In 1922, NACA had 100 employees. By 1938, it had 426. In addition to formal assignments, staff were encouraged to pursue unauthorized "bootleg" research, provided that it was not too exotic. The result was a long string of fundamental breakthroughs, including "thin airfoil theory" (1920s), "NACA engine cowl" (1930s), the "NACA airfoil" series (1940s), and the "area rule" for supersonic aircraft (1950s). On the other hand, NACA's 1941 refusal to increase airspeed in their wind tunnels set Lockheed back a year in their quest to solve the problem of compressibility encountered in high speed dives made by the Lockheed P-38 Lightning.