Researchers at NASA's Armstrong Flight Research Center (AFRC) recently completed a study of the feasibility of aerial towing an unpiloted, sub-scale vehicle to supersonic flight conditions to examine the sonic boom that is produced — or, preferably, not produced.
The United States Federal Aviation Administration (FAA) currently restricts commercial supersonic flight over land because the sonic booms can disturb people on the ground and can damage property. Innovators in NASA and industry have identified low-boom aircraft designs and other strategies for reducing the noise and disturbance from sonic boom. NASA's current research focus is getting these technologies out of the wind tunnel and into flight where their performance can be verified in realistic conditions. NASA's goal is to design new sub-scale vehicles aerodynamically shaped from nose to tail to create a quiet thump sound to replace the sonic boom. There is much to be learned about the new supersonic waveform through testing in the real atmosphere. However, building even a sub-scale research aircraft is an expensive proposition. Recently NASA studied alternate approaches to verifying elements of these new designs.
Supersonic aerial towing may provide a low-cost, repeatable means to safely fly a low-boom vehicle design to supersonic conditions, and measure the shape and intensity of its supersonic acoustic signature on the ground. The technique would provide valuable data to validate analytic tools and inform model development. In addition, it could help increase the accuracy of computer simulations of a new supersonic aircraft's acoustic signature and unlock key information about the effects the atmosphere, such as turbulence, has on how these sounds are perceived on the ground.
AFRC's initial study concluded that the supersonic towing concept using a commercial off-the-shelf tow reel with an F-15 airplane is feasible, and a towed shape could be designed to replicate the supersonic “thump” of a larger aircraft. Control system requirements for the towed vehicle and a means of protecting the towline from the airplane exhaust plume require further study.
NASA's recently announced New Aviation Horizons Initiative contains a proposal to jump directly into the design and construction of a larger, piloted, quiet supersonic technology demonstrator aircraft. However, the concept of supersonic towing remains a viable low-cost approach that could be revisited for specific flight research needs.