In last week’s blog post, we discussed one of the great impediments to commercially successful supersonic aircraft: the sonic boom. A theory based on shaping of supersonic booms in order to reduce the pressure wave—the noise—began to emerge in the late 1960s.
The theoretical models—first developed by two Cornell University aerospace engineers, Richard Seebass and Albert George—focused on techniques for the reduction of the first or front part of the supersonic N-wave. Despite the development of Seebass-George theory in the late 1960s, it wasn’t until the early 1990s that the computational resources—in the form of more capable Computational Fluid Dynamics software enabled by faster hardware—necessary to design the shape of the test aircraft in accordance with the dictates of Seebass-George theory became available. Nearly thirty-five years passed before this theory was subjected to flight-testing in August 2003 and January 2004 as a part of the DARPA Quiet Supersonic Platform (QSP) program.
During the QSP program, Seebass-George theory eventually met practice in the guise of the SSBD aircraft, a heavily modified F-5E. The F-5E was chosen after flight test program proposals based on modifying a Firebee II drone or an SR-71 were rejected for technical risks and costs. The F-5E worked because of the wide range of nose shapes already flown as a part of the F-5 family (the nose of the reconnaissance version RF-5 differs from the F-5E, and the two-seat F-5F is different still) and because of the familiarity of one of the QSP contractors, Northrop Grumman, with the F-5. Prior to its merger with Grumman, Northrop manufactured more than 900 of the F-5E/F series of aircraft and more than 2000 of the closely related T-38 and first-generation F-5 airframes.
SSBD design work began in late 2001. Construction of the Seebass-George glove to replace the F-5E’s nose took place at Northrop Grumman’s El Segundo operation in California, and the glove was installed on the F-5E airframe in January 2003 at Northrop Grumman’s St. Augustine facility in Florida. Prior to testing, the SSBD’s fuselage was emblazoned with a paintjob that graphically depicted two N-waves superimposed upon each other, one, in red, an unmodified waveform and the other, in blue, with the “flat-top” signature that indicates a reduced sonic boom.
Most of the SSBD flight test program consisted of identical runs through Edwards Air Force Base airspace by the SSBD and an unmodified Navy F-5E. The two aircraft, flying at Mach 1.36 and 32,000 feet, were separated by 45 seconds, a timeframe deemed long enough to allow the shockwave from the SSBD to dissipate, but short enough so that the unmodified F-5E passed through an atmosphere that hadn’t evolved enough to invalidate comparisons between the two runs. Other test runs involved collecting pressure measurements from a NASA F-15B flying in the SSBD’s shockwave. A glider flying beneath the test flight path also collected test data. By the end of the two test sequences, more than 1300 sound and pressure measurements were taken on the ground and in the air.
The flight test sequence confirmed the nearly one-third reduction in the leading portion of the pressure wave by the reshaped nose (the glove), as predicted by Seebass-George theory. The test team exhibited a high degree of confidence in the theory from the beginning of the program. The results indicated that the shape of the new nose prevented the bunched pressure waves from forming into one large shock wave.
After completion of the flight tests, the SSBD aircraft was given over to the Valiant Air Command (VAC) Warbird Museum located just a stone’s throw from NASA’s Kennedy Space Center on the grounds of the TICO airport in Titusville, Florida. The VAC’s mission dictates that its collection only include warbirds. VAC Public Affairs Officer Terry Yon, a retired Army colonel and helicopter pilot who flew in Vietnam, says that the museum happily made a “squishy argument” based on the SSBD’s origins as a Navy aggressor aircraft to include it in the museum’s collection. After all, few truly unique aircraft exist, and this modified F-5E is indeed one of a kind.
If supersonic transports and business jets are ever to reach the air, let alone their potential, it must be demonstrated that they can fly over land at supersonic speeds without causing a ruckus. By confirming the potential for shaping supersonic shockwaves in a manner that diminishes their impact, the SSBD program took the first step toward accomplishing sonic boom-lite flight. As such, the SSBD program is destined to have long-lasting effects.
Bernard Roussett, COO of HyperMach, one of the companies announcing an super-sonic business jet at Le Bourget 2011, told us in an email about the SonicStar: “Yes, our solution for reducing significantly the sonic boom at high mach number (still supersonic!) is partly inspired from the DARPA program.” Only months before Concorde flew its final commercial flights, the SSBD aircraft made a supersonic future seem possible again.
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