It’s a bit counter-intuitive, but if you have a supersonic aircraft it often performs better at high speeds if it’s tail end is “draggier” than it is at low speeds. This is due to the fact that as the vehicle pushes past the speed of sound, the airflow over the vehicle, or at least over its control surfaces, can get disrupted and basically turn to mush. Consequently, the tail surfaces need to be larger, or extend further out, in order to “grab” onto some proper airflow.
This seems to be especially pronounced with lifting body designs, where a fat fuselage forms a fat, widely-expanding flowfield of mush that the control surfaces need to project past. For the X-24A, this was accomplished by having large body flaps that would angle further and further out as speed increased, turning the vehicle into a “shuttlecock” configuration.
