Since 2003, when the last Concorde was withdrawn from service, there has been no commercial supersonic transport (SST) to whisk well-to-do passengers across the Atlantic. That could change within a few years, as several companies – Aerion Corp. (https://www.aerionsupersonic.com), Boom Aerospace (https://boomsupersonic.com), Spike Aerospace (http://www.spikeaerospace.com) – are developing supersonic business-jets and 55-seat jetliners for transoceanic routes because supersonic flight is prohibited over the United States.

One SST effort hopes to find a way around the ban. NASA, in partnership with Lockheed Martin’s Skunk Works, seeks to advance quiet supersonic technology (QueSST) that reduces the objectionable sonic footprint generated by surpassing the sound barrier. The goal is to reduce noise levels heard on the ground from a boom as loud as a chainsaw (105dB) to a gentle bump no louder than a dishwasher (65dB). An acceptable noise level would allow the supersonic ban to be lifted, opening new markets over land and water with aircraft that can cut travel time in half.

Construction has started on the X-59 QueSST test aircraft, also known as the Low-Boom Flight Demonstrator (https://www.lockheedmartin.com/en-us/products/quesst.html). Engineers have been working on preliminary designs for the X-59 since 2016, trying to minimize its noise signature.

I learned more about these efforts during an American Institute of Aeronautics and Astronautics (AIAA) distinguished lecture at the Ohio Aerospace Institute given by NASA Glenn Research Center’s Ray Castner, an aeronautical engineer specializing in propulsion (engine nozzles, specifically). I was surprised to learn that despite all the advances in computational analysis and airflow simulation, there is still a solid need for wind tunnel work. Castner and his team tested a stainless-steel model that filled NASA Glenn’s 8ft x 6ft supersonic wind tunnel to validate assumptions about air flow around the engine inlet. Positioning the engine above the wing distributes the shock wave, reducing sharp pressure changes that create loud sonic booms. Castner says that so many factors influence air flow – the fuselage, canards, wing, conventional and T-tail – that testing an inlet in isolation would not do. Only testing the complex aerodynamic shapes together would give meaningful results. The team tested 50 flight configurations and angles of attack for aerodynamics (some with flaps extended, rudder deflected) and two inlet designs, measured with 40 probes. The researchers discovered that small vanes – vortex generators – added to the fuselage aft of the canopy were critical to managing air flow into the engine inlet.

Photo courtesy of Lockheed Martin

The team used computational analysis, but Castner emphasized, “The only way to get the integrated inlet effect was to use the wind tunnel.”

Test results from the single-engine X-59 may not apply directly to a twin-engine design necessary for commercial operations, but they are a start in finding a way to reduce prohibitive sonic booms to barely noticeable bumps and return SSTs to the skies. – Eric