Christopher C. Daniels

Characteristics of Elastomer Seals Exposed to Space Environments

Abstract A universal docking and berthing system is being developed by the National Aeronautics and Space Administration (NASA) to support all future space exploration missions to low-Earth orbit (LEO), to the Moon, and to Mars. The Low Impact Docking System (LIDS) is being designed to operate using a seal-on-seal configuration in numerous space environments, each having unique exposures to temperature, solar radiation, reactive elements, debris, and mission duration. As the LIDS seal is likely to be manufactured from an elastomeric material, performance evaluation of elastomers after exposure to atomic oxygen (AO) and ultraviolet radiation (UV) was conducted, of which the work presented herein was a part. Each of th e three candidate silicone elastomer compounds investigated, including Esterline ELA-SA-401, and Parker Hannifin S0383-70 and S0899-50, was characterized as a low outgassing compound, per ASTM E595, having percent total mass loss (TML) less than 1.0% and collected volatile condensable materials (CVCM) less than 0.1%. Each compound was compatible with the LIDS operating environment of –50 to 50 °C. The seal characteristics presented include compression set, elastomer-to-elastomer adhesion, and o-ring leakage rate. The ELA-SA-401 compound had the lowest variation in compression set with temperature. The S0383-70 compound exhibited the lowest compression set after exposure to AO and UV. The adhesion for all of the compounds was significantly reduced after exposure to AO and was further decreased after exposure to AO and UV. The leakage rates of o-ring specimens showed modest increases after exposure to AO. The leakage rates after exposure to AO and UV were increased by factors of up to 600 when compared to specimens in the as-received condition.

Performance of Subscale Docking Seals Under Simulated Temperature Conditions

Abstract A universal docking system is being developed by the National Aeronautics and Space Administration (NASA) to support future space exploration missions to low Earth orbit (LEO), to the moon, and to Mars. The candidate docking seals for the system are a composite design consisting of elastomer seal bulbs molded into the front and rear sides of a metal ring. The test specimens were sub-scale seals with two different elastomer cross-sections and a 12-in. outside diameter. The seal assemblies were mated in elastomer seal-on-metal plate and elastomer seal-on-elastomer seal configurations. The seals were manufactured from S0383-70 silicone elastomer compound. Nominal and off-nominal joint configurations were examined. Both the compression load required to mate the seals and the leak rate observed were recorded while the assemblies were subjected to representative docking system operating temperatures of –58, 73, and 122 °F (–50, 23, and 50 °C). Both the loads required to fully compress the seals and their leak rates were directly proportional to the test temperature.

The Mechanical Performance of Subscale Candidate Elastomer Docking Seals

The National Aeronautics and Space Administration is developing a Low Impact Docking System (LIDS) for future exploration missions. The mechanism is a new state-of-the-art device for in-space assembly of structures and rendezvous of vehicles. At the interface between two pressurized modules, each with a version of the LIDS attached, a composite elastomer-metal seal assembly prevents the breathable air from escaping into the vacuum of space. Attached to the active LIDS, this seal mates against the passive LIDS during docking operation. The main interface seal assembly must exhibit low leak and outgas values, must be able to withstand various harsh space environments, must remain operational over a range of temperatures from -50 C to 75 C, and perform after numerous docking cycles. This paper presents results from a comprehensive study of the mechanical performance of four candidate subscale seal assembly designs at -50, 23, 50, and 75 C test temperatures. In particular, the force required to fully compress the seal during docking, and that which is required for separation during the undocking operation were measured. The height of subscale main interface seal bulbs, as well as the test temperature, were shown to have a significant effect on the forces the main interface seal of the LIDS may experience during docking and undocking operations. The average force values required to fully compress each of the seal assemblies were shown to increase with test temperature by approximately 50% from -50 to 75 C. Also, the required compression forces were shown to increase as the height of the seal bulb was increased. The seal design with the tallest elastomer seal bulb, which was 31% taller than that with the shortest bulb, required force values approximately 45% higher than those for the shortest bulb, independent of the test temperature. The force required to separate the seal was shown to increase with decreasing temperature after 15 hours of simulated docking. No adhesion force was observed at 75 C, while magnitudes of up to 235 lbf were recorded at the refrigerated temperature. In addition, the adhesion force was observed to increase with bulb height. When compared with the LIDS program requirements, the measured compression force values were found to be below the maximum allowable load allotted to the main interface seal. However, the measured adhesion force values at the refrigerated test temperature were found to exceed the program limits.

