Proof at Last: The Shaped Sonic Boom Demonstration

After the HSR program dropped plans for an overland supersonic air­liner, Domenic Maglieri compiled a NASA study of all known proposals for smaller supersonic aircraft intended for business customers.[501] In 1998, one year after the drafting of this report, Richard Seebass (now with the University of Colorado) gave some lectures at NATO’s von Karman Institute in Belgium. He reflected on NASA’s conclusion that a practical, commercial­sized supersonic transport would have a sonic boom that was not accept­able to enough people. On the other hand, he believed the recent high-speed research "leads us to conclude that a small, appropriately designed super­sonic business jet’s sonic boom may be nearly inaudible outdoors and hardly discernible indoors.” Such an airplane, he stated, "appears to have a significant market. . . if. . . certifiable over most land areas.”[502]

At the start of the new century, the prospects for a small supersonic aircraft received a shot in the arm from the Defense Advanced Research Projects Agency, well known for encouraging innovative technologies. DARPA received $7 million in funding starting in FY 2001 to explore design concepts for a Quiet Supersonic Platform (QSP)—an airplane that could have both military and civilian potential. Richard W. Wlezien, a NASA official on loan to DARPA as QSP program manager, wanted ideas that might lead to a Mach 2.4, 100,000-pound aircraft that "won’t rattle your windows or shake the china in your cabinet.” It was hoped that a shaped sonic boom signature of no more than 0.3 psf would allow unrestricted operations over land. By the end of 2000, 16 companies and laboratories had been selected to participate in the QSP project, with the University of Colorado and Stanford University to work on sonic boom propagation and minimization.[503] Support from NASA would include modeling exper­tise, wind tunnel facilities, and flight-test operations.

Although the later phase of the QSP program emphasized military requirements, its most publicized achievement was the Shaped Sonic Boom Demonstration (SSBD). This was not one of its original components.

Resurrecting an idea from the HSR program, Domenic Maglieri and col­leagues at Eagle Aeronautics recommended that DARPA include a flight – test program using the BQM-34E Firebee II as a proof-of-concept for the QSP’s sonic boom objectives. Liking this idea, Northrop Grumman Corporation (NGC) wasted no time in acquiring the last remaining Firebee IIs from the Naval Air Weapons Station at Point Mugu, CA, but later determined that they were now too old for test purposes. As an alternative, NGC aerodynamicist David Graham recommended using different versions of the Northrop F-5 (which had been modified into larger training and reconnaissance models) for sonic boom compari­sons. Maglieri then suggested modifications to an F-5E that could flat­ten its sonic boom signature. Based largely on NGC’s proposal for an F-5E Shaped Sonic Boom Demonstration, DARPA in July 2001 selected it over QSP proposals from the other two system integrators, Boeing Phantom Works and Lockheed Martin’s Skunk Works.[504]

In designing the modifications, a Northrop Grumman team in El Segundo, CA, led by David Graham, benefited from its partnership with

a multitalented working group. This team included Kenneth Plotkin of Wyle Laboratories, Domenic Maglieri and Percy Bobbitt of Eagle Aeronautics, Peter G. Coen and colleagues at the Langley Center, John Morgenstern of Lockheed Martin, and other experts from Boeing, Gulfstream, and Raytheon. They applied knowledge gained from the HSR with the latest in CFD technology to begin design of a nose exten­sion and other modifications to reshape the F-5E’s sonic boom. The mod­erate size and flexibility of the basic F-5E design, which had allowed different configurations in the past, made it the perfect choice for the SSBD. The shaped-signature modifications (which harked back to the stillborn SR-71 proposal of the HSR program) were tested in a supersonic wind tunnel at NASA’s Glenn Research Center with favorable results.[505]

In further preparation for the SSBD, the Dryden Center conducted the Inlet Spillage Shock Measurement (ISSM) experiment in February 2002. One of its F-15Bs equipped with an instrumented nose boom gathered pressure data from a standard F-5E flying at about Mach 1.4 and 32,000 feet. The F-15 did these probes at separation distances ranging from 60 to 1,355 feet. In addition to serving as a helpful "dry run” for the planned demonstration, the ISSM experiment proved to be of great value in val­idating and refining Grumman’s proprietary GCNSfv CFD code (based on the Ames Center’s ARC3D code), which was being used to design the SSBD configuration. Application of the flight test measurements nearly doubled the size of the CFD grid, to approximately 14 million points.[506]

