NERVA has about 250 kN of thrust at 841 seconds of specific impulse.
Merlin 1D has about 1000 kN of thrust at 311 seconds of specific impulse.
That suggests NTRs have around 4x less thrust, but around 2.7z more efficiency than chemical rockets. At a large enough scale craft, the weight of the engine could be considered irrelevant. But for smaller crafts, the Merlin has a thrust to weight ratio of 184:1, whereas NTRs only ever achieved 7:1 (they hoped to get up to around 35:1 eventually).
I imagine the thrust to weight could be substantially improved by making lightweight reactors, but I'm wondering if anybody knows anything about concepts for higher thrust NTRs?
https://en.wikipedia.org/wiki/NERVA
https://en.wikipedia.org/wiki/SpaceX_Merlin
Here's the relevant Atomic Rockets page.
If you're sticking with NTRs, the main way to get more power out is to increase the reactor temperature. That inevitably leads you to liquid-core NTRs, and even gas-core (though these are either open-cycle, or have an operational lifetime measured in minutes).
But most NTR designs center around higher exhaust velocity (so, higher ISP), instead of higer thrust. This is because, unless you have a torch drive, you must choose between high thrust (by throwing more mass out the back) and high ISP (by throwing less, but faster).
Since high thrust needs more reaction mass, it invokes the tyranny of the rocket equation, so we prefer high ISP, and just take longer to get places. But ISP is limited by how hot/fast you can make the propellant (chemical engines max out at roughly 600 seconds). That's why we even want to use NTRs in the first place.
Also, TWR is only really relevant on/near a planet, and you do not want to be using powerfull nuclear engines anywhere near that. Regular NTRs should be fine, but unless you're thinking of taking off or landing with them, don't worry about the TWR.
save