Mach 3 refers to a speed that is three times the speed of sound in air. The speed of sound, known as Mach 1, varies with temperature and altitude but is approximately 1,125 feet per second (or 343 meters per second) at sea level. When an object travels at Mach 3, it moves at about 2,535 miles per hour (or 4,080 kilometers per hour), making it a supersonic speed. Aircraft and missiles designed to operate at Mach 3 are engineered to withstand extreme aerodynamic forces and heat generated by air friction. Notable examples include the Lockheed SR-71 Blackbird, a reconnaissance aircraft, and the Boeing X-51 Waverider, an experimental hypersonic vehicle.