For use in the Shaped Sonic Boom Demonstration, the Navy loaned Northrop Grumman one of its standard F-5Es, which the company began to modify at its depot facility in St. Augustine, FL, in January 2003. Under supervision of the company’s QSP program manager, Charles Boccadoro, NGC technicians installed a nose glove and 35-foot fairing under the fuselage (resulting in a "pelican-shaped” profile). The modi­fications, which extended the plane’s length from 46 to approximately 50 feet, were designed to strengthen the bow shock but weaken and stretch out the shock waves from the cockpit, inlets, and wings—keep­ing them from coalescing to form the sharp initial peak of the N-wave signature.[507] After checkout flights in Florida starting on July 25, 2003, the modified F-5E, now called the SSBD F-5E, arrived in early August at Palmdale, CA, for more functional check flights.

On August 27, 2003, on repeated runs through an Edwards super­sonic corridor, the SSBD F-5E, piloted by NGC’s Roy Martin, proved for the first time that—as theorized since the 1960s—a shaped sonic boom signature from a supersonic aircraft could persist through the real atmosphere to the ground. Flying at Mach 1.36 and 32,000 feet on an early-morning run, the SSBD F-5E was followed 45 seconds later by an unmodified F-5E from the Navy’s aggressor training squadron at Fallon, NV. They flew over a high-tech ground array of various sensors manned by personnel from Dryden, Langley, and almost all the organi­zations in the SSBD working group. Figure 9 shows the subtle but sig­nificant difference between the flattened waveform from the SSBD F-5E (blue) and the peaked N-wave from its unmodified counterpart (red) as recorded by a Base Amplitude and Direction Sensor (BADS) on this his­toric occasion. As a bonus, the initial rise in pressure of the shaped sig­nature was only about 0.83 psf as compared with the 1.2 psf from the standard F-5E—resulting in a much quieter sonic boom.[508]

During the last week of August, the two F-5Es flew three missions to provide many more comparative sonic boom recordings. On two other mis­sions, using the technique developed for the SR-71 during HSR, a Dryden F-15B with a specially instrumented nose boom followed the SSBD-modified F-5E to gather near-field measurements. The data from the F-15B probing missions showed how the F-5E’s modifications changed its normal shock wave signature, which data from the ground sensors revealed as persist­ing down through the atmosphere to consistently produce the quieter flat – topped sonic boom signatures. The SSBD met expectations, but unusually high temperatures (even for the Antelope Valley in August) limited the top speed and endurance of the F-5Es. Because of this and a desire to gather more data on maneuvers and different atmospheric conditions,

Peter Coen, Langley’s manager for supersonic vehicles technology, and researchers at Dryden led by SSBD project manager David Richwine and principal investigator Ed Haering, began planning a NASA-funded 4 Shaped Super Boom Experiment (SSBE) to follow up on the SSBD.[509]

NASA successfully conducted the SSBE with 21 more flights during 11 days in January 2004. These met or exceeded all test objectives. Eight of these flights were again accompanied by an unmodified Navy F-5E from Fallon, while Dryden’s F-15B flew four more probing flights to acquire additional near-field measurements. An instrumented L-23 sailplane from the USAF Test Pilot School obtained boom measurements from 8,000 feet (well above the ground turbulence layer) on 13 flights. All events were pre­cisely tracked by differential GPS receivers and Edwards AFB’s extensive telemetry system. In all, the SSBE yielded over 1,300 sonic boom signature recordings and 45 probe datasets—obtaining more information about the effects of turbulence, helping to confirm CFD predictions and wind tun­nel validations, and bequeathing a wealth of data for future engineers and designers.[510] In addition to a series of scientific papers, the SSBD-SSBE accomplishments were the subject of numerous articles in the trade and popular press, and participants presented well-received briefings at vari­ous aeronautics and aviation venues.