Leak Rates of a Candidate Main Interface Seal at Selected Temperatures

The leak rate of a 54 inch diameter composite seal assembly was quantified at selected temperatures as part of developmental work for the National Aeronautics and Space Administration Low Impact Docking System (LIDS). The 54 inch diameter composite seal assembly studied was representative of the 58 inch diameter LIDS main interface seal. This near-full-scale seal assembly consisted of elastomer seals (Parker Hannifin S0383-70) molded into an aluminum retainer. To mimic the mission operational configuration, the seal assemblies were mated in a seal-on-plate test configuration. Tests were completed for fully compressed and partially compressed seals. The composite seal assemblies were subjected to test temperatures of -30◦C, +20◦C, and +50◦C (-22◦F, +68◦F, and +122◦F) which are representative of the docking system operating temperatures. In addition, comparisons of the 54 inch diameter composite seal assembly leak rates to those of a similarly designed 12 inch diameter subscale composite seal assembly were made to determine how well the near-full-scale leak rates were predicted by subscale tests. Observed leak rates were exponentially proportional to the test temperature due to the relationship between permeability and temperature. In the fully compressed test configuration, the near-fullscale leak rate increased by a factor of 3.89 from the lowest test temperature to the highest test temperature whereas the subscale seal assembly leak rate increased by a factor of 3.37 over the same temperature range. Leak rates were normalized per linear inch of seal for a direct comparison between the full-scale and the subscale assemblies; normalized leak rates of the near-full-scale seals were 2.21 to 2.65 times greater than the subscale leak rates. The difference in the normalized leak rates between the near-full-scale and subscale assemblies was attributed to an additional leak path in the near-full-scale test hardware that was not present in the subscale test hardware. At each temperature and compression level, the leak rate of the near-full-scale composite seal assembly was below the LIDS main interface seal leak rate allocation of 2.5x10−3 lbm,air per day.

Adhesion of Silicone Elastomer Seals for NASA's Crew Exploration Vehicle

Abstract Silicone rubber seals are being considered for a number of interfaces on NASA’s Crew Exploration Vehicle (CEV). Some of these joints include the docking system, hatches, and heat shield-to-back shell interface. A large diameter molded silicone seal is being developed for the Low Impact Docking System (LIDS) that forms an effective seal between the CEV and International Space Station (ISS) and other future Constellation Program spacecraft. Seals between the heat shield and back shell prevent high temperature reentry gases from leaking into the interface. Silicone rubber seals being considered for these locations have inherent adhesive tendencies that would result in excessive forces required to separate the joints if left unchecked. This paper summarizes adhesion assessments for both as-received and adhesion-mitigated seals for the docking system and the heat shield interface location. Three silicone elastomers were examined: Parker Hannifin S0899-50 and S0383-70 compounds, and Esterline ELA-SA-401 compound. For the docking system application various levels of exposure to atomic oxygen (AO) were evaluated. Moderate AO treatments did not lower the adhesive properties of S0899-50 sufficiently. However, AO pretreatments of approximately 10

Experimental Investigation of Elastomer Docking Seal Compression Set, Adhesion, and Leakage

Abstract A universal docking and berthing system is being developed by the National Aeronautics and Space Administration (NASA) to support all future space exploration missions to low-Earth orbit (LEO), to the Moon, and to Mars. An investigation of the compression set of two seals mated in a seal-on-seal configuration and the force required to separate the two seals after periods of mating was conducted. The leakage rates of seals made from two silicone elastomer compounds, S0383–70 and S0899–50, configured in seal-on-seal mating were quantified. The test specimens were sub-scale seals with representative cross-sections and a 12 in. outside diameter. The leakage rate of the seals manufactured from S0899–50 was higher than that of the seals made from S0383–70 by a factor of 1.8. Similarly, the adhesion of the 50 durometer elastomer was significantly higher than that of the 70 durometer compound. However, the compression set values of the S0899–50 material were observed to be significantly lower than those for the S0383–70.

