While I was designing my interplanetary vessel, I decided to make a small change and have the engine section actually be a docked SSTO craft (with a few quantum struts to keep it from shaking apart). However, in trying to design this, I'm coming up short.

I want a vertical liftoff, runway landing craft. I've got enough jet engines to get it off the ground and heading into orbit with my "first stage" liquid engines ready to kick in when the jets start to fail from a lack of oxygen (I have a "second stage" designed for the interplanetary part). However, I'm having trouble figuring out how to stop the jet engines and turn on the liquid fuel/rocket engines quickly.

Is there a custom button I can make to do this? I haven't really played around with that part of KSP much. Its a slow, shallow ascent to orbit to be able to use the jet engines as much as possible, but I'd love to make it a bit faster if I could switch more easily.

Any help would be appreciated.