Nasa_Pyros (Раздаточные материалы), страница 3

Furthermore, manufacturers may have adifferent view of success than does the user. Should a failure occur, there is a conflict between findingout the exact cause and getting on with the program schedule.3Statistical reliability and confidence is usually based on information compiled by the manufacturer infunctional evaluations on any particular device and on its predecessor designs.

To achieve a statisticalbasis for a 99.9% reliability with a 95% confidence level, more than 2000 identical devices would haveto be evaluated. Such a number is often cost prohibitive.4Chapter 3.- PYROTECHNIC FUNCTIONAL PRINCIPLES••••Majority of pyrotechnically actuated functions accomplished through piston/cylinder devicesOther functions accomplished by linear explosivesFigures 1 through 14 show basic principlesTables I and II show past applicationsReferences 1, 5, 6, 7, 8 and 9Figure 1. Cross sectional views of pyrotechnically actuated linear actuators, describing function.5Figure 2.

Cross sectional views of pyrotechnically actuated valves, describing function.Figure 3. Cross sectional views of pyrotechnically actuated separation nuts, describing function.6Figure 4. Cross sectional views of explosive and pyrotechnically actuated separation bolts, describingfunction.Figure 5. Cross sectional views of pyrotechnically actuated cutters of guillotines.7Figure 6.

Cross sectional views of mild detonating cord (MDC)-actuated severance and separationapproaches.Figure 7. Cross sectional views of flexible linear shaped charge (FLSC) severance.8Table I. Major Past and Current Pyrotechnic Applications in AeronauticsTable II. Major Part and Current Pyrotechnic Applications in Astronautics9Figure 8. Cross sectional view A-A on right of F-111 crew module severance system.Figure 9.

Functional depiction of Rotor Systems Research Aircraft (RSRA) in-flight escape system.10Figure 10. Depiction of pyrotechnic devices used on Project Mercury.Figure 11. Depiction of pyrotechnic devices used on Project Gemini.11Figure 12. Depiction of pyrotechnic devices used on the Command Module and escape system, ProjectApollo.Figure 13. Depiction of pyrotechnic devices used on the Lunar Excursion Module, Project Apollo.12Figure 14.

Shuttle Transportation System pyrotechnics.13Chapter 4.- PYROTECHNIC, PROPELLANT AND EXPLOSIVE MATERIALS ASENERGY SOURCES• Energy delivery affected by burn rate• Time delay trains (pyrotechnic mixes): inches/second• Double-base propellant: inches/second• Metal/metal oxides: hundreds of feet/second• Primary explosives: less than 10,000 feet/second• Secondary explosives: over 20,000 feet/second• Wide range of energy characteristics• Energy delivered in various forms: heat, light, gas• Minimal to intense heat production• Gas evolution: gasless to millions of psi• Time to peak pressure less than microsecond to seconds• Combustion affected by:• Density and particle size of burning material• Initial free volume• Confinement• Shape of the volume• Heat transfer characteristics• Changing volume (stroking piston)• Energy delivery can be tailored to meet a very wide range of performance by adjusting the aboveparametersReference 4The primary influence of energy deliverable by pyrotechnics, propellants and explosive is burn rate.

Asthe burn rate changes, so do the products of the combustion. The forms of energy delivered are heat, gasand light, depending on the material or combination selected. Pyrotechnic compositions produce intenseheat and often intense light with very little gas production. Propellants are used to deliver high- pressurevolumes of gas, often with only moderate heat. Primary explosives rapidly (microseconds) develop hundreds of thousands of psi of gas pressure, while high explosives develop millions of psi in even a shortertime frame, with very little heat production.The combustion of these materials is affected by a number of parameters.

Compacted small particleswill burn faster than larger particle sizes at the same density. Of course, with loose-particle combustion,the larger the surface area, the more rapid the combustion. Gas producing materials are generallyaffected by ambient pressure; the higher the ambient pressure, the faster the burn rate. Consequently,propellants ignited in a large free volume must first pressurize the volume, before the ambient pressureis raised sufficiently to increase the burn rate. The shape of the volume in which reactions occur affectheat transfer within the combustible material itself, as well as transferring heat to surrounding structure.The greater the surface area and thermal transfer properties of the container, the more heat loss andreduction in burn rate of the material.

An increasing volume, such as a stroking piston reduces burn rateby lowering ambient pressure, as well as increasing the surface area exposed to the hot gas. Energydelivery characteristics can be tailored over a wide range by adjusting the above parameters. Tables III14through VIII describe the properties of several widely used pyrotechnic, primary explosive and secondary explosive materials.TABLE III.- Properties of a Time-Delay Mix (D-16, MIL-M-21383)• Formula:••••Ingredient% by WeightManganese37Barium Chromate20Lead Chromate43Burn rate: 8.7 seconds/inchVirtually gasless outputStability: Extended service life results in longer delayApplication in sequencing pyrotechnic functionsReference 10TABLE IV.- Properties of a Gas-Generating Material (Hercules Hi-Temp)• Formula:Ingredient% by WeightRDX80Nitrocellulose20• Gas Composition:CO33.5CO215.1H20.8H2O17.2N232.4Other1.0• Stability: less than 1% weight loss in 5 hr.

