In an effort to reduce launch costs, satellite manufacturers have been trying to build smaller, lighter satellites. (It can cost more than $10,000 a pound to launch a satellite.) One major stumbling block has been the size of the satellite's momentum wheels. Used to keep the satellite stable and pointed in the right direction, traditional momentum wheels have a mass of 5-10 pounds, and some are much larger. A momentum wheel built with superconducting technology, on the other hand, would have a mass of less than two pounds.

The project started when a NASA official was touring the lab of Wei-Kan Chu, who heads TCSUH's Applied Research Division's efforts to develop superconducting applications. Upon seeing Chu's superconducting flywheel, which set a world record as the longest continuously spinning flywheel, the visitor asked if the technology could be applied to satellite momentum wheels.

"At that time I was not very convinced because superconductors require cryogenic refrigeration. But in the last year, the refrigeration community has made significant progress," says Chu. Appropriate refrigeration units are now the size of a soda can and require little energy, making the superconducting momentum wheel an economically feasible alternative for satellites.

A momentum wheel works on the same principles as a gyroscope. Wheels, each oriented in a different direction, spin simultaneously. The spinning wheels hold considerable angular momentum (inertia) and resist being reoriented. Thus, when outside forces buffet the satellite, the momentum wheels keep the satellite stable. A typical satellite has four wheels, one for each axis plus a spare.

Traditional satellite wheels were relatively large, about ten inches in diameter, but spun slowly. Satellite makers have moved to shrink momentum wheels to about four inches in diameter, but that requires a faster spin rate to maintain an equivalent effect.

Unfortunately, faster spinning wheels encounter more friction and therefore wear out faster and require more power, which is scarce on board a satellite.

In contrast, a superconducting wheel can be as little as an inch in diameter, can spin faster without encountering much friction and uses little power. The decrease in size, when combined with the weight reduction and lower power consumption, makes this technology economically more attractive than those in current use.

There is considerable demand in the satellite industry for smaller momentum wheels, says Rick Fleeter, president of AeroAstro, a Herndon, Virginia based company that manufactures smaller satellites. Fleeter critiqued the superconducting momentum wheel for UH.

"I was very enthusiastic about it because it is a way to package a lot of momentum in a small package," says Fleeter, noting that a superconducting momentum wheel would be smaller, weigh less, use less power and fit into a smaller satellite than conventional momentum wheels.

He cautions, however, that the satellite industry is unlikely to adopt a new technology until it is proven in space.

With that in mind, the TCSUH research team is now gearing up to move beyond the laboratory and produce a momentum wheel system for a real satellite.

The superconducting momentum wheel gains its advantage from a unique property of superconducting materials called "flux pinning," in which a superconductor's magnetic field holds other magnetic fields in a stable bond. (This is often demonstrated by placing a magnet above a superconductor so that it "floats.")

UH researchers added a magnet to the center of the momentum wheel and bracketed the hub with superconducting materials a short distance away. The magnet is trapped by the superconductors and resists moving closer or farther away, but it can spin freely. This free-spinning phenomenon is the basis of the superconducting momentum wheel. With no contact between moving parts, the superconducting bearings are nearly friction-free, especially in the vacuum of space.

"We can't eliminate friction completely, but we're making better use of the laws of physics," quips senior research scientist Ki Bui Ma, who helped design the momentum wheel.