Thunder claps, the ear-piercing sound of high-speed jets flying overhead, and even the snapping of a bullwhip are all examples of sonic booms. These loud noises are caused when an object moves faster than the speed of sound and can range from small and quick to earth-shaking and long. While scientists have been able to observe what goes on during a breach of the sound barrier, they had never observed what happened when two objects surpassed the sound barrier right next to one another.
Fluid Dynamics 101
Understanding the impact of this discovery first requires a basic understanding of fluid dynamics and how sonic booms happen. Fluids are not just liquids. A fluid is any substance that has no fixed shape, which means that the air all around you is a fluid. Just like a boat moving through water, a jet moving through the air leaves wakes in the fluid around it. Aircraft rely on areas of high and low pressure to remain aloft. In flight, the air in front of the plane is compressed compared to the air behind the wings. When the speed of the aircraft increases to the speed of sound, or Mach 1, the ripple in the air and the craft will be moving at the same speed. As soon as the plane goes any faster, it generates a shock wave carrying incredible amounts of energy, which are then released in the form of an explosive sonic boom.
In the case of a bullwhip, the crack is a single instance, as the tip of the whip, or the cracker, only briefly breaks the sound barrier. High-speed jets, on the other hand, continue to travel faster than Mach 1, generating a continuous supersonic disturbance in the fluid around them. This disturbance takes the shape of a cone, radiating out from the nose of the plane and fanning out behind at an angle relative to the speed of the vehicle. The sonic booms we hear from passing supersonic jets are when that pressurized cone of high-energy fluid meets us, the observers. Using specialized equipment, it is possible to photograph these pressure wakes.
Until very recently, two supersonic jets had never been flying close enough to one another for their shock waves to interact. As a result, there were plenty of photos of solo shockwaves but no record of what happens when two of them meet. Predicting the fluid motion outside of a controlled environment is extremely difficult due to the number of variables: Atmospheric pressure, countercurrents, and temperature, just to name a few. Now, scientists have a leg up after NASA was able to photograph the merging shockwaves of two T-38 supersonic jets.
Knowing how shock waves interact is a crucial step in designing quieter supersonic aircraft. Currently, due to the noise level, even when flown at high altitudes, planes traveling faster than Mach 1 are not permitted to fly over land. Engineers hope to use the new information about the interaction of shock waves to create high-speed jets that don’t produce as much disruptive noise pollution. Lockheed Martin and Skunk Works are working to build the X-59 Quiet Supersonic Technology or QueSST. The plane is planned to be capable of flying at roughly Mach 1.3 at 55,000 feet. Instead of producing a sonic boom, the engineers working on the plane anticipate the noise of its shockwave to be no louder than a car door closing. Designing a piece of technology capable of getting the overland flight ban lifted is no small task. The end result will be a commercial aircraft capable of carrying passengers across the world in nearly half the time.