No, extra mass is needed to make them operational, not politically acceptable.
First is engine mass. (All numbers from memory)
A SpaceX Raptor weighs around 4,000 lbs, and generates over 400,000 lbs of thrust (100x). NERVA weighed around 40,000 lbs and only generated 50,000 lbs of thrust (1.2x).
A fully fueled orbital Starship in would weigh around 2.5M with about 25,000 lbs of engine, 150,000 lbs of ship and 2.3M lbs of fuel generating 1.2M lbs of thrust out of three vacuum Raptors.
For a similar level of thrust from NERVA would add nearly 1M lbs of dry mass. Newer designs can cut that engine mass, but at most in half. And your nuclear ship could get by with less thrust and just burn the engine longer. But that brings two new questions
Where does all the heat go?
NTR send almost all of the heat out the back, but not all. Running that engine for hours on end in the vacuum of space (Instead of the very conductive earth NERVA was tested on) is going to heat stress your engine and everything it’s connected to. Do you need to add massive radiators and insulation to run the engines longer?
How do you take off on Mars?
If you have a lot less thrust, you will only be able to lift a lot less fuel and cargo. And you can’t land or take off on Earth. Unlike Starship NTRs may only be usable in between planets, requiring carrying heavy and expensive landers.
Then there is propellant. A 900 ISP is only achievable with hydrogen (H2), using Methane or water or any other feasible propellant drops ISP below 600, making overall system performance far worse than burning methane in Starship Raptors.
OK, let’s use Hydrogen then. Well unfortunately Hydrogen is a shitty propellent in every other measure. It doesn’t compress well so yet requires larger, heavier tanks. Also H2 is a tiny slippery molecule, forcing you to freeze it near absolute zero in order to keep it from leaking, which makes those tanks even heavier. And guess what? It still leaks. Will you have enough fuel left to fire a retro burn to match orbits with Mars? Better leave with at least 30% more than you need.
Now, you probably don’t want to be killed by your engine on the way to Mars, so let’s add some radiation shielding between the crew and the NTR engines, or put the on a long spar sticking out behind the ship. Both add even more mass.
A NTR ship will likely require at least 2-3x more dead mass than a chemical rocket ship, so the question becomes, what’s the point? Why jump through political hoops when Starship VP can already fly crew to Mars in only 90 days and deposit over hundred tons of cargo to the surface of Mars, and cheaply?
A lot of your math is way off, brother. Sure, NERVA was close to 1.0 thrust to weight ratio, but that says a lot more about the state of technology at the time than about the capabilities of nuclear thermal rockets.
The Dumbo / STNP engines were order of magnitude better than NERVA - way more than "half the mass" and I'm pretty sure we can do better than that these days. The quote is "Better than a conventional engine for any given amount of fuel", implying that even with balance-of-system mass issues they're still ahead of chemical rockets using virtually any fuel.
Also, I don't know whether anybody is suggesting crewed missions, or at least I'm not. But "Why bother", well, cost and efficiency and size, frankly. Starship is a bold move, but to me, it looks pretty small. I'd like them to add a zero onto the end of the mass budget. Think Gerald R. Ford class aircraft carrier going to space and back, with 5000 ton cargo capacity to LEO, or in that neighborhood.
My math on a NTR Starship: (1 ton = 1000kg throughout)
Two-stage Starship mass budget: 5000 tons, assuming max payload.
Dry mass: 180 tons
Current payload to LEO: 100 tons (my math says closer to 155t but whatever)
Even if you assume that an NTR version would be more than 5x the dry mass (quite a bit more than your estimate)
NTR mass budget (unchanged) 5000 tons
NTR dry mass: 1000 tons
Resulting payload to LEO: 800 tons
Plus, the NTR equivalent would be SSTO, straight up and back, only the tankage needing a refill, possibly for years. Isp really matters a ton and it outweighs a lot of the admittedly significant concerns you raise. Same thrust, more than 5x the dry mass, and it's still enough to 8x the cargo.
And of course this is without getting too sci-fi, no salt water Zubrin rockets or Project Orion or Project Pluto nonsense. SSTO using air as a working fluid until it gets up over ~30km is another nice concept.
This is all so theoretical though, and many decades in the future in the best case scenario. Someone with your opinions arguing about this in the 1970s would've argued we'd have had all this long before now, yet we don't. It's not clear that any progress will actually be made in the coming decades either; no one is seriously working on it like SpaceX and Blue Origin are working on their Mars rockets.
Chemical rockets have had thousands of successful missions. NTRs not only have zero, they don't even have a single complete rocket that's ever been built. If you want to get to Mars any time soon, don't pin your hopes on NTRs. The people who are actually doing the thing certainly aren't.
First is engine mass. (All numbers from memory)
A SpaceX Raptor weighs around 4,000 lbs, and generates over 400,000 lbs of thrust (100x). NERVA weighed around 40,000 lbs and only generated 50,000 lbs of thrust (1.2x).
A fully fueled orbital Starship in would weigh around 2.5M with about 25,000 lbs of engine, 150,000 lbs of ship and 2.3M lbs of fuel generating 1.2M lbs of thrust out of three vacuum Raptors.
For a similar level of thrust from NERVA would add nearly 1M lbs of dry mass. Newer designs can cut that engine mass, but at most in half. And your nuclear ship could get by with less thrust and just burn the engine longer. But that brings two new questions
Where does all the heat go? NTR send almost all of the heat out the back, but not all. Running that engine for hours on end in the vacuum of space (Instead of the very conductive earth NERVA was tested on) is going to heat stress your engine and everything it’s connected to. Do you need to add massive radiators and insulation to run the engines longer?
How do you take off on Mars? If you have a lot less thrust, you will only be able to lift a lot less fuel and cargo. And you can’t land or take off on Earth. Unlike Starship NTRs may only be usable in between planets, requiring carrying heavy and expensive landers.
Then there is propellant. A 900 ISP is only achievable with hydrogen (H2), using Methane or water or any other feasible propellant drops ISP below 600, making overall system performance far worse than burning methane in Starship Raptors.
OK, let’s use Hydrogen then. Well unfortunately Hydrogen is a shitty propellent in every other measure. It doesn’t compress well so yet requires larger, heavier tanks. Also H2 is a tiny slippery molecule, forcing you to freeze it near absolute zero in order to keep it from leaking, which makes those tanks even heavier. And guess what? It still leaks. Will you have enough fuel left to fire a retro burn to match orbits with Mars? Better leave with at least 30% more than you need.
Now, you probably don’t want to be killed by your engine on the way to Mars, so let’s add some radiation shielding between the crew and the NTR engines, or put the on a long spar sticking out behind the ship. Both add even more mass.
A NTR ship will likely require at least 2-3x more dead mass than a chemical rocket ship, so the question becomes, what’s the point? Why jump through political hoops when Starship VP can already fly crew to Mars in only 90 days and deposit over hundred tons of cargo to the surface of Mars, and cheaply?