Errr...seriously? That's pretty much completely missing the point of the things. The Nerva is a nerva and, as a result, it has the second best ISP of anything on standard tanks and due to its size, it generally has more delta-v than the best IPS engine on the stuff as well. Thrust is not what you get a NERVA for. And as far as the aerospike is concerned, again, it has better ISP than most the stuff you can get but this one is made to not care about altitude differences. And the one non-nuclear engine with better efficiency is the stock aerospike, which has less thrust. Sure it has less thrust and more mass than the other engines and is, as such, not exactly what you'll usually want on a rocket but I'm guessing it does wonders on a spaceplane. Now someone come kick me in the shins for all the mistakes I made and fix them for me.