RTV Silicone Sealing Method for Component Interfaces

Approximately 50 inches in diameter, Glenn's unique sealing system consists of multiple elements installed in a recessed rectangular sealing groove. The main sealing function is provided by an elastomer element (e.g., silicone) comprising one or more sealing wall(s) connected by a web. The wall(s) extend above the top of the sealing groove so they are compressed by the opposing mating surface during the sealing process. The retractable shroud element is installed between the wall(s), with its base resting atop the web of the sealing element. The shroud is typically composed of an elastomer material to allow for flexibility (which is essential to retraction), but it can also be made from thin metal or plastic materials. When the seal is no longer in use, a pair of V-shaped shroud "arms" extend upward from the base of the seal to cover the wall(s). A thin metal retainer is installed on top of the shroud, and fasteners pass through holes in the retainer, shroud, and sealing elements to secure the system to the base of the sealing groove. Metal washers are installed in these holes to provide a load path between the metal retainer and the surface of the sealing groove. The system can seal against either a flat metal surface or another seal of the same design. This sealing system has been designed to accommodate multiple sealing cycles and has exhibited extremely low leak rates, making it an attractive solution within a variety of industries from aerospace to agriculture.

For extremely high-temperature sealing applications, Glenn researchers have devised novel methods for fabricating single-crystal preloaders. NASA's high-temperature preloaders consist of investment cast or machined parts that are fabricated in various configurations from single crystal superalloys. Machined preloaders include a variety of spring configurations, compressed axially or radially, fabricated from single crystal slabs. Before machining, the slabs are carefully oriented in a special goniometer using x-diffraction techniques. This helps to maintain proper crystal orientation relative to the machined part and the applied loads. For more complex geometry components which cannot be easily and economically machined, an investment casting approach would be used. Complex preloader geometries include wire coil springs of various configurations. These single crystal preloaders would be designed with the appropriate stiffness for the intended thermal barrier/seal application and placed underneath, or integrated within, the seal/barrier. At extrememly high temperature, the preload device keeps the seal/barrier mated against the opposing surface as the gap between the two surfaces changes, maintaining contact between surfaces and preventing convective heat transfer.

NASA's technique simplifies the seat installation process by requiring less installation equipment, eliminating the need for unnecessary apparatus such as fasteners and retainers. Multiple seals can be installed simultaneously, saving both time and money. NASA has tested the long-term performance of a solenoid actuated valve with a seat that was fitted using the new installation technique. The valve was fabricated and tested to determine high-cycle and internal leakage performance for an inductive pulsed plasma thruster (IPPT) application for in-space propulsion. The valve demonstrated the capability to throttle the gas flow rate while maintaining low leakage rates of less than 10-3 standard cubic centimeters per second (sccss) of helium (He) at the beginning of the valves lifetime. The IPPT solenoid actuated valve test successfully reached 1 million cycles with desirable leakage performance, which is beyond traditional solenoid valve applications requirements. Future design iterations can further enhance the valve's life span and performance. The seat seal installation method is most applicable to small valve instruments that have a small orifice of 0.5 inches or less.

This approach allows water recycling, air treatment, thermal control, and solids residuals treatment and recycle to be removed from the usable habitat volume and placed in the walls of a radiation-shielding water wall. It also provides a mechanism to recover and reuse water treatment (solids) residuals to strengthen the habitat shell. Water-wall treatment elements are a much-enlarged version of the commercially available X-Pack hydration bag. Some water bags have pervaporation membranes facing inward that provide the capability to remove H0, C0, and trace organics from the atmosphere. Ideally the water wall is composed of a series of membrane bags packed as dry elements integrated into an inflatable habitat structure wall. After launch and deployment, it is filled with water and maintained as both a freshwater supply and radiation shield. As the initial water supply is consumed, the depleted treatment bags are filled with waste water and take on a dual role of active forward osmosis (FO) water treatment and water-wall radiation shielding.

This NASA innovation applies the concepts of electrodynamic dust shielding (EDS) to develop seals (e.g., O-rings) with active self-cleaning capabilities. NASA’s self-cleaning seals are manufactured in the following manner: A seal with a conductive surface (or otherwise fabricated to be conductive) is generated and an electrical connection, lead or electrode is attached. Next, a dielectric material is coated or placed over the conductive surface of the seal. (NOTE: Using conductive elastomer materials eliminates the need for a conductive cover layer) A high voltage (nominally >1kV) power supply is connected to the conductive layer on the seal and grounded to the metallic groove or gland that houses the seal. Given the design, dust accumulates on the outer dielectric layer (a high-voltage insulator) of the seal. To clean the seal, a time varying alternating voltage is applied from the power supply, through the high voltage lead and onto the conductive layer of the seal. When this voltage is applied, the resulting electric field produces Coulomb and dielectrophoretic forces that cause the dust to be repelled from the sealing surface. In practice, NASA’s self-cleaning seals could be operated in continuous cleaning mode (actively repelling dust at all times, preventing it from ever contacting the seal surface) or in a periodic cleaning cycle mode (removing dust from the seal surface at regular intervals). NASA’s self-cleaning seals have been prototyped and demonstrated to be highly effective at dust removal. The invention could serve as the basis of an active, self-cleaning seal product line marketed for in-space and/or terrestrial applications. Additionally, companies developing space assets destined for operation on dusty planetary surfaces (e.g., the Moon) may be interested in leveraging the technology to protect seals from dust/regolith accumulation, ensuring continuous low leakage operations.