Description of Driving Event:
Use of "Fire-Evident" Pyrotechnic Initiators
Lesson(s) Learned:
Many deployable structures on the ISS and satellites on which astronauts must work during EVA's 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 structure 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 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 it 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, 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 to the electrical wiring, 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 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 mechanism 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 visual 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 temperature 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 initiator 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 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 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. Those 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.
Documents Related to Lesson:
N/A
Mission Directorate(s):
- Space Operations
- Exploration Systems
Additional Key Phrase(s):
- Aerospace Safety Advisory Panel
- Energetic Materials - Explosive/Propellant/Pyrotechnic
- Flight Operations
- Human Factors
- Research & Development
- Risk Management/Assessment
- Spacecraft
Additional Info:
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