The former United Airlines Boeing 747SP earmarked for conversion to NASA's SOFIA (Stratospheric Observatory For Infrared Astronomy) project has been ferried from United's San Francisco maintenance base to Waco, Texas, for modification by Raytheon E-Systems.

The aircraft was brought to San Francisco in mid-February from Las Vegas, Nevada, where it had been in open storage for two years, says United's SOFIA programme manager, Ann Blundell.

E-Systems Airborne Systems Division is planning to instrument the aircraft, N145UA, with a range of accelerometers, strain gauges and boundary layer rigs to assess the baseline performance of the unmodified aircraft during tests later this year.

Using the data, the company plans to develop computer models of the aircraft that will be used to help design the extensive structural modifications needed to accommodate the 2.5m-diameter infra- red telescope. This will be housed in an unpressurised cavity in the aft section of the fuselage, forward of the pressure bulkhead location.

Raytheon is designing a new pressure bulkhead which will be located between fuselage stations 1720 and 1740, close to the trailing edge of the wing. The area aft of this will be occupied by the telescope, mounted on a cradle assembly which will rotate about the aircraft's longitudinal axis. Aft of the cavity will be a second bulkhead, the main purpose of which is to thermally insulate the telescope area from the tail section.

The telescope, and its cavity, will be cooled to -40¼C before missions to reduce the effect of thermal interference on the mirror. The door will then be opened at high altitude when the air temperature matches that of the telescope.

NASA and Raytheon also plan to conduct more windtunnel tests on a new door design for the telescope cavity. This will be more than 3m in width and affect around 90% of the fuselage circumference.

Windtunnel tests of the first design, a barrel-type inset door, produced unexpected aero-acoustic disturbances which caused the mirror to resonate. The new design is a flush door which would move inside the fuselage and rotate internally to lie against the inside of the upper starboard fuselage roof section. The fuselage skin would have to be "bulged" on one side of the fuselage by more than 100mm to accommodate the door, but NASA believes that the drag will be less severe than on the inset door.

Conversion work is expected to begin in the third quarter of 1998, with the first scientific flights taking place in late 2001.

Source: Flight International