Live From...the Stratosphere
SOFIA Development

(This text was originally written by Allan Meyer and Terry Duncan on 14-April-95) Last Modified - 12/13/1995

Included in President's 1996 Budget Request.

The airborne astronomy community was delighted to see that President Clinton's budget for Fiscal Year 1996, released February 6, includes $48.7 million to initiate development of the Stratospheric Observatory for Infrared Astronomy (SOFIA). As noted in the budget request, this sum incorporates nearly $10 million in savings gained by ceasing KAO operations. If SOFIA is developed on schedule, the first research flights would occur at the end of 2000. The budget notes that
"...SOFIA will be a cooperative program with the Germans..."
Final approval will depend on congressional budget deliberations to occur over the next few months.

YOU can help design the SOFIA

Check it out HERE!

The SOFIA team has been busy preparing for a development start in FY96. SOFIA's inclusion in the President's budget request has increased the hope that their hard-worked plans will soon bear fruit. Chris Wiltsee, Manager of the SOFIA Project Office, recently provided an overview of accomplishments and work in progress for the final SOFIA definition effort. Some of the documents and outside review activities are listed below:

The Implementation Phase Project Plan, a roadmap of SOFIA's development, is complete, and an updated Phase B Definition Document, known more simply as the "Blue Book", is being assembled by Tom Maiello and Mike Shovlin for release in February. The baseline aft cavity concept was verified by extensive wind tunnel tests at Ames last summer, and a short final test is now in progress

At the January Tucson AAS meeting, two dozen "friends of SOFIA" met to review a draft Science Operations Plan. As expected, the discussion was lively and productive.

One important review was recently completed and another will occur soon. An Independent Project Review by the Aerospace Corporation concluded that Project plans were in "good shape" and supported very near term procurement of the aircraft. They also concluded that an operation of 160 flights per year was "aggressive, but feasible". The reviewers suggested that the SOFIA team consider installing multiple focal-plane instruments on the telescope. The SOFIA team anticipates a productive and satisfactory Non-Advocate Review in the near future.

The figure below was kindly provided by Dane Elliot. He is using Pro/Engineer to develop an authentic model of the SOFIA telescope cavity as a three-dimensional specification of the volume available for the telescope assembly. This cavity envelope model will be used to check the fit and movement clearance of the telescope design as it is developed.

Wind Tunnel Tests

Recent tests of a SOFIA model in Ames' 14-foot Transonic Wind Tunnel successfully demonstrated the viability of installing the telescope in an open port cavity aft of the wing. The tests determined the choice of a passive flow- control geometry which eliminates significant cavity resonances and provides ample aerodynamic stability and control margins. Measured telescope wind loads suggest a truss structure may be preferable to a closed tube design.

The first wind-tunnel tests for the SOFIA definition effort were completed in 1990, with a telescope aperture located forward of the left wing in a Boeing 747 model. However, the much lower cost of an aft cavity telescope intallation prompted a second test series which was com-pleted in August 1994. The model used in this 12- week long test series had a full tail assembly, to evaluate the effect of the telescope opening on the tail surfaces ('empennage'). Both the 747-200 and the 747-SP were modeled during the summer 1994 tests. An actual Boeing 747-200 is 225 feet long, so the 7% model was 16 feet long in that configuration.

During the summer 1994 tests, over 100 distinct configurations of the two versions of 747 were tested for characteristics relevant to flight safety and telescope cavity environment (some baseline tests were run with the cavity closed). The wind tunnel was run at Mach 0.76 to Mach 0.86, using up to 80 megawatts of electrical power, so the test runs were usually done in the evening. Measurement techniques included pitot static taps, dynamic pressure kulites, two separate 6 degree of freedom force balances (one for the telescope and one for the tail), boundary layer rake probes, oil paint flow visualization, accelerometers, pressure sensors within the telescope cavity, and pressure-sensitive paint on the empennage. The optical reflectivity of pressure-sensitive paint is sensitive to the partial pressure of oxygen, providing a continuous map of the pressure distribution on the painted surfaces.

The selected design for the cavity opening is described as an internal D-shaped aperture, which uses a semi-circular ramp within the aft part of the opening to re- attach the descending turbulent shear layer. It was also established that the disturbance created by the telescope opening did not reduce the effectiveness of the empennage for flight control on either Boeing model. Because the SP has a slightly higher rudder and longer horizontal stabilizers, the overall ≥margin of immunity≤ is better for the SP. The measured acoustic noise level in the model telescope cavity was 135 decibels. This scales to a predicted noise level in the SOFIA cavity of 123 decibels at 41,000 feet, significantly less than the noise levels measured in the KAO cavity.

During the 1994 tests, the telescope model in the cavity had a solid tube from the primary mirror to the top end of the telescope. Beginning in January, 1995, a short test will be performed to determine whether there would be any reduction in the aerodynamic loads on the telescope with an open-truss metering structure.

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