Guy Norris/WACO, TEXAS
Twenty-four years ago, the Boeing 747SP was designed to be flown higher and further than anything in its class. These attributes, plus the large payload capacity of the "Special Performer", have now landed one aircraft in the small world fleet the unique role of the SOFIA, the world's most advanced airborne space observatory.
The SOFIA (stratospheric observatory for infra-red astronomy) is designed to replace NASA's Kuiper Airborne Observatory (KAO), which is housed in a now-grounded Lockheed C-141 Starlifter. Between 1974 and 1995, astronomers used the KAO's 910mm telescope to view space in the infra-red (IR) spectrum, which is inaccessible to ground-based observatories because of water vapour in the lower levels of the Earth's atmosphere.
Telescope task
With the C-141 reaching the end of its design life and the IR regime still largely unexplored, NASA and the German space agency DARA jointly agreed to develop a new and more capable successor. The heart of the SOFIA is a 2.5m-long IR telescope to be designed and developed in Germany. The sheer scale of the instrument, and the requirements for up to ten times the sensitivity and three times the angular resolution of the KAO, means that only a widebody aircraft would be fit for the task of carrying the telescope about.
NASA, which has responsibility for the provision, refurbishment and modification of the platform, began the search for the right aircraft. "We considered the [Lockheed Martin] C-5, but it could not climb as high or as well as we needed," says NASA Ames-based SOFIA project manager Chris Wiltsee. "We even looked at buying the Antonov An-225 as part of a complex US Government deal involving the disposal of nuclear weapons, but that ended because we wanted to be able to fly worldwide at a moment's notice. The L-1011-500 was the only other serious contender, and the Boeing 767 was somewhere in there, but neither could match the 747. Both had between 2.5h and 3h less time at altitude, which was the main criterion," he says. "The 747 became the aircraft of choice, and that's when the SP price came down to between $5 million and $25 million," he adds.
NASA selected a relatively "young" SP from among a batch recently retired by United Airlines to storage in Las Vegas, Nevada. With the airframe selection accomplished, NASA moved ahead, and in December 1996 awarded the Universities Space Research Association and a team including United Airlines and Raytheon E-Systems a ten-year, $484 million contract to design, assemble, test and operate the SOFIA. Other team members include The Astronomical Society of the Pacific, the University of California, the SETI Institute and Sterling Software.
After a ferry flight in mid-February to United's maintenance base in San Francisco, the SP (N145UA) was prepared for its handover to NASA and to Raytheon E-Systems. The latter was given the task of performing the extensive structural modifications at its site in Waco, Texas. In early May, the 747SP was flown to Texas, where Raytheon began the task of fitting the aircraft out with a wide range of instrumentation, including accelerometers and strain gauges.
These will be used to assess the baseline performance of the unmodified aircraft during a series of flight tests planned for September. Using the data, the company plans to develop finite-element computer models of the aircraft which will be used to help design the structural modifications required to house the telescope and related hardware.
The telescope will be located in an unpressurised cavity in the aft (Section 46) of the fuselage, forward of the current pressure-bulkhead location. Raytheon is designing a new pressure bulkhead which will be between fuselage stations 1720 and 1740, close to the trailing edge of the wing. The telescope, weighing around 18,000kg with related instruments, will be mounted on a cradle assembly which will rotate about the aircraft's longitudinal axis. The cradle will be supported on a shaft running through the forward bulkhead to the main cabin. A spherical air-bearing rotation isolation system consisting of 24 pads, 1.2m in diameter, will be used to keep the telescope fully moveable. A two-element rotation-drive system will be used to point the telescope: a coarse drive for elevation and a fine drive (brushless direct-current spherical segment motors) for elevation, cross-elevation and line-of-sight.
