> 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.
Oxygen in an NTR provides an ISP barely better than chemical rockets, and how reactive would 1500K oxygen be with your reactor? with the additional mass requirements of massively heavy NTR engines, shielding and cooling oxygen doesnt make sense.
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.
It doesn't take an NTR to land on Mars. If you're going to pin all your hopes on NTRs I think it's still going to take decades. No one is even seriously working on them.
I'd much rather pin my hopes on chemical rockets in the form of SpaceX's Starship.
I didn't say it did. But it's one of the two most feasible options, the second involving lunar ISRU and orbital propellant depots. NASA's Mars Design Reference Mission has consistently included NTR studies.
Going past Mars and into the Belt, an NTR is practically essential.
> If you're going to pin all your hopes on NTRs I think it's still going to take decades.
Anything we try is going to take decades if you're talking about real time and not Elon time. But that's beside the point.
The point I was trying to make is that there has been approximately zero serious investment in human spaceflight beyond LEO once Apollo wrapped up. The Soviets decided they didn't want to land on the Moon after all, the US decided they would rather build a flying space truck that goes to LEO than build on Apollo, and everyone else spent decades just catching up. Since an NTR is only useful for interplanetary flight, no interplanetary flight means no need to develop NTR.
> 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.