You can copy the LV-N's part folder and dupicate it. Then, edit the LV-N's thrust to 30, mass to 1, and its Isp SL to whatever you want. Do the same for the aerospike. Build the engine ingame and now you have a LV-N with a plug nozzle.
There is a thread for that: http://forum.kerbalspaceprogram.com/showthread.php/29701-The-Nukespike-Engines-An-Experimental-Stock-Part-Engine-Design Also, can you send me the .craft? I want to see how you did it.
It's 449s for 235kN. The Aerospike greatly increases the TWR of the rocket and can be toggled off to save fuel. The engine is for heavy lifting interplanetary tugs and is lighter and more realistic than having 5 NTRs attached to a fuel tank. It may also be useful for landers once I figure out how to make it stack attachable. It's based off of a LANTR: http://www.projectrho.com/public_html/rocket/enginelist.php#id--Nuclear_Thermal--Solid_Core--LANTR And this: http://www.projectrho.com/public_html/rocket/realdesigns.php#id--Basic_Solid_Core_NTR--Shadow_Shield
In your KSP folder, there's a "saves" folder. Double click on it and copy the save folder you want and paste it into the "saves" folder of your other KSP folder.
Jettisoning RTGs won't be a good idea if you're in the atmosphere. IRL, it'll either shatter when it hits the ground or burn up. Either way, they will not be good for your space program.