Scott Crossfield – the first test pilot to fly in excess of Mach 2, in 1951 – took part in the X-15 programme. An ambitious project, it began in 1956 and aimed to develop new-generation, more powerful engines that paved the way for the propulsion of the first space rockets. During testing, Crossfield made 14 test flights. In October 1962, he wrote to Rolex declaring his watch to have worked perfectly in temperatures ranging from –54 °C (–65 °F) to 75 °C (170 °F), as well as at altitudes of 76,000 metres (249,000 feet) simulated in a hyperbaric chamber and 28,000 metres (92,000 feet) in real flight conditions.
One of the programme’s test pilots went on to be known as the fastest man of all time by commentators of the era. On 3 October 1967, in the rocket-powered X-15, US Air Force lieutenant and engineer William J. Knight set the highest speed ever recorded, which is unequalled to this day: 7,274 kilometres (4,520 miles) per hour – Mach 6.7. On his wrist was a GMT-Master.