Dassault Continuing Supersonic Inquiry Despite Sobering News on Engine Front

Dassault will continue its studies of a supersonic Falcon business jet, the company says, despite bad news on the propulsion front.

When the company discussed details of its supersonic design in May, at the annual Falcon operators' meeting, its engineers expected to use modified versions of current production fighter engines-either the Snecma M88 or General Electric F414. Since then, Dassault has determined that the fighter engines will need more extensive changes than the company had expected.

The problem is heat and durability. At supersonic speed, an efficient jet inlet compresses the incoming air, so that it's hot before it enters the engine. A modern, high-pressure-ratio fighter engine further increases the pressure and temperature of the air before it reaches the combustor, and consequently runs at very high temperatures. A fighter engine sees such temperatures for only minutes at a time in any sortie, and is unlikely to run at maximum temperatures for more than 500 to 600 hours in an 8,000-cycle design lifetime. A transport, however, would accumulate more supersonic time than that in a single year.

There are three ways to solve the problem, but none of them are cheap. One is to redesign the fighter engine, including major changes in materials, structures and cooling systems in the aft end of the compressor, the combustor and the turbine. The second is to design a new engine optimized for the business jet's flight cycle. The third is to adopt an oversized engine running at a lower pressure ratio-but that is likely to result in a larger and less efficient aircraft.

The challenge is to pay for engine development, within a realistic estimate of the size of the corporate jet market, while keeping the aircraft affordable.

Dassault is still working with engine manufacturers to solve the problem. (NASA is working on the same issues in its High-Speed Research program, and is reporting good progress.) The French company has spent about $5 million on the project up to now, and will continue that investment. Some additional key features of the future supersonic business aircraft have been defined. It will have an automatic fuel transfer system for zero-drag trim. The airframe structure will combine composites and titanium alloys, like Dassault's Rafale fighter, and the crew will have a system of mirrors and lenses to improve the over-the-nose view.

By Bill Sweetman