The details of something mysteriously known only as Project 506 was revealed in 1950 to be a hypersonic wind tunnel, with a test section of just eleven inches, but capable of subjecting models to wind speeds close to Mach 7. That test facility, and a large complex under construction called the Gas Dynamics Laboratory, which would be capable of wind tunnel tests up to Mach 18, tipped the agency’s interest in flight so fast that it could occur only at the limits of Earth’s atmosphere.
