Guy Norris/LOS ANGELES
Lockheed Martin is studying new turret design options to improve the performance of the Airborne Laser (ABL) ballistic missile defence system following the completion of windtunnel tests by programme leader Boeing.
The YAL-1 ABL will be based on a modified Boeing 747-400F, and the design of the nose-mounted ball turret is critical to the successful performance of the laser with which missiles will be destroyed during the post-launch boost phase. Windtunnel tests at Boeing's transonic site in Seattle were aimed at "-checking the exhaust impingement from the laser and the aero-optics", says Boeing senior research engineer Victor Buonadonna. "Particularly important is the dynamic pressure environment, how it affects acoustic levels and how much the design of the laser will have to accommodate for that," he adds.
In the tests, methods of reducing the dynamic pressure and further optimisation of the passive flow-control system (which is designed to preserve a smooth boundary layer around the ball turret) were examined. The system essentially consists of a hole cut into the back side of the roll shell enclosing the turret. Earlier tests showed that, in particular positions, the boundary-layer flow separated "at the shoulder of the turret into the look-back cavity", creating a turbulent shear layer. This affected the strength of the TRW-developed chemical oxygen-iodine laser and had to be corrected, says Boeing.
As part of the tests, Boeing tried a "suction bump", located by the inlet to the passive flow-control duct. The bump was intended to generate a low-pressure area to raise the suction rate. "The best solution, however, turned out to be no bump at all, which was a big surprise to all of us," says Buonadonna. A total of 12 separate flow-control-device configurations was devised for future trade studies.
"We've also given Lockheed Martin data for a couple of options to reduce dynamic pressure," Buonadonna adds. One is redesigning the sphere with faceted surfaces, resembling a golf ball. "This is basically a roughened sphere everywhere, except for the laser window. It produced a very thick boundary layer and very quiet shear layer," says Buonadonna. The second is a splitter plate in the look-back cavity. "Looking at the turret face-on, it would show up as a line. It works well, quietening the acoustics around the back end of the ball," he adds.
A larger-than-normal (6% scale) model of the 747 forebody was used to produce higher Reynolds numbers. Ironically, the model came from the former Rockwell site at Seal Beach, California, where it had been made to support the company's abortive ABL bid before its take-over by Boeing. Test results will now be used by the ABL team to revise the turret design before the full-scale, flight-weight, laser-module demonstration preliminary design review in the second quarter of this year.
The first ABL aircraft will be flown from the Everett production line to Wichita, Kansas, in 2000 for modification. In 2002, it will demonstrate the laser by shooting down a theatre ballistic missile. Pending success, the programme enters the engineering, manufacturing and development phase in 2003 with a planned fleet of seven becoming operational by 2008.
Source: Flight International