Complex sentence: The delta planform had good internal volume for support systems and fuel; plus a low wing loading (for airframe size), that permitted supersonic flight in the mid-stratosphere (50--70,000 feet).
Split into simpler sentences: The delta planform had good internal volume for support systems and fuel. It also had low wing loading (for airframe size), thus permitting supersonic flight in the mid-stratosphere (50--70,000 feet).