The quiet return of supersonics

When an aircraft exceeds the speed of sound, the air pressure forms a shock wave that is heard as a sonic boom when it reaches the ground. However, shock waves at higher altitudes travelling beyond the speed of sound reflect off the warmer, denser air at 1 500 m. Waves that continue to the ground are moving slower than the speed of sound and do not produce a boom.
Date:31 December 2008

Dedicated engineers are seeking solutions to again push business jets beyond the sound barrier.

Civilian aircraft designers have been trying to get back into the supersonic business ever since the 2003 forced retirement of the Concorde, but they have been hampered by the Federal Aviation Administration and international regulations that prohibit sonic booms over inhabited areas. Now, in a bid to bring Mach-busting jets to wealthy travellers, aircraft vendors – including Gulfstream and Lockheed Martin – are designing aircraft with features such as retractable nose spikes that may reduce these bone-rattling noises.

But instead of relying on hardware alone, a small company called Aerion, based in Reno, Nevada, is focusing on a physical phenomenon that pilots can use to reduce these booms. The speed of sound varies with altitude and is affected by air temperature and pressure. For example, whereas Mach 1 is around 1 200 km/h at sea level, it might be as slow as 1 060 km/h in the cold, thin air of the stratosphere. If atmospheric conditions between the aircraft and the ground are right, the shock wave created by the aircraft will drop to subsonic speed as it propagates to lower altitudes.

A pilot using real-time data on air temperature and atmospheric conditions could fly at a speed that ensures that the boom will go subsonic at around 1 500 m.

Another limitation on supersonic flight is fuel use – the Concorde once ran out of fuel while taxiing to a hangar. Aerion’s R800 million jet will reduce aerodynamic inefficiencies caused by travelling on both sides of the sound barrier. Its short, angled wings will help the craft fly up to Mach 0,99 without fuel-guzzling performance penalties.