Gulfstream is designing a new engine exhaust to reduce the sonic boom of a supersonic business jet. The design will ensure the shape of the exhaust plume is as smooth as possible.

"The exhaust plume has a great deal to do with the sonic signature," says Pres Henne, senior vice-president programmes, engineering and test. The so-called "twin stream" nozzle ensures the exhaust is fully expanded to avoid shockwaves in the plume.

"There should be no over- or under-expansion of the plume that would create shocks," says Henne. "It has to be as smooth as possible." This is achieved by mixing the fan and core exhaust streams with bypass air that flows through a duct around the engine.

Henne says Gulfstream, which is developing technology for a low-boom Quiet Supersonic Jet, now has a design for the complete engine nacelle, including an external-compression "spike" inlet and the plug-type twin-stream nozzle.

In February, NASA completed supersonic flight tests of Gulfstream's patented Quiet Spike, a telescoping noseboom designed to minimise sonic signature by reducing the strength of the bow shockwave. Tests on a Boeing F-15 at speeds up to Mach 1.8 confirmed the structural and actuation concepts, Henne says.

Although boom reduction was not possible because of the fighter's "huge" signature, near-field probing confirmed the Quiet Spike divided the bow wave into a series of weaker shocks. Measurements taken 95ft (29m) below the F-15 at M1.4 showed "excellent correlation" with Gulfstream's computational fluid dynamics analysis, he says.

Aerion plans to test its supersonic natural laminar-flow wing in the European Transonic Windtunnel in 2008 to confirm the performance of its Mach 1.6 business-jet design. Tests of the wing on a supersonic rocket sled ran into instrumentation problems.

Tests at Cologne-based ETW have confirmed the tunnel is quiet enough to measure the extent of laminar flow over the wing at actual flight Reynolds numbers, although the tunnel's speed is limited to below M1.3, says Aerion chief operating officer Mike Henderson.