"Amongst the pilots who came to rely on the GMT Masters were the cadre of probably the finest (certainly the bravest) pilots in the world; those chosen by NASA to fly the North American X 15. The history of the X-15 deserves more than a throwaway paragraph; so I have taken the liberty of quoting from NASA's official history below.

An unofficial motto of flight research of the 1940s and 1950s was "higher and faster." By the late 1950s the last frontier of that goal was hypersonic flight (Mach 5+) to the edge of space. It would require a huge leap in aeronautical technology, life support systems and flight planning. The North American X-15 rocket plane was built to meet that challenge. It was designed to fly at speeds up to Mach 6, and altitudes up to 250,000 ft. The aircraft went on to reach a maximum speed of Mach 6.7 and a maximum altitude of 354,200 ft. Looking at it another way, Mach 6 is about one mile per second, and flight above 265,000 ft. qualifies an Air Force pilot for astronaut wings. The plane was air launched by NASA's converted B-52 at 45,000 feet and a speed of 500 mph. Generally there were two types of flight profiles: high-speed, or high-altitude. High-speed flights were usually done below an altitude of 100,000 feet and flown as a conventional aeroplane using aerodynamic controls. High-altitude flights began with a steep, full-power climb to leave the atmosphere, followed by up to two minutes of "coasting up" to the peak altitude after the engine was shut down. "Weightless" flight would last for 2 - 5 minutes as it made a ballistic arc before re-entering the atmosphere. A reaction control system was used to maintain attitude above the atmosphere. The reaction controls employed hydrogen peroxide thrusters located on the nose and wings. A typical research flight lasted about 10 or 11 minutes while covering nearly 400 miles along a course that stretched from Smith Ranch, Nevada to Edwards Air Force Base. The X-15 program made many accomplishments; here is list of some of its contributions to space flight:

First use of a full-pressure suit for spaceflight.
First use of reaction controls for manoeuvring in space.
First use of a flight control system that automatically blended aerodynamic and reaction controls.
Development of thermal protection for hypersonic re-entry.
Development of the first large, restartable, and throttleable rocket engine.
Development of an inertial guidance system.
Demonstration of a pilot's ability to operate in "micro-gravity".
Demonstration of the first piloted re-entry-to-landing from space.
Acquisition of hypersonic acoustic measurements, which influenced structural design criteria for Mercury capsule.
Verification of the validity of hypersonic wind tunnel data, which were later used in the design of the Space Shuttle.

One of those pilots, Pete Knight, wrote the following to Rolex after one flight. “I finally flew on October 3, 1967 to a speed of 4,534 mph (7,269 kph) or Mach 6.72 and all systems functioned properly with the exception of some local heating damage on the lower ventral. I have been wearing my Rolex GMT-Master for a period of months now and have calibrated it to within a few seconds a day” The throwaway phrases “Mach 6.72” and “some local heating damage on the lower ventral” disguised the real facts. Knight had flown the X-15 to a world record speed that still stands over 40 years later and had brought the plane home after temperatures later determined to have been above 1650°C (3000°F) burned a ramjet engine off its pylon and seared a hole measuring 18 by 8 centimetres into the ventral fin's leading edge. An airscoop effect channelled hot air into the lower fuselage and damaged the propellant jettison system meaning that Knight eventually had to land the plane 680 kilograms heavier than planned because he could not jettison the residual fuel. If the heat had damaged the craft's hydraulics, Knight might have had to abandon the plane. Fortunately, that did not happen. Knight landed at Edwards Air Force Base with the plane resembling burnt firewood. It seems amazing that the plane made it and even more so that despite acceleration of more than 3.5G the Rolex GMT Master on his wrist also performed perfectly. The other X-15 pilots also wrote to Rolex about their watches but the story of Pete Knight’s final flight is perhaps the most interesting.

It is worth noting that while Knight wore a Rolex GMT Master on his world record flight he was following in the grand tradition of Edwards Air Force Base, for it was there in 1947 that Chuck Yeager first broke the “Sound Barrier” in a Bell X-1 and did so wearing a Rolex Oyster; the same one he had worn all through World War II. "