WILL LOW-BOOM FLY?

High speed without a sonic boom is a cherished goal of supersonic business jet designers, and four teams are working to further the technology under NASA contracts, with results to be reviewed early next year.

Aerion Corp. and Supersonic Aerospace International will be presenting updates on their SSBJ designs at the National Business Aviation Assn. show in Orlando, Fla., Nov. 9-12. Both companies are proposing Mach 1+ airplanes.

The utility of a supersonic aircraft would be greatly increased if it could fly overland unrestricted by sonic boom. But penalties of drag, weight and cost could easily make it a non-starter. Some think it possible, but there is not the proof to justify such an expensive project.

Recent changes in NASA's priorities have set back tests that would help answer basic questions. The agency's plan was to build a second manned low-boom demonstrator aircraft, and it wanted to issue a request for proposals as early as last September. It would have been a follow-on to the successful Shaped Sonic Boom Demonstration (SSBD) aircraft that flew about two years ago.

BUT BARELY TWO months into the July-awarded concept exploration contracts, Lisa Porter, NASA's new associate administrator for aeronautics, told the teams on Aug. 30 that there no longer was funding for a demonstrator. Team members are trying to devise cheaper alternatives for the next phase of research, but turmoil continues in the agency's aeronautics plans.

A sonic boom is a pair of closely-spaced sharp pressure rises, each usually 1 psf. or more. The Concorde made about 2 psf. overpressure at Mach 2, whereas an F-5 fighter at Mach 1.4 makes about 1.2 psf. It's still unclear what is acceptable, but the Defense Advanced Research Projects Agency (Darpa) was using 0.3 psf. as its goal recently and other experts agree that is the right ballpark.

NASA's Dryden Flight Research Center has taken steps to see what the public finds acceptable. It devised a "Low-Boom" Mach 1.1, 53-deg. dive technique that can produce booms lower than 0.1 psf. on the ground from an F/A-18 that normally makes a 1.5 psf. boom, to simulate a quiet aircraft. Observers are 20 mi. ahead of the trajectory's ground intercept point, which greatly reduces intensity. The dive keeps the sound waves from being refracted away from the ground by the atmosphere's temperature profile.

Low Boom/No Boom Principal Investigator Edward A. Haering says a 0.5-0.6 psf. boom may not be heard among urban noise. At 0.1 psf., it may not be noticed in normal conversation. Not only are the pressures low, but the rise time slows, making the wave less perceptible.

The main focus of boom reduction efforts is to shape the pressure wave along the length of the aircraft so it won't coalesce into the standard sharp N-wave by the time it hits the ground. Spreading pressure over the signature's length reduces the abrupt changes at the beginning and end of the signature, which are what humans hear.