at 275°F(Source: Hercules Incorporated)• RDX sublimes under vacuum (shouldn’t be used for deep- space applications; container seal is asingle-point failure)• Sensitive to ambient pressure for ignition and burning (higher rate at higher pressures)• Application as gas generating source for cartridgesTABLE V.- Properties of Boron/Potassium Nitrate (B/KNO3)•••••••Gas generating materialBurn rate minimally affected by ambient pressureHigh-temperature combustion, hot particlesThermally stableVacuum stableLong shelf lifeApplication as rocket motor igniter and gas generator15References 11 and 12TABLE VI.- Properties of NASA Standard Initiator (NSI) Mix• Zirconium/potassium perchlorate (Zr/KClO4)• Burn rate of hundreds of feet/second• Rapid pressure rise• Output = hot particles, little gas• Electrostatically sensitive• Good hotwire initiation interface• Thermally and vacuum stable• Long shelf life• Application as an initiator and as an energy sourceReferences 4 and 13TABLE VII.- Properties of Lead Azide• Transfers from a deflagration to detonation, short distance (about 0.1 inch)• Detonation rate of about 7,000 feet/second• Thermally stable (except for desensitizing agents: dextrin)• Vacuum stable (except for dextrin)• Long shelf life• Sensitive to impact, friction and electrostatics• Application in detonators to initiate a high-explosive outputReferences 14, 15 and 16TABLE VIII.- Properties of Hexanitrostilbene (HNS)• Detonates at a propagation velocity of 22,000 feet/second (32,000 psi compaction pressure)• Thermally stable• Vacuum stable• Insensitive to non-explosive stimuli• Application in detonators, linear explosives and bulk chargesReferences 16, 17 and 1816Chapter 5.- INITIATION SYSTEMS/INITIATORSThe basic initiation systems for aerospace systems are:••••••ElectricalExplosive transferMechanicalShock tubeHot GasLaser5-1 Electrical Firing System Characteristics• Provide reliable electrical energy to initiator• Direct current• Capacitor discharge• Protect against inadvertent initiation• Shielding: lightning, static electricity, radio frequency, electromagnetically induced energy• Two-fault tolerant switches• Control/sequence firing commands• Provide electrical isolation from other electrical circuits• Greatest safety consideration is final connection to device• Assure no energy in circuit• Remove shield from device and install final connectorReferences 19, 20 and 215-1-1 Electrical Initiator Characteristics (NASA Standard Initiator (NSI), figure 15)• Convert electrical energy to heat to ignite “first- fire” through high-resistance bridgewire• Direct current• Capacitor discharge• Provide reproducible initiation characteristics• No-fire energy; 1-amp/1-watt, five minutes• Predictable ignition delay for recommended firing energy• Prevent inadvertent initiation• 1-amp/1-watt no-fire dissipation• Stray energy (transients, radio frequency, electromagnetic)• Electrostatic discharge• Provide electrical isolation from structure• Provide ignition for pyrotechnics, propellants and explosive trains• Sometimes used as sole energy source for small mechanisms• Provide post-fire sealReferences 4 and 13175-1-2 Exploding Bridgewire (EBW) Initiator• Uses low-resistance conductor (gold) bridgewire• Uses internal spark gap to prevent conducting low voltage and current levels through bridgewire• Uses several thousand-volt capacitor discharge firing system, which couples through internal sparkgap••••Bridgewire vaporizes (explodes) to provide an impulse to directly initiate secondary explosivesEliminates the need for sensitive initiation materials and primary explosivesProvides post-fire sealMajor drawbacks are bulky, heavy power supplies, capacitors, switches and cables5-2 Mechanical Initiation System Characteristics• Provide mechanical input to initiator (primer)• Spring compression/release• Pneumatically driven• Impact driven• Provide firing pin interface to primer• Prevent inadvertent functioning of initiation handles• Two-step operation (squeeze/pull or rotate/pull)• Minimum force and stroke required• Assure adequate energy to initiate primer• Threshold pneumatic and impact energy• 2 × (50% firing energy level + 5 standard deviations)• Provide post-fire sealReference 225-2-1 Mechanical Initiator Characteristics (M42 Percussion Primer example)• Convert mechanical energy to ignite primer mix• Primer composition ignited by crushing/friction between cup and anvil• Provide reproducible initiation characteristics• No-Fire (1.92-ounce ball drop):50% firing level drop height minus 2 standard deviationsshall not be less than 2 inches (3.84 inch-ounces)• All-Fire (1.92-ounce ball drop):50% firing level drop height plus 5 standard deviationsshall not exceed 13 inches (25.49 inch- ounces)• 50% firing level approximately 10 inch-ounces• Provide ignition output (heat, gas, light, burning particles)• Provide post-fire seal• Primers themselves not sealed; must be sealed by assembly into which it is installedFigure 16 shows percussion primer designs.Reference 2218Figure 15.