Nasa_Pyros (Раздаточные материалы), страница 5

For example, lead azide often hasdesensitizing agents mixed into it, and it is shipped under water, to reduce the opportunity of inadvertentinitiation. Also, loading facilities are designed to accommodate inadvertent initiations; no matter whatcare, procedures and logic are applied, initiations can occur. However, once lead azide is properlyloaded in sealed, electrically conductive containers made of compatible materials, it is very stable.6-2 Safe/Arm Devices• Safe/arm devices provide configurations for:• Input isolation (safe)• Input transfer (arm)• Actuation accomplished by electrical input, mechanical input, or both• Electrical• Safe = ganged electrical switches to short circuit and electrically ground firing leads tocomponents• Arm = same switches open shorts and connect to electrical energy source• Electrical command (manual backup) moves switches• Verified visually and electrically• Mechanical• Safe = Physical barrier interposed to prevent transfer of explosive, gas, laser or other initiationstimuli• Arm = Barrier removed to allow stimulus to transfer• Commands can be manual, electrical and/or pyrotechnic• Verified visually and electricallyReferences 19, 20 and 2125Figure 21.

Cross sectional view of manually operated, explosive transfer safe/arm.Safe/arm devices (figure 21) provide positive means for assuring that stray energy or an inadvertent firing command does not initiate the entire ballistic train. That is, in the safe mode, a firing command(electrical, hot gas, explosive, etc.) cannot be transmitted. Conversely, in the arm mode, a firing command can be transmitted. Electrical safing disconnects the firing circuit from the pyrotechnic device, aswell as provides a short circuit across the bridgewire. Arming allows the firing circuit to be connectedand the short disconnected.

These safe/arm commands are usually provided with stepping motors todrive rotating shafts to the desired position. The shaft location is verified by electrical contacts in a separate circuit, as well as visually. Mechanical barriers can block initiation signal transfer by rotating ashaft or sliding a plate across an interface to seal a passage or prevent explosive transfer through a transfer charge or cavity. Again, safing and arming commands can be electrical, manual and/or pyrotechnicwith visual and electrical verification. One of the worst nightmares at a launch site is that a safe/arm unitdoes not properly cycle through its functions.

Consequently, elaborate care is applied to seal thesedevices to prevent contamination of moving interfaces and to maintain electrical contacts. Adding tothis complexity is the use of built-in explosive transfer charges, which require special handling, storageand assembly procedures as a pyrotechnic device.6-3 Component Safegaurds• Components have additional safeguards• Hazardous material safety data sheets (OSHA)• Safing pins, Remove Before Flight• Shear pin strength set to withstand highest level inadvertent input• Protective caps/connectors for resisting RF, EMI and electrostatic energies26• Procedures provide final safety protection• Handling, transport and storage• Inspection of components• Checkout of firing systems• Final assemblyReferences 19, 20 and 2127Chapter 7 - TEST METHODS AND FUNCTIONAL PERFORMANCE7-1 Non-destructive Tests• Non-destructive test inspection required to assure single- shot item is properly assembled• Dimensions of components and final assembly• X-ray to image high-density materials• N-ray to image organic compounds (explosive materials)Reference 297-2 Functional Tests• Test methods should represent the function of device• Shape, size, volumes, masses, materials• Stroke• Resistance (friction, shear pin strength, mass, mechanical force)• Measure input initiation parameters• Electrical• Mechanical• Pneumatic• Explosive• Laser• Measure output• Work/energy• Pressure• Force• Stroke• Industry standard measurement for cartridge output, closed bomb, does not represent performance ina deviceReferences 4, 30, 31, 32 and 33In order to understand functional performance, test hardware must accurately represent the devicebeing tested.

As described in Chapter 4, a number of interrelated parameters affect combustion efficiencies and, consequently, the performance achieved. The test program should evaluate both input (initiation), as well as output performance. The key to evaluation tests is to reduce the expense of testing flighthardware by using a controlled, reproducible simulation. The widely used closed bomb firing system isshown in figure 22. Although electrical initiation evaluations can be made, such as in figures 23 and 24,the closed bomb’s use in measuring the output of cartridges cannot predict performance in a device.That is, firing a cartridge into a closed, fixed volume accomplishes no work, and the parameters affecting combustion efficiencies in a device are not simulated.

As shown by the typical pressure traces in aclosed bomb, figure 25, it is not at all apparent that the two NSI-derived Gas Generating Cartridges(NGGC) can produce more than twice the energy of the two essentially equivalent initiators, the VikingStandard Initiator (VSI) and the NASA Standard Initiator (NSI). The energy delivered by the NGGC, asdetermined by a specific output test described in section 7-3-1, was 750, versus 340 inch-pounds for theVSI.28Figure 22. Closed bomb firing and monitoring system.Figure 23.

Typical current versus bridgewire break function time curves to evaluate electrical initiationcharacteristics of cartridges.29Figure 24. Typical current versus first pressure indication function time curves to evaluate electricalinitiation characteristics of cartridges.Figure 25. Typical pressures produced by cartridge firings in a closed bomb.307-3 Types of Functional Performance Tests and Determination of Functional Margin• Examples of functional performance tests• Piston/cylinder configurations• Ignitability• Explosive transfer or initiation of explosive acceptors• Explosive severance/fracture• Flexible linear shaped charge (FLSC)• Lockheed’s “Super*Zip” separation joint• Structural containment• Functional margin (“how well” something performs) is determined by measuring and comparing• Energy required to accomplish function to Energy deliverable by pyrotechnic source• Relative rate of ignition produced by one initiator type to That produced by other initiators underconsideration• Determining minimum explosive load to accomplish function (while maintaining flight configuration) to flight explosive load• Determining key functional parameters, measuring their performance at limits of functionality toflight configuration(For example, plate severance is enhanced by the bending of the plate during fracture; tests wouldbe conducted at the thinnest and thickest limits of the plate.)• Uniformity of performance is key to understanding• Conduct multiple tests, 5 to 10 minimum• Provides adequate definition if standard deviation is a small percentage of the meanSerious shortcomings exist with “go/no-go testing” and the widely cited +/- 15% “margin demonstration criteria.”“Go/no-go testing” is accomplished by assembling devices and firing them without measuring functional parameters.

That is, they either do or do not accomplish the desired function. The shortcoming isthat there is no way to determine how close the device was to failing, either through inadequate functional or structural containment margins. In testing identical devices, more than 2000 successful, consecutive functional tests would have to be conducted to obtain a simplistic reliability prediction of99.9%.Margin criteria for pyrotechnically actuated devices were first created for the Gemini Program in theearly 1960’s. These criteria are the go/no-go firing of a few devices at 15% under and over-load to demonstrate that an 85% pyrotechnic load would still function the device and a 115% overload would notcause rupture of the device.

Although these criteria implied some confidence in performance, no quantitative information is produced. Also, under certain conditions, the performance of pyrotechnic devicesactually increased with an 85% load.The following test methods are recommended to overcome these shortcomings.317-3-1 Recommended functional tests for piston/cylinder configurations• Energy required measured by dropping mass onto piston to find minimum energy required toaccomplish function• Energy deliverable determined by measuring• Crush of honeycomb: Energy = Crush Distance X Strength• Velocity of moving mass: Energy = 1/2mv2References 4, 30, 31, 32 and 33The energy required to accomplish the stroking of a piston in a mechanical function is determined bycontrolled drop testing of small masses onto the piston to be stroked. The energy required to accomplishthe function is the drop height multiplied by the weight to provide a value in inch-pounds. Ideally, ahigh velocity of the falling mass simulates the dynamics of the pyrotechnically driven piston.