Scaling up low thrust to weight engines with large cooling and shielding requirements doesn’t create many mass efficiencies. Maybe in the shielding, but that’s

Scaling up low thrust to weight engines with large cooling and shielding requirements doesn’t create many mass efficiencies. Maybe in the shielding, but that’s the least of your concerns. The other problem is that low thrust to weight means an NTR spade ship can’t land on Mars, or on any body with a significant gravity well. So you need to bring chemical rocket landers, increasing your mass duplication and tech complications. A multipurpose chemical rocket powered space ship Luke Star Ship is far more practical and nearly as fast.