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.

Large nose was grafted onto F-5 Shaped Sonic Boom Demonstrator (SSBD) (above) so area could be pinched in sharply near inlet and wing to weaken their shock waves and prevent them from joining leading shock wave.

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.

The N-wave is formed by shock strength and temperature variations in the pressure wave. The net result is shock waves--created by components such as the cockpit, inlets, wing and tail--that march either forward or aft in the signature as it moves away from the aircraft, and coalesce into single strong shocks at the front and back--the N-wave. The leading and trailing edges are slightly divergent because they are operating in different upstream conditions, and the N-wave on the ground may be twice or greater the length of the aircraft.

It may take 10,000 ft. or more for the N-wave to coalesce. Haering led an experiment in 1995 that measured the details of the pressure wave at various distances from a 107-ft.-long Lockheed SR-71. The canopy waves could still be identified from 10,000 ft. away. This is the mid-field region. The far field is where pressures have resolved to an N-wave, and the near field is immediately adjacent to the aircraft.

In the late 1960s, Cornell University Profs. A. Richard Seebass and Albert R. George devised a method to design near-field pressures to reduce the sonic boom by extending the mid-field region down to the ground. The uncoalesced signature would have weak leading and trailing edges and sound quiet. The idea is to control the strength of shock waves from aircraft components so that they travel parallel to the leading and trailing waves and delay coalescence. A part with an unavoidably strong shock, like the wing, may be compensated for with an adjacent region of expansion, perhaps by pinching in the fuselage, to reduce net shock strength.

Low-boom design took a big step under the Quiet Supersonic Platform (QSP) program that Darpa started in 2000. The notional aircraft was close to a business jet: 6,000-naut.-mi. range, Mach 2.0-2.4 cruise, 100,000-lb. maximum takeoff weight and a high payload fraction of 20%. Phase 2 study and test contracts worth about $2.5 million each were given in March 2002 to Lockheed Martin Advanced Development Co. and Northrop Grumman.

Graph of sonic boom on ground shows the modification (blue line) worked by changing shape of leading wave at upper left.

A separate $3.4-million contract for the SSBD was awarded to Northrop Grumman in 2002 to modify an F-5E fighter for the first flight test of a low-boom design, using the Seebass-George principles (see photo above). SSBD was a culmination of the QSP program, which ended in late 2003.

The F-5E flew in the summer of 2003 under SSBD, then made 21 follow-on flights in January 2004 under the NASA-funded shaped sonic boom experiment (SSBE). The key result is illustrated in the graph of signatures measured on the ground (see above). The plateau at the top of the blue line shows the experimenters were able to prevent the leading edge of the signature from fully coalescing. This resulted in a 30% drop of the pressure spike, to 0.85 psf. from 1.2 psf., and, equally as important, a halving of the spike rise rate.

Lockheed Martin concept for business jet shows low-boom design doesn't need to be ugly like the SSBD. Lift and fuselage area are distributed to reduce shock waves. Tail structure supports engine weight.

For practical considerations, only the front of the F-5E was modified, leaving the trailing wave to be coalesced. As an example of the changes, the aircraft inlet made a strong shock wave that would normally march forward and join the leading wave. Engineers pinched the fuselage bottom where the inlet shock went by, creating a region of expansion that reduced shock strength, slowing forward motion of the wave.

Opinions vary about the ability to keep drag down. Kenneth J. Plotkin, an expert on boom propagation and low-boom design with Wyle Laboratories, thinks that the quiet shape is different than one designed for low drag. But Thomas M. Hartmann, the Lockheed Martin project manager for quiet supersonic transports, says "there was an aero penalty five years ago, but we say there is no aero penalty now. Improvement in design tools make low boom compatible with low drag."

NASA's sonic boom mitigation project (SBMP) is the successor to SSBE and is the one whose plans to build a second demonstrator have been cut short. It is a joint project of Dryden and the Langley Research Center. The four teams that received contracts in July, worth about $1 million each, are Boeing Phantom Works, Raytheon Aircraft, Northrop Grumman/ Gulfstream Aerospace, and Lockheed Martin Advanced Development Projects/Cessna Aircraft.

The contracts are to determine whether a supersonic aircraft with an acceptable boom (about 0.3-0.5 psf. initial overpressure) is feasible, and to produce design requirements for a demonstrator. "Most approaches were new aircraft," says Joseph Pawlowski, the Northrop Grumman project manager. Radical shaping may be the only way to achieve the tough goals. He thinks a demonstrator would weigh 30,000-50,000 lb. But it may be that a modified aircraft will be the only thing cheap enough to have a chance of restarting the program.

The QSP program concluded that no one technology could make a practical low-boom aircraft. Shaping is important, but so is light weight, achieved through a combination of better lift-to-drag ratio, improved engine performance and advanced materials. For example, Darpa was looking at natural laminar flow aided by distributed roughness to inhibit crossflow instabilities, and foamed metals. Sounds clever--and expensive. A lot of costly things may have to come together to make a low-boom business jet.