Supersonic speed would allow travelers to cut significantly their travel time. However, because of the resulting sonic booms, the Federal Aviation Administration (FAA) and similar associations restrict supersonic travel to transoceanic only. Gulfstream Aerospace Corp. (Savannah, GA) partnered with NASA Dryden in a commercial investigation of the Quiet Spike, a sonic-boom suppression system. Leslie Molzahn was part of Dryden’s investigative team
NASA Tech Briefs:What is the Quiet Spike, and how does it work?
Leslie Molzahn: The Quiet Spike is a commercial investigation, a structural proof-of-concept experiment that Gulfstream Aerospace Corp. put together to investigate the feasibility of this type of object. We partnered together, Gulfstream and NASA Dryden. The Quiet Spike is designed to mitigate supersonic booms, and was mounted onto one of NASA Dryden’s F15-Bs. It extends approximately 14' in front of the F15-B testbed’s nosecone when it is in the retracted position, and it is a telescoping, composite tube mechanism that will extend and partition strong shockwaves into a series of weaker shockwaves at supersonic speeds.
When extended, the Quiet Spike is an additional 10' long, for a total 24' length. The spike is a series of nested tubes, with the largest diameter tube being 16" and the smallest diameter being 4". When it's retracted, the 16" tube is all that is visible. In the extended configuration, a 10" tube and a 4" tube are also exposed. The concept is to initiate a series of less intense sonic boom shock waves as the flow transitions past the Quiet Spike sections, instead of having one strong shock wave. This technique could mitigate the adverse effects of a strong sonic boom shock wave.
NTB: Why was it developed?
Molzahn: Right now, supersonic flight over land isn’t feasible. It isn’t allowed. When it flew, the Concord went supersonic over the ocean only. There could be a huge commercial application if the sonic boom can be mitigated to have less impact on people and resources on the ground. One could potentially get the FAA to lift their restrictions and be able to fly supersonic over, say, the continental US. I personally think it would be incredible to go from Los Angeles to New York in the matter of a couple of hours, instead of the time it takes now.
The sonic boom is a phenomenon that occurs when a vehicle goes the speed of sound, about 769.5 mph, and it is a large pressure differential. When that pressure differential hits the ground, that is what you hear. These are strong shockwaves, very loud, and what the Quiet Spike does is make the one shockwave into a series of smaller shockwaves to help mitigate the pressure differential into a weaker shock. A lot of different companies are looking into ways of mitigating sonic booms. In the, past at NASA Dryden, this F15-B was involved with probing near field shockwaves off of another aircraft that did a flight test program just a few years ago that was an F5-shaped sonic boom investigation. This was done with Northrop-Grumman. What that test did was, it had a different forward fuselage shape to mitigate the pressure wave that comes from the sonic boom. Current technologies trying to minimize the shockwave hitting the ground involve only flying technique. There are techniques of different air speeds, altitudes, and maneuvers where the shockwave won’t hit the ground.
There are, in fact, investigations underway looking into the effects of sonic booms of structures on the ground. It is a significant pressure differential, and depending on how big that is, you can have different levels of interaction. Some sonic booms sound like thunder, some sound like gunshots. And obviously, the walls of buildings and structures react to that pressure differential. There are a lot of human factors: if you heard a gunshot every five minutes, you might go a little crazy. But part of the Quiet Spike mitigation technology is what level sound one might be able to deal with. What if we could mitigate the sound to something like distant thunder or a train going by? Human factors-wise, how would people react to that? There are a lot of questions that need to be answered still.
NTB: Describe how a test with the Quiet Spike was conducted.
Molzahn: It was about a yearlong investigation. Research testing of the Quiet Spike consisted of numerous ground and flight operations. Each step in the process had unique objectives, and involved numerous test team members from NASA Dryden and Gulfstream Aerospace Company. Once we had the hardware, and we were ready to put it on an aircraft, it was a yearlong process of ground and flight testing integration and ground and flight testing. Obviously, there was a lot of Gulfstream work prior to the point of integrating it onto an aircraft. They spent years designing, developing, and building this technology.
