NASA Jet Bears Nose that Grows for Sonic Boom Tests

NASA's F-15B testbed aircraft with Gulfstream Quiet Spike sonic boom mitigator attached. CREDIT: NASA/DFRC/Lori Losey.

With its extendable nose, a NASA research jet may resemble some sort of high-flying Pinocchio, but this supersonic aircraft tells no lies.

The F-15B research jet's elongated nose - dubbed the Quiet Spike - is part of an experiment by Gulfstream Aerospace Corp. and NASA's Dryden Flight Research Center (DFRC) to determine ways to hush the sonic booms from aircraft flying faster than the speed of sound.

"We're really focused on more of the civilian applications," said researcher Michael Toberman, NASA's DFRC project manager for the Quiet Spike experiment at Edwards Air Force Base in California, in an interview. "It's really just fundamental research into fully understanding sonic booms and how to mitigate them so that we can one day fly over the United States without having any concerns."

Current Federal Aviation Administration (FAA) regulations prohibit commercial supersonic flights over land and restrict military flights to special corridors to limit the effects of humans and the environment below.

Military aircraft the world over have been flying at supersonic speeds since U.S. test pilot broke the sound barrier aboard the rocket-powered X-1 aircraft in 1947. It was the Russian Tu-144 and the joint British-French Concorde supersonic planes that carried commercial passengers around the globe at twice the speed of sound, or Mach 2.

The Concorde, which carried passengers through 2003, was limited to subsonic speeds while flying over land.

"We believe we can reduce the sound made, for example, by the Concorde to the tune of 10,000 times," Gulfstream spokesperson Robert Baugniet told SPACE.com.

'Spiking' sonic booms

Sonic booms result from shockwaves that build up around supersonic aircraft as they approach Mach 1, or about 760 miles per hour (1,223 kilometers per hour) at sea level.

"In standard [supersonic] aircraft, there's usually two booms because the shockwaves coalesce into two large shocks," Toberman said.

The Savannah, Georgia-based Gulfstream developed and patented the Quiet Spike technology to determine how best to control or suppress the intensity of aircraft sonic booms for potential supersonic commercial aircraft over land.

Three conical fairings along the length of the Quiet Spike are designed to produce three parallel shocks that head Earthward independently - rather than folding into a two large shocks - where their effects are muted, compared to the average sonic booms.

"You'll still have the booms," Toberman said. "They'll just be really small and they won't make much of an impact."

NASA has conducted past research to suppress sonic booms, including test flights in 2003 and 2004 to show that aircraft shaping can reduce the intensity of sonic booms.

"I think this is really building up a lot of interest in the industry into this sort of technology," Toberman said. "I'm hoping that it'll feed into a larger research [project] so that we can develop a full vehicle that will actually have a low signature...a low boom."

One long nose

The current F-15B tests are aimed at determining the structural integrity of grafting a telescoping spike to the nose of a supersonic aircraft.

"You're putting this large structure protruding out from the front of the aircraft and it could have some destabilizing effects," Toberman said. "We want to make sure that the stability and control is acceptable for the pilot."

Built of a carbon composite structure over an aluminum frame, Gulfstream's Quiet Spike measures a total length of about 24 feet (seven meters) when fully extended. The spike consists of three primary segments - a four-foot (1.2-meter) tip that tucks into a six-foot (1.8-meter) section which slides into a 14-foot (4.2-meter) trunk protruding from NASA's F-15B aircraft.

The entire spike can be fully extended with electric motors in 21 seconds, though the deployment can be controlled during that process as well, Toberman said.

To date, test pilots have flown six subsonic flights with the Quiet Spike-equipped F-15B, reaching speeds of Mach 0.8, or about 608 miles per hour (978 kilometers per hour).

Over the next two weeks, project managers plan to push the Quiet Spike up to speeds of about Mach 1.8, or about 1,370 miles per hour (2,205 kilometers per hour), but don't expect any supersonic silence from their F-15B aircraft.

"That's really because the actual aircraft itself is rather noisy," Toberman said, adding that the research F-15B has not undergone modifications to quiet its inlets.

Instead, NASA test pilots will fly a second F-15 aircraft alongside the test vehicle to measure the structural integrity of the Quiet Spike at supersonic speeds.

Subsonic tests have proven favorable according to test pilots, Toberman added.

"Actually, the pilot's comments have all been very benign," he said of the subsonic Quiet Spike tests. "He doesn't even know it's on there for the most part."

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