On August 14 (1958), on its 38th flight, the prototype reached Mach 2 (twice the speed of sound) for the first time. Highest speed attained was Mach 2,39, but remarkably even at this speed the aircraft continued to accelerate, going Mach 0.1 faster every 17 seconds! Only a seemingly small detail kept the F8U-3 from going faster: the cockpit glazing fitted, and especially the frontal part, was not designed for speeds faster than Mach 2.2. Going faster than that and the windscreen would soon turn opaque and melt, all due to the friction and subsequent heat generated by the onrushing air. Of course developing new cockpit glazing that would eliminate this restriction was a high priority item, but was never implemented. Otherwise the true potential of the Crusader III would have been revealed. Experience gained from the flight test program predicted a (rather pessimistic) maximum speed of Mach 2.6 at an altitude of 35,000 ft (10,700 m), but near Mach 2.9 was more likely. With some modifications Mach 3 would have been within easy reach, but even without the Crusader III would have become the fastest fighter aircraft in the world. Not only the maximum speed was remarkable, the acceleration too was simply breathtaking, even without the proposed rocket engine. At an altitude of 35,000 ft (10,700 m) at Mach 0.98 the first prototype needed only 3 minutes and 54 seconds to accelerate to Mach 2.2. Only a few fighters of today can match that! The second prototype with its slightly lower powered engine however needed no less than 9 minutes, and it was predicted that the production aircraft's figure would lie somewhat in the middle. Maximum altitude was also not bad, the F8U-3 could sustain 60,000 to 65,000 ft (almost 20,000 m). With a ballistic "zoom climb" trajectory the aircraft could top 90,000 ft (more than 27 kilometers).