Here is what I read about the X-15 spaceplane:
The X-15 had a thick wedge tail to enable it to fly in a steady manner at hypersonic speeds. This produced a significant amount of drag at lower speeds;
A wedge shape was used because it is more effective than the conventional tail as a stabilizing surface at hypersonic speeds. A vertical-tail area equal to 60 percent of the wing area was required to give the X-15 adequate directional stability.
The X-15 does indeed have flat trailing edges on its stabilizers. Some photos show this.
So why does this produce better stability at hypersonic speeds? I thought stabilizers were more about the total surface area anyway. It just seems so counter-productive to make a flat end like that giving huge drag, especially for hypersonic stuff.