An ablative composite coating has been developed to protect a steel substrate against a short-duration exposure to supersonic stream of hot gases and molten ceramic particles. In the original application, the short-duration hot, abrasive flow is resultant from the shuttle solid-fuel rocket motor exhaust during launch. The steel substrate to be protected is a holddown-post blast shield on a space-shuttle mobile launcher platform. The maximum temperature in the rocket blast exceeds 5,500 °F (≈3,000 °C), and the heat load on the blast shield can exceed 8,000 Btu/ft2s (≈91 MW/m2s). Other components and launch accessories exposed to similar intense, short-duration, heat loads could also be protected by use of similar composite coatings.
The basic architecture of this ablative composite coating is a high-density, room-temperature-cured silicone (RTV), reinforced with layers of glass fabric. By chemically preparing the substrate and fabric laminates and by use of vacuum consolidation during cure, a coating is created that not only provides the known ablative properties of silicone resins, but also provides the physical strength to withstand the enormous pressures and shear forces encountered during exposure with delamination or debonding from the substrate.
In this application, the substrate is prepared by sandblasting an application of a hydrolyzable silicone primer. If necessary, the thick trowelable putty is made from RTV and a ceramic powder. Layers of glass fabric are preprocessed to remove the sizing and chemically treated to ensure a strong resin-to-fiber bond. The laminate is then built up to the required thickness by applying alternating layers of fabric and silicone resin. After sufficient layers have been applied, caul plates are applied and the entire blast cover cured under moderate vacuum pressure to squeeze out excess resin and ensure a strong coating-to-substrate bond.
The figure shows a newly coated blast shield and a blast shield after an exposure to the rocket exhaust. The average thickness of the ablative coating when new was 0.542 in. (≈13.8 mm); the average thickness after exposure was 0.165 in. (≈4.2 mm). The thickness of the coating was found to be adequate everywhere except at two corners. Ceramic particles in the rocket exhaust did not adhere to the silicone rubber. The silicone rubber at the steel substrate and the laminate exhibited no visible signs of deterioration. This observation was interpreted as signifying that the ablative coat succeeded in keeping the temperature at the bond surface below 600 °F (below approximately 320 °C). Except for a minor surface crack at one corner, the blast shield was undamaged.
This work was done by Robert C. Dyer, Martin J. Wilson, Jean M. Charvet, and Burton J. Pelkey, Jr., of United Space Alliance for Kennedy Space Center.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Technology Programs and Commercialization Office,
Kennedy Space Center,
Refer to KSC-12285.