Cryogenic Selective Surfaces

Reentry heating comes from two primary sources: Convective heating from both the flow of hot gas past the surface of the vehicle and catalytic chemical recombination reactions at the surface; and Radiation heating from the energetic shock layer in front of the vehicle. The magnitude of stagnation heating is dependent on a variety of parameters, including reentry speed, vehicle effective radius, and atmospheric density. As reentry speed increases, both convective and radiation heating increase. At high speeds, radiation heating can quickly dominate and as the effective vehicle radius increases, convective heating decreases, but radiation heating increases. Radiation during reentry is a function of reentry speed which occurs very early in reentry for very specific wave-lengths and is dependent upon atmospheric composition. The Biologically-Inspired Radiation-Reflecting Ablator (BIRRA) approach converts amorphous silica of diatoms to a more refractory material, Silicon carbide (SiC), and incorporates it into a Thermal Protection System (TPS), especially near the surface to provide enhanced reflection during the initial stage of reentry into planetary atmospheres at high speeds. Different diatom species reflect different wavelengths. This conversion is performed in a fluidized bed reactor or other type of reactor to convert naturally ordered structures to higher temperature materials with a photonic structure that can better reflect radiation. TPS that shields vehicles from both radiative and convective heat would allow reduction of the mass fraction of the TPS, which will increase the mass fraction available for payload or reduce the overall mass and thus launch and propulsion requirements.

The thin film sensor’s principal advantage lies in its potential to take high frequency temperature measurements from the surface of a reentering spacecraft while simultaneously withstanding the high temperature and oxidizing environment encountered. This data provides engineers with operational phase measurements used to refine the spacecraft’s operational envelope and track flight hardware behavior in addition to providing high frequency temperature measurements that can inform the physics of a boundary layer. Mismatches in coefficients of thermal expansion (CTE) are expected in TPS-based sensor applications because the metallic materials used for temperature sensing have thermal expansion rates that differ from the rates of the substrate and coating materials in the TPS. At high temperatures during reentry, this mismatch in CTE can create a significant strain differential between the metallic sensor, sensor leads, and the materials to which the sensor and leads are bonded. High frequency response temperature measurements on the surface of entry spacecraft are not currently possible above ~700 F with existing measurement capabilities. This shortcoming is primarily due to the need for robust sensor behavior at temperatures of several thousand degrees F. The sensor design of this technology preserves the integrity of sensor components while enhancing its high temperature functionality. The thin film temperature sensor has a technology readiness level (TRL) 5 (Component and/or breadboard validation in relevant environment) and is now available for patent licensing. Please note that NASA does not manufacture products itself for commercial sale.

In applying PS400 using the plasma spray-coating process, a 0.010 inch layer is deposited onto a metal surface. This composite coating often includes a metallic-based binder, a metal-bonded hardener, a high-temperature lubricant, and/or a low- temperature lubricant. PS400's improved metallic-based binder alloy greatly increases the structural strength and durability of the composite with respect to the operating temperature and the bearing load, and provides superior dimensional stability. PS400's metal-bonded oxide hardening agent provides additional hardness, wear resistance, and thermal stability, while also exhibiting a low coefficient of friction when used in sliding contacts. It is also significantly less expensive in terms of both acquisition and grinding processes. Depending on the desired environment, high- and low-temperature lubricants may be added to the composite coating. The preferred high-temperature lubricant is a metal fluoride and the optional optional low-temperature lubricant is composed of metals, such as silver or copper, that are soft enough to provide lubrication at low temperature while maintaining oxidation resistance with a sufficiently high melting point. These qualities permit the materials to be used over a broad temperature range. Once the spray coating has been applied, the metal surface is ground and polished to produce a smooth, self-lubricating surface before use. Unlike some coatings that must be diamond-ground, PS400 is readily ground with a substantially less expensive abrasive, such as silicon carbide. This grinding process generally yields a coating thickness of 200 to 400 micrometers. In instances when a coating is not convenient or possible, powder metallurgy techniques using PM400 can be used to make freestanding self-lubricating components such as bushings and wear plates.

The difference between Layered Energy Depletion Radiation Shielding (LEDRS) and Stacked Energy Depletion Radiation Shielding (SEDRS) is how the piece of matter, or shield, is analyzed as radiation passes through the matter. SEDRS involves using a defined and ordered stack of layers of shielding with different material properties such that the thickness and chemical properties of each material maximizes the absorption of energy from the radiation particles that are most damaging to the target. The SEDRS shielding method aims to provide the maximum level of energy absorption while still keeping shielding mass and volume low. The process of LEDRS involves using layers of shielding material such that the thickness of each material is designed to absorb the maximum amount of energy from the radiation particles that are most damaging to the target after subsequent layers of shielding. The more energy is absorbed by the shielding material, the less energy will be deposited in the target minimizing the required mass to achieve a resulting lower dose for a given geometrical feature. The LEDRS shielding method aims to provide the maximum level of energy absorption. The process for designing LEDRS views potential radiation shields as a cascade of effects from each shielding layer to the next and is helpful for investigating the particular effects of each layer. SEDRS and LEDRS can improve any technology that relies on the controlled manipulation of a radiation field by interaction with a material element.

Oxide coatings have been used in thermal and environmental barrier layers for coatings for hot section turbine applications, among other uses. With the PS-PVD method, Glenn researchers observed the formation of a minority phase of a metastable oxide (zirconium oxide) that is usually found only in a vapor state. They found that the high temperatures and fast deposition process of the PS-PVD system incorporated nonequilibrium phases in the coating and retained them at room temperature as well as at high temperature in the absence of oxygen. The material is vaporized and condensed on the surface via a rapid quenching, essentially &#34trapping&#34 this phase in the deposited coating. The coating microstructure and composition can also be manipulated by changing the processing parameters, allowing the thickness of the coating to be tailored to a given application. Since this metastable phase is conductive, this coating can be used as (for example) an extremely sensitive (thermal or temperature) sensor. It also has very good durability and erosion resistance, making it useful as a protective and conductive coating for electronics and microelectronics. This is an early-stage technology requiring additional development, and Glenn welcomes co-development opportunities.