Nicholas G. Garafolo

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.

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).

Compressible Advection Through an Elastomer Seal: A Porous Media Approach to Seals for Space Applications

Gas permeability characterization is of the utmost importance in space seals applications. Space seals must maintain acceptable mass losses in harsh environments where temperatures widely vary under vacuum conditions. Silicone elastomers are commonly used in space as they offer significant sealing performance at temperature extremes and are capable of meeting stringent outgassing requirements necessary for vacuum environments. Traditional models of leak rates solely rely on a diffusive transport mechanism; mass is transported across a membrane through molecular flow induced by a concentration gradient under isostatic conditions. In the application of space seals, the pressure gradients are large, resulting in advection dominated transport. Conventional applications of advection utilize Darcy’s law; however, the fluid is assumed incompressible and fails to capture the nonlinear pressure gradient under compressible situations. Consequently, employing Darcy’s law incorrectly predicts the leak rate. A novel model in compressible advection through an elastomer seal is presented. A phenomenological approach is taken to determine the specific discharge. Through the conservation of mass, the governing equation for pressure is derived. An exact analytical solution exists for one-dimensional flow in the form of a Generalized Emden-Fowler equation and as a result, an analytical expression for mass flow is developed. A series of experiments is presented to deduce permeability constants and Klinkenberg parameter of silicone S0383-70 under one-dimensional flow conditions. The leak rates of the model and experiments are compared. Through the presented compressible advection model, the mass leak rate of any candidate seal geometry can be evaluated.Copyright

Evaluation of a Novel Seal for Space Applications

To meet the requirements of future exploration missions to low Earth orbit, to the moon, and to Mars, a new docking system was developed by the National Aeronautics and Space Administration. A gas pressure seal is positioned at the interface between the twomating halves of the newdocking system. This interface sealmust operate across a wide range of temperatures while maintaining acceptable performance levels of both leak rate and the load required to compress the seal. A novel subscale test article was designed andmanufactured for evaluation purposes. The candidate seal was composed of a primary and a redundant elastomer seal retained by a metallic ring. The elastomer used to manufacture the seal was Esterline ELA-SA-401. The seal performance was characterized at environmental conditions across the representative system operational temperature range of 58 to 167 F.With a pressure differential of 14.7 psid applied, themass leak rate of air was quantified when the seal was fully compressed and at two levels of partial compression. The highest leak rate observed, 13:1 10 7 lbm=day-in:, was recorded when the seal was partially compressed at the highest test temperature. The force required to compress the seal increased with the test temperature and the largest force, 3700 lbf , was recorded at the highest test temperature. When compared with the system leak rate and compression force requirements, the seal met all of the requirements for all of the configurations tested.

Experimental Investigation of Leak-Rate Performance of a Subscale Composite Elastomer-Retainer Docking Seal

A novel docking seal was investigated for application to the main interface seal of NASA’s low-impact docking system. This interface seal was designed to maintain acceptable leak rates while being exposed to the harsh environmental conditions of outer space. In this experimental evaluation, the leak rate of a candidate docking-seal assembly was characterized and evaluated. The composite seal was manufactured from silicone elastomer S0383-70, vacuum molded into a metal retaining ring. Four seal designs were considered with unique characteristic heights. The leak-rate performance was characterized through a mass point leak-rate method by monitoring gas properties within an internal control volume. The leak rates of the seals were measured at representative docking temperatures of −58, +73, and +122°F for all four seal designs and characterized at 100, 74, and 48% of full closure. For all conditions, considered the candidate seal assemblies met the leak-rate criteria derived from the approximate mission requ...

Mass Point Leak Rate Technique with Uncertainty Analysis

The mass point leak rate technique is often the methodology of choice for quantifying leak rates as it uses simple elementary measurements, applies to gas systems of low mass, proves effective for low leak rates, and does not rely on test-gas conversions. In this methodology, a number of instantaneous mass measurements are calculated through samples of volume, pressure, gas composition, and temperature measurements over time. A regression analysis of the corresponding mass-time sample set yields the leak rate of the system. A detailed uncertainty analysis is paramount for a complete, experimental characterization of the leak rate and previously was not fully implemented in the mass point leak rate method. Recent advancements in regression uncertainty analysis by propagation of errors afford the ability to quantify the uncertainty with estimates of covariance in the regression results. The mass point leak rate technique with the associated detailed measurement uncertainty analysis offers the ability to quantify both the leak rate and the uncertainty associated with the leak rate value. Detailed herein is the development of the methodology and a detailed uncertainty analysis that includes both precision (repeatability) and bias (systematic) error. Alternative leak rate methods are also discussed for comparison purposes. An example in the methodology is presented.

