THE SINGLE BIGGEST AERODYNAMIC CHALLENGE NASA engineers faced was modifying the airplane so it could fly safely even with an enormous hole in its fuselage. That challenge broke down into several key imperatives, notably: not disrupting clean airflow over the fuselage and control surfaces; restoring the strength and stiffness of the fuselage after structural members were removed and a heavy telescope assembly was added; and minimizing acoustic vibrations.
Plenty of airplanes open doors in horizontal flight—to release bombs, parachutists, or cargo. But they don’t continue to fly with open cavities for eight to 10 hours at a stretch, they typically don’t fly in the stratosphere, and they certainly don’t fly with a door as big as SOFIA’s. This telescope’s cavity is so huge in proportion to the size of the aircraft that the aerodynamic challenges in designing it were “in a different universe [from the KAO],” says former project director Ken Szalai of NASA’s Dryden Flight Research Center at Edwards, California.
Even to the untrained eye of a visitor walking around the outside of the aircraft, as I did last March at the 747’s new home in Palmdale, California, the scale of the challenge is startlingly evident. The full cut-out measures 14.5 feet wide and more than 16 feet circumferentially—although only about half of that circumference is exposed in flight because a rigid upper door and a lower flexible door (like the front of a roll-top desk) slide in tracks to follow the 22-ton telescope’s aperture as it points high or low. “It’s the largest cavity we’re aware of that’s ever been flown,” says Robert Meyer Jr., a former flight test crew member who is now SOFIA program manager at Dryden.
The cavity for the KAO’s reflector was forward of the C-141A’s wing and right behind the pilots, in what on a commercial flight would be first class. But in SOFIA, “the telescope flies economy,” quips Erickson, because a far aft position for such a large open port reduced engineering complexity and expense. Only a third of the control cables—those for the tail—would need to be rerouted around the unpressurized and unheated telescope cavity. Most crucially, an aft position minimizes the chances of high-speed, energetic air dipping down into the cavity and creating destructive acoustic resonances—literally tones. “Think of blowing across the open top of a Coke bottle to make it hum,” says William Rose, who made open-cavity aero-optics and aero-mechanics improvements for the KAO at Ames and now as president of Rose Engineering in Nevada is doing similar work for SOFIA. The cavity was placed at the rear of the aircraft to keep it away from the most highly compressed and highest energy airflow—around the nose of the vehicle, the forward fuselage, and the wing leading edges.
The task of modifying the airplane was contracted to what is now L-3 Platform Integration in Waco, Texas. “We were brought in because we make heavy aircraft mods. We don’t touch anything smaller than a C-130,” says Greg Bruich, technical director for SOFIA’s mission controls and communications system from 1995 through 2003. (During the creation of SOFIA, the company was acquired and changed names four times—from Chrysler Technologies Airborne Systems to E-Systems to Raytheon Aircraft Integration Systems to L-3—so NASA personnel “took to calling it just ‘Waco,’ ” Nans Kunz says.)
First, L-3 had to reverse-engineer every aspect of the aircraft’s existing design. The 747SP was designed in the late 1960s, long before the days of computational fluid dynamic models. Sure, Boeing had aircraft drawings, specifications, and analyses, but NASA needed a detailed structural model of the airplane to understand how loads were carried through the structure. “So we covered the unmodified aircraft with 350 strain gauges and other instrumentation, and then collected data as we repeatedly put it through wind-up turns, side slips, and other baseline maneuvers to measure loads on fuselage, wings, and tail,” recalls Albert Ruggles, L-3’s lead structures analyst for SOFIA. From that data, the L-3 engineers painstakingly built a computer model that represented every frame, sill, stringer, and other structural member—some 150,000 elements in all. The model gave the engineers a powerful tool to plan SOFIA modifications and simulate how various configurations would affect structure and aerodynamics.
For one thing, the engineers needed to model what would happen when the telescope cavity left only three-quarters of the fuselage holding onto the tail. A quick try-this-at-home experiment reveals why: Wearing gloves, grab the ends of an empty aluminum soda can in both hands and try to twist it. As thin as the aluminum is, it resists a surprising amount of force. Next, cut a square hole in the can that extends a quarter of the way around the circumference. Now, crumpling the can takes far less twisting force. Why? “The skin of an airplane is more than just a big balloon with ribs that holds in the cabin air pressure,” explains Dwight Doty, L-3’s engineer in charge of aerodynamic analysis. “The airplane’s fuselage is a semi-monocoque construction, meaning the skin is mounted to an underlying skeleton. The skin carries pressure and shear loads—that is, forces that would tend to stretch, compress, or twist the fuselage—and distributes these loads to the stronger skeleton.”
Even with the cavity door closed, I could clearly see that the door and the tracks along which it runs make part of the fuselage asymmetrical, raising the aft left side a bit and making it more cylindrical. The asymmetry in turn changes the airflow, stresses, and structural loading on the rest of the airframe. Moreover, Doty adds, with all the telescope’s weight concentrated in the back, “this structure is not loaded like a regular aircraft.” Thus, L-3 needed to calculate how to redistribute strength, stability, stiffness, and shear stresses around the telescope cavity by strengthening the underlying skeleton structure, which is made up of hoop-like frames encircling the fuselage, longitudinal beams, and other internal supports. The engineers also needed to thicken the skin itself. “Some places right around the cavity [now] have several skin layers totaling more than half an inch thick,” Doty points out.
The interior of the telescope cavity also had to be mathematically modeled. When the doors on most bomb bays, or even on the KAO, open, the airflow is disturbed, causing the airplane to shake noisily. With SOFIA’s huge cavity, design engineers did not want shaking. Worse, says Kunz, “we didn’t want a 747-sized organ pipe.”
Under the right conditions, all cavities resonate acoustically, just like a musical wind instrument. “We were really concerned about organized tones,” said aero-acoustics expert Rose. “You can’t have that, because structures can be torn apart by pressure changes.” The amplitude—or loudness—of the pressure variations changes with the square of the speed; the pressure fluctuations at 500 mph are 100 times greater than those at 50 mph. If fluctuations of that magnitude happened in a car with a window down driving at freeway speeds, “blood would be running out of your eardrums,” says Rose. Such powerful vibrations would “quickly eat up the fatigue life of the aircraft’s metal structure in a matter of minutes, not hours,” says Dryden’s Meyer.