AIAA 95-2702, Proceedings of 31st Joint Propulsion Conference, San Diego, CA, July 1995.

USE OF VISAR TO REPLACE DETONATOR DENT TESTS

Barry T. Neyer, Member AIAA
EG&G Star City, Inc.
Miamisburg, OH 45343-0529

Current Address
Barry T. Neyer
Excelitas Technologies
1100 Vanguard Blvd
Miamisburg, OH 45342
(937) 865-5586
(937) 865-5170 (Fax)
Barry.Neyer@Excelitas.com

Abstract

The standard test procedure for determining the output of detonators is the dent block test. A detonator is placed against a metallic block of known hardness and fired. The depth of the dent produced is used as a measure of the strength of the detonator. The dent depth is a function of the integrated pressure pulse of the detonator.

In many applications, a detonator is used to detonate a next stage explosive assembly. Often times the detonation of the next assembly is governed, not by the time integral of a pressure pulse, but by the instantaneous shock pulse. A typical example of such an "instantaneous" detonator is a device that throws a flying plate. Thus, a dent block test may not adequately characterize the performance of these types of detonators.

The Velocity Interferometer System for Any Reflector (VISAR) is a diagnostic technique that is capable of measuring the velocity of a flying plate. In addition, it is also capable of measuring the instantaneous pressure pulse produced by a detonator affixed to a (transparent) witness plate.

This paper will show that the dent block test and VISAR test measure different characteristics of the output of detonators. Moreover, the output measured by a VISAR in many cases may be a more meaningful measure of the performance of the detonator than that provided by a dent test.

Technical Papers of Dr. Barry T. Neyer

Introduction

This paper will describe the application of VISAR diagnostics to the development of a new detonator, the High Voltage Detonator (HVD). The Velocity Interferometer System for Any Reflector (VISAR) (Barker and Hollenback 1972, Neyer 1987, Neyer 1993v) measures the velocity of a moving target by determining the Doppler shift imparted to a laser beam reflected from its surface. This paper will describe the VISAR, the High Voltage Detonator, VISAR measurements of the HVD, and a comparison of the VISAR with standard Dent Block measurements (DOD-E-83578A, MIL-STD-331B, MIL-STD-1576).

Description of a VISAR

The VISAR (Velocity Interferometer System for Any Reflector) is a commonly used tool for determining the velocity of rapidly moving projectiles. The system consists of a Michelson Interferometer with unequal path lengths. (See Figure 1). Light from a laser is reflected from the surface of the projectile. The reflected light has a Doppler shifted frequency:

(1)

where w0 is the laser frequency, v is the velocity of the moving projectile, and c is the speed of light. Due to the extra time delay, t, in one leg of the interferometer, the light in one beam of the interferometer has a phase shift of F=wt with respect to the other. For all reasonable velocities the ratio v/c is small, allowing the Doppler shifted frequency to be approximated by:

(2)

The interferometer determines the incremental phase shift,

(3)

by measuring the fringes, and determines the velocity by solving the above equation.

A typical VISAR setup is shown schematically in Figure 1. The figure shows the "Fixed-Cavity" design (Sweatt et al. 1991) , but the other geometries are possible. For maximum precision and to avoid ambiguities, VISAR signals are quadrature coded. An eighth wave plate is placed in one leg of the interferometer with the optical axis aligned vertically and the light entering the interferometer is polarized at 45. The vertical polarization component of the light has 90 more delay than the horizontal component. Measuring the horizontal component, proportional to cos wt, and the vertical component, proportional to sin wt, allows the experimenter to use a double argument arc tangent to determine the phase, and thus the velocity, uniquely. Most VISARs use the refinement introduced by Hemsing [1983] of recording the difference between the two output beams with a given polarization.

Figure 1: Schematic VISAR Diagram

Description of the HVD

The HVD is a hermetically sealed, glass-ceramic filled, Exploding Bridge Wire (EBW) detonator with an internally sealed EG&G spark gap. The HVD, illustrated in Figure 2, is being developed by EG&G Star City for Lockheed-Martin for use on the Titan IV launch vehicle. The HVD design is based on two other components that were made at EG&G Mound. One is the High Voltage Initiator that was made for the Navy's Trident II gas generator launch system, and is described in this conference. The other is the MC4217, an EBW that was designed by Sandia National Laboratories for the Department of Energy.

The HVD uses the glass-ceramic and spark gap design of the HVI, and the hermetically sealed EBW powder column of the MC4217. The powder column uses an initial pressing of 2-(5-cyanotetrazolato)-pentaamine cobalt (III) perchlorate (CP) and an output pellet of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX).

