NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.

Apollo Tower Jettison Motor Qualification Issue

Tower-Jettison Motor Qualification and Production Problems

During static test firing, the thrust-rise times of two tower-jettison motors exceeded the specification. The static test performance of these motors was normal except for a long ignition delay time, which resulted in a prolonged thrust-rise time and total burn time. The cause of these unusually long ignition-delay times was traced to low input current being applied to the initiator bridgewires. The low input current was caused by an improperly used firing harness (ground-support equipment) that connects the igniter cartridges to the ignition circuit (ground-support equipment). A firing current of 5 amperes should have been applied to each of two bridgewires that were attached to each initiator. Instead, the improperly used firing harness resulted in the application of approximately 2.5 amperes of firing current to each of two bridgewires that were attached to each initiator. The motor manufacturer did not have adequate definition of current application requirements to design the wiring harness properly. Clarified instructions were provided and the low-amperage condition was corrected for all subsequent testing by making two special firing harnesses. One of these special firing harnesses was used for static test firings of tower-jettison motors that were assigned to ignition test categories 1 and 3; that is, both ignition categories necessitated duplication of a failed igniter cartridge. The other special firing harness was used for static test firings of tower-jettison motors that were assigned to ignition test categories 2 and 4; that is, both of these ignition categories necessitated normal ignition conditions. The failures of the two tower-jettison motors were a direct result of improperly used ground-support equipment rather than the result of a tower-jettison motor malfunction, and the results were deleted from the performance evaluations.

The thrust-rise time that was obtained from another tower-jettison motor during static test firing exceeded the specification. The static test firing performance of this motor was normal except for an unusually long igniter ignition-delay time, which resulted in prolonged motor ignition-delay, thrust-rise, and total times. A thorough check of the electrical and instrumentation systems led to the conclusion that the proper firing current, igniter-cartridge resistances, and igniter harness were used. The precise cause of this failure could not be determined from the available information. However, possible factors that could have contributed to the malfunction were inert debris from the igniter cartridge, relatively small-diameter flame ports in the igniter case, premature expulsion of the booster powder charge from the igniter cartridge, and deflection of the igniter-cartridge flame by the igniter-cartridge closures into the heat-sink area of the igniter case.

Corrective action to prevent future igniter failures of the types discussed included modification of the igniter assembly to permit a greater tolerance for the debris associated with the igniter cartridge. The diameter of the flame ports in the igniter case was enlarged from 0.375 to 0.500 inch to preclude flame-passage blockage. The two-layer vinyl-tape cover on the boron/potassium nitrate pellet container was reduced to a single-layer-tape cover to permit easier tape burnthrough. This igniter ignition-delay malfunction occurred during the time that the tower-jettison motor was duplicating a double-failure allure mode (a failed initiator and a failed nozzle closure), and there is no requirement for successful demonstration with the tower-jettison motor under these conditions. Because the igniter assembly was modified and because a double-failure mode was being duplicated, the test results of the failed motor were deleted from the performance evaluation.

After the completion of the tower-jettison motor qualification-test program, an igniter-test program was conducted to verify that the igniter modifications were successful. This program was completed without any failures.


NASA Technical Note TN D-7083
"Apollo Experience Report - Launch Escape Propulsion Subsystem"
Neil A. Townsend
Manned Spacecraft Center
National Aeronautics and Space Administration
March, 1973

Pyrotechnic Devices and Firing Circuits

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