When NASA Dryden integrated the Quiet Spike onto our testbed, it actually took a couple of months. We then did a series of ground tests to look into predictions of how it would behave structurally in flight — loads testing, ground vibration testing, and structural mode interaction testing. Once we got through all of the ground testing, we started flight testing. A series of ground tests with the Quiet Spike installed on the F15B included loads calibration tests, drag loads capability tests, a ground vibration test (GVT), and a structural mode interaction test (SMI). These ground tests evaluated the structural instrumentation, mechanical function, and predicted dynamic response of carrying the Quiet Spike on the aircraft during flights. Each of these ground test operations provided valuable data on the road towards preparing for Quiet Spike research flights. We had about 30 flights. We performed maneuvers in the retracted configuration, where the Quiet Spike measures 14', and then at the extended configuration, where it telescopes out another 10', making for a total of 24' out in front of the aircraft. We did envelop expansion for those configurations out to Mach 1.8, which is almost twice the speed of sound. At Mach 1.4, we did an investigation where we had another aircraft, a NASA A-36, fly next to the testbed in an investigation of near-field pressure distribution to see how effectively the Quiet Spike broke up the shockwave. The first flight was flown on August 10, 2006, with the Quiet Spike retracted and the F15B landing gear extended for the duration. Envelope expansion, utilizing a risk reduction build-up methodology, commenced on the second flight of Quiet Spike. Subsonic flight envelope expansion was completed on October 17, 2006, for the Quiet Spike in the extended position. Envelope expansion included numerous maneuvers to assess aerodynamics, stability and control, flutter, aeroservo-elastic effects, and structural loads. These maneuvers included push-over pull-ups, wind-up turns, lateral and longitudinal stick raps, rudder pedal kicks, pitch and roll doublets, and beta sweeps. Engineers in the NASA Dryden Mission Control Center evaluated the maneuvers in real time, in order to progress through envelope expansion.
Quiet Spike entered the supersonic flight regime on October 20, 2006. Once the envelope was cleared through Mach 1.4 at 40,000 feet, a flight to investigate the near-field shock signature of Quiet Spike was performed. Another NASA Dryden test asset, the F15 NASA 837, was configured with pressure sensing equipment to probe the shock waves created by the F15B NASA 836 with Quiet Spike installed. The probing maneuvers, with the NASA 836 and NASA 837 in formation, were performed on December 13, 2006. In January 2007, the spike-extended envelope clearance was completed for flight out to Mach 1.8. Directly following spike-extended envelope clearance completion, the test team evaluated the static and dynamic structural envelope up to Mach 1.4 with the Quiet Spike in the retracted position. During the envelope expansion research test flights, the Quiet Spike was extended and retracted at flight conditions between 5,000 to 15,000 feet at approximately 225 KIAS. A flight test engineer in the aft cockpit initiated the Quiet Spike extension and retraction operations through a control panel. In addition to system status feedback available in the aft cockpit, the engineers in the Mission Control Center had instrumentation information available to assess the operation of the Quiet Spike mechanisms. Following both the extended and retracted envelope clearance out to Mach 1.4, the Quiet Spike transition capabilities were demonstrated with the aircraft at airspeeds up to Mach 1.4.
Structurally, the Quiet Spike was sound. Mechanically, the telescoping mechanism worked well. It looked very promising; it is an exciting technology. It’s only the first step. There are a lot of people in commercial aerospace looking into sonic boom mitigation. It would be incredible to get on a supersonic jet and get to wherever you are going in a quarter of the time, or half the time.
NTB: Is it in commercial use now?
Molzahn: It is not. This was just a developmental, one-step investigation into the feasibility of mitigating a sonic boom to the point where it might be feasible to implement it in a commercial application, to press forward with the possibility of supersonic flight in the commercial industry.
The readiness level of going on to a commercial airliner is not feasible. This was just the first step. This was an investigation into the technologies to see if it was feasible structurally to fly on the forward section of an aircraft. This would just be one piece of a supersonic jet that could fly over the continental US. A new airframe has to be developed for it; a commercial airliner can’t go supersonic. We would first have to get over the hump of getting a commercial airliner to go supersonic, and then move on to breaking the stronger shockwaves up into weaker shockwaves.
Looking into the aerodynamics of it, the Quiet Spike looked to match predictions, what it was hoped to do. There will be formal papers coming out in the near future with the data. But it looks like a feasible solution.