NASA wants to take the blast out of sonic booms

As the military and occasionally commercial aviation researchers look to fly aircraft higher, farther and faster - much faster, sonic booms such planes can create can cause problems on the ground.

That's why NASA and the Japan Aerospace Exploration Agency (JAXA) this week announced a partnership to jointly research sonic boom. NASA said sonic boom modeling is one of the key technologies needed to let a next generation supersonic aircraft quiet enough that it can fly supersonically over land without significant disturbance to the people or damage to property under such noise.

The sonic boom work will include a look at what JAXA has done with its "Silent Supersonic Technology Demonstration Program" that is researching key technologies to realize a silent supersonic transport. JAXA's program looks at all manner of flight technologies - such as aircraft body and wing composition as well as wing design to reduce sonic booms. Because of sonic boom intensity, the Federal Aviation Administration prohibits supersonic flight over land, except in special military flight corridors.

NASA has a long history of supersonic flight and sonic boom testing. Last year it noted that researchers had spent some 200,000 processor-hours on the Columbia supercomputer, screening various new control concepts for a tailless supersonic aircraft. Exploration of non-conventional controls today may someday translate to better fuel economy, increased flight range, and sonic boom reduction.

According to NASA, an aircraft, flying supersonic at 50,000 feet can produce a sonic boom about 50 miles wide. The sonic boom, however, would not be uniform. Maximum intensity is directly beneath the aircraft, and decreases as the lateral distance from the flight path increases until it ceases to exist because the shock waves refract away from the ground. The lateral spreading of the sonic boom depends only upon altitude, speed and the atmosphere -- and is independent of the vehicle's shape, size, and weight, NASA said.

NASA describes sonic booms like this: "Air reacts like a fluid to supersonic objects. As objects travel through the air, the air molecules are pushed aside with great force and this forms a shock wave much like a boat creates a bow wave. The bigger and heavier the aircraft, the more air it displaces. The shock wave forms a cone of pressurized air molecules which move outward and rearward in all directions and extend to the ground. As the cone spreads across the landscape along the flight path, they create a continuous sonic boom along the full width of the cone's base. The sharp release of pressure, after the buildup by the shock wave, is heard as the sonic boom."

The change in air pressure associated with a sonic boom is only a few pounds per square foot -- about the same pressure change experienced riding an elevator down two or three floors. It is the rate of change, the sudden onset of the pressure change, that makes the sonic boom audible, NASA said.

All aircraft generate two cones, at the nose and at the tail. They are usually of similar strength and the time interval between the two as they reach the ground is primarily dependent on the size of the aircraft and its altitude. Most people on the ground cannot distinguish between the two and they are usually heard as a single sonic boom. Sonic booms created by vehicles the size and mass of the space shuttle are very distinguishable and two distinct booms are easily heard.

A few years ago the Defense Advanced Research Projects Agency (DARPA) demonstrated through its Shaped Sonic Boom Demonstration (SSBD) Program that sonic booms can be substantially reduced by incorporating specialized aircraft shaping techniques. Some of the results of those tests have been incorporated into its X Plane development.

The agency in March opened competition for contract reportedly valued at $750 million for a hypersonic aircraft known as Blackswift that can take-off and land on a runway and of course handle that barrel roll maneuver. The ultimate flight demonstration shall consist of a powered take-off, climb and acceleration to a Mach 6+ cruise speed, sustain this Mach 6+ cruise speed in level flight for at least 60 seconds, and demonstrate maneuverability by executing an aileron roll and land under power. It is envisioned that flying this reusable hypersonic testbed in a relevant, flight environment will permit the future development of enhanced-capability reusable hypersonic cruise vehicles for intelligence, surveillance, reconnaissance, strike or other national need missions."

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