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In this case, each system works independently without any influence on the other.However, as shown in example B, the two pin pullers could be designed to directly accomplish the45function without an interface. In example C, two independent systems through the cartridge level couldprovide inputs into a pin puller; in this case, either cartridge has sufficient energy to independently actuate the pin puller and accomplish the function. Finally, independent SBASIs could be used to ignite asingle cartridge to actuate the pin puller.

Single point failures occur in the system in example B, the system and the pin puller in example C, and the system, device, and cartridge in example D. The level ofredundancy depends on the criticality of the function and is achieved by design. That is, simplifieddesigns may actually be an improvement over total redundancy. For example, system complexity interms of the number of components and interfaces can increase the opportunity of failure. Furthermore,the possibility of inadvertent functioning increases with twice as many firing systems.

In any case, theuse of redundancy does not alleviate the need for designing, demonstrating and producing componentsand interfaces of the highest quality and with the ability to resist inadvertent functioning or failure tofunction. Redundant systems should not be counted on to accomplish the function; each system shouldbe expected to accomplish the function, even if the other did not exist.Figure 39. Levels of pyrotechnic redundancy established by the Apollo Program.A special word of caution is warranted for the use of cross- overs in explosive transfer lines, figure 40.Designers have fallen into the trap of putting cross-overs at the output tips of devices, such as timedelays.

The concept is that, since the output of time delays won’t function simultaneously, the earliestarrival will trigger the system. Or, if one path had stopped functioning, a cross-over would reinitiate thatpath. However, the fallacy lies in the possibility that the first input to arrive may cause the cross-over tomalfunction, which in turn can destroy the second time delay output, causing a loss of propagation inthe second path.46This discussion on success and failure leads to the contradictory problem faced by the pyrotechnic specialist. As shown in figure 41, the need to prevent inadvertent initiations must be balanced against theneed to assure that initiation has been achieved. To prevent initiation, all of the safety features describedabove are employed.

These features may be so successful that initiation is prevented. A great deal ofeffort must be made in assuring that these safety features are successfully disabled to properly “arm” thesystem to allow an initiation command to be transmitted. Conversely, to assure initiation is achieved,the sensitivity of devices to initiation commands must be increased to a reasonable level without undulyimpacting safety features.47Figure 40.

Example of false redundancy in the use of explosive crossovers.Figure 41. Depiction of the need for the pyrotechnic specialist to meet both safety and reliabilityrequirements.48Chapter 9 - SYSTEM APPROACH FOR APPLICATION OF PYROTECHNICS• Appoint a pyrotechnic manager• Compile system requirements• Performance• Physical envelope• Interface with flight vehicle• Environments• Schedule• Reliability/simplicity• Service• Cost• Project experience• Management preferenceThe most important task for project management is to recognize that pyrotechnics must be a separateengineering discipline from those that are obvious, such as structure, electrical power, propulsion,telemetry and thermal control.

Pyrotechnic subsystems accomplish a wide variety of critical functions,such as initiation, staging, ejection and release, and, consequently, must interface with most other onboard systems.A compilation of the requirements on a quantitative basis is the most valuable framework for driving thedesign, development, qualification and integration of pyrotechnic systems. Realizing that tradeoffs willhave to be made throughout the effort, flexibility in requirements and approach must be emphasized. Aclassic example is establishing a requirement for emergency escape from an aircraft over a range of zerovelocity/zero altitude through supersonic speed at high altitude.

The problem is that escape under supersonic conditions may require many times the cost, time and effort, than for subsonic escape. Historically, the preponderance of in-flight escapes for supersonic aircraft have been at subsonic conditions.The two statements on project experience and management preference cannot be discounted. It is oftenwise to continue the use of hardware and logic that is well proven and is familiar to project personnel.Project management often has a preference, based on experience, which becomes the baseline fromwhich all concepts are compared. That is, when another approach is offered, advantages and disadvantages should be compared to the project experience and preference.• Compile history of similar past applications• NASA LeRC compiling catalog• Contact past program managers/designers• Drawings• Functional performance/understanding• Environmental demonstrations• System performance• Negotiate/select approach and functions that most closely meet requirements• Cannot be unilateral decision• Interfacing system managers may not appreciate pyrotechnic engineering logicThis kind of homework is valuable in preparing trade studies and supporting recommendations.

Unfortunately, few projects have the time to document the logic used in making selections of devices. Even49more important is determining the “pedigree” of the selected hardware: 1) development history, including functional margin demonstrations, 2) qualification demonstration, and 3) lessons learned from failures. Since there is very little engineering training in the unique energy sources offered by pyrotechnics(reference 3), personnel with limited hands-on experience are often assigned and reluctantly accept program responsibility. Few positions within a project require such preparation (starting from a modesttechnology baseline with limited definitive standards or specifications for design, development, qualification and integration). Consequently, when the project concludes, these personnel are grateful to havesuccessfully “survived” the experience and few stay on in the field for the next project.

NASA LewisResearch Center is compiling a catalog of pyrotechnic applications and devices, which should be usefulin beginning the search cycle. Most of the information on pyrotechnic devices is from manufacturers.They have listings of past successful applications of their devices and often the qualification reports.Unfortunately, since current specifications do not require quantitative functional margin demonstrations, the insight into how these devices and systems were developed is often lacking.

Pyrotechnicdevices are presented by manufacturers as “off- the-shelf,” like nuts and bolts, needing only “minimal”modification to meet the specific requirements of the current mission. The pyrotechnic manager, whiledeveloping requirements, is faced with digesting this myriad of information from the manufacturers ontheir hardware.After compiling needed information, conducting trade studies and making recommendations to theproject, the pyrotechnic manager must be prepared to negotiate the final approaches. These negotiationsmust be made with the managers that are responsible for systems that interface with the pyrotechnic systems. These negotiations will likely continue throughout the life of the project.

Recognizing that littleengineering logic has been available for pyrotechnic applications, it is necessary to assist the managersof interfacing systems with the specific information needed to achieve integration.• Compile detailed performance requirements• Determine integration approach• Obtain approval from interface managersOnce general agreement has been achieved regarding the logic and principles to be used in applyingpyrotechnic systems within the project, detailed specifications can then be compiled. It is important toprovide information in the specifications that maintains an overview of what the objectives of the pyrotechnic subsystems are and how the pyrotechnic devices are to be integrated into the system.

Managersthat interface with pyrotechnics must approve the specifications, and be kept abreast of the developmental status.50Chapter 10 - GENERATE PERFORMANCE-BASED SPECIFICATIONS••••Describe approach to systemDefine system physical and performance requirementsDefine component physical and performance requirementsRequire demonstration of functional margin for components and systems contracted; go/no-go testingalone unacceptable• Require demonstration of environmental resistance (qualification)• Confirm functional margin in lot acceptance testing: require performance demonstration at a levelgreater than worst-case flight conditionsPyrotechnic specifications should be constructed to insure that the functions to be accomplished and thelogic to be used are clearly understandable. The function for each pyrotechnic device, as well as theoverall system, should be readily apparent.