Forecasts 2010: Sonic boom

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As NASA enters the fifth year under its fundamental aeronautics programme, supersonic-focused research continues at a steady pace on the environmental, performance, structures and integrated design aspects of future-generation aircraft.

The work plan, funded at roughly $50 million a year, calls for bits and bobs that, taken together, are beginning to move the USA and the international community ever so slowly and deliberately closer to a return to commercial supersonic flight in the neighbourhood of 2025. NASA expects private industry to develop the first of the new generation civil supersonic aircraft in the form of a business jet, potentially in this decade.

While continuing to focus on some basic research elements that will be key to designing the second- and third-generation supersonic airliners post-2025, the agency will perhaps more importantly continue to aid the international community in defining a defendable modification of rules that today severely limit the economic viability of the technology - the ban on overland supersonic flight by the US Federal Aviation Administration and others.

Technically, the solution to the noise problem includes developing integrated aircraft design technologies that reduce the pressure wave, hence the sonic boom noise, that the vehicle will produce at the surface while simultaneously minimising interference and wave drag for best high-speed cruise efficiency, says Peter Coen, principal investigator for NASA's supersonic fundamental aeronautics programme.

nasa supersonic aircraft nasa
 © NASA
NASA is leading efforts to return supersonic aircraft to the world's skies

Perhaps a more daunting effort is coming to grips with how much noise the public will tolerate, which could deter potential builders of supersonic jets.

Flight-test work in 2010 will largely focus on reducing the contribution of the engine inlet to the sonic boom contribution. Earlier tests in 2003 and 2006 proved methods for reducing the contribution of the front end of a jet through shaping, including extending a spike forward from the nose at supersonic flight.

Experiments in 2009 with the NASA Boeing F-15B research aircraft delved into the back end of the jet, looking at the effects of shock waves formed by the engine exit nozzle, with test results validating computational models, says Coen.

"Eventually we want to be able to include nozzle effects into the design of the supersonic signature," he adds. "As we continue to reduce the [boom] signature, particularly in the aft end, the presence of those shocks would change the close-out shock, potentially increasing it enough to lose the benefit of the overall configuration shaping.

Ideas to reduce inlet shocks include "relaxed compression" designs that use "micro ramp flow control" devices to control boundary layer stability to set up multiple internal and external oblique shock waves at the inlet rather than using the typical non-optimal method of using bleed air to suck off the boundary layer to stabilise the normal shock wave the decelerates the inlet air to subsonic speeds for combustion.

While no flight tests are planned for micro ramps in 2010, another technology - a high-performance inlet that features a channelled centre body with a rotating component that opens up a large slot symmetrically down the centre of the inlet - will be tested on the F-15B.

The work plan for 2010 includes finalising a technology roadmap with the FAA and the International Civil Aviation Organisation's's committee on aviation environmental protection in advance of the committee's triennial meeting in Montreal in February.

"We've put together a roadmap for the technology needs that would support development of a standard for a noise-based rule for supersonic overland flight," says Coen.

As a sign of momentum toward a rule change, he says the ICAO task force set up to monitor the boom reduction activities has asked the organisation to change its work guidelines to include actively promoting and directing development research in support of standards for sonic boom community noise.

"What we're hoping is that [the change] will result in more international co-operation on some of the research into sonic boom acceptability," says Coen. "Member states would become aware of it and some new programmes would likely be started."