Behind the Scenes: NASA's Flying IR Observatory
In the wonderful documentary, In the Shadow of the Moon, there's a revealing scene in which astronaut Alan Bean openly wonders if the engineers who designed the Saturn V booster really got the load calculations right, given how the thing shook and vibrated during the first-stage ascent. I thought of that last week when I was touring another of NASA's programs, the airborne observatory called SOFIA for Stratospheric Observatory for Infrared Astronomy.
Basically, NASA got hold of a retired 747 SP, ripped out the interior and carved a two-car garage-sized hole in the side of it, aft of the left wing. Into the cavity, they installed a 17-ton infrared telescope. They climb the airplane into the low 40s, open that door at Mach .8, and stare back into the universe for millions of years. You can see a video on the project here.
The Alan Bean moment came when I was given a tour of the aft cabin and got a look at how they engineered the telescope mount. The telescope is really two major elements: the optical/mirror section which inhabits the cavity and is open to the unpressurized environment and the electronic sensor and related equipment, which is in the pressurized side of the cabin. The trick part is this: because the IR sensor is inside the pressure hull, the telescope's faint light has to be carried through a pressure bulkhead via a fat metal pipe that doubles as the telescope's pivoting azimuth pointing system. If you know anything about the nature of large pressurized hulls, you know that next to wing spars, the pressure bulkhead at the rear of the cabin is a critical structure. It carries enormous loads—more than a million pounds—and if it fails, the resultant explosive decompression can take the airplane with it. And it has. Recall the 1985 crash of JAL 123, in which a botched repair on the pressure dome blew off the airplane's vertical fin and took out all of the airplane's hydraulics.
So in the SOFIA airplane, not only did the engineers have to design a pressure bulkhead with a big, load-carrying pass through, they also had to carry the flight loads around that giant opening just ahead of the empennage area. The control cables are also routed around the opening. During flight test, the airplane was peppered with so many strain gauges, they could measure the weight of the pilot's foot on the rudder pedals. As you'd expect, the airplane is festooned with structural add-ons to provide a margin of safety. In the video, aerodynamicist Brent Cobleigh explains how the engineers teased the airflow to keep that giant opening from becoming a destructive resonant cavity.
Vibration in telescopes can significantly degrade image quality and diminish the scientific value of the observations, even if the optics are mounted on earth. In a moving airplane, the challenge is magnified, especially because no matter how well the airflow is manipulated, there will be some vibration from air flowing past a huge opening at Mach 0.8. Not to mention the kind of turbulence any airplane encounters in flight and even subtle engine vibration. James Mills, the avionics lead for SOFIA, told me that the engineers addressed this in a couple of ways. That big light-tube pivot pipe rides in a giant oil bearing pressurized by robust, oil-field-type pumps down in the forward equipment bay. To damp the high-amplitude bumps, the circular mount is ringed by pneumatic dampers and the entire rotating mount is aimed by magnetic repulsion torque motors—conventional servo motors have too much gear lash. The assembly is balanced accurately enough to be easily rotated by light finger pressure; miniscule amperage will point it. The telescope is stabilized and pointed with the same type of ring laser gyro technology used in missile guidance systems. But it has gimbal limits which, if exceeded, mean than the telescope assembly is locked down until the flight crew can find quieter air.
As of mid-2013, SOFIA has flown about 100 missions and the pace is accelerating as infrared observation becomes an increasingly important part of the larger field of astronomy. Typically, the airplane takes off around sunset and flies a 10- to 12-hour profile, each encompassing specific data collection goals. The project accepts observational proposals from all over the world and the basic optics can be equipped with a variety of sensor packages. It can also fly to other parts of the world, as it will next month to New Zealand to observe targets not visible from the northern hemisphere. SOFIA itself is engineered to have at least a 20-year lifespan.
I'm sure Alan Bean would be happy with it.