Nasa_Pyros (Раздаточные материалы), страница 6

While thissimulation may not be perfect, it is certainly better than the current practice in which sellers and users ofpyrotechnic devices often have no idea of the energy required for functioning.The energy delivered by a gas generating cartridge can be measured by the McDonnell Energy OutputTest Fixture (Energy Sensor) or by measuring the velocity of the piston. The Energy Sensor, figure 26,represents an application of working against a constant force, using calibrated aluminum honeycombagainst which the cartridge-driven piston strokes.

Energy is obtained by multiplying the amount ofcrush in inches by the honeycomb’s crush strength to provide a value in inch-pounds. Several examplesare shown for various cartridges in table IX: the Viking Standard Initiator (VSI) and the NSI-derivedGas Generating Cartridge (NGGC).Figure 26. Cross sectional view of McDonnell Energy Output Test Fixture.32TABLE IX - ENERGY SENSOR PERFORMANCE DATA ON TEST CARTRIDGES(Average/Standard Deviation)CartridgeNo. FiredEnergy Delivered inch-poundsPerformance Baseline (No Environments)VSI5466/21Hi-Shear NGGC5815/99UPCO NGGC5812/90Post EnvironmentsHi-Shear NGGC16869/80UPCO NGGC12927/58To determine energy by measuring the velocity of the piston, the LaRC Dynamic Test Device and a PinPuller have been employed. The Dynamic Test Device, figure 27, represents a jettisoned mass application and employs a one-inch diameter, one-pound mass that strokes one inch to clear the o-ring.

Thevelocity of the mass is measured by an electrically grounded needle, mounted on the face of the mass,sequentially contacting five, 0.25-inch spaced aluminum foil “make” switches. The 0.25-inch spacing,divided by the time interval, yields velocity. The pressure traces produced by several different cartridges (the NSI-derived Gas Generating Cartridge (NGGC) and the Viking Standard Initiator (NSI) areshown in figure 28. These pressure measurements cannot be used as a direct indicator of energy delivered by the cartridge.

Once the total energy is measured for any particular pressure trace, the integral ofa different trace, which was produced in the same device, can be used as a relative indicator of energyfor the second trace.Figure 27. Cross sectional view of NASA LaRC Dynamic Test Device.33Figure 28. Typical pressure traces recorded and the energies produced in firing cartridges in theDynamic Test Device.The NASA Pin Puller, figure 29, was developed for a spacecraft function, and because it has ruggedsteel construction, it has been useful for comparative testing. In this case, energy was measured by thevelocity of the pin and the amount of crush in the calibrated Energy Absorbing Cup, which crushed atthe end of the stroke.

Typical pressure traces for the VSI and NGGC in the Pin Puller are shown in figure 30. The test setup for this data collection allowed the piston to jettison, rather than stopping at theend of stroke. A logical question in comparing the energy delivered by the same cartridges in three different test devices is “Why isn’t the energy the same?” The Energy Sensor measures more of the energy,because it doesn’t vent like the other two test methods. The Dynamic Test Device has a very large piston face exposed to the working gas, compared to the Pin Puller. The Pin Puller has a tortuous path forthe hot gas to pass from the cartridges to the piston; the cartridge starts the flow at 90o from the axis ofthe piston, is forced through a 0.10-inch diameter orifice and then pressurizes a narrow working face ofthe piston.

It is also clear that the initial free volumes among the three test methods were completely different, which caused considerably different combustion of the gas generating materials and pressuresproduced. For example, the area exposed to the hot gas in the Pin Puller produced a considerable heatsink, changing both the temperature at which the combustion occurred, and, consequently, the burn rateand the quantity of gas produced. The amount of residue (unburned fuel) following the firings in thethree test methods was testimony to these effects; the Pin Puller had the most.34Figure 29. Cross sectional view of NASA Pin Puller.Figure 30. Typical pressure traces recorded and the energies produced in firing cartridges in the NASAPin Puller.35These results clearly demonstrate why closed-bomb tests cannot predict performance in a device. Caremust be taken in the selection of a cartridge energy-measuring method, so that the test closely simulatesuse in the production device.7.3.2 - Recommended test for determining ignitability (output initiation performance of cartridgesand ignition sensitivity of materials)• Pressure at one millisecond• Peak pressureReferences: 34, 35 and 36The principle for determining the output ignition performance of initiating devices, such as percussionprimers and cartridges, is to fire the devices onto a controlled bed of combustible material (referred tohereafter as ignition material) and measuring the way this ignition material responds (ignites and burns).Conversely, to determine the sensitivity of materials to be ignited a controlled initiator is fired onto thecombustible material under evaluation.

