A flight experiment was conducted at the NASA's Dryden Flight Research Center in Edwards, California, using an SR-71, an F-16XL, and a YO-3A airplane (see Figure 1) to study the propagation of sonic booms. This work was geared toward developing a high-speed civil transport (HSCT) aircraft for operational use early in the next century.

At supersonic speeds, an aircraft generates numerous shock waves that emanate from such major components as the nose, canopy, inlets, wings, and vertical tails. These multiple shock waves tend to merge into a strong bow shock and a strong tail shock as they propagate through the atmosphere. At present, supercomputers and wind tunnels are used to predict the structures of these shock waves, but only within a few body lengths of the aircraft. Other computational techniques are then used to determine the propagation and merging of these shock waves down to ground level. To verify and enhance the quality of these computational propagation techniques, a database of sonic-boom measurements at various altitudes was gathered for use by the aerospace industry, universities, and NASA research centers. These organizations will use the enhanced computational techniques in the design of the HSCT. Varying the design of a HSCT could help soften the intensity of sonic booms at ground level.

In the experiment, an SR-71 (commonly known as the Blackbird) airplane, was used to generate the sonic booms. The SR-71 flew at speeds from mach 1.25 to 1.60, altitudes from 31,000 to 48,000 ft, (9.4 to 14.6 km), and gross weights from 73,000 to 118,000 lbf (325 to 525 kN) in steady, level flight. The F-16XL flew below and behind the SR-71, probing in and out of the shock waves generated by the SR-71. Sensitive pressure transducers measured the pressure changes from these shock waves. By flying the F-16XL in formation with the SR-71, the spatial resolution of the shock wave measurements was maximized and the F-16XL gathered 105 probings during only 7 flights. These probings were measured from close to the SR-71 to more than 8,000 ft (2.4 km) below. Both aircraft carried differentially corrected carrier-phase Global Positioning System (GPS) receivers. Subtracting the position of the F-16XL from that of the SR-71 gave relative position and distance with an accuracy on the order of 1 ft (0.3 m).

These signatures are based on preliminary measurement data acquired by flying the F-16XL at several distances below the SR-71. The SR-71 was flying at mach 1.25 and an altitude of 31,000 ft (9.4 km).

A large array of sonic-boom recorders was deployed on the ground under the flight track of the SR-71 to measure the sonic-boom signatures. During eight SR-71 flights, 172 separate ground-level signatures were recorded. Sonic booms are affected by temperature, wind, humidity, and turbulence as they propagate through the atmosphere. In particular, the turbulent atmospheric layer near the ground distorts the sonic-boom signatures. To provide undistorted data at low altitude, the low-speed YO-3A airplane was flown at an altitude of 10,000 ft (3 km) to record the sonic booms above the turbulent atmospheric layer. Sensitive microphones were located at each wing tip and the tip of its vertical tail. The YO-3A recorded 17 passes of the SR-71. Sonic booms that had first reflected from the ground and then propagated upward to the YO-3A were also recorded. Atmospheric data were gathered for use in analysis of flight data and in computing codes for predicting the propagation of sonic booms. The flight-test techniques used in this experiment to measure pressures and relative aircraft separation accurately are also under consideration for use in other flight research unrelated to sonic booms.

Preliminary analysis of the data shows the attenuation and lengthening of the sonic boom signatures as they propagate through the atmosphere, as well as the merging of individual shock components. The mach number, altitude, and gross weight (which translates into lift) all affect the propagation characteristics. Analysis of the flight data and ground-recorded signatures continues, but preliminary results have been published in papers from the NASA High Speed Research Program's 1995 Sonic Boom Workshop.