NASA hopes to test Gulfstream's Quiet Spike telescoping nose boom on its Boeing F-15B at speeds above Mach 1 for the first time as the US National Business Aviation Association (NBAA) convention gets under way. The flight tests aim to validate the sonic-boom shaping design tools Gulfstream would use in development of any future supersonic business jet (SSBJ).
NASA Quiet Spike project manager Michael Toberman says: "We have achieved five flights so far, and plan to go to Mach 0.95 next week and Mach 1.2 the week after." The Gulfstream-built three-section nose spike first flew on the F-15B on 10 August, and was tested initially for structural response. The boom, which measures 9.15m (30ft) from the radar bulkhead to the tip, telescopes from 4.3m in length at take-off to a maximum of 7.3m in flight.
In-flight measurements are being made of the shockwave signature close to the aircraft, for comparison with Gulfstream's predictions. "The shockwaves come off at the three conical fairings of the spike, are non-coalescing and run in parallel to each other. We are using another F-15 [the F-15 ACTIVE - advanced control technology for integrated vehicles - testbed] to fly into the shock, and we have already installed pressure sensors on that other aircraft," says Toberman.
As the Quiet Spike is mounted on a standard F-15 that is a "non-optimised" shape for low sonic booms, the second F-15 therefore measures the supersonic signature in the near field, to see if the wave system develops from the spike as predicted. The Quiet Spike is designed to divide the bow shock into several less-intense pressure waves, turning the characteristic N-wave sonic boom into a softer S-shaped signature.
The spike is asymmetric in cross-section and, in an operational SSBJ, would probably have four sections and be smooth rather than stepped when extended, says Gulfstream. The nose spike, which artificially extends the whole length of the aircraft in flight, could also be complemented by an aft-spike that would counter the effect of shocks generated by the empennage.
"We're looking at a slow build-up approach to ensure that at Mach 1.2 things are okay. At faster speeds you can get more dynamic pressure and a lot less of a stability margin," says Toberman, who says the plan is to go beyond Mach 1.2 and get data at "much faster speeds".