Bernadette J. Puleo

Effects of Atomic Oxygen and Grease on Outgassing and Adhesion of Silicone Elastomers for Space Applications

An investigation of silicone elastomers for seals used in docking and habitat systems for future space exploration vehicles is being conducted at NASA. For certain missions, NASA is considering androgynous docking systems where two vehicles each having a seal would be required to: dock for a period of time, seal effectively, and then separate with minimum push-off forces for undocking. Silicone materials are generally chosen for their wide operating temperatures and low leakage rates. However silicone materials are often sticky and usually exhibit considerable adhesion when mated against metals and silicone surfaces. This paper investigates the adhesion unit pressure for a space rated silicone material (S0383-70) for either seal-on-seal (SoS) or seal-on-aluminum (SoAl) operation modes in the following conditions: as-received, after ground-based atomic-oxygen (AO) pre-treatment, after application of a thin coating of a space-qualified grease (Braycote 601EF), and after a combination of AO pre-treatment and grease coating. In order of descending adhesion reduction, the AO treatment reduced seal adhesion the most, followed by the AO plus grease pre-treatment, followed by the grease treatment. The effects of various treatments on silicone (S0383-70 and ELA-SA-401) outgassing properties were also investigated. The leading adhesion AO pre-treatment reduction led to a slight decrease in outgassing for the S0383-70 material and virtually no change in ELA-SA-401 outgassing.

Compliant Foil Journal Bearing Performance at Alternate Pressures and Temperatures

An experimental test program has been conducted to determine the highly loaded performance of current generation gas foil bearings at alternate pressures and temperature. Typically foil bearing performance has been reported at temperatures relevant to turbomachinery applications but only at an ambient pressure of one atmosphere. This dearth of data at alternate pressures has motivated the current test program. Two facilities were used in the test program, the ambient pressure rig and the high pressure rig. The test program utilized a 35 mm diameter by 27 mm long foil journal bearing having an uncoated Inconel X-750 top foil running against a shaft with a PS304 coated journal. Load capacity tests were conducted at 3, 6, 9, 12, 15, 18, and 21 krpm at temperatures from 25°C to 500°C and at pressures from 0.1 to 2.5 atmospheres. Results show an increase in load capacity with increased ambient pressure and a reduction in load capacity with increased ambient temperature. Below one-half atmosphere of ambient pressure a dramatic loss of load capacity is experienced. Additional lightly loaded foil bearing performance in nitrogen at 25°C and up to 48 atmospheres of ambient pressure has also been reported. In the lightly loaded region of operation the power loss increases for increasing pressure at a fixed load. Knowledge of foil bearing performance at operating conditions found within potential machine applications will reduce program development risk of future foil bearing supported turbomachines.

Rotor Support Technology Developments for Long Life Closed Brayton Cycle Turbines

Power conversion systems based upon the Closed Brayton Cycle (CBC) turbine are under consideration for space power generation applications. Using this approach, inert gas, heated with a nuclear reactor or other means, is used to drive a turbine‐generator in a recirculating flow path. As a closed system, contamination of the working fluid, for instance with bearing lubricating oil, cannot be tolerated. To prevent this possibility, compliant surface gas film bearings are employed that use the working fluid as their lubricant. Foil gas bearings are in widespread use in turbocompressors and microturbines in aeronatuics and terrestrial applications. To successfully implement them for space power CBC systems, research is underway at NASA’s Glenn Research Center to assess foil bearing start‐up torque requirements, bearing thermal management and the effects of high ambient pressures in inert gases on performance. This paper introduces foil gas bearing rotor support technologies and provides an update on bearing performance testing and evaluations being conducted to integrate foil bearings in future CBC turbine systems.Power conversion systems based upon the Closed Brayton Cycle (CBC) turbine are under consideration for space power generation applications. Using this approach, inert gas, heated with a nuclear reactor or other means, is used to drive a turbine‐generator in a recirculating flow path. As a closed system, contamination of the working fluid, for instance with bearing lubricating oil, cannot be tolerated. To prevent this possibility, compliant surface gas film bearings are employed that use the working fluid as their lubricant. Foil gas bearings are in widespread use in turbocompressors and microturbines in aeronatuics and terrestrial applications. To successfully implement them for space power CBC systems, research is underway at NASA’s Glenn Research Center to assess foil bearing start‐up torque requirements, bearing thermal management and the effects of high ambient pressures in inert gases on performance. This paper introduces foil gas bearing rotor support technologies and provides an update on bearing p...