These rocket-powered experimental aircraft were designed to test pilots’ ability to withstand the effects of extreme speeds and suborbital flight, measuring accelerations, pressures, shocks, vibrations, temperatures and other data. The programme was also aimed at developing new-generation jets for the first space rockets.

In a letter to Rolex in October 1962, Crossfield spoke of the flawless functioning of his GMT-Master despite the extreme conditions to which it was exposed.