Complex sentence: The role was finally filled by the AGM-28 Hound Dog, a much larger supersonic missile, so large that only two could be carried by a B-52 and only if it removed all other weapons.
Split into simpler sentences: The role was finally filled by the AGM-28 Hound Dog, a much larger supersonic missile. However, Hound Dog was so large that only two could be carried by a B-52, and only if it removed all other weapons.