Nasa_Pyros (Раздаточные материалы), страница 4

Cross sectional view of NASA Standard Initiator (NSI).Figure 16. Views of commercial or military center-fire percussion primers.195-2-2 Stab Primer•••••Used where full containment not necessarySharp firing pin penetrates through cupAchieves ignition by fracture/friction of primer compositionInput energy generally less than needed for percussion primerOutput comparable to percussion primerThe Separation Plane Initiator Assembly (figure 17) has a number of valuable design features to assureoperational safety and reliability. The “remove before flight” Pip Pin prevents any motion of the FiringPin. The expanded diameter of the Sear assembly shoulders on the bore of the Primer Pin Yoke, preventing Firing Pin displacement and contact with the Percussion Primer. The Spring was preloaded tokeep the Sear assembly shoulder engaged, preventing rattle.

The energy content in the Spring in inchpounds is determined by the equation 1/2kX2, where k is the spring constant and X is the compressiondistance. When the Wire Rope pulls on the Sear, the Spring is compressed. The Sear releases the firingpin, as the Sear clears the Primer Pin Yoke, allowing the Sear’s angular interlocking finger to slide offthe corresponding finger on the Firing Pin. The Support Disc prevents too great a penetration by the Firing Pin into the Percussion Primer.Figure 17. Cross sectional view of separation plane initiator for Delta Launch Vehicle.205-3 Hot Gas System Characteristics• Gas generators pressurize pneumatic system plumbed to initiate each aircraft system function (canopyjettison, crewmember retraction, seat ejection, etc)• Mechanically initiated primer to gas generator• Device shear pin retains firing pin prior to stroke• Gas pressure at device 1000 psi minimum; shear pin fails at about 500 psi• Gas generator boosters used for large volumes and for long lengths of tubing• System pressure integrity critical5-4 Explosive Transfer Line Characteristics• Explosive impulse is transmitted to each system function by mild detonating fuse within steel tubing• Identical booster charges in thin-walled cups (tips) at each end of line• Initiation of input booster causes detonation propagation through explosive cord to initiate output tip• This tip creates a pressure impulse and fragments to initiate the next line• Tip output can also be used to accomplish work• Explosive transfer lines can also be flexible with fiberglass and steel overbraid for containment ofexplosive products• Transfer lines plumbed together like high-pressure tubing; free rotating nut on inside shoulder of eachtip allows threading/seating into receiving ports• Steel tubing allows assembly to be hermetically sealed by welding cups to tube• Insensitive to initiation by lightning, RF, EMI and bullet impact• Will burn in fire, but will not build to detonationReferences 5, 23, 24 and 25Figure 18.

Cross sectional view of rigid explosive transfer line, also called shielded mild detonatingcord (SMDC).21Rigid explosive transfer lines (figure 18) for high- temperature environments and the technology fortheir use were developed for the F-111 escape system in the early 1960’s. Explosive transfer fromdonors to acceptors is primarily accomplished by cup fragments. The sketches in figure 19 show theexplosively driven shrapnel patterns produced as the end tip cup explodes. The sketches in figure 20show the gaps at which explosive transfers were accomplished. The shrapnel fragments produced offthe end (bottom) of the cup proceed across the gap through the air in a contoured front with approximately a one-degree divergence.

As these fragments impact against an acceptor tip, a sufficiently highimpulse is introduced into the acceptor tip to initiate detonation in the explosive. The donor cups mustbe fully annealed after forming to produce the fragment patterns shown in figure 19, so as to maximizethe reliability of transfer. Although gaps to five inches are indicated between donor and acceptor, amaximum gap of 0.250 inch is recommended to accommodate the limits of variables, such as cup wallthickness and properties, cup breakup, and explosive density.

In the “end to side” configuration, thecylindrical side of the acceptor tip is a poorer target; on- coming fragments can be deflected, reducingthe amount of energy transferred into the acceptor. In the side-to-end transfer mode, the fragments produced off the sides of the cylindrical cup are strips, like barrel staves. With the radial expansion of thesefragments, appreciable gaps occur between fragments.

Note, gaps must be at least 0.006 inch to allowthe formation of fragments; this phenomenon can be explained through the realization that the cup hasto expand slightly before it bursts to create high-velocity fragments.Figure 19. Views of SMDC tip fragment dispersion.22Figure 20. Gaps at which SMDC fragments initiated acceptors; donors and acceptors identical.5-5 Shock Tube System Characteristics (Thin-Layer Explosive (TLX))• Shock pulse (6500 feet/second) is transmitted to each function in the system through plastic tube withinterior thin layer coating composed of HMX and aluminum (50/50)• Deflagrating material in input of assembly is initiated by primer or other means• Output cup can contain deflagrating material to transfer the impulse to the next line or can containexplosive material to initiate high explosive functions• Tube can be initiated by some projectile impactsReference 26The main advantage of the Thin-Layer Explosive (TLX) approach over explosive transfer lines is inweight.

Since the shock wave is less energetic, explosive containment is more easily accomplished.However, this approach lacks the maturity of explosive transfer lines, and difficulties have been experienced in its application. Care must be taken to assure adequate margins in line initiations in multi-portmanifolds.235-6 Laser System Characteristics•••••Coherent light transmitted to all functions within a system through fiber optic cablesCoherent, high-intensity light generated by lasing gas, rods and diodesLight transmitted through connectors and window in initiator or pigtail fiberWindow and pigtail are sealed to prevent ventingFiber optic cables insensitive to lightning, RF, EMI and bullet impactReferences 27 and 28The major thrust of laser-initiated pyrotechnic devices is the virtual impossibility of initiating deviceswith anything but the coherent light.

This allows installation of the pyrotechnic charges early in theassembly of subsystems and systems, such as is accomplished with explosive transfer lines and exploding bridgewire devices. Laser systems can safely allow early installation with weight savings over theabove-mentioned systems. However, a price is paid; system complexity and electrical energy consumption is increased. Although laser diodes are small enough to provide a lasing source for each pyrotechnicdevice, eliminating the need for complex mirrors or switching mechanisms, the electronics necessary topower the diodes are more sophisticated and consume considerably more energy than those needed todirectly fire a hot bridgewire system.

This system also lacks a significant flight history. Definitions andstandards are currently being developed to assure functional margins at field connectors and at interfaces to the pyrotechnic charges. Range safety personnel have yet to establish safety and operational criteria. For example, is a physical light barrier or an open electrical switch required to assure the system is“safed,” as has been required for electrical and explosive initiation systems?24Chapter 6 - PYROTECHNIC SYSTEM SAFETY6-1 Properties of Materials• Explosive/pyrotechnic and propellant material safety properties established by:• Impact sensitivity• Thermal stability (autoignition temperatures and rates)• Vacuum stability• Electrostatic sensitivity - Friction sensitivity• Compatibilities with other materials• Aging characteristics• Explosive input initiation sensitivity• Safeguards for initiation systems mentioned aboveReferences 14 and 15Elaborate procedures have been established (usually with military specifications) to provide comparative information on each of the above properties for raw materials.