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To compare the shock produced by various pyrotechnic devices, a Hopkinson bar setup should beemployed.The complex shock waves generated by individual pyrotechnic systems, monitored on a Hopkinson barwith high-response (80 khz) strain gages, have demonstrated a remarkable degree of reproducibility.The Hopkinson bar is a one-inch diameter, 10-foot long steel bar, which represents a simplified structure. The shock wave propagates undisturbed down the length of the bar with only a simple reflectedwave at the end of the bar opposite to the end on which the pyrotechnic device was mounted.

The lengthof the bar allowed the complete initial transient pulse to be recorded without interference from thereflected wave. Multiple firings (reference 39) of separation nuts produced strain signals that overwroteone another.However, pyrotechnic shock simulation is questionable, since the dynamics of a mechanism are difficult to duplicate and the accelermometer-based data acquisition cannot provide linear response. Therefore, it is difficult to be assured of good simulations, risking the possibility of over or under- testing.56Chapter 14 - FAILURE MODES AND EFFECTS ANALYSES• Consider and list what can “go wrong,” such as:• Improper assembly (mechanical hardware and firing system)• Inadvertent command• Out-of-sequence command• Mechanical hangup• What can result if incidents occur• Put emphasis on preventing incidents and demonstrating system capabilities• Device and system design/functional margins• Personnel awareness• System schematics/diagrams• Procedures• Use mockups of devices and subsystemsFailure Modes and Effects Analyses (FMEAs) are more than just a paperwork game to be played after apyrotechnic device and its application have been designed, developed and qualified.

FMEAs (what can“go wrong,” and what can result) should be considered throughout the effort, from the earliest conceptualizations, through qualification. In fact, considering FMEAs at the conclusion of the design process is amistake, since it is almost certainly too late to modify devices or system designs. The emphasis shouldbe placed on preventing incidents and demonstrating device and system capabilities.

The best way ofpreventing failures is through design, using logic, such as the positioning of components to assureproper assembly and the demonstration of functional margins. Equal in importance is the awareness ofpersonnel concerning what the system is supposed to accomplish and how, by way of device and subsystem schematics, as well as system diagrams and assembly and checkout procedures.57Chapter 15 - INSTALLATION, INTEGRATION AND CHECKOUT• Final assurance system will accomplish the required task• Require end-to-end schematics/diagrams• Show all commands (software)• Electrical wiring and switching• Required end function and location• Labeled interfaces• Simplified labeling of all components and interfaces• Provide detailed mockups of final functions (since flight pyrotechnics cannot be functioned)• Require end-to-end checkouts• Power up electrical firing circuits• Verify proper firing signals, related to commands• Verify signals reached the correct point for functionReference 43Installation, integration and checkout is not only the final assurance the system will accomplish therequired task, it is the last opportunity to detect and correct any flaws in the hardware or software of thesystems.

Since pyrotechnic devices generally cannot be functioned during checkout of electrical orother firing systems, it is imperative that personnel are aware of what they are testing and what isexpected to occur on any given firing command. This information should be included on end-to-endsystem schematics, diagrams and procedures.

One drawing should contain sufficient information,(referring to supporting drawings, hardware and procedures), to allow personnel to trace an entire command subsystem from generation (what the command is supposed to look like), through how it is transferred to the pyrotechnic device, and what is supposed to occur when the command is received.Simplified, standardized labeling should be continuous throughout all of the components of a subsystem; do not allow labeling to be independent among individual subsystems and designers. Accuratemockups (with appropriate sensors and indicators to acknowledge the arrival of the command) of thedevices and subsystems into which these firing systems are to be installed are also valuable in assuringthat the system will function as required and that the correct installations will be accomplished.

That is,personnel should be able to understand that command A should be transmitted through electrical circuitA to fire the primary energy source, not just that a final connection was made. End-to-end checkoutsshould be performed on the assembled subsystems by powering up electrical (or other) firing circuits toverify the proper firing signals were generated, and related to the commands at the correct sequence, aswell as to verify that the signals were correct and reached the correct point to accomplish the function.The problem encountered, particularly by large projects, is the lack of ownership of the pyrotechnichardware.

