BYLINE:Chuck Yeager climbed into the cockpit of his X-1 aircraft on the morning of October 14, 1947. Ahead of him lay a frontier that many believed impossible to cross: the sound barrier. But the 24-year-old American air force captain thought different.

He was aware of the risk he was taking: Second World War fighter pilots had increasingly encountered the problem as they attempted to push their aircraft to ever greater speeds. As they approached the sound barrier, they battled to keep control as their planes were violently buffeted. Often, the aircraft simply shattered. Sound waves were simply too slow to get out of the way. They built up in a “plug” of chaotic air molecules in front of the aircraft, combining to form a violent shock wave that could be heard as a thunderous boom. The aircraft had to bludgeon its way forwards, while previously it had cut through the air like a knife.

In the picture of the modern American F-18 Hornet, the shock wave has caused a cloud of water droplets to condense from the air. The trauma was simply too much for standard aircraft - new and radical designs were vital if supersonic flight was to be realised. But attempts often ended in catastrophe. The British programme ground to a halt early in 1947 when Geoffrey de Havilland’s tailless DH 108 Swallow aircraft disintegrated during a high-speed dive.

The shape of the X-1 was based on something everyone knew could fly faster than sound: a bullet. Powered by a liquid-fuel rocket engine that mixed ethyl alcohol and liquid oxygen, the X-1 had a unique, movable tail that allowed the pilot to maintain control as the plane broke the sound barrier. But would this be enough? Yeager was the ideal man to find out. He thrived onthe danger.

He vomited during his first flight. Inside the X-1‘s cockpit, conditions were cramped and uncomfortable. Yeager sat in front of the liquid oxygen tank, which was cooled to a temperature of - 149C, while outside the air was - 58C. The X-1 was launched from the belly of a Boeing B-29 bomber at 6,400 metres. Yeager powered up the four rockets, felt his aircraft gripped by the familiar turbulence, throttled up still further - and then suddenly the flight smoothed out.

The speed of sound varies depending on conditions such as temperature and air pressure. At ground level it is about 750mph, while at an altitude of 13,100 metres - the height Yeager reached - it is approximately 700mph. Soon after he passed this speed, his colleagues back on the ground heard a boom as the shock wave travelled towards them. Yeager became a national hero, lauded by presidents and immortalised in books and movies.

For several decades, the American military competed with the Soviets for faster aircraft, and by 1967 was building planes that could reach speeds of more than six times the speed of sound. By the time Yeager broke the sound barrier again in 1997, to mark the 50th anniversary of his historic achievement, supersonic flight had become routine.

Web links

Interview with Chuck Yeager: www.achievement.orgautodocpageyea0pro-1

Fifty years of supersonic flight:

www.boeing.comdefensespacemilitaryf15barrierSonic booms:

www.discovery.comareaskinnyonskinnyon.html

This picture appears in the World Press Photo Exhibition, Royal Festival Hall, London, from today until October 15, 10.30am-10.30pm. Admission is free

steve farrar

Steve Farrar is science reporter for the Times Higher Education Supplement