Brake Failure from Residual Magnetism in the Mars Exploration Rover   Lander Petal Actuator  

Louise Jandura *  

Abstract 

In January 2004, two Mars Exploration Rover spacec raft arrived at Mars. Each safely delivered an  identical rover to the Martian surface in a tetrahedral lander encased in airbags. Upon landing, the  airbags deflated and three Lander Petal Actuat ors opened the three deployable Lander side petals  enabling the rover to exit the Lander. Approximately nine weeks prior to the scheduled launch of the first  spacecraft, one of these mission-critical Lander Peta l Actuators exhibited a brake stuck-open failure  during its final flight stow at Kennedy Space Center. Residual magnet ism was the definitive conclusion  from the failure investigation. Although residual magnet ism was recognized as an issue in the design, the  lack of an appropriately specified lower bound on brake drop-out voltage inhibited the discovery of this  problem earlier in the program. In addition, the brakes  had more unit-to-unit variation in drop-out voltage  than expected, likely due to a larger than expected va riation in the magnetic properties of the 15-5 PH  stainless steel brake plates. Failure analysis and s ubsequent rework of two other Lander Petal Actuators  with marginal brakes was completed in thr ee weeks, causing no impact to the launch date. 

Introduction  Two Mars Exploration Rover (MER) spacecraft were sent to Mars, each with a rover to explore the  Martian surface with its suite of instruments. After entering the Mart ian atmosphere in an aeroshell, the  rovers were delivered to the surface in a Lander cove red in airbags. Once the landing system came to  rest on the Martian surface and the airbags defla ted, three Lander Petal Actuators (LPAs) opened the  three deployable Lander side petals, enabling the rove r to exit the Lander. Approximately nine weeks  prior to the scheduled launch of the first spacecraft, one L PA exhibited a brake failure during its final flight  stow at Kennedy Space Center. The failure analysis of this mission critical actuator and the subsequent  rework of two other marginal flight LPAs were  all done without causing the launch date to slip.  The MER spacecraft, carrying the rovers called Spirit and Opportunity, were launched on June 10, 2003  and July 7, 2003. These spacecraft successfully  landed on Mars on January 3, 2004 and January 24,  2004 respectively. All six LPAs operated without any problems.  Figures 1 and 2 show the LPAs installed in the Lander in both the stowed and deployed configurations.  The tetrahedral Lander shape with its three LPA-depl oyed side petals is inherited from the 1997 Mars  Pathfinder (MPF) program [1]. With this arrangement, the Lander can right itself from any side petal onto  its base petal by opening that side pet al until the Lander center of gravit y tips the entire system onto the  base petal. The LPA torque requirements for MER were much higher due primarily to the larger mass of  the landed system, making a re-fli ght of the MPF LPA design impossible. The same MPF volume  constraints for the LPA were applied to MER so the Lander would fit inside the aeroshell. Maintaining the  same volume and nearly the same mass as MPF wh ile producing three times the output torque was a  significant challenge for the MER LPA. Each LPA had to  develop sufficient torque to lift, overturn and right  the Lander should it come to rest on a side petal rather than the base petal. Both the first MER lander and  the MPF lander stopped on the base petal. However, the second MER lander came to rest on a side  petal, causing that side petal LPA to right the lander . In addition, each LPA had to be able to over-deploy  its petal to assist in leveling the Lander for a safe rover egress should it come to rest on uneven terrain.  Thus, the actuator’s unpowered holding torque (or backd rive torque) had to exceed the reaction load from  the weight of the Lander supported on a petal with that petal in the fully deployed position. Petal  adjustments were made on each MER lander to aid the rover egress. During petal opening, LPA position                                                    * Jet Propulsion Laboratory, California In stitute of Technology, Pasadena, CA 

Proceedings of the 37th Aerospace Mechanisms Symposium, Johnson Space Center, May 19-21, 2004   221