Charles Marfre, Jr., age 10, of North Brunswick, New Jersey, for his question:
What actually is the sound barrier?
Until 1947, no pilot had ever flown at 700 miles an hour and lived to tell about it. The airplanes designed in the early 1940s shuddered and shattered as they reached what we call the speed of sound. Many people despaired and said that no plane would ever fly faster than this mysterious sound barrier.
The sound barrier and the thermal barrier are problems for the experts who design fast planes. Both of these barriers are related indirectly to the speed of sound and the way in which sound travels through the air. A sonic boom from a high flying jet tells us that the aircraft has reached a speed as fast or faster than the speed of sound. It has broken through the sound barrier. A plane at supersonic speeds must also cope with the thermal or heat barrier. If the plane is not properly designed, the heat caused by supersonic shock waves may melt the metal on its nose and wings.
Sound travels out from a vibrating object in waves. Its pulsing energy keeps pace with the vibrating drum or other object that starts it and its energy gets weaker as it goes. It must travel through a substance such as air, water or steel or some other medium. And it travels at different speeds through different substances. At sea level, it travels through ordinary air at about 1,100 feet a second, or 750 miles an hour. Sound waves may travel four times faster than this through water and more than 15 times faster through steel.
The sound barrier that concerns a plane designer is the speed of sound through the air. Something very dramatic occurs when a plane reaches this speed. At slower speeds, streams of gaseous air molecules flow smoothly around the graceful surface of the plane. They form cushions of pressure that lift the flying plane and help to keep it from falling. But this friendly assistance breaks down as the plane reaches the speed of sound. Its nose seems to meet a sudden resistance like a stone wall. The smooth air currents become shuddering shock waves that punch the plane like giant invisible fists.
This is the so called sound barrier and supersonic planes are designed to cope with it. It occurs because air molecules can stream smoothly around a moving object so fast and no faster. When the object reaches that certain speed called the speed of sound, the air molecules cannot move around fast enough. Instead, they pile up in front of the onrushing plane forming a strong barrier of resistance. Shock waves are produced when the plane forces its way through the resisting air barrier and the result is what we hear as a sonic boom.
As we have noted, the speed of sound at sea level is about 750 miles per hour and at this speed a low flying plane may crash into the sound barrier. Sound travels slower through the cool, thin air at higher altitudes. A plane at 40,000 feet may reach the sound barrier at 660 miles per hour. Supersonic speed is figured in Mach numbers. A pilot reaches Mach One at the crashing moment of the speed of sound and this varies with altitude and temperature. He reaches Mach Two at double and Mach Three at triple this speed.