A nutation damper was developed for the Radiation Belt Storm Probes (RBSP) program. The passive damper is fully filled with a custom-blended silicone and comprised of 1-inch titanium tubing formed into a racetrack shape measuring approximately 35 inches wide and 27 inches high. One of the critical components of this nutation damper is the reservoir, which provides sufficient space for the fluid to expand and contract throughout the damper's operational temperature range, while maintaining positive fluid volume in the damper. The reservoir contains a piston-and-spring mechanism that applies pressure generated by the spring compression to maintain a fully-filled state, and to prevent bubbles from forming. The reservoir's piston-and-bore interface is sealed with two inline O-rings that are compatible with the silicone oil and with temperature conditions ranging from -30 to 60°C. The design of the reservoir involves the complex relationships of fluid volume, thermal expansion, O-ring compression, friction, spring stiffness, and spring stroke. This paper will address the development of the reservoir and its mechanism, the engineering model tests at various developmental stages, the leak failure mitigation efforts, and the successful mechanical qualification tests of the flight units.
