Hypersonic flight is achieved at speeds at or above Mach 5, or five times the speed of sound. In the 1940s and 1950s the goal of aeronautical research was to design and build aircraft that could fly at that speed and reach altitudes at the edge of space.

Spacecraft Re-Entry

When orbiting spacecraft reenter Earth's atmosphere, they are traveling at many times the speed of sound and they generate high temperatures because of friction with the air. Ballistic re-entry vehicles such as the Gemini and Apollo capsules have a thick heat shield that slows the spacecraft and dissipates heat. Aircraft designers have always considered this solution practical but primitive. They would prefer to build a spacecraft that could act like an aircraft as it reentered the atmosphere, flying through the atmosphere to a safe landing.

The X Planes

High-speed aircraft design began with rocket-powered craft. Many...