1、Lessons Learned Entry: 1111Lesson Info:a71 Lesson Number: 1111a71 Lesson Date: 1999-02-01a71 Submitting Organization: HQa71 Submitted by: David M. LengyelSubject: International Space Station Program/Extravehicular Activity (EVA)/Pyrotechnic Initiators Description of Driving Event: Use of “Fire-Evide
2、nt“ Pyrotechnic InitiatorsLesson(s) Learned: Many deployable structures on the ISS and satellites on which astronauts must work during EVAs use pyrotechnic initiators. There is often no simple way for an EVA astronaut to know by visual inspection whether or not an initiator has fired when a structur
3、e has failed to deploy properly.Recommendation(s): NASA should develop and require the use of pyrotechnic initiators that leave clear visual evidence that they have fired. These “fire-evident“ initiators should be required for all applications that may be encountered by an EVA astronaut.Evidence of
4、Recurrence Control Effectiveness: The NASA Standard Initiator (NSI) is required for use in all electrically initiated pyrotechnic systems whether the application may be encountered by an EVA astronaut or not. The NSI does not provide any means for external visual inspection of fired condition when i
5、t is installed in a mechanism. Currently, the only test being performed to verify that the initiator fired, without disassembly of the pyrotechnic mechanism, is to measure firing circuit resistance before and after firing. This function can be built into the firing unit. It is not foolproof however,
6、 since it cannot detect a smart short. To date this has not been a problem with the NSI since in nearly 100,000 units produced and certified there are no documented failures. That is why the NSI carries a reliability of 0.999 at a 90% confidence level. All failures to fire have been traced directly
7、to the electrical wiring, Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-connectors, firing unit or flight computer. Breaks in the electrical firing circuit can be identified by a pre-fire circuit resistance check.The desire of visual identification
8、 is further compounded by the physical location of the initiator. In many applications it is located internal to a mechanism and is not directly accessible or visible. For those applications where it is external to the mechanism it is still not visible since half the device is torqued into the mecha
9、nism and the other half is covered by the electrical connector. Stretching the device to make a portion of it visible would require a re-design and re-qualification of the initiator at an extremely high cost as well as making it larger and heavier in a size and weight conscious world.Two types of vi
10、sual indicators have been considered for incorporation into the initiator. The first is a temperature sensitive tape that could be placed on the outside of the initiator body that would change colors due to temperature rise generated from firing the initiator. This is not considered practical. The t
11、emperature rise of the NSI body is small and further effected by heat sinking of the mechanism it is inserted into. The actual temperature rise that would result is lower than the temperature rise generated by direct solar radiation. It would be unknown whether the color change was due to the initia
12、tor firing or the sun. The tape would also not be visible due to coverage by the electrical connector without redesign of the initiator body. The second possibility of a visual indicator is a pop-up pin that would be pressure driven by the NSI firing. Incorporating the pin into the NSI would be both
13、 complex and expensive. The NSI is a hermetically sealed device, there is no way to incorporate a pop-out pin without violating the hermetic seal. The size of the NSI would have to be greatly expanded to accommodate the pin/piston, which would have to withstand pressures from 600 psia to as high as
14、25,000 psia. The pin/piston orientation would also affect the pressure output and function time of the initiator. One final consideration that is very significant is that there are over 1000 pyrotechnic devices and mechanisms that have been flight qualified and certified to function with the NSI. Th
15、ose devices are in repeated use on numerous crewed and uncrewed programs. The intrusion of a new initiator would not only be a reduction in reliability but would require re-qualification of associated components at a tremendous cost. Currently, there are no plans to pursue recommendation #16.Documen
16、ts Related to Lesson: N/AMission Directorate(s): a71 Space Operationsa71 Exploration SystemsAdditional Key Phrase(s): a71 Aerospace Safety Advisory PanelProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 Energetic Materials - Explosive/Propellant/Py
17、rotechnica71 Flight Operationsa71 Human Factorsa71 Research & Developmenta71 Risk Management/Assessmenta71 SpacecraftAdditional Info: Approval Info: a71 Approval Date: 2002-02-12a71 Approval Name: Bill Loewya71 Approval Organization: HQa71 Approval Phone Number: 202-358-0528Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
