SOFIA (the Stratospheric Observatory for Infrared Astronomy) passed a major milestone last month when the telescope assembly arrived in Waco, TX to be integrated into the SOFIA aircraft. Due to its considerable size and weight, the telescope was disassembled into three components and shipped on an Airbus Beluga Cargo aircraft (see figure 1), specially designed to handle oversize loads. The three components, including the suspension assembly (~11,400 kg), the primary mirror assembly (~2,000 kg) and the metering structure for the secondary mirror (~900 kg) arrived from Germany on September 4 after short stops in Scotland, Iceland, eastern Canada, and western New York. The major components were successfully unloaded (figure 2) and are now ready for integration into the aircraft later this year.

figure1: Airbus Beluga Cargo aircraft on Waco runway	figure2: Unloading the telescope assembly components
Click on image for close-up view

In this news note, we discuss the aircraft modification necessary for operation of the SOFIA telescope. The work on the aircraft is being performed at the L3 Communications Integration System in Waco, TX. L3 is being assisted in the modification effort by United Airlines (UAL) and NASA Ames Research Center. Both L3 and UAL are under subcontract to USRA, which has the development and operations contracts from NASA.

A Boeing 747-SP was selected as the SOFIA platform due to its proven track record in commercial service, its large fuselage, which can accommodate the 2.5m telescope and mission systems, and its ability to allow observations above 41,000 ft for at least six hours and above 45,000 ft for over an hour.

figure3: Cavity door Click on image for close-up view

The most significant aircraft modification is the construction of an open port that allows unobstructed viewing from the visible to the submillimeter. Wind tunnel tests were conducted to verify the aerodynamic feasibility of such a large open port. As a result, an aperture concept was developed as part of the cavity door system that provides an opening just large enough to allow the telescope an unvignetted view while moving with the telescope through the desired elevation angle range of 20 to 60 degrees. This 5.5m (arc length) by 4.1m wide curved cavity door provides many of the same functions that a ground-based telescope dome does, but also provides a thermal barrier to allow the telescope to be pre-conditioned to stratospheric temperatures (typically -40 degrees C) as well as provide a quiet cavity and stable aircraft. The sophisticated cavity door, designed by NASA Ames Research Center, consists of three major components Ð an Upper Rigid Door, a Lower Flex Door and the Aperture (see figure 3). Once the desired observing conditions have been achieved, the Upper door will open, exposing the telescope. After this, the remaining door components will move together to track the telescope as it moves to the desired elevation angles, closing off the portions of the structural opening not required for the observation.

figure 4: Interior of observatory Click on image for close-up view

figure5: Forward bulkhead Click on image for close-up view

The second major modification is the forward bulkhead, which serves two purposes. It provides a pressure seal between the open-air cavity and the pressurized cabin for the crew, scientists and education specialists (see figure 4). Secondly, the forward bulkhead functions to support the 17,000 kg telescope assembly with instruments in the aircraft. The forward bulkhead, now attached in the SOFIA aircraft, is shown in (figure 5). An aft bulkhead is required to minimize the overall cavity size for aero-acoustic purposes during the open-door flight operations and for thermal requirements to facilitate pre-cooling of the telescope and cavity prior to an observing flight.

The structural reinforcement for the aircraft (sill beams and frame modifications) required for the cavity cutout is near completion. All replacement skins have been installed, the aft bulkhead installation is complete and the forward pressure bulkhead installation is in place. Integration of the first major telescope subsystem into the modified aircraft is planned later this fall, and flight testing is scheduled before the end of 2003.

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