The Friction Behavior of Individual Components of a Spark-Ignition Engine During Warm-Up

The research presented herein fills a void in the published literature through investigation of transient friction contributions by individual internal combustion engine components during simulated engine warm-up. Currently, engine manufacturers design internal combustion engines primarily for use at steady-state operating conditions with little design consideration for transient engine warm-up. Using the motoring torque waveform and cycle-averaged data of a spark-ignition internal combustion engine, the present work determined the friction behavior of individual engine component assemblies, including the valve train, pistons and connecting rods, oil pump, and crankshaft of a modern internal combustion engine. A common criticism of the standard motoring method is that the engine does not warm up, so lubricant temperature and viscosity does not model that of a fired engine. In the present study, the lubricant and coolant were warmed from 25 to 85°C. Observations were presented as to the effect of engine speed and the temperature of the coolant and lubricant on total engine friction. Contributions of individual engine components to total engine losses were examined, as well as their variation with engine temperature. The added knowledge of the transient effects of engine temperature can help future designers to mitigate friction and component wear, thus improving overall maintenance costs, specific fuel consumption, and emissions. Presented at the STLE Annual Meeting in Las Vegas, Nevada May 15-19, 2005 Review led by Gary Barber

Full-Scale System for Quantifying Leakage of Docking System Seals for Space Applications

NASA is developing a new docking and berthing system to support future space exploration missions to low-Earth orbit, the Moon, and Mars. This mechanism, called the Low Impact Docking System, is designed to connect pressurized space vehicles and structures. NASA Glenn Research Center is playing a key role in developing advanced technology for the main interface seal for this new docking system. The baseline system is designed to have a fully androgynous mating interface, thereby requiring a seal-on-seal configuration when two systems mate. These seals will be approximately 147 cm (58 in.) in diameter. NASA Glenn has designed and fabricated a new test fixture which will be used to evaluate the leakage of candidate full-scale seals under simulated thermal, vacuum, and engagement conditions. This includes testing under seal-on-seal or seal-on-plate configurations, temperatures from -50 to 50 C (-58 to 122 F), operational and pre-flight checkout pressure gradients, and vehicle misalignment (plus or minus 0.381 cm (0.150 in.)) and gapping (up to 0.10 cm (0.040 in.)) conditions. This paper describes the main design features of the test rig and techniques used to overcome some of the design challenges.

Influence of temperature on cyclic stress response and fracture behavior of aluminum alloy 6061

Abstract The cyclic stress response and fracture characteristics of aluminum alloy 6061 was studied at different temperatures. The specimens were cyclically deformed using tension-compression loading under total strain-amplitude control. The alloy showed evidence of softening at all test temperatures. The degree of cyclic softening was observed to increase with an increase in test temperature. The presence of shearable matrix precipitates in the alloy results in a local decrease in resistance to dislocation movement, thereby causing a progressive loss of strengthening contribution. At the elevated temperatures, localized oxidation and embrittlement at the grain boundaries are promoted by the applied cyclic stress and play an important role in accelerating crack initiation and subsequent crack propagation. The fracture behavior of the alloy is discussed in terms of competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, plastic strain amplitude and concomitant response stress, and test temperature.

Adhesion of an Elastomer Seal to Metal and its Mitigation with Atomic Oxygen Pretreatment

Silicone elastomer compounds are a reliable option for gas pressure seals used in space applications. Seals fabricated from these compounds effectively meet typical leak rate, compression force, and outgassing requirements. One negative characteristic of silicone elastomer compounds use in space applications, however, is their adhesive tendency. When configured in the face-seal orientation of a reusable docking seal, an elastomer seal needs to readily release from its mating counter-face. Elevated adhesion between a seal on one spacecraft and its counter-face on a mating spacecraft is unacceptable, as it may damage or remove the seal, cause an uneven release between the two vehicles, or prevent undocking from occurring altogether. To characterize the adhesion of a candidate docking seal, a subscale silicone elastomer seal assembly was evaluated at −50 ◦ C, 23 ◦ C, and 75 ◦ C. Tests showed that adhesion between the seal and its counter-face was greatest at −50 ◦ C, at an unacceptably high 1.4 kN. To investigate a potential mitigation technique, additional seal assemblies were exposed to different levels of atomic oxygen (AO) fluence, and then evaluated at −50 ◦ C for comparison to results from tests of the untreated assembly. AO pretreatment of 1.3×10 20 atoms/cm 2 showed a complete mitigation of adhesion. In addition, the durability of the surface pretreatment was verified by completing over 150 simulated docking-undocking cycles.