Sonic Booms

Just as the X-15 program was winding down, researchers noted that airplanes flying faster than Mach 3 (the YF-12C/SR-71A and XB-70A) did not generate a sonic-boom noise. NASA had made some measurements at Mach numbers up to 16 during the liftoff and reentry of Apollo spacecraft, but did not consider this representative of future aerospace vehicles, such as the Space Shuttle. Therefore, researchers made measurements during several X-15 flights at Mach numbers up to

5.5 and compared these with results obtained by theoretical methods of determining overpressures.-12121

For flight 1-70-119, instruments were set up at Mud Lake to record the boom generated at Mach 5.3 and 92,000 feet. Researchers obtained satisfactory data even though the airplane was about 6 miles east of the monitoring site. The sonic boom was a typical far-field signature with some slight atmospheric distortion, although this was less than predicted. The boom peak overpressure was about 0.34 psf.12131

environmental conditions based on data obtained at Edwards. When the airplane arrived over Goldstone at Mach 4.8, the engine was operating at 50% thrust and the speed brakes were extended; at Cuddeback the engine had already shut down, the speed brakes had retracted, and the airplane was at Mach 3.5. Although the flight plan called for the airplane to fly directly over the microphone arrays, in reality it passed 1.7 miles south of the array at Goldstone and 7.9 miles south of the Cuddeback array-not an unusual amount of error for an X-15 flight.-1214!

Researchers scaled and corrected the data collected from Goldstone so they could compare it with similar data obtained from an SR-71 flight. The two sets of data were in general agreement. Researchers did not evaluate the data from Cuddeback because of the X-15 miss distance. The results of the experiment also compared favorably to theoretical results, and no unusual phenomena related to the overpressure were encountered.12151