Why is a blunt trailing edge a better stabilizer at hypersonic speeds?

Question

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.[14] 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.

Answer 1

A converging shape at hypersonic speed in a low-pressure medium will produce close to vacuum pressure on its surfaces (hypersonic shielding). A small sideslip angle will only result in a very small pressure difference between both sides.

Contrast this with a diverging shape which produces higher than ambient pressure on both sides. In hypersonic flow this pressure grows approximately with the square of the inclination angle. The same sideslip angle, therefore, will result in a much higher pressure difference, rendering the diverging shape more effective in sideslip.

A more detailed discussion can be found here, page 11-6 to 11-8.

A wedge shape was chosen since it allowed to predict forces and temperatures with greater confidence than would had been possible with a more complex shape.

Answer 2

Thanks for @PeterKampf's answer and reference for the illumination. This is to complement the results drawn within its reference, which I don't think are completely accurate.

As shown in Anderson, Fundamentals of Aerodynamics, the Newtonian theory is an ok first cut at hypersonic aerodynamics: due to the vast difference between the speed of sound and the airspeed, air flow is basically deflected by whatever shape it encounters. For a flat plate at an angle of attack ($\alpha$) with the hypersonic airflow, the bottom surface (incident with the airflow) will have a coefficient of pressure:

$$C_{p,l}=2\sin^2\alpha$$

while the upper surface will have no effect:

$$C_{p,u}=0$$

Combined together, this makes a coefficient of lift:

$$C_l=sgn(\alpha)*2\sin^2\alpha\cos\alpha\approx sgn(\alpha)*2\alpha^2$$

Therefore, for a flat plate, the lift is nonlinear with respect to the flow incidence. For a vertical stabilizer, the directional stability would vanish at small perturbation and would also be nonlinear.

With a wedge shape making a half angle $\theta$, and for flow incidence smaller than $\theta$, the lift is:

$$C_l=2\sin^2(\alpha+\theta)\cos(\alpha+\theta)-2\sin^2(\alpha-\theta)\cos(\alpha-\theta)\approx2\theta\alpha$$

Therefore, for flow incidence smaller than $\theta$, a wedge shape has a linear lift with respect to flow incidence and a fairly constant stability derivative even at small perturbations.