An Experimental Investigation of Leak Rate Performance of a Subscale Candidate Elastomer Docking Space Seal

A novel docking seal was developed for the main interface seal of NASA s Low Impact Docking System (LIDS). This interface seal was designed to maintain acceptable leak rates while being exposed to the harsh environmental conditions of outer space. In this experimental evaluation, a candidate docking seal assembly called Engineering Development Unit (EDU58) was characterized and evaluated against the Constellation Project leak rate requirement. The EDU58 candidate seal assembly was manufactured from silicone elastomer S0383-70 vacuum molded in a metal retainer ring. Four seal designs were considered with unique characteristic heights. The leak rate performance was characterized through a mass point leak rate method by monitoring gas properties within an internal control volume. The leakage performance of the seals were described herein at representative docking temperatures of -50, +23, and +50 C for all four seal designs. Leak performance was also characterized at 100, 74, and 48 percent of full closure. For all conditions considered, the candidate seal assemblies met the Constellation Project leak rate requirement.

A Compressible Permeation Approach to Elastomeric Space Seal Characterization

The development of elastomeric face seals is imperative for NASA’s manned space flight program. Lacking in the development of state-of-the-art space seals was a technique for predicting the performance of candidate designs prior to experimental characterization. To this end, a physics-based model for compressible permeation in elastomeric face seals was developed to provide a predictive methodology for designers and researchers. In this novel approach for seal research, compressibility effects and the dependence of permeability on pressure was retained. Two independent permeation parameters arose from an exact, analytical solution to the one-dimensional permeation transport equations. The application of the derived transport equations and the developed permeability coefficients resulted in a noteworthy and practical tool for seal researchers to predict the leak rate of alternative geometries. For an example in the methodology, the characterization of a candidate space seal material, silicone elastomer S0383-70, was performed. Results illustrated the model’s capability for capturing the permeation leak rate of elastomeric seals for various pressure differentials.

Durability Testing of Docking System Seals for Space Applications

NASA is developing a new docking system to support future space exploration missions to low-Earth orbit, the Moon, and other destinations. This system will be used to dock future vehicles with the International Space Station or with each other. A key component of this system is the seal at the main docking interface which inhibits the loss of cabin air once docking is complete. Depending on the mission, the seal must be able to dock in either a sealon-flange or seal-on-seal configuration. Seal-on-flange mating occurs when a docking system equipped with a seal docks to a system with a flat metal flange. This would occur when a vehicle docks to a node on the International Space Station. Seal-on-seal mating occurs when two docking systems equipped with seals dock to each other. During and after the docking process, the seals will be subjected to scrubbing, or sliding, at the docking interface due to mechanical and thermal movements. To evaluate the effects of scrubbing on seal performance, durability tests were performed on subscale docking system seals manufactured from two candidate silicone elastomer compounds. These tests were performed under anticipated worst-case conditions of seal-on-flange mating at warm temperatures while under high compression levels. Seal performance was evaluated via leak and adhesion tests before and after the durability tests. Leak testing revealed no statistical differences in seal leak rates before and after durability testing. However, adhesion testing of the seals before and after the durability tests indicated that seal scrubbing during a mission could lead to increased adhesion during undocking.

Effect of system variables on the uncertainty of the mass point leak rate methodology using first-order regression

Advances in nondestructive testing for leak rate characterisation are rare because procedures are well accepted; however, for many critical applications, an understanding of the accuracy and reliability of a reported leak rate is equally as important as the leak rate itself (e.g., manned spacecraft and nuclear power industry). Although it is known that the mass point leak rate method can achieve high accuracies, the measurement uncertainty and limitations of the method are less understood. Using a least-squares regression on a mass–time population and a statistical analysis of regression uncertainty, this study investigated the influence of (1) differential pressure, (2) steady-state and transient temperature, (3) volume size, (4) gas type, (5) sampling time duration, and (6) sampling interval on the reported mass point leak rate value. The analyses accounted for all significant sources of error. An experimental evaluation and validation of the method were conducted on a capillary-type, National Institute of Standards and Technology traceable leak standard and, where appropriate, simultaneous measurements were taken using a helium leak detector for comparison. The mass point technique was shown to provide high-accuracy results for various gases and volume sizes. Across the range of temperatures and pressures, the uncertainty measurements of the mass point technique were between ± 0.34% and ± 0.72%. When the temperature of the test section was varied ± 5°C during the experiment, the mass point technique results deviated 2–4% from those of the standard.