The secret to making a stable SSTO spaceplane is to turn on the indicators for center of mass and center of lift while constructing. When you are finished, you should have a plane with a center of lift that is inside the center of mass, but just behind it. If that's not clear: The mid point of your center of lift, should be nearly touching the midpoint of your center of mass, but still just behind it. This will create a stable, easy to fly plane every time if you positioned your control surfaces correctly. But even if you haven't, it will still fly perfectly when you're not touching the controls. Keep in mind though, that while you fly you burn fuel, and this will alter your center of mass - so your plane may be a completely different bird that it started once you burn down most of your fuel - take that into account when making your reentry. Typically though the front end will want to be a bit squirrely - your plane can be made more manageable by installing RCS thrusters, necessary thought if you want an SSTO.