252 With a piston guide of reasonable length, friction and the allowable radial clearance are extremely  important design parameters because this mechanical configuration is very sensitive to these variables. The best option, where possible, is to use a lubricant to control the sliding friction. Otherwise, the lubrication function must be provided by careful material selections of the moving components. Polymers like Teflon ® are great, but their CTE is large compared to metals and the applied contact pressure  between the sliding surfaces must be limited. The slotted guide presented here worked exceptionally well to accommodate the large CTE difference.   Table 3.  Results from the load testing of the final guide design  (all bellows pressure values were well below the 38 kPa limit) 

References 

1. Bhandari, P., Birur, G., Pauken, M., Paris, A., Novak, K., Prina, M., Ramirez, B., and Bame, D., “Mars  Science Laboratory Thermal Control Architecture,” SAE 2005-01-2828, 35th International Conference on Environmental Systems, Rome, Italy, July 2005.  2. Bhandari, P., Birur, G.C., et al, “Mechanically Pumped Fluid Loop Heat Rejection & Recovery  Systems For Thermal Control on Martian Surface – Case Study of The Mars Science Laboratory,” 36th International Conference on Environmental Systems, Norfolk, Virginia, July 2006.  3. Flexial Corporation provided experienced advice on the details of bellows analysis methods within the  confined space of the accumulator housing. Flexial Corporation, 1483 Gould Drive, Cookeville, TN, 38502-3105, USA, www.flexial.com  

Acknowledgements 

This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement by the United States Government or the Jet Propulsion Laboratory, Pasadena, California. Copyright 2012 California Institute of Technology. Government sponsorship acknowledged. The authors wish to acknowledge the many individuals working on the MSL project, of which the thermal subsystem is a part of the greater whole. Dave Bame and AJ Mastropietro from the MSL Thermal team provided excellent support for the testing of the bellows at JPL. Pacific Design Technology (PDT) in Goleta, CA is acknowledged for their excellent work in developing the pump assemblies and providing significant support for this investigation. N u m b e r  o f  t e s t  p l a t e s  0 1 42 22 93 7   Added mass [kg] 0.000 1.540 2.368 3.093 3.921  Total mass [kg] 0.680 2.220 3.048 3.773 4.601  Total applied load [N] 6.671 21.778 29.901 37.013 45.136  Cantilever length [m] N/A 0.069 0.065 0.072 0.096  Applied moment [Nm] 0.000 1.022 1.492 2.160 3.647               Equivalent Radial load [g's] 0.5 1.6 2.1 2.6 3.2  Equivalent Moment load (x Flight) 0.0 1.5 2.2 3.2 5.4    Bellows Pressure, kPa gage  Guide started to slide, extending 0.34 2.4 5.2 9.3 7.9  Guide is halfway, extending 1.4 4.5 8.3 10.7 13.4  Guide is almost at hardstop, extending 3.1 7.2 10.3 13.4 16.9  Guide started to slide, contracting 2.4 0.34 † † †  Guide is halfway, contracting 1.4 † † † †  † the drag was high enough to prevent the bellows from contracting on its own