Feasibilty Assessment of Thermal Barrier Seals for Extreme Transient Temperatures
Bruce M. Steinetz
Glenn Research Center, Cleveland, Ohio
Patrick H. Dunlap, Jr.
Modern Technologies, Corp., Middleburg Heights, Ohio
July 1998
Abstract
Assembly joints of modern solid rocket motor cases are generally sealed using conventional O-ring seals. The 5500°F combustion gases produced by rocket motors are kept a safe distance away from the seals by thick layers of phenolic insulation. Special compounds are used to fill insulation gaps leading up to the seals to prevent a direct flowpath to them. Design criteria require that the seals should not experience torching or charring during operation, or the sealing ability would be compromised. On limited occasions, NASA has observed charring of the primary O-rings of the space shuttle solid rocket nozzle assembly joints due to parasitic leakage paths opening up in the gap-fill compounds during rocket operation. NASA is investigating different approaches for preventing torching or charring of the primary O-Rings. One approach is to implement a braided rope seal upstream of the primary O-ring to serve as a thermal barrier that prevents the hot gases from impinging on the O-ring seals.
This paper presents flow, resiliency, and thermal resistance for several types of NASA rope seals braided out of carbon fibers. Burn tests were performed to determine the time to burn through each of the seals when exposed to the flame of an oxyacetylene torch (5500°F), representative of the 5500°F solid rocket motor combustion temperatures. Rope seals braided out of carbon fibers endured the flame for over six minutes, three times longer than the solid rocket motor burn time. Room temperature compression tests were performed to assess seal resiliency and unit preloads as a function of compression. The thermal barrier seal was tested in a subscale "char" motor test in which the seal sealed an intentional defect in the gap insulation. Temperature measurements indicated that the seal blocked 2500°F combustion gases on the upstream side with very little temperature rise on the downstream side.
Photos
Candidate thermal barrier seal for Shuttle SRM before flame exposure (5500°F oxyacetylene flame) |
Candidate thermal barrier seal for Shuttle SRM 5500°F oxyacetylene flame on seal |
Candidate thermal barrier seal for Shuttle SRM 5500°F oxyacetylene flame on seal viewed through welding glass minimal damage after 2 min 20 sec) |
Candidate thermal barrier seal for Shuttle SRM Carbon-3 average burn through time = 6 min 27 sec (3x longer than solid rocket burn time) |