There are, in fact, investigations underway looking into the effects of sonic booms of structures on the ground. It is a significant pressure differential, and depending on how big that is, you can have different levels of interaction. Some sonic booms sound like thunder, some sound like gunshots. And obviously, the walls of buildings and structures react to that pressure differential. There are a lot of human factors: if you heard a gunshot every five minutes, you might go a little crazy. But part of the Quiet Spike mitigation technology is what level sound one might be able to deal with. What if we could mitigate the sound to something like distant thunder or a train going by? Human factors-wise, how would people react to that? There are a lot of questions that need to be answered still.

NTB: Describe how a test with the Quiet Spike was conducted.

Molzahn: It was about a yearlong investigation. Research testing of the Quiet Spike consisted of numerous ground and flight operations. Each step in the process had unique objectives, and involved numerous test team members from NASA Dryden and Gulfstream Aerospace Company. Once we had the hardware, and we were ready to put it on an aircraft, it was a yearlong process of ground and flight testing integration and ground and flight testing. Obviously, there was a lot of Gulfstream work prior to the point of integrating it onto an aircraft. They spent years designing, developing, and building this technology.

When NASA Dryden integrated the Quiet Spike onto our testbed, it actually took a couple of months. We then did a series of ground tests to look into predictions of how it would behave structurally in flight — loads testing, ground vibration testing, and structural mode interaction testing. Once we got through all of the ground testing, we started flight testing. A series of ground tests with the Quiet Spike installed on the F15B included loads calibration tests, drag loads capability tests, a ground vibration test (GVT), and a structural mode interaction test (SMI). These ground tests evaluated the structural instrumentation, mechanical function, and predicted dynamic response of carrying the Quiet Spike on the aircraft during flights. Each of these ground test operations provided valuable data on the road towards preparing for Quiet Spike research flights. We had about 30 flights. We performed maneuvers in the retracted configuration, where the Quiet Spike measures 14', and then at the extended configuration, where it telescopes out another 10', making for a total of 24' out in front of the aircraft. We did envelop expansion for those configurations out to Mach 1.8, which is almost twice the speed of sound. At Mach 1.4, we did an investigation where we had another aircraft, a NASA A-36, fly next to the testbed in an investigation of near-field pressure distribution to see how effectively the Quiet Spike broke up the shockwave. The first flight was flown on August 10, 2006, with the Quiet Spike retracted and the F15B landing gear extended for the duration. Envelope expansion, utilizing a risk reduction build-up methodology, commenced on the second flight of Quiet Spike. Subsonic flight envelope expansion was completed on October 17, 2006, for the Quiet Spike in the extended position. Envelope expansion included numerous maneuvers to assess aerodynamics, stability and control, flutter, aeroservo-elastic effects, and structural loads. These maneuvers included push-over pull-ups, wind-up turns, lateral and longitudinal stick raps, rudder pedal kicks, pitch and roll doublets, and beta sweeps. Engineers in the NASA Dryden Mission Control Center evaluated the maneuvers in real time, in order to progress through envelope expansion.