catalogia 6y ago
        If they're launching it over the ocean (they would be, if they were launching from Kennedy or Vandenberg) they could just say "YOLO" and let the reactor fall into the ocean. It certainly wouldn't be the first time a nuclear reactor was dumped into the ocean (either by accident or deliberately.) It's obviously not a popular thing to do, but ocean water does provide a lot of shielding.. https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593) https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593) https://en.wikipedia.org/wiki/Lenin_(1957_icebreaker)#Nuclear_accidents https://en.wikipedia.org/wiki/Lenin_(1957_icebreaker)#Nuclea...
        CydeWeys 6y ago
        Doesn't solve the problem of it blowing up on the pad, or near the pad while still over land, or crashing into land. Rockets have gone the wrong way before and been intentionally self-exploded mid-air.
        catalogia 6y ago
        While true, a reactor blown up over land would also be simpler to clean up; while a reactor scattered over the ocean bed would be easier to leave in place.
- deleted 6y ago
  [deleted]
- bryanlarsen 6y ago
  > The reactor isn't going to be running during launch DennisP was talking about SSTO, which implies the reactor is running during launch.
  - DennisP 6y ago
    I also said I didn't expect to actually see it used for launch. In reality you'd only use it in space. During launch you've just got uranium, which isn't very radioactive before you start getting fission products.
philwelch 6y ago
This is all overstated IMO. > You'd need heavy radiation shielding, heavy shielding around the reactor itself so that it wouldn't be blown to bits in the event of a launch explosion, etc. Not nearly as much as you think. You need some shielding between the reactor and any radiation-sensitive payloads (including humans) but you don’t need anything around the reactor; radiating into outer space is kind of like pouring water into the ocean. This requires the reactor not to be operating while on the ground, of course, and probably not even fueled until it reaches orbit. (Note that I said “fuel”—ie uranium—and not “propellant”.) But since the purpose of NTR’s is to enable deep space travel beyond cislunar space, that’s not a blocker. > Plus, hydrogen is the least dense propellant so your fuel tank needs to be much bigger than e.g. the RP-1/LOX fuel tanks NTR’s are competing with hydrolox upper stage engines for propulsion in space. They aren’t competing with high-thrust RP-1 or methane engines for launch. In fact RP-1 isn’t even in the picture after you reach LEO. SpaceX is going with methane over hydrolox for Starship largely because liftoff from Mars is a requirement and methane can be synthesized on Mars. > You end up in a much worst place than even hydrolox, because at least in hydrolox the liquid oxygen fuel tank at least can be relatively small. With NERVA it's only hydrogen. This makes zero sense. There’s no reason you couldn’t use oxygen as propellant in an NTR; it’s just that hydrogen works better. Again I think you’re missing the point—density is important for launching from the ground because during launch, you need to be able to produce a TWR > 1. But once you’re in orbit, none of that matters anymore. Upper stages—especially ones for going anywhere past LEO—are already predominantly hydrolox because the better specific impulse of hydrolox, combined with its low mass, more than compensates for the added dry mass of tankage.
- CydeWeys 6y ago
  We'll see ... personally I think the proof is in the pudding that despite having had these designs on the drawing board for 60+ years, there hasn't even been a single complete rocket that's ever materialized for any of them, let alone a successful mission. Contrast that with the thousands of successful missions using chemical rockets over that time period. So it's not overstating it to say that there are some problems with nuclear thermal rockets, otherwise they would be commonplace by now.

arethuza 6y ago

The reactor isn't going to be running during launch - so it's not going to be highly radioactive. Also I'd expect the reactor to be fairly small, dense and robust - I doubt if it would be "blown to bit" in the event of a problem with the rest of the launcher. e.g. During the 1980 Damascus Titan missile accident the missile exploded underground in a bunker and the warhead was thrown a fair distance but was recovered relatively intact: https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_explosion https://en.wikipedia.org/wiki/1980_Damascus_Titan_missile_ex... And that was something that was intended to explode!

