Page 24 - January 2006

prototypes optimized to produce minimal sonic booms, and these booms might be non-obtrusive for small vehicles. Because aircraft weight is a contributing factor to the ampli- tude of the sonic boom, the only currently viable “low-boom” designs are for small jets. Large supersonic airliners with “low-boom” designs are not yet possible with current tech- nology.
In addition to sonic boom there are, of course, many
other considerations one must take into account for civil
supersonic flight—take off and landing noise, emissions,
operational issues, etc., as well as the tradeoffs one must
make between the environment and the desire of humans to
go faster. Many of these issues have been discussed in-depth
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by Fisher, et al. , so the reader is directed toward their work
regarding such tradeoffs. However, the “main issue” of con- cern for civil supersonic flight is sonic boom.
Theory says “low-boom” design is possible, but does it really work in a real atmosphere?
Defense Advanced Research Projects Agency/National Aeronautics and Space Administration (DARPA/NASA) breakthrough
Fig. 3. F-5E (left) and SSBD (right) aircraft. (Courtesy Northrop Grumman Corporation.)
There were exciting developments in 2003 and 2004 by the DARPA/NASA Quiet Supersonic Platform Program, and this has been reported in the popular media12 as well as in
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technical publications . A Navy F-5E aircraft was physically
retrofitted to have a nose which was especially shaped to change the aircraft’s sonic boom signature (see Fig. 3). The airplane, called the Shaped Sonic Boom Demonstrator (SSBD), and an unmodified F-5E airplane were flown back to back supersonically to produce ground signatures. The intended area shaping of the SSBE did indeed produce a sonic boom pressure versus time waveform whose shape persisted all the way to the ground, validating the CFD and optimization predictions (see Fig. 4). In blue the SSBE acoustic pressure versus time signature has a “flat top” on the positive portion of the waveform. The unmodified F-5E sig- nature in red was the usual N-wave shape. Clearly the design process followed to shape the SSBD ground signature worked well.
This breakthrough grabbed the attention of the worldwide aerospace industry as well as U.S. and foreign governments. The result has been a renaissance in sonic boom research.
Fig. 4. First measurement of shaped sonic boom, August 27, 2003. Sonic booms compared for SSBD and F-5E. (Courtesy Northrop Grumman Corporation and Wyle Laboratories.)
Results from the International Sonic Boom Forum
As noted, the ASA co-sponsored the 17th
International Symposium on Nonlinear Acoustics
(ISNA17) and the International Sonic Boom Forum (ISBF)
in State College, PA, USA in July 2005. This is the most
recent major meeting on sonic boom in the U.S. in the last
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seven years . Summaries of some of the technical presen-
tations given at the ISBF appear in this article as an appen- dix.
Recent NASA low-boom flights
Two papers at the ISBF discussed a series of new exper- iments recently completed at NASA Dryden Flight Research Center. Peter Coen, of NASA Langley Research Center, pro- vided an overview of NASA's Vehicle Systems Program efforts related to the reduction of sonic boom and explained that the purpose of the low-boom experiments is to deter- mine the threshold of acceptability of low overpressure N waves. These low overpressure N waves, as will be described in a moment, can be created by careful maneuvering of existing aircraft. The first goal of this testing is to show the feasibility and repeatability of generating the low overpres- sure N waves (< 0.6 psf) within a specific geographic area with F-18 aircraft in a low-lift flight condition.
In a separate paper providing details of the effort, Ed Haering of NASA Dryden presented a paper with coauthors James Smolka and James Murray of NASA Dryden and Ken Plotkin of Wyle Laboratories. Haering indicated that these low amplitude N-waves have been produced from the top of an F-18 in a supersonic dive. This boom from the upper por- tion of the aircraft does not include lift, and thus has a lower boom amplitude that the usual boom off a supersonic air- craft’s lower side. Maximizing the aircraft altitude and the sonic boom propagation distance also minimizes the over- pressure.
Figure 5 shows the path of a typical flight experiment to measure low overpressure booms. The aircraft does a 180° roll, then a supersonic dive, then another 180° roll. This puts the airplane in a steep enough dive so that one can measure the signature from the top of the aircraft. The aircraft then
22 Acoustics Today, January 2006