It's because the LV-N is too massive to see any benefit from the better efficiency. The Terrier has an Isp of 345 and mass of 0.5, whereas the Nerv is 800 and 3.0. So, the respective delta V's are: 345 * 9.81 * ln([m,full]/[m,empty]) for the 909 and 800 * 9.81 * ln([m,full]/[m,empty]) for the LV-N. This is a pretty complex equation if you're trying to solve for the masses, but if you look at it in terms of the mass ratios, it becomes a little simpler. The LV-N is only more efficient if the following holds true: [m,full,909]/[m,empty,909] < ([m,full,LV-N]/[m,empty,LV-N])^2.3188 So, if, with the 909, you have a mass of 2 t initially and 1 t after the burn, you'll have more delta V than you would if you add the nuclear engine. If, however, you have a mass of 3 t at start and 2 t at the end of your burn, you'll be better off with the LV-N. In general, the more fuel you're burning, the more beneficial it is to use the LV-N. You'll get more use out of your fuel, so with longer burns you're saving more. However, the LV-N weighs a lot. So, if you have a small amount of fuel on your vessel, a lighter engine will help you more than a higher efficiency engine. A heavy engine behind a heavy fuel tank doesn't change the mass significantly, but a heavy engine behind a light fuel tank makes a world of a difference.