Ensign Bickford Aerospace Company (EBAC) has over ten years of experience in the design and development of laser ordnance systems. Recent efforts have focused on the development of laser diode ordnance systems for space applications. Because the laser initiated detonators contain only insensitive secondary explosives, a high degree of system safety is achieved. Typical performance characteristics of a laser diode initiated detonator are described in this paper, including all-fire level, function time, and output. A finite difference model used at EBAC to predict detonator performance, is described and calculated results are compared to experimental data. Finally, the use of statistically designed experiments to evaluate performance of laser initiated detonators is discussed

Ewick, David W.

Graham, J. A.

Hawley, J. D.

English

NASA Technical Reports Server

N93-20147
LASER DIODE INITIATED DETONATORS FOR SPACE APPLICATIONS
D. W. Ewick, J. A. Graham, and J. D. Hawley
Ensign Bickford Aerospace Company
Simsbury, CT 06070
ABSTRACT
Ensign Bickford Aerospace Company (EBAC) has over ten years of experience in the design and
development of laser ordnance systems. Recent efforts have focused on the development of laser
diode ordnance systems for space applications. Because the laser initiated detonators contain
only insensitive secondary explosives, a high degree of system safety is achieved. Typical
performance characteristics of a laser diode initiated detonator are described in this paper,
including all-fire level, no-fire level, function time, and output. A finite difference model used at
EBAC to predict detonator performance, is described and calculated results are compared to
experimental data. Finally, the use of statistically designed experiments to evalute performance
of laser initiated detonators is discussed.
INTRODUCTION
Work on all-secondary explosive laser initiated detonators began at Ensign Bickford Aerospace
Company (EBAC) in the early 1980's. Although initial efforts were based on solid state (Nd:YAG)
laser initiation, the more recent emphasis has been in the area of laser diode initiated detonators.
The feasibility of laser diode initiation is due largely to dramatic improvements in diode output
power and, to a lesser extent, improvements in powder blending technology for laser grade
explosives.
Because they have relatively low autoignition temperatures, secondary explosives such as RDX
and HMX are excellent candidates for laser diode initiation. Since the optical absorption of these
materials at typical laser diode wavelengths is relatively poor (Ref. 1) additives such as graphite,
carbon black, and boron are used to enhance powder absorptivity and, therefore, lower detonator
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all-fire level.
PETN has been found to be susceptible to temperature cycling induced effects and should not
be considered for laser diode initiation applications. The temperature cycling problems with PETN
are not unique to laser initiation, but have also been reported with other forms of thermal ignition,
such as hot bridgewire (HBW) and semiconductor bridge (SCB) initiation.
Initial detonator designs at EBAC were based on the concept of flying plate initiation (Ref. 2). In
this approach, a confined donor explosive charge is ignited by the laser pulse and burns until
sufficient pressure has been built up to rupture a confinement disc. The ruptured disc, or flying
plate, is then propelled down a barrel by the high pressure gases produced by the donor charge
at a velocity in excess of 1 km/sec. The velocity of the flying plate is sufficient to shock initiate
a secondary explosive acceptor charge at the end of the barrel.
More recent efforts relative to laser initiated detonators have been based on the principle of
deflagration-to-detonation transition (DDT). As with the flying plate detonator, a confined
secondary explosive donor charge is ignited by the laser pulse, causing it to deflagrate and
rupture the confining disc. In this type of device, however, the "barrel" contains a relatively long
column of secondary explosive termed the "transition charge". As the high pressure gases and
the ruptured disc drive into the transition charge, the reaction transitions into a high order
detonation.
Confinement is an extremely important parameter in all-secondary explosive laser initiated
detonators. As a general rule of thumb, the more insensitive the explosive, the higher the degree
of confinement needed for reliable function.
Important design requirements for laser diode initiated detonators include all-fire and no-fire
levels, output, function time, and environmental ruggedness. Typical environmental requirements
include hot and cold temperature function, as well as temperature cycling, thermal shock,
vibration, and pyrotechnic shock. For DDT detonators, run-to-detonation distance is also an
important parameter, since it directly related to detonator size and weight, as well as margin.
248
Performance of laser diode initiated DDT detonators can be predicted and optimized using tools
such as computer modeling as advanced experimental design techniques. At EBAC, finite
difference modeling has been used to predict ignition-related aspects of detonator performance,
including all-fire and no-fire levels, and effects of temperature and laser diode pulse width.
Advanced experimental design techniques have been used to optimize detonator design
parameters, such as powder density, particle size, and column diameter.
PERFORMANCE CHARACTERISTICS
Important performance characteristics of laser diode ignited detonators include all-fire level, no-fire
level, function time, and output. All-fire and no-fire levels are determined by performing sensitivity
tests, such as Bruceton, Langlie, or Neyer (Ref. 3) and are conducted at a fixed laser diode pulse
width. The data reported in this paper are based on a pulse width of 20 ms. Detonator all-fire and
no-fire tests are normally conducted at "worst case" temperature levels, which are typically -54°C
and 74°C, respectively.
Twenty (20) each all-secondary explosive laser diode initiated detonators were used to determine
all-fire and no-fire levels. As was noted above, the laser diode pulse width was 20 ms. The
detonators were coupled to the laser diodes via a 200 I_m diameter optical fiber. The .999/95%
all-fire level was found to be 478 mW at -54°C. The .001/95% no-fire level, at 74°C, was 116 mW.
It should be noted that these tests were performed with a single SMA type fiber optic connector
between the laser diode and detonator. The all-fire and no-fire level, therefore, are actually for the
detonator plus one connector.
For electro-explosive devices (EED's) no-fire testing is normally conducted using a 5 minute
duration constant current pulse as a worse case scenario rather than the actual firing pulse, which
may be only tens of milliseconds in duration. This requirement stems from concerns relative to
potential sources of RF energy in proximity to EED's, where the firing circuit can act as an
antennae and induce an electrical current in the bridgewire. In general, a 5 minute no-fire
requirement may not be applicable to laser initiated detonators, however, it certainly represents
a conservative approach to no-fire testing.
249
Twenty additional laser diode initiated detonators were assembled and utilized in a 5 minute laser