I think that design is really cool and I actually have had success with it in the past. what you need to do is first build the space plane and make sure that the center of thrust lines up with the center of mass, (the 3 little buttons under the part index toggle these on and off) after that you should get an orange tank and strap it on, now you will notice that your center of mass and center of thrust are not lined up at all, to counter this place large solid fuel boosters so that the center of mass and center of thrust are lined up again, however when you jettison the solid fuel boosters it will make your rocket lean, you can use this to your advantage just like in real life and use this turn as your gravity turn, so just rotate your rocket so that the orange tank faces east (or witch ever direction you want your rocket to go) and blast off!