This change could be partially explained by some new ease of filing assignments or registering a corporate form. However, I see two other major factors driving the split. First, the PTO & FTC crack-down on any attorney associated with an invention promotion company means that the patent attorneys who have historically filed the most design patents on behalf of individual inventors are no longer available. Most importantly, however, is the relatively small set of corporations who have figured out how to successfully use design patents.
There are two big technical problems that a hypersonic airplane would face. Hypersonic flight a hard nut to crack for engineers because of the incredible heat that the aircraft must face while cutting through the atmosphere at this speed. At Mach 10, the air in front of the plane is compressed by the travel speed and heats up to more than 1600 Celsius, a temperature that can melt iron. HyperSoar would avoid excessive heating by bouncing into the cold of space.