Aerion's trump card is its ability to use an existing engine to reduce development cost and timescale. SAI's larger, faster, quieter aircraft needs substantially modified powerplants now being studied by the big three engine manufacturers.

Aerion's SSBJ has two Pratt & Whitney JT8D-219s - engines that power the Boeing MD-80 - with minor modifications for durability at sustained supersonic speed. "We will not certificate a new model," says Peter Robertson, advanced commercial engines programme manager. With a bypass ratio of less than two, "the JT8D-219 is very close to a good supersonic cycle".

The engine will be derated to 18,200lb thrust (81kN) for take-off to provide margin below Stage 4 noise limits, but will operate at its 22,000lb maximum thrust throughout climb, acceleration and supersonic cruise. At Mach1.6, Mach number at the fan face will be about M0.6. "The engine does not know it is flying supersonic," says Robertson. "The issues are cooling air for the turbine and the sustained higher temperatures." P&W's GG8 marine derivative of the JT8D operates with a similar sustained maximum-power cycle, and incorporates material changes that could be certificated for Aerion.

"There will be a hit on turbine life, but not enough to kill the engine," says Robertson. Aerion's goal is a minimum 2,000h time between overhauls (TBO), but engine life could be as high as 3,000h depending on how much time the aircraft spends at supersonic speed.

Coming in below Stage 4 noise limits will require treatment of the inlets and nozzles. Aerion has selected a simple external-compression inlet, raked to produce an oblique shock in supersonic cruise and with a hinged lip to increase airflow on take-off. The overwing location provides shielding of fan noise, says Robertson.

Aerion plans to use both a core muffler and an advanced exhaust mixer now under development for the JT8D, with backing from Bass, to enable the MD-80 to meet Stage 4. P&W has recommended a plug-type nozzle, says Richard Tracy. This provides more area for acoustic treatment and is "multi-Mach capable without moving parts", he says.

Lockheed Martin is working with General Electric, Pratt & Whitney and Rolls-Royce to define an engine for SAI's QSST that is based on an existing commercial core. The aircraft needs non-afterburning turbofans of 33,000-35,000lb thrust with a minimum 2,000h initial TBO. "We have several chances at a new-technology engine that will exceed Stage 4 with the fuel efficiency to meet our range," says Michael Paulson. GE's proposed medium-bypass, variable-cycle engine is based on its Tech56 advanced core; P&W's on a PW4000 core matched to a V2500 fan; and R-R's on a Trent core.

"We are looking at tailored engines based on existing cores and technologies, but modified significantly to meet the performance, life and noise requirements," says Lockheed Martin's John Hartmann. The engines, tucked under the QSST's gull wing, have external-compression conical-shock inlets with translating cowls to increase airflow on take-off.

Source: Flight International