Simulated Sonic Booms Shock Silently
I could tell something was wrong. From the cockpit window of my F-16 fighter jet, the horizon froze. My co-pilot, Air Force Captain Eric Rokke, told me I had lost a little elevation.
"You're flying under the ground right now," he said through the headset, explaining to me that I'd made a mistake in reading the altimeter.
I didn't die because I was in the F-16 flight simulator at Eielson Air Force Base, researching a column on sonic booms with Rokke, a pilot with whom I spent two weeks near the Charley River two summers ago. While camping on a hilltop there, we tried to monitor the sonic booms of his squadron-mates and other pilots involved in Cope Thunder, a military exercise that included the airspace above Yukon-Charley Rivers National Preserve, where I was a ranger.
The sonic booms we heard and felt on the Charley River ranged from what sounded like a distant thunder clap to those that sounded as if someone behind you had unexpectedly fired a 12-gauge shotgun into the air.
After researching numerous journals and scientific encyclopedias, I learned that sonic booms are not a "boom" at all, but a continuous air-pressure disturbance caused by any object exceeding the speed of sound.
When an object is moving slower than the speed of sound, which is about 1,100 feet per second at sea level (about 768 miles per hour), sound waves precede the object like ripples in a pond. A moving car, for example, sends out waves of noise that can alert a raven standing on the road to its approach, allowing the raven to fly away before getting hit.
Because a supersonic jet is moving faster than the sound waves it's producing, air molecules don't have time to get out of the way. The leading edges of the jet---the nose, wings and tail fins---don't disturb the air molecules until they approach within a half-inch. The jet then displaces the air within a few millionths of a second, and the resultant, dramatic air-pressure change produces a shock wave.
The shock wave created by a supersonic jet resembles a cone, with the pointed end at the nose of the jet and the wide end flaring out behind. The jet drags this cone along behind it the entire time the pilot is flying faster than the speed of sound. A sonic boom is felt on the Earth's surface as the cone of the shock wave fans out to intersect the ground. Because people are stationary in relation to the jet, they only feel the brief, explosive impact of the air pressure change.
The intensity of a jet-produced sonic boom depends largely on the jet's elevation when it's moving faster than the speed of sound. When a jet flies at 1,000 feet, for example, the shock wave cone it produces is much stronger than the same cone produced at 20,000 feet, because much of the energy of the shock wave created at high altitudes is dissipated in the atmosphere before it hits the surface.
The F-16 simulator showed how easy it is for a pilot to inadvertently break the speed of sound. The pilots don't feel the boom; the only indication that they've broken the sound barrier is the silent change of a Mach number gauge from .99 to 1.00. To add to a pilot's challenge, in the thin air of 20,000 feet, an F-16 breaks the sound barrier with much less speed than it takes at 1,500 feet, where the atmosphere is much denser and much more resistant to the jet's movement.
Rokke also pointed out that during war simulation, a pilot is thinking about avoiding or pursuing the enemy and other things besides breaking the sound barrier.
During peace-time drills, however, sonic booms are a concern to people who get their windows broken and their peace shattered by shock waves, so the simulator is useful in quietly showing how sonic booms are produced. In my case, it also saved the Air Force a few F-16's.