Figure 2: Cross Section of an HVD

The HVD is designed to throw a flyer into the next assembly, a Confined Detonating Cord (CDC). The HVD was designed to minimize the pressure on the next assembly, because the CDC was prone to wall "blow out" if the pressure was greater than 50 Kpsi. Because of the HVD design, the most important performance parameter is the velocity of the flying plate. The "best" design would have a high flyer plate velocity, while having a reduced integrated pressure pulse. Thus a diagnostic tool such as the VISAR would give a more meaningful measure of the HVD performance than a dent test.

HVD VISAR Measurements

 

Figure 3: HVD Velocity Measurement

A typical VISAR measurement on a HVD component is shown in Figure 3. The corresponding flyer velocity as a function of the flight distance is shown in Figure 4. The flyer accelerates rapidly, reaching a constant velocity of 2.8 km/s at a distance of 1.5 mm. (The oscillations found in the curve are due to a longitudinal sound wave in the flyer material.) It was established from previous work that a velocity of 1.0 km/s was needed to reliably detonate the CDC. Thus, the VISAR data was used to set the flight distance for the design at 1.5 mm. It also showed that the velocity would be sufficient at half and four times the design distance, indicating that the HVD had sufficient design margin to pass the qualification tests.

Figure 4: HVD Velocity vs Distance Measurement

Comparison of VISAR and Dent Measurements

As part of a HVD margin study, HVDs were made with various lengths of the HMX output pellet. The goal of the study was to verify that the design length of a 100 mil HMX length produced sufficient output to meet the product specification (a dent of 10 mils) with sufficient margin. Both modeling and a CP cutback study had indicated that a full detonation was achieved in the initial CP charge. The HMX cutback study was to ensure that there was sufficient HMX to allow the wave to grow into a full detonation wave in the HMX.

Table 1: Dent and VISAR Comparison

HMX Length (mils)

Average Dent (mils)

Average Velocity (km/s)

50

9.21.0

2.440.06

75

12.11.0

2.770.06

100

14.31.0

2.760.06

Table 1 shows a listing of the results of the study. Several conclusions can be drawn from the data. One conclusion is that the VISAR is more consistent, with a relative precision of approximately 3%, than the dent block tests with a relative precision of approximately 10%. Furthermore, the VISAR data shows that the flyer velocity is essentially the same at both larger HMX distances, while the Dent Depth test shows an increase in dent as a function of HMX length.

The fact that the dent depths show increasing dent with increasing HMX is not surprising. The dent block responds to a time integral of the pressure pulse. A larger quantity of HMX thus produces a correspondingly larger dent. The VISAR data, conversely, shows that the HMX has achieved full detonation. A longer column of HMX would have the same detonation characteristics, and thus throw the flyer with the same velocity.

Because the HVD is a flying plate device designed to promptly detonate the next assembly, the VISAR data are a more meaningful characterization of the function of the HVD then the Dent Depth data.

Acknowledgments

The author thanks Mike Saemish and Don Jackson of Lockheed-Martin for their efforts on the HVD program and for encouraging this work.

References

DOD-E-83578A, "Explosive Ordnance for Space Vehicles, General Specification for."

MIL-STD-331B, "Fuze and Fuze Components, Environmental and Performance Tests for."

MIL-STD-1576, "Electroexplosive Subsystem Safety Requirements and Test Methods for Space Systems."

L. M. Barker and R. E. Hollenback (1972), "Laser Interferometer for Measuring High Velocities of any Reflecting Surface," J. Appl. Phys., Vol. 43, pp. 4669-4675.

Barry T. Neyer (1987), "Velocity Interferometer System for Any Reflector (VISAR)," in Photonics: High Bandwidth Analog Applications, J. Chang, editor, Spie Vol 648, pp 301-313.

Barry T. Neyer (1993v), "VISAR Error Analysis and Enhancements," in Ultrahigh- and High-Speed Photography, Videography, and Photonics `93, Paul Roehenbeck, editor, SPIE, Vol 2002, pp. 107-115.

W. C. Sweatt, P. L. Stanton, and O. B. Crump (1991), "Simplified VISAR System," in Untrahigh- and High Speed Photography, Videography, and Velocimetry `90, L. L. Shaw, P. A. Jaanamagi, and B. T. Neyer, editors, SPIE Vol 1346, pp. 151-159.

W. F. Hemsing (1983), "VISAR: Some Things You Should Know", in High Speed Photography, Videography, and Photonics, D. L. Paisley, editor, SPIE Vol 427, pp. 144-148.