In the near future, SKF bearings will go where few bearings have gone before, as part of the University of Pennsylvania’s Balloon-Borne Large Aperture Sub-Millimeter Telescope (BLAST) project. Funded partially by the National Aeronautics and Space Administration (NASA), BLAST will address some of the most important galactic and cosmological questions being researched by leading astronomers and physicists.
   
“Our universe is 14 billion years old,” says Associate
Professor of Physics and Astronomy Mark Devlin. “Because it takes light so much time to travel a distance, we can actually look at light that originated during the birth of the universe. We’ll be looking at the first galaxies and learning about our history in terms of star formation.”

   Although NASA has a sophisticated satellite system in development, it will not be operational for seven to eight years. “The BLAST system will provide valuable data, while allowing us to test many of the components slated for inclusion in the later NASA project,” Devlin explains.
   
The challenge for the BLAST team is in getting a telescope above the earth’s atmosphere and precisely controlling its position and movements so scientists can capture the images they’re after. Engineering students working on the project will gain invaluable experience in designing and building the BLAST system – a real-life opportunity that goes far beyond textbook theory and classroom instruction.
Up, up and away
The BLAST system essentially consists of a telescope fitted in a gondola which is suspended from an enormous 28-million-cubic-foot helium balloon. When fully inflated, the balloon is big enough to fill an entire football stadium. After the
balloon is launched, the reduced pressure in the upper atmosphere causes the balloon to inflate. The balloon stops rising when it reaches equilibrium at about 130,000 feet. At this point the telescope is decoupled from the balloon, suspended from a pivot and put into
operation – all with the help of SKF bearings.

SKF spherical roller thrust bearings assure stability

A crucial component in the success of the BLAST project, the pivot must smoothly decouple the gondola from the balloon and suspend the
4,000-lb. load. Most important, it must work to stabilize the gondola. To achieve these operational imperatives,
University of Pennsylvania engineers specified SKF spherical roller thrust bearings for the pivot assembly.
   
According to Devlin, the telescope must be pointed accurately to about 0.002 of a degree, a task made difficult by outside forces acting on the balloon. The pivot must counteract these forces to keep the gondola from rotating. “It’s like a rope ladder connecting the gondola to the balloon,” he explains. “We twist the pivot that connects to the ladder and it winds up to counteract the force.” He points out that in applying torque to the ladder, the tug against the bearings can cause what he calls bearing “stiction.” “We needed to eliminate the coefficient of static bearing friction,” he says. “This is a non-linear force that we see when we take the bearing from 0 rpm to moving. We don’t want this non-linear effect on the system,” he notes, “and we don’t want the clicking sound caused by friction. It interferes with our readings.”

   Devlin notes that he chose SKF spherical roller thrust bearings for this application because they are designed to correct for errors of alignment. He built a back-to-back configuration with a motor that keeps the bearings rotating. “If we don’t keep the bearings rotating we have problems,” he notes. “Basically, the ladder rotates until it winds up enough that it breaks the static bearing friction. The bearings release and unwind the ladder. This sends a vibration throughout the telescope. By rotating the bearings, we avoid the wind-up of the ladder and the abrupt release. It’s a very smooth system.”v
In addition to the spherical roller thrust bearings, university engineers used SKF deep groove ball bearings to aid in pivot shaft alignment.

Bearings assist in pointing accuracy
Telescope pointing accuracy is aided by a 200-pound flywheel located at the bottom of the telescope. Running at speeds of up to 180 rpm, the flywheel provides the torque needed to turn the telescope and to maintain the telescope’s attitude. The SKF angular contact bearings used in the flywheel suspension enable “a smooth, non-friction operation,” notes Devlin. SKF deep groove ball bearings were also used to keep the flywheel shaft from pivoting.

An environment of extremes
SKF bearings were also
chosen for BLAST because of their strength and reliability even under extreme conditions. In the BLAST project, extremes are the rule rather than the exception.

   “When it’s time to bring the telescope back,” says Devlin, “it comes flying down in a free fall. At the right altitude, the chute opens very quickly, which creates a shock load of 60,000 pounds. Every component must be capable of withstanding this instantaneous shock.”

   Other challenges relate to the extremes of temperature common to space and designated launch areas. BLAST will be launched twice – once from Texas and again from Antarctica.
    In Texas, the telescope is launched from a very hot climate into the sub-zero environment of the stratosphere. In Antarctica, equipment and systems must withstand frigid conditions from the ground up.
   
“Reliability is crucial in a project like this,” notes Devlin. “We’ve always relied on the outstanding precision of SKF bearings.” He points out that a single point failure could make it impossible to aim the telescope. If that happens, the entire project is in trouble. “When your system is in space you can’t send a maintenance guy up to fix it,” he notes.

SKF donations open doors

While SKF bearings are only a small part of the BLAST design, it is corporate donations and sponsorships like these that provide opportunities for students. “SKF’s donation saves us money. The money we save enables us to compensate our undergraduates for their work on the project,” notes Devlin. “This allows them to both satisfy their need for an income and their desire to learn.”

BLAST is scheduled to be launched from NASA’s Texas facility in May of 2002.