It's true. Specific impulse is only one part of the equation. Tankage mass is really bad with pure hydrogen, which is usually the reaction mass of choice. Hydro

It's true. Specific impulse is only one part of the equation. Tankage mass is really bad with pure hydrogen, which is usually the reaction mass of choice. Hydroden's density is abysmally low. And the reactor weighs a lot too, compared to chemical rocket engines which have very high power densities and thrust to weight ratios. So nuclear thermal would tend to only win over chemical propulsion for long duration low thrust large delta vee missions. But then, at the top end, it's squeezed by solar electric propulsion which has much higher specific impulses (but even worse thrust to weight ratio). Solar cells and power electronics have improved a lot since the sixties...