Sonic booms that accompany travel beyond the speed of sound are a major environmental challenge to civil supersonic transportation. Others include airport noise, and high-altitude emissions, In the near term, the project is emphasizing the understanding and overcoming of barriers that stand in the way of achieving acceptable levels of sonic boom.

Sonic Boom Modeling
Successful supersonic civil aircraft must be capable of supersonic flight over land. In order to achieve this end result, questions associated with sonic boom noise must be addressed. Existing knowledge of the impact of sonic boom on the community is based primarily on experiments conducted during the 1960s (Concorde and the U.S. SST) and the 1980s (NASA’s High Speed Research (HSR) Program.  In the former work it was concluded that high-amplitude sonic booms (Concorde ~2 psf) were clearly unacceptable to a large segment of the population, and overland flight was prohibited. Although much progress was made in modeling the sonic boom and its effects during HSR, the boom levels achieved were still unacceptably loud. The Supersonics Project is developing technologies that potentially will lower the boom to acceptable levels. There is very little data on the effects of such booms, and therefore, the project has adopted a three-pronged approach to studying the atmospheric effects, transmission into structures, and human reaction to these booms.

The ability to model sonic boom propagation from the aircraft to the ground is a necessity, with all relevant physical phenomena included in such models. This will enable the accurate prediction of sonic boom levels on the ground under realistic atmospheric conditions and for all flight conditions. In addition, in order to fully understand human reaction to these low-intensity sonic booms, it is necessary to be able to predict the transmission of booms into buildings. Both interior noise levels and structural vibration are of interest since both are important characteristics of the indoor environment.

Studies of human response to sonic booms heard indoors and outdoors have identified several factors that may contribute to an increased annoyance indoors, including reverberation and rattle noise. Investigation of these factors through laboratory and field studies is critical to understanding and developing prediction models for human response to low-intensity sonic booms.

The Project recognizes that in the past there have been other elements of sonic boom that have received considerable study. These include reaction of animals and structural damage. Past studies have indicated that if boom noise is low enough to be acceptable to humans is achieved, there will be minimal other impact. The project has therefore chosen to defer research into such topics until the human reaction is better understood.

Airport Noise Reduction
International regulations regarding allowable aircraft noise around airports are in effect for the commercial jet fleet and will be enforced for supersonic civil aircraft. Significant noise reduction must be obtained in order to achieve supersonic aircraft that are as quiet as subsonic aircraft operating at the same airport. For propulsion noise, current subsonic aircraft meet requirements through the use of high-bypass turbofan designs. But high-bypass engine systems do not lend themselves to a supersonic cruise regime, due to large installed drag penalties. The supersonic aircraft requires the characteristics of a large bypass engine in the airport/community regions and the characteristics of the turbojet during the supersonic portions of the flight.

The challenge for the Supersonics project is to develop a combined cycle or variable-bypass engine cycle that retains the characteristics required for low noise at takeoff and landing and the high efficiency required for the supersonic portion of the flight. These cycles will require variable geometry inlets and exhausts, and will push the limits of acceptable noise. Noise prediction methods used by engineers must handle non-conventional geometries, including embedded engines, and be accurate enough for design to within a very small margin of error. Current empirical prediction methods are ill suited for this task. Better tools for unique geometry nozzles are those that have an input that is Reynolds-Averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD), and relies on theoretically based acoustic analogies. Better still are high-resolution, time-dependent simulations of jets that show the mechanisms by which turbulent energy converts into sound. Such simulations guide efforts with jet control actuators that would reduce noise by manipulating the turbulent vortices in the jet.

In addition to noise improvements via engine design, other noise-reduction concepts should be investigated for important engine and airframe sources. Innovative low-noise nozzles and propulsion integration concepts must be studied. Improvements in low-speed aerodynamic performance can also result in reduced airport community noise.

High-Altitude Emissions
Supersonic aircraft cruise more efficiently at higher altitudes than subsonic aircraft. For lower-speed supersonic vehicles (~Mach 1.5 - 2.5) typical of a future commercial aircraft, flight will occur near 50,000 feet, or in the stratospheric ozone layer. This will cause increased amounts of aircraft emissions, such as nitrogen oxides (NOx), carbon dioxide, water vapor, and particulates to be released into the stratosphere. These cruise emissions may cause ozone destruction and possibly influence other factors affecting the global climate. Compared to subsonic aircraft, effects of these emissions are exacerbated at the higher altitudes needed for a supersonic aircraft. Continuing research is critical to establishing a fundamental understanding of combustion chemistry, liquid fuel atomization and vaporization, turbulence-chemistry interaction, fuel-air mixing, and particulate formation and transport that will allow the development of new combustor designs to minimize these harmful cruise emissions. The key challenge will be to achieve supersonic cruise emission levels that are lower than those currently targeted for commercial subsonic aircraft in order to mitigate environmental concerns. It should also be emphasized that emissions in the airport community are local air quality concerns as well and it is expected that supersonic aircraft will need to meet all subsonic aircraft emissions standards.