20thcentury accidents aerodynamics aeronautics airbases aircraftaccidents airplanes antigravity casestudies classified defenseinformation designandconstruction flighttesting history humanfactors hypersonic juvenileliterature military militaryweapons prototypes research rocketplanes sightingsandencounters spacevehicleaccidents spacevehicles transportation unidentifiedflyingobjects unitedstates x-15(rocketaircraft)