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.
> 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.
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:
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.
The nuclear fuel doesn't really need to be shielded any better during launch than the existing RTGs used on space missions. Scaremongering notwithstanding, those are pretty much impossible for a mere launch accident to "blow to bits" in a way that exposes the PuO, and so would the fuel rods of a proper reactor. At least unless the reactor itself suffers a catastrophic excursion and blows itself to bits à la Chernobyl No. 4, but we've become pretty good at building reactors that don't do that.
And the RTGs use an isotope that provides significant energy just from radioactive decay. By contrast, uranium in a reactor is barely radioactive at all, before the reactor has been turned on.
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..
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.
"Simpler" here is relative. It would probably still cost billions of dollars and would create enough of a PR disaster to permanently forestall an NTR from ever launching again.
Water is not remotely as good as lead is by mass as a radiation shield. If you aren't already hauling many cubic meters of water for other reasons (and you wouldn't be), it absolutely wouldn't make sense to bring it along solely for that purpose. Water also has the severe problem of being liquid at habitable temperatures, so you either have to continuously spend energy to freeze it or deal with absolutely ruinous slosh.
The linked article is talking about the radiation shielding properties of what essentially amounts to a large swimming pool's worth of water. You know how much that would weigh?? Water is not a good radiation shield, it's just cheap here on Earth, so we use large quantities of it in applications where weight doesn't matter.
> It is going to be highly radioactive at some point (when it's in use), so it does need shielding for that point.
We can use two things as shielding - propellant and distance. For the first, we would want to make sure we don't use all the propellant with the reactor running hot, but decrease power output as we run out of propellant. As for distance, we may want to add a foldable structure between the propulsion section and the habitable section that would be extended to its full length prior to starting the nuclear reactor.
The long foldable structure would add considerable weight* and complexity (and thus risk), and would also be unsuitable for use as a first or second stage. So, this would maaaaybe be acceptable to handle the Hohmann transfers on a crewed mission to Mars, but not for much else.
* It needs to be strong enough to handle the full acceleration of the engines.
> It needs to be strong enough to handle the full acceleration of the engines.
I don't think we can expect to have high accelerations with NTRs anyway, so I don't think it'd need to be particularly robust. Also, propellant tank walls can be structural elements here too. If a foldable structure isn't practical, we could just assemble it from rigid elements lofted on other launches.
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.