Development of thermal barriers for solid rocket motor nozzle joints
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The Space Shuttle solid rocket motor casc assembly joints are sealed using conventional ()-ring seals. The 5500+°F combustion gases are kept a safe distance away from the seals by thick layers of insulation. Special jointfill compounds arc used to fill the joints in the insulation to prevent a direct flowpath to the seals. On a number of occasions, NASA has observed in several of the rocket nozzle assembly joints hot gas penetration through defects in the joint-fill compound. The current nozzle-to-case joint design incorporates primary, secondary and wiper (innermost) O-rings and polysulfidejoint-fill compound. In the current design, I out of 7 motors experience hot gas to the wiper ()-ring. Though the condition does not threaten motor safety, evidence of hot gas to the wiper ()-ring results in extensive reviews heft)re resuming flight. NASA and solid rocket motor manufacturer Thiokol are working to improve the nozzle-to-case joint design by implementing a more reliable J-leg design and a thermal barrier. This paper presents burn-resistance, temperature drop, flow, and resiliency test results for several types of NASA braided carbon-fiber thermal barriers. Burn tests were pertormed to determine the time to burn through each of the thermal barriers when exposed to the flame of an oxyacetylene torch (5500 °F), representative of the 5500 °F solid rocket motor combustion temperatures. Thermal barriers braided out of carbon fibers endured the flame tbr over 6 rain, three times longer than the solid rocket motor burn time. Tests were performed on two thermal barrier braid architectures, denoted Carbon-3 and Carbon-6, to measure the temperature drop across and along the barrier in a compressed state when subjected to the flame of an oxyacetylene torch. Carbon-3 and Carbon-6 thermal barriers were excellent insulators causing temperature drops through their diameter from 25(_) to 2800 °F. Gas temperatures 1/4" downstream of the thermal barrier were within the downstream Viton ()-ring temperature limit of *Senior Research Engineer. Mechanical Components Branch, Member AIAA. Resean:h Emgineer. 600 °F. Carbon-6 perlormed extremely well in subscale rocket "char" motor tests when subjected to hot gas at 3200 °F fl)r an l 1-see. rocket firing, simulating the maxinmm downstream joint cavity fill time. The thermal barrier reduced the incoming hot gas temperature by 22(X) °F in an intentionally oversized gap defect, spread the incoming ,jet flow, and blocked hot slag, thereby offering protection to the downstream O-rings.