How do I reuse a launch vehicle design?

[saskwach]

I don't want a reusable launch vehicle itself, just the design. I basically want to build the equivalent of the SLS and use it to orbit kerbins, fuel tanks, space stations, etc. But if I start out with a command pod mk-1, I can't seem to replace it with a stayputnik mk-2 without rebuilding the whole rocket. Am I missing some obvious button?

[Noticeably FAT]

It doesn't exist in the stock game, but there are a couple mods that do what you want. Look for Payloader and Subassembly Saver/Loader. Payloader does a better job of joining ships, but it runs out side of KSP (though you don't have to close KSP to run it), and there are a few things you have to do while building (simple procedures, but it doesn't work without them). SS/L is faster, easier, and works right from the VAB, but it has issues with staging. Stage 0 in the subassembly gets placed in Stage 0 in the ship you are loading it into, so you are about guaranteed to have to fix your staging.

[waerth]

I was looking for this to, if fixing the staging is all you have to do than Subassembly Saver/Loader sounds the easier way. No other hidden caveats?

[Noticeably FAT]

That's the only consistent problem I've noticed with it. I haven't gotten around to trying it, but I think I may have a solution. In your subassembly (I'm thinking of a lifter subassembly here, since that's what I use it for the most), add 4-6 dummy decouplers to the top of the stack, and assign them to stages 0-5. When the subassembly is loaded, those decouplers will be in those stages in your ship, and can easily be deleted. Then the rest of the subassembly parts will be in stages 6-X. It should work in theory, but I haven't put it into practice yet.

[Flixxbeatz]

so basically you want to use the same lifter design on your payloads without rebuilding it again from scratch?

get the subassembly loader plugin. but if you don't want plugins, better rebuild everything from scratch.

[Procyon]

I have used it and it works well. Just the staging issues previously mentioned and I have found I need to make minor parts changes on some things, to make look nice