Crucial bulkhead
A second aft bulkhead, again well forward of the present pressure bulkhead, will be positioned close to the point where the dorsal fin begins to emerge from the fuselage. This essentially non-structural bulkhead forms the aft wall of the cavity, and is being designed to insulate thermally the telescope area from the tail section. The telescope, and its cavity, will be cooled to around -40íC before every mission to reduce the effect of any thermal interference on the mirror. The door will then be opened at high altitude when the outside air temperature matches that of the telescope.
The last remaining, and one of the most fundamental, unknowns about the SOFIA is the final design of the door which will cover the telescope cavity. "We still have two door options and will conduct windtunnel testing at Ames in November to choose the preferred one," says NASA SOFIA chief engineer Nans Kunz. One of the options proposed for the aircraft is a barrel-door concept which "rotates circumferencially" within the fuselage.
"We discovered that the overall spectrum of noise or sound pressure created by the cavity was causing some disturbances," says Wiltsee. It was inducing fairly significant activity of the telescope and, "-although we could probably live with it, the pointing stability would be degraded", he adds.
The team attempted to fine tune the aerodynamics of the fuselage plug which had been designed to exercise shear-layer control over the barrel door and the opening through which the telescope would peer. Despite four major windtunnel tests, the design continued to produce disappointing results with acoustic resonance in the cavity itself, evidence of high fatigue on the fuselage skin, larger air loads which would affect the pointing stability of the telescope and, lastly, a possible impact on the control of the aircraft.
Part of the problem was traced to the disturbance created by the 400mm deep, aft-facing step of the cavity door. This also created a substantial incremental drag penalty, estimated at up to 0.45m2 (4.8ft2), produced aeroacoustic loads on the door and telescope and aero-buffeting. Finding a solution would not only reduce wear and tear on the airframe and scientific equipment, but would potentially improve the data and increase the science time at altitude.
Five alternatives
Engineers therefore designed alternatives which were boiled down to five finalists. In April, NASA selected the competing design which is called the "partial external door". This is enclosed by a fairing which would be built to wrap around the starboard upper side of the fuselage. The door would "-roll out from under the fairing and roll out on top of the fuselage", says Kunz. The partial external door would be external to the aircraft's outer mould line, being raised by around 200mm. Despite the bulge, NASA believes that the overall drag would be lower for this design than for the barrel, but it will wait for windtunnel tests to be completed before making a final assessment. The door design would still need the shear-layer control aperture, but would not require the complex fuselage plug used in the barrel- door design. "Everybody feels it won't be a problem, and we are keeping the other really as a back-up," explains Wiltsee.
The cavity door also performs the essential role of a thermal barrier. "You simply cannot operate unless you make that work", adds Wiltsee. "You get problems if the telescope is warmer than the air because of the formation of convection currents that disturb the images." The telescope and its cavity will be pre-cooled for between 4h and 5h before take-off with a liquid-nitrogen system. "It's a fairly major operation because the thermal mass of the telescope is quite large," Wiltsee explains. The German telescope team, which includes MAN Gutehoffnungshutte, MAN Technologie and Kayser-Threde, are incorporating a fan which will blow cold air below the mirror, helping to reduce the temperature even further.
The door will then be opened once the aircraft is at its cruising altitude at 41,000-45,000ft (12,500-13,700m) and the outside temperature equates to that of the cavity. NASA plans to be able to fly for at least 7.5h at altitudes of 41,000ft and above on each mission. With up to 160 observing flights a year, this is expected to result in around 1,000h of total research flight hours a year. "We are actually hoping for up to 8h, and have written an incentive into the contract for every minute over 6h that we get at the altitude and payload we've specified." The KAO, in comparison, managed to achieve between 4h and 5h at 41,000ft.
Astronomic hopes
NASA and literally hundreds of astonomers and scientists will be hoping for a positive outcome to the flight-readiness review planned for March 2001. Raytheon, meanwhile, does not expect to begin "cutting metal" on the conversion until late 1998 at the earliest because of the length of time involved in the development of the telescope. If all goes to plan, the 747SP will by 2001 be carrying scientific teams eager to bring unprecedented clarity to the dark reaches of our own and other galaxies.
Source: Flight International
