NASA recently awarded Lockheed Martin Aeronautics Co. a contract to build a quiet supersonic jet. The $247 million contract is an addition to the aerospace company’s preliminary agency contract which was awarded in 2016. The bold idea behind the new jet is to make it a platform for a supersonic airliner which will have a “quiet” sonic boom, unlike the Concorde.
The NASA Quiet Supersonic Transport (QueSST) project will build upon prior developments at Lockheed’s secretive Skunk Works facility, the project aims to have the first test flight by the end of 2021. The plane’s specifications include travelling at Mach 1.5 (approximately 990 mph) at the cruising height of 55,000 feet. This would allow flights to cross the Atlantic in roughly three hours. Current subsonic airline flights take eight hours from New York to Paris.
Aiming for the Sound of Thunder
A key element of the design for the aircraft is for the sonic boom to be within acceptable limits of only half the sound at that speed. In a written statement from NASA, the resulting sonic boom would be “so quiet it hardly will be noticed by the public, if at all.” It has been likened to the sound of “distant thunder” or “the sound of your neighbor forcefully shutting his door outside while you are inside.”
The loud noise of the sonic boom was the cause for contention regarding the Concorde flying supersonic over the continental United States. The resulting regulation was one of the reasons Boeing was forced to cancel their development of the Boeing 2707. It was supposed to be the first American SST. It had already won a government-funded project to build the SST at its Seattle, Washington facility. The 2707 was designed to have seating for 250 to 300 passengers and cruise at speeds close to Mach 3. This would have been a larger and faster SST compared to the Concorde.
In September, 2017, NASA unveiled the basic design of the QueSST. According to the launch, NASA and Lockheed have already invested £290 million or more than $415 million in research for the plane. They have been testing the mockups in wind tunnels.
Working on the “Boom”
As a plane approaches the speed of sound or Mach 1, the air in front of the plane compresses. The compression stays in front of the plane while it is flying at less than Mach 1. When the plane reaches Mach 1, the compression waves are no longer able to travel faster than the plane, and they accumulate and pile up, producing a booming sound like an explosion. The sonic boom created by supersonic flight is a double boom, with the first explosion occurring as the compression waves pile up on the leading edge of the wing, and the second wave occurs at the tail, when the pressure goes back to normal. The sonic boom is continuous while the plane is in supersonic flight. The sound of the rolling boom across the ground underneath the plane’s path is called the “boom carpet.”
The sonic boom is a result of a plane flying faster than the speed of sound. It can be as loud as 200 decibels, and rattle windows and homes miles away. The sonic boom is within the range of 0.1 to 100 hertz, the lower range of human hearing. The sound is below the range of conventional aircraft and most industrial noise. The duration also characteristically brief, being less than a second. It can be as long as a 100 milliseconds or 0.1 second for fighter planes, and around 500 milliseconds or half a second in length for the Concorde, the space shuttle or any large supersonic vehicle.
The boom is a cone of sound with the aircraft at its leading point. The higher the plane flies, the wider the boom is on the ground. However, with greater altitude, there is less overpressure on the ground. Due to the nature of sound and compression waves, there are various factors which affect how it is heard or the perception of the boom, how loud and where it is heard. The carpet boom follows the flight path of the plane on the ground. This weakens farther away from the flight path. The boom exposure area is estimated to be about 1 mile wide, for each 1,000 feet altitude of flight. Flying supersonic at 30,000 feet, the carpet boom would be 30 miles wide on the ground. The plane’s behavior also affects the boom. Diving and accelerating can focus the boom, whereas deceleration and climbing can reduce the effects of the shockwave. Weather conditions and disturbances can also distort booms.
Supersonic Transports in History
There have only been two supersonic transports. The most successful was the Concorde which was jointly developed by France and the United Kingdom. It had an average cruising speed of Mach 2.02 or about 1,334 mph, with a service ceiling of 60,000 feet. It was first flown in 1969 and entered commercial service in 1976. It had a capacity of 98 to 128 passengers and flew from New York to Paris in less than 3.5 hours. It was in service until 2003. The restrictions on supersonic flight are for commercial SSTs over land, specifically over populated areas. The Concorde was only allowed to fly at supersonic speeds over the Atlantic and the Pacific.
The only other SST to fly commercial routes was the Russian TU-144. It had a shorter commercial life flying passengers. It flew freight and passengers on the Moscow to Alma-Ata and Moscow to Khabarovsk routes. Its first regular passenger service was in November 1977, and the last was in June 1979. The TU-144 development program was cancelled in 1983. Unlike the Concorde, the supersonic overland route was allowed because the Soviet Union did not have any regulations pertaining to the sonic boom.
The QueSST is the latest incarnation in the development of a low noise supersonic transport. There have been various research including DARPA’s Quiet Supersonic Platform. The government agency also funded the Shaped Sonic Boom Demonstration (SSBD) aircraft. Using a modified F-5 Freedom Fighter aircraft, the SSBD concept was tested in 21 flights over a two-year period. This extensive study of the sonic boom included 1,300 recordings of the shock wave and sonic boom. The SSBD was able to reduce the sonic boom volume by about one-third. If the developments on the SSBD were used on the Concorde, it would have been capable of supersonic flight at acceptable noise levels.
One interesting outcome of the various research into finding a low-noise sonic boom is the development of theoretical designs that seem to have no sonic booms at all. However, even without it, there would still be a shockwave if the aircraft were to generate aerodynamic lift. Earlier studies involving the North American B-70 Valkyrie showed that the sonic boom was still a problem even if it flew at a much higher altitude of 70,000 feet.
The contract between NASA and Lockheed aims to have a working prototype of the QueSST. It would be a single-pilot plane ready for flight by 2021. Testing the aircraft would help determine a design which could be used for commercial aircraft of the future.
While Lockheed is developing supersonic noise abatement designs for aircraft, NASA will also look into the human and legal side of the equation. The noise regulations which limited the overland flight of SST was the result of public clamor during the late 1960s. The regulations were based on the understanding of science at the time. NASA will be conducting tests around urban areas to determine if the stipulated regulations regarding noise levels could be amended.
There is excitement in the development of the plane. According to a Lockheed Martin spokesman, the QueSST is “NASA’s first X-plane in a generation.” Lockheed and NASA are not the only ones developing new generation SSTs. Japan Airlines and Richard Branson of Virgin Airlines are backing Boom Supersonic in the development of SST for transoceanic flights.
In a separate project, Lockheed is working with Aerion Corp. to develop the AS2 business jet class SST with a planned first flight in 2023.
Supersonic flights could very well be in people’s travel options soon.