AIAA 97-2825, Proceedings of 33rd Joint Propulsion Conference, Seattle, WA, July 1997.
Douglas E. Benner, Member AIAA
David J. Haas
Richard J. Massey
Alan C. Munger
Barry T. Neyer, Member AIAA
Miamisburg, OH 45343-0529
Barry T. Neyer
1100 Vanguard Blvd
Miamisburg, OH 45342
(937) 865-5170 (Fax)
EG&G Optoelectronics has an internally funded R&D Program designed to bring a laser ignited pyrotechnic actuator system to the market place. The design incorporates miniature lenses to efficiently focus the laser light output, from a high power EG&G Optoelectronics InGaAs laser diode, into the explosive powder. The focusing of this light into a small spot causes a rapid temperature rise in the explosive material resulting in device function. The output lens is sealed to the metal shell using DOE developed Glass-Ceramic sealing technology. In addition to providing hermeticity and a high pressure seal, the Glass-Ceramic, lens, metal shell material set presents an "Incompatibility Free" (no solders, glues, etc.) environment for the powder. The powder used in the actuator version of this device is TiH1.65/KClO4 material developed at Sandia National Laboratories and used by the DOE for many years. Ambient threshold tests (conducted on twenty two test devices at a pulse duration of 10 ms) have shown the threshold for this device to be 226 mW with a standard deviation of 12 mW. The actuators have also been successfully fired at -65°F and +176°F and produce 800 psig into a 10 cc pressure vessel. Included are the results of the development tests as well as the design features of the actuator.
Laser initiated ordnance has been a technology under development for 20 years. Most of the attractiveness of this technology has evolved from the fact that optical ordnance is insensitive to the dangers of Electromagnetic Radiation and Electrostatic Discharges (Merson and Salas 1995). In 1996 EG&G provided internal R&D funds to bring to the market place a laser ignited pyrotechnic actuator. The design goal was to build a highly reliable, efficient, laser actuated device capable of being ignited with a laser diode/100m fiber. The charge cavity of the test unit was to remain hermetic after firing. Additionally, the output of this device was to generate a pressure pulse output of at least 650 psig into a 10 cc pressure vessel.
Figure 1: SC 313 Laser Actuator
Figure 1 shows a picture of the EG&G Laser Ignited Actuator. It measures 0.625 inch in length has a 0.375-24 thread on the output end and accepts a standard SMA906 connector on the input end. The charge cavity is 0.169 inch in diameter and 0.150 inch deep. When loaded to a density of 2.1 g/cc the output charge is 118 mg.
The shell itself is manufactured as two pieces. The two halves are then laser welded to form the final shell.
The optical system is composed of two lenses. The first lens collects light from 0.22 or 0.37 NA fibers and provides near collimated light output. The second lens takes the near collimated light and focuses it to a spot less than 80 m in diameter. The input lens is made of a low loss material and is captured between the input and output shells before the laser weld is applied. The second lens is sealed to the metal shell using ceramic sealing technology developed and used by the DOE. The material set in the shell is Inconel 718, 304L Stainless Steel, or Hastelloy C276.
To determine the strength of the charge cavity seal, the unit was hydrostatically burst tested to 100,000 PSI with no leaks. To further test the strength, a test unit was built up and functioned. The unit was then helium leak tested with no leaks detected to a background leak rate of 2 x 10-10 std atm cc sec-1. The test unit was cleaned, reloaded, and refired six times. Post test helium leak tests still failed to detect a leak above the background leak rate.
Certainly one of the most important features of this device is the hermetic, "Incompatibility Free" charge cavity. No solders, welds, etc. are introduced. This type cavity prevents the degrading effects incompatible materials can have as the component ages (a problem which has plagued bridgewired and other optically initiated items).
The pyrotechnic material used in this device is a Titanium Subhydride Potassium Perchlorate blend. The material is blended in a 33:67 weight ratio and compacted to an average density of 2.1 g/cc. This particular form of titanium is impervious to static discharge and the blends made from the fuel are also insensitive. This is of particular interest in electrically fired actuators but is of lesser importance in a laser design that has no electrical conductors into the charge cavity. The material is safer to handle during manufacture of the actuators and therefore is a good choice of material. If the output of a pyrotechnic device such as the one described in this paper would be used to ignite other pyrotechnic or explosive material, then a coarser non-hydride material may be a better choice.
The pyrotechnic cavity is hermetically sealed by the use of a laser welded closure disk. The unit is helium leak checked as an optical feed through and again after the final closure disk welding.
The laser diode that has been used for the development tests is a C86155E-10, see Figure 2. This device is a product of EG&G Optoelectronics, Canada and is described in a companion AIAA paper 97-2829. The diode is housed in an eight pin DIP package and can easily be mounted on a Printed Wiring Board. The device is categorized as a 1 Watt diode however the unit used for these tests generated an output greater than 1.3 Watts with a drive current of approximately 2 Amps. The output of the diode is coupled into a 100/140 step index fiber. An SMA-906 connector was placed on the end and the fiber polished. With the addition of the drive electronics to interface with the diode (provided by EG&G Miamisburg) a complete laser ignited ordnance system is available all from within one company, EG&G.
Figure 2: C86155-E10 Laser Diode
Actuators were loaded and tested into a 10 cubic centimeter closed vessel and the pressure profiles captured. An example of one of these profiles can be seen in Figure 3. The profile contains a high frequency ring which is generated by the transducer and the passage way the transducer is mounted. This ringing quickly decays and on the average produces no useful work.
Figure 3: Actuator Pressure Output
The average (filtered) pressure developed from the actuator is approximately 800 psig over the first 5 msec. The rise time on the pressure pulse is 60 s. When functioned with 782 mW, the device functions in 353 s. (from rise of laser output to first pressure output). The data can be filtered with an analog filter that is resident in the analog pressure capturing equipment or by digital processing methods after the data is captured. In many cases the analog filters are used, but for this data the waveforms were captured without filtering.
Threshold testing, using a Neyer D-Optimal test (Neyer 1994) protocol to choose the stimulus levels, has been completed on twenty two units at ambient conditions. The results of these tests are summarized in Figures 4 and 5. Figures 4 gives the stimulus level and whether the unit fired or failed. Figure 5 shows the probability of firing at various confidence limits Vs input energy. The mean for these twenty two tests was measured at 226 mW with a standard deviation of 12 mW. The tests were conducted as follows: The energy out of the fiber was measured twice prior to connecting the fiber onto the test units. During the function test, the internal photodiode (internal to the laser diode and calibrated prior to testing) was measured. The energy out of the fiber was also measured after the test to ensure the output had not changed. The pulse duration from the laser was 10 ms. on each test.
Figure 4: Threshold Results Ambient
Figure 5: Probability Plot
The design goal for this R&D project was to bring to the marketplace a highly reliable, efficient, laser actuated device. This device meets that goal. It produces an average of 800 psig into a 10 cc pressure vessel and has a mean firing value of 226 mW. The low standard deviation also attests to the successful design. The test unit is robust and reliable. It remains hermetic even after being fired several times and has an "incompatibility free" charge cavity. This device provides the base for a family of other pyrotechnic devices tailored to customer needs. One such application could be the generation of persistent burning particles for ignition purposes. Future activities include building a detonator version of the device, as well as fine tuning a Built In Test scheme to be used in this device.
John A. Merson and F. J. Salas (1995), "Safety Analysis of Optically Ignited Explosive and Pyrotechnic Devices," Applications of Laser Initiated Ordnance Technology Transfer Workshop.