As the projectile slowed from its initial speed and began to approach the speed of sound, it would undergo a rapid increase in drag and slow much more rapidly. This could be seen in tests using projectiles fired from guns, a common method for checking the stability of various projective shapes. In particular, the very simple theory of thin airfoils at supersonic speeds produced a curve that went to infinite drag at Mach 1, dropping with increasing speed. The existence of the sound barrier was evident to aerodynamicists before any direct in aircraft evidence was available. Meteorites in the Earth's upper atmosphere usually travel at higher than Earth's escape velocity, which is much faster than sound. This finding is theoretical and disputed by others in the field. Some paleobiologists report that computer models of their biomechanical capabilities suggest that certain long-tailed dinosaurs such as Brontosaurus, Apatosaurus, and Diplodocus could flick their tails at supersonic speeds, creating a cracking sound. The sound barrier may have been first breached by living beings about 150 million years ago. Firearms made after the 19th century generally have a supersonic muzzle velocity. Some common whips such as the bullwhip or stockwhip are able to move faster than sound: the tip of the whip exceeds this speed and causes a sharp crack-literally a sonic boom. By the 1950s, new designs of fighter aircraft routinely reached the speed of sound, and faster. In 1947, American test pilot Chuck Yeager demonstrated that safe flight at the speed of sound was achievable in purpose-designed aircraft, thereby breaking the barrier. These difficulties represented a barrier to flying at faster speeds. The term came into use during World War II when pilots of high-speed fighter aircraft experienced the effects of compressibility, a number of adverse aerodynamic effects that deterred further acceleration, seemingly impeding flight at speeds close to the speed of sound. Flying faster than sound produces a sonic boom. The term sound barrier is still sometimes used today to refer to aircraft approaching supersonic flight in this high drag regime. When aircraft first approached the speed of sound, these effects were seen as constituting a barrier, making faster speeds very difficult or impossible. The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound.
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