>Sure you can make one that's ⅓ of mass, but that doesn't help much if you will still end up needing 3 of them. You know

The three final engine designs in the SNTP program all had thrusts of ~45,500lbf - about 80% of NERVA's 55,500lbf, while weighing between 3000lb and 4200lb - only about a tenth of NERVA's 40,000lb.

The best all around design was probably the partial flow expander, which offered an isp of around 935s and weighed 3300lb for 45,700lb of thrust. Two of those would weigh 6600lb - only 1/6th as much as a single NERVA, while producing ~65% more thrust and ~11% better isp.