Ive managed to make a space plane awhile back and surprisingly it can make it to laythe. Not sure if this helps but here are the specs in the image below. I think it stays stable because of the 2 SAS parts and strut parts. Ive recently converted this design below into a kethane based spaceplane. I also find it funny how our designs look similar. (I normally add struts to the wings) Im actually having the same issue as you are with this guy displayed below Here are some extra pics http://imgur.com/a/mcas9#0 (Im also starting a sync space program where everyone can join together on one save game.