That is, no one assumes the responsibility of assuring the success of the hardware in meetingthe required function from design through the final installation and checkout. Unfortunately, the samepersonnel generally do not follow the hardware throughout its lifetime, and it is handed from one organization to another to accomplish each phase of the program. At each interface, the next team mayassume that their predecessors did their job, and details in understanding the hardware can be lost.58Chapter 16 - SHELF AND SERVICE LIFE EVALUATION• Shelf and service survivability demonstrated by performance• Determine/utilize key performance parameters• Establish a performance baseline• At original manufacturing cycle• Or use youngest available, identical units• Sample and function units from shelf/service• Compare to baseline• Frequency of sampling dependent on• Design• Performance• Criticality of functionReferences 37 and 38The survivability of pyrotechnic devices under long-term shelf and service conditions should be evaluated by demonstrating performance before and after such exposures.

The long-term stability of devicescannot be accurately predicted, nor demonstrated through accelerated aging techniques. Past assumptions have been made that 1) all reactive materials have a predictable deterioration mechanism to allowpredictions of allowable lifetimes, and 2) elevating the temperature would predictably accelerate thedeterioration.Key performance parameters must be determined and a baseline established for comparison to the performance produced by devices removed from the shelf or after a long time in service. For example, inthe evaluation of rigid explosive transfer lines, functional measurements included the velocity of explosive propagation down the explosive cord, and the velocity and pattern of the explosively acceleratedfragments off the end and side of the transfer cups. The purity of the explosive materials was evaluated,and degradation studies were conducted, which corroborated functional performance decreases withloss in chemical purity.

However, for percussion primer-initiated devices, a chemical analysis of themulti-ingredient primer mix would be not only complex, but would be difficult to correlate to functionalperformance. Furthermore, the failure modes of percussion primers are contamination and displacementof the mix, due to mechanical inputs, such as vibration. Therefore, the evaluation emphasis was placedon determining functionality with the highly sensitive ignitability test method.The frequency of removing samples from service for evaluation depends on the design, the performanceof the device and the criticality of its function.

To use the rigid explosive transfer line example, thedesign of the line provides for hermetic sealing of all explosive materials, the materials selected wereextremely pure and very stable under time and temperature, and the assemblies were very rugged withthe explosives compacted at 32,000 psi.

The performance of transfer lines is highly reproducible, producing standard deviations of 1 to 5% of the mean values of performance. Obviously, transfer linesserve a critical function, initiating virtually all the escape system functions in a wide variety of fighteraircraft.

Therefore, the original allowable service life was 18 months before changeout and further evaluation was required. However, as experience increased, service life was extended with the finding thatlines removed from 9 years of service on Army helicopters could not be functionally or chemically differentiated from new lines. The Army is leaving all these lines in their fleet, removing a sample of unitsat periodic intervals for evaluation, and expecting no changes for the lifetime of their aircraft.

For theAir Force’s B-1B aircraft, service life was extended from 3 to 20 years in a single step.59Chapter 17 - PAST EXPERIENCE AND LESSONS LEARNEDTwo examples of past failures of completely different pyrotechnic devices are provided to present logicfor evaluation and correction.17-1 - Failure Investigation of Viking Pin Puller• Successful performance on Mars Lander• Planned to utilize original hardware lot, 20 years later• Second user had failure• Duplicate unit• Same manufacturer• Same design/drawings• Contributors to failure• Lack of o-ring seals• NSI combustion inefficiency/quenching• Deformation of cylinder bore• After redesign, pin puller went from inadequate functional margin to a capability of 6 times theenergy required to stroke• Confinement margin demonstrated by dual-cartridge firing at 200oF under vacuumReference 4Following the successful use of the Viking Lander pin puller (figure 42) on the surface of the planetMars, two other programs selected it for further application.