NASA GODDARD SPACE FLIGHT CENTER OPTIMIZES SATELLITE REACTION WHEEL WITH RANDOM VIBRATION SOFTWARE

The redesigned reaction wheels were launched successfully on the Transition Region and Coronal Explorer (TRACE, shown left) in April 1998, a satellite mission that is studying the sun's coronal region.

December 30, 1998, Pittsburgh, Pennsylvania -- NASA scientists rely on reaction wheels to maneuver observation satellites in space. Based on information gathered by sensors, four reaction wheels position the satellite to face constellations of interest. The reaction wheels must withstand rocket launch vibrations to operate effectively in orbit. Engineers at the NASA Goddard Space Flight Center, Greenbelt, Maryland, used random vibration stress analysis software from ALGOR, Inc. to test the structural integrity of a redesigned reaction wheel that can position satellites more quickly. NASA simulated vibration forces during a rocket launch and analyzed deflection in the reaction wheel's outer housing structure. NASA then optimized the housing's design on the computer to reduce deflection that would otherwise cause the reaction wheel to fail.

The new reaction wheels were launched successfully on the Transition Region and Coronal Explorer (TRACE) in April 1998, a satellite mission that is studying the sun's coronal region, and are scheduled to be used on two upcoming space expeditions.

Redesigned Reaction Wheel Controls Satellite's Position More Efficiently

A reaction wheel is an actuator that is part of the satellite's Attitude Control System (ACS). The reaction wheel changes the satellite's position based on torque commands issued by the satellite's exterior sensors. Each reaction wheel is comprised of a motor, flywheel, bearings and a printed circuit (PC) board that supports its electronic parts. The PC board is attached to ribs at the base of an outer housing, the component that protects the reaction wheel's parts from radiation and the pressure differential in orbit.

The reaction wheel's motor, flywheel, bearings, printed circuit (PC) board and electronic parts attach to the base of an outer aluminum housing (shown left with the top cover removed) that protects the components from radiation and the pressure differential in orbit. NASA used ALGOR's random vibration stress analysis software to analyze the housing component's deflection during the 8- to 12-minute launch from Earth.

NASA redesigned its reaction wheel in 1993 to maneuver satellites more quickly. Engineers enlarged the motor, bearings and flywheel to provide more torque and momentum storage and altered the aluminum housing to fit them. They first used ALGOR's linear static stress analysis software to optimize the geometry of the revised flywheel and housing component. Then they used ALGOR's random vibration stress analysis software to analyze the housing component's deflection during the 8- to 12-minute launch from Earth.

Random Vibration Software Simulated Launch Conditions

To operate effectively during the reaction wheel's three-year life span, its outer housing must not deflect enough during launch to fracture the solder joints that attach electronic parts to the PC board. The reaction wheel would then lose functionality. The housing also must not deflect against the PC board or rotating flywheel, causing them to malfunction. NASA used ALGOR's random vibration stress analysis software to determine the housing's response to simulated launch vibration loads.

NASA engineers designed a 3-D finite element model of the housing with ALGOR's Superdraw program. They used beam elements to represent six radial ribs at the base of the housing and a combination of 3-D plate elements and 3-D brick elements for the remainder of the housing model.

After conducting a modal analysis with ALGOR software to determine the housing's natural frequencies and modes of vibration, NASA performed a random vibration analysis that simulated actual rocket launch vibration levels. Engineers applied a 14.1-G root mean square (RMS) acceleration to the model, a combination of the accelerations experienced in the X-, Y- and Z-directions at one time. The first set of analysis results showed excessive deflection within the housing's radial ribs, indicating that the ribs would fracture the solder joints. NASA thickened the radial ribs to increase their stiffness.

This final ALGOR model of the reaction wheel's housing illustrates the response due to random vibration. The base has been constrained in all directions to replicate its attachment to the satellite. (One-half of the model is shown for better viewing.) In earlier iterations, excessive deflection was found within the housing's radial ribs, indicating that they would fracture the solder joints that attach electronic parts to the PC board. NASA thickened the ribs to increase their stiffness and decrease deflection. The white lines at the housing's base represent beam elements that NASA used to represent the ribs.

After three iterations, NASA created a physical prototype and affixed it to a vibration machine in its laboratory that simulated 14.1 G's (RMS) in the X-, Y- and Z- directions. The physical prototype test results correlated closely with ALGOR software results.

Reaction Wheels Launched Successfully with TRACE Satellite

The redesigned reaction wheels were successfully launched with the TRACE satellite in April 1998 and are scheduled to operate on the Submillimeter Wave Astronomy Satellite (SWAS) leaving Earth in December 1998 and the Wide Field Infrared Explorer (WIRE) satellite launching in April 1999. The expeditions will explore multiple star fields to give scientists a better understanding of star formation.