Veraster

So I have a ship with 25 of those LV-N atomic rocket engines. Here is a picture of it. It's got a total delta v of like 16000 m/s. That big array of engines is separated into 3 stages. However, I was trying to find a way to get that 16 km/s of delta v into one stage. When I just merge all the engines into ONE STAGE, it says I only have 7km/s of delta v. WHY?!? They all have the same isp! Each engine has it's own fuel tank. All the fuel tanks are exactly the same size. How can combining a bunch of these together be less efficient than only using some of them at the same time?

I also can't seem to get it to work on .24 x64. I'm using Windows 7 Ultimate x64 if that makes any difference. Here's my crash log in case it helps. link

Hey could you please explain how to do that? I've tried adding a colored atmosphere to a PF CE planet I made but haven't been able to figure it out and make it work.