CydeWeys 6y ago
It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point. It needs to be able to survive any kind of catastrophic detonation/break-up/crash landing you can think of, because the alternative is spreading a large amount of radioactive material directly into the atmosphere/onto the Earth's surface. For example, the Challenger orbiter itself was fine, but when the SRB blew up it took up the orbiter with it. So if there'd been any nuclear materials in the orbiter, even if they weren't used at launch, they still would've needed serious shielding. And it doesn't matter if the consequences of said radioactive material release aren't actually that bad in the grand scheme of things (like Fukushima looks not to have been) -- what matters is that the public reaction to such an event would preclude the possibility of ever launching it again.
- arethuza 6y ago
  I suspect the public should be worried more about the thousands of other rockets sitting around with nuclear materials on top! Edit: Regarding the Challenger accident - the crew compartment survived the explosion and the crew were still alive for a while after :-( https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster#Cause_and_time_of_death https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disas...
  - CydeWeys 6y ago
    It didn't survive the crash though, which is what you'd need to prevent a radioactive materials release.
    - catalogia 6y ago
      If they're launching it over the ocean (they would be, if they were launching from Kennedy or Vandenberg) they could just say "YOLO" and let the reactor fall into the ocean. It certainly wouldn't be the first time a nuclear reactor was dumped into the ocean (either by accident or deliberately.) It's obviously not a popular thing to do, but ocean water does provide a lot of shielding.. https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593) https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593) https://en.wikipedia.org/wiki/Lenin_(1957_icebreaker)#Nuclear_accidents https://en.wikipedia.org/wiki/Lenin_(1957_icebreaker)#Nuclea...
      - CydeWeys 6y ago
        Doesn't solve the problem of it blowing up on the pad, or near the pad while still over land, or crashing into land. Rockets have gone the wrong way before and been intentionally self-exploded mid-air.
        catalogia 6y ago
        While true, a reactor blown up over land would also be simpler to clean up; while a reactor scattered over the ocean bed would be easier to leave in place.
deleted 6y ago
[deleted]
bryanlarsen 6y ago
> The reactor isn't going to be running during launch DennisP was talking about SSTO, which implies the reactor is running during launch.
- DennisP 6y ago
  I also said I didn't expect to actually see it used for launch. In reality you'd only use it in space. During launch you've just got uranium, which isn't very radioactive before you start getting fission products.

philwelch 6y ago

This is all overstated IMO. > You'd need heavy radiation shielding, heavy shielding around the reactor itself so that it wouldn't be blown to bits in the event of a launch explosion, etc. Not nearly as much as you think. You need some shielding between the reactor and any radiation-sensitive payloads (including humans) but you don’t need anything around the reactor; radiating into outer space is kind of like pouring water into the ocean. This requires the reactor not to be operating while on the ground, of course, and probably not even fueled until it reaches orbit. (Note that I said “fuel”—ie uranium—and not “propellant”.) But since the purpose of NTR’s is to enable deep space travel beyond cislunar space, that’s not a blocker. > Plus, hydrogen is the least dense propellant so your fuel tank needs to be much bigger than e.g. the RP-1/LOX fuel tanks NTR’s are competing with hydrolox upper stage engines for propulsion in space. They aren’t competing with high-thrust RP-1 or methane engines for launch. In fact RP-1 isn’t even in the picture after you reach LEO. SpaceX is going with methane over hydrolox for Starship largely because liftoff from Mars is a requirement and methane can be synthesized on Mars. > You end up in a much worst place than even hydrolox, because at least in hydrolox the liquid oxygen fuel tank at least can be relatively small. With NERVA it's only hydrogen. This makes zero sense. There’s no reason you couldn’t use oxygen as propellant in an NTR; it’s just that hydrogen works better. Again I think you’re missing the point—density is important for launching from the ground because during launch, you need to be able to produce a TWR > 1. But once you’re in orbit, none of that matters anymore. Upper stages—especially ones for going anywhere past LEO—are already predominantly hydrolox because the better specific impulse of hydrolox, combined with its low mass, more than compensates for the added dry mass of tankage.

CydeWeys 6y ago
We'll see ... personally I think the proof is in the pudding that despite having had these designs on the drawing board for 60+ years, there hasn't even been a single complete rocket that's ever materialized for any of them, let alone a successful mission. Contrast that with the thousands of successful missions using chemical rockets over that time period. So it's not overstating it to say that there are some problems with nuclear thermal rockets, otherwise they would be commonplace by now.