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Thomas Keilig manages SOFIA’s telescope and science instruments. (Chad Slattery)

A 747 for Star-gazing

How engineers altered a jumbo jet to carry the world's biggest airborne telescope.

Because mathematical modeling is less reliable for unsteady airflow than for, say, structural loads, the team collected data from wind tunnel tests on a 7-percent-scale model. “We knew we had only one shot at modifying the real thing,” says Doty, “so we bought another Section 46 from a sister 747SP from an aircraft storage yard in Oklahoma to practice on.” Section 46, the fuselage segment between the wing and tail, is the part that would hold the telescope cavity and new pressure bulkhead (separating the heated and pressurized crew and astronomers’ cabin from the unpressurized telescope cavity). L-3 flew the section back to Waco in NASA’s Super Guppy transport.

L-3 did not saw a hole in the side of the 747SP. Instead, engineers removed structural elements and panels and replaced them a few at a time, while taking an approach analogous to shor¬ing up the walls of a house before digging a new basement. Ruggles says that for the modification, “the plane needed to be in traction, so we put it in jig position,” a term for supporting the aircraft so there are no loads anywhere on the structure.

Once the aircraft was in position, Ruggles says, “we started by installing as much of the new skeleton first before taking out the old skeleton.” The installers first replaced frames, stringers, and other supporting members with beefed-up versions, then removed the originals. Same for the skin: on with the new, off with the old. At the telescope cavity, says Ruggles, “off came a regular panel, on went a panel with part of the cutout.”

After finishing the modification, the engineers faced a trial that was more challenging emotionally: giving the reconstructed airframe a “proof pressure” test by inflating it to 12.5 pounds per square inch (psi), 33 percent above maximum operational cabin pressure, to prove it could be safely pressurized in flight. “At first we heard a few light pops as skin panels pillowed out,” Ruggles recalls. “Then at about 12 psi, suddenly a loud bang made us all jump. But it turned out only to be severe thunderstorm winds slamming a door behind someone.”

WHILE L-3 WAS MODIFYING THE AIRCRAFT in Texas, the German DLR center was overseeing construction of SOFIA’s telescope in Bonn, Germany. The instrument was built primarily by MAN Technologie AG and Kayser-Threde GmbH. After being integrated and tested in Augsburg, the telescope was disassembled and shipped to Waco in a Beluga (Airbus A300-600ST Super Transporter). A crane gently lifted each major subsystem and lowered it into the modified aircraft’s telescope cavity. “It barely shoehorned into the opening,” Doty recalls. “It was a remarkable feat—almost like giving birth in reverse.”

In May 2007, after some significant structural test flights at L-3 were completed, SOFIA flew from Waco to Dryden for interior completion. In January 2008, the flying observatory was relocated to the new Dryden Aircraft Operation Facility in Palmdale, about 40 miles from Edwards, which allows easy access for non-NASA and international personnel. Dryden will oversee SOFIA’s flight operations even after SOFIA program management moves to the SOFIA Science Center at Ames.

On December 18, 2009, SOFIA passed a major aerodynamic milestone: a daytime test flight with the telescope cavity door fully open, all under the watchful eyes of chase planes and ground cameras. And on the night of May 25-26, 2010, SOFIA passed a major astronomical milestone: while airborne at 35,000 feet, the telescope achieved what astronomers call first light, taking a look at both Jupiter and the galaxy Messier 82.

Both achievements were huge moments for NASA, L-3, and the German Aerospace Center. “If compared to human surgery, that aircraft went through the equivalent not just of a heart-lung transplant, but also replacing your liver, kidneys, and part of your brain,” says Ken Szalai. SOFIA has run far beyond NASA’s originally projected schedule and budget; its current development cost, $1.1 billion, is about triple the budget NASA originally projected in 1997. But Szalai is not bothered: “In doing something this close to the edge of the unknown, there is nobody who could accurately estimate it,” he says.

Last summer, the SOFIA Science Center announced the first 75-hour block of observing time for basic research between March and August 2011. The center received 59 proposals from 31 institutions for more than triple the number of observations that can be made during the first block of time.

Trudy E. Bell is a former editor at Scientific American.

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