Patrick H. Dunlap

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.

Toward an Improved Hypersonic Engine Seal

SUMMARY AND CONCLUSIONS NASA GRC developed a variety of high temperature structural seals during the NASP program, but those seals fell short of program goals and due to program termination could not be adequately matured. Current requirements for advanced hypersonic engines are even more demanding, and the current SOA seals do not meet these requirements. To overcome the shortfalls of SOA seals, GRC is developing advanced seals under NASA™s NGLT program. Shortfalls that were investigated in the current study included a loss of seal resiliency with load cycling at high temperatures, lack of seal flexibility, and seal flow blocking ability. In an effort to address these shortfalls, two seal designs and two types of seal preloading devices were evaluated in a series of flow tests at room temperature and compression tests at room temperature and 2000 °F. Based on the results of these tests, the following conclusions are made: 1. Both the NASP-generation AC1 seal design with its core of uniaxial fibers and the BC1 seal design with its braided core structure lost resiliency with repeated loading at high temperatures. After 20 load cycles at room temperature, these seals had a residual interference of about 26 percent of the total linear compression applied to them. At 2000 °F, the residual interference dropped to 5 to 6 percent after 20 load cycles. Additional work will need to be done to develop seals that have more resiliency after repeated load cycling. 2. Although the BC1 seal design had about the same amount of resiliency as the AC1 design, its braided core allowed it to be as much as 30 times more flexible than the AC1 design. This added flexibility makes the BC1 design able to seal around corners in locations of a hypersonic engine where the AC1 design would not be able to. 3. Canted coil springs are promising seal preloading devices. Adding a canted coil spring behind the seals improved seal residual interference at room

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.

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.

Evaluation of High Temperature Knitted Spring Tubes for Structural Seal Applications

Control surface seals are crucial to current and future space vehicles, as they are used to seal the gaps surrounding body flaps, elevons, and other actuated exterior surfaces. During reentry, leakage of high temperature gases through these gaps could damage underlying lower temperature structures such as rudder drive motors and mechanical actuators, resulting in impaired vehicle control. To be effective, control surface seals must shield lower temperature structures from heat transfer by maintaining sufficient resiliency to remain in contact with opposing sealing surfaces through multiple compression cycles. The current seal exhibits significant loss of resiliency after a few compression cycles at elevated temperatures (i.e., 1900 F) and therefore would be inadequate for advanced space vehicles. This seal utilizes a knitted Inconel X-750 spring tube as its primary resilient element. As part of a larger effort to enhance seal resiliency, researchers at the NASA Glenn Research Center performed high temperature compression testing (up to 2000 F) on candidate spring tube designs employing material substitutions and modified geometries. These tests demonstrated significant improvements in spring tube resiliency (5.5x better at 1750 F) through direct substitution of heat treated Rene 41 alloy in the baseline knit design. The impact of geometry modification was minor within the range of parameters tested, however trends did suggest that moderate resiliency improvements could be obtained by optimizing the current spring tube geometry.

Further Investigations of Control Surface Seals for the X-38 Re-Entry Vehicle

Patrick H. Dunlap, Jr., and Bruce M. SteinetzGlenn Research Center, Cleveland, OhioDonald M. CurryJohnson Space Center, Houston, TexasCharles W. Newquist and Juris VerzemnieksThe Boeing Company, Seattle, WashingtonPrepared for the37th Joint Propulsion Conference and Exhibitcosponsored by the AIAA, ASME, SAE, and ASEESalt Lake City, Utah, July 8--11, 2001National Aeronautics andSpace AdministrationGlenn Research Center

Investigations of Control Surface Seals for Re-Entry Vehicles

Re-entry vehicles generally require control surfaces (e.g., rudders, body flaps) to steer them during flight. Control surface seals are installed along hinge lines and where control surface edges move close to the vehicle body. These seals must operate at high temperatures and limit heat transfer to underlying structures to prevent them from overheating and causing possible loss of vehicle structural integrity. This paper presents results for thermal analyses and mechanical testing conducted on the baseline rudder/fin seal design for the X-38 re-entry vehicle. Exposure of the seals in a compressed state at the predicted peak seal temperature of 1900 F resulted in loss of seal resiliency. The vertical Inconel rudder/fin rub surface was re-designed to account for this loss of resiliency. Room temperature compression tests revealed that seal unit loads and contact pressures were below limits set to protect Shuttle thermal tiles on the horizontal sealing surface. The seals survived an ambient temperature 1000 cycle scrub test over sanded Shuttle tiles and were able to disengage and re-engage the tile edges during testing. Arc jet tests confirmed the need for seals in the rudder/fin gap location because a single seal caused a large temperature drop (delta T = 1710 F) in the gap.

Rudder/Fin Seal Investigations for the X-38 Re-Entry Vehicle

Patrick H. Dunlap, Jr. and Bruce M. SteinetzGlenn Research Center, Cleveland, OhioDonald M. CurryJohnson Space Center, Houston, TexasPrepared for the36th Joint Propulsion Conference and Exhibitcosponsored by AIAA, ASME, SAE, and ASEEHuntsville, Alabama, July 16-19, 2000National Aeronautics andSpace AdministrationGlenn Research Center

Pressure Decay Testing Methodology for Quantifying Leak Rates of Full-Scale Docking System Seals

NASA is developing a new docking system to support future space exploration missions to low-Earth orbit and the Moon. This system, called the Low Impact Docking System, is a mechanism designed to connect the Orion Crew Exploration Vehicle to the International Space Station, the lunar lander (Altair), and other future Constellation Project vehicles. NASA Glenn Research Center is playing a key role in developing the main interface seal for this docking system. This seal will be relatively large with an outside diameter in the range of 54 to 58 in. (137 to 147 cm). As part of this effort, a new test apparatus has been designed, fabricated, and installed to measure leak rates of candidate full-scale seals under simulated thermal, vacuum, and engagement conditions. Using this test apparatus, a pressure decay testing and data processing methodology has been developed to quantify full-scale seal leak rates. Tests performed on untreated 54 in. diameter seals at room temperature in a fully compressed state resulted in leak rates lower than the requirement of less than 0.0025 lbm, air per day (0.0011 kg/day).