The approach for these determinations is to enclose the ignitionmaterial in a sealed volume and monitor the rise in pressure, created from the burning material. Intuitively, the better the initiator performs, the more rapidly the ignition material ignites, burns and pressurizes the volume.The NASA Ignitability Test Bomb, as shown in figure 31, was initially designed to evaluate percussionprimers. However, the configuration can be modified to incorporate any type of initiator. The ignitionmaterial is placed in a hemispherical cavity in the ignition material holder. This holder has vent holes toallow the gas to vent to the lower portion of the volume, where pressure is measured.

The percussionprimer is installed in the primer holder, which is sealed within the adaptor. A firing pin is installed intoa port within the adaptor, and rests on the percussion primer. A weight is dropped onto the firing pinfrom a controlled height to assure adequate initiation of the primer. The data recorded on a high-speedmagnetic tape recorder consists of the strike of the firing pin, as measured by an accelerometer mountedon the drop weight, and two pressure traces. Figure 32 shows the pressure produced as a response to theinput of two different percussion primers, the M42C1 and M42C2 fired into 200 milligrams of FFG particle size black powder. Clearly, the M42C1 ignites the black powder more quickly.36Figure 31.

Cross sectional view of NASA LaRC Ignitability Test Method bomb.Figure 32. Typical pressure traces produced by the M42C1 and M42C2 percussion primers in theNASA Ignitability Test Method, using 200 mg of FFG black powder.37Figure 33 shows how these data were compiled for analysis. The time from the firing pin strike to anindication of 100 psi pressure was defined as primer output delay.

This is a slightly longer time intervalthan that required for the primer to be initiated by the firing pin. Because the first indication of pressurerise is often difficult to detect, the 100 psi level was arbitrarily selected. This level provides a more precise start point. The pressure achieved within the first millisecond, following the 100-psi pressure level,was selected for ratioing to the peak pressure achieved, and was defined as ignitability. The initiator thatproduces a higher pressure at one millisecond, as compared to other initiators, indicates a greater ignitability and thus, a faster initiator. The initiator selected for any particular application does not necessarily depend on a high rate of ignition; some applications, such as initiating time delays, require a soft,slow initiation, so as to not to damage the delay columns.Figure 33.

Percussion primer ignitability performance definitions.The ignitability Test Method can be similarly applied to any initiator, whether electrical, explosive orlaser.For evaluating the relative sensitivity of various ignition materials, figure 34 shows the response ofthree different materials to initiation inputs from M42C1 and M42C2 percussion primers. Clearly, theFFG black powder was more sensitive to ignition than was the much coarser A cannon black powder.The most difficult to ignite was the Type I particle size BKNO3.

The same ignitability definition, ratioing the pressure achieved at one millisecond to the peak pressure, applies.38Figure 34. Ignitability comparison of three ignition materials, each ignited by the M42C1 and M42C2percussion primers.7.3.3 - Recommended Tests for Explosive Transfer• For explosive transfer from a donor to an acceptor, measure• Fragment velocity/energy delivered by the donor• Fragment velocity/energy required to initiateReferences 5, 23, 24, 29, 30, 31, 37 and 38Initiation of high explosives across hermetically sealed interfaces is accomplished primarily by highvelocity fragments from the donor. In the case of explosive transfer lines (described in Chapter 5) the0.005-inch wall thickness 302 stainless steel cup fragments, and the particles accelerated to velocities of8,000 to 10,000 feet/second as the 65-milligram explosive load within the cup explodes.

The shape,impact pattern and velocity of the fragments depend on parameters such as: 1) cup material, propertiesand thickness, 2) explosive material, particle size and loading pressure, and 3) the medium throughwhich the fragments pass (usually air). The test setup used to monitor the donor- delivered fragments isshown in figure 35. Fragments are created off the end and off the side of the cup. Aluminum foil “make”switches, spaced known distances from the cup provide time intervals to calculate velocities.