Business jet designer hopes to exploit 'Mach cutoff' effect to achieve acceptable overland supersonic flights

Supersonic business jet designer Aerion is looking at the potential for boomless overland flight at speeds up to Mach 1.2 by exploiting an atmospheric effect that prevents the shockwave reaching the ground. NASA experimented with the so-called "Mach cutoff" effect in the mid-1960s, and recently conducted new flight tests using a Boeing F/A-18.

The speed of sound increases with air temperature, causing the shockwave from an aircraft flying supersonically at altitude to be refracted as it enters warmer air closer to the ground. Below a threshold Mach number - about M1.2 for an aircraft flying above 35,000ft (10,700m) in standard conditions - the shockwave will not reach the ground and no sonic boom will be heard.

"We have begun a study of boomless supersonic flight to see if it is robust enough," says chief operating officer Mike Henderson. The effect is influenced by weather, and exploiting it operationally will require in-flight knowledge of atmospheric conditions. "We need to know how conservative we need to be in flight operations to guarantee no boom," he says.

© Aerion   
The ability to fly at low supersonic speed over land would improve the business case for Aerion's SSBJs

NASA collected data on Mach cutoff in the mid-1960s using a Lockheed F-104, and conducted the recent F/A-18 tests to confirm the conditions at altitude that result in no boom on the ground, says chief technology officer Richard Tracy. Aerion is aiming for Mach cutoff at 5,000ft to provide a buffer. This equates to a M1.15 cruise.

Boomless supersonic flight is of interest to Aerion because its aircraft is not a low-boom design and must fly subsonically over land to comply with existing rules. The ability to fly at low supersonic speed over land would improve the business case for the aircraft now being discussed with potential industrial partners.

Convincing the US Federal Aviation Administration that boomless flight is possible is critical, Aerion acknowledges. "The USA is the only country with a law banning supersonic flight," says Henderson.

The market forecast for Aerion's business jet assumes no supersonic flight over the USA, says chief executive Brian Barents, adding: "Boomless supersonic flight would be an add-on." Aerion hopes to form a consortium next year to develop the aircraft (Flight International, 1-7 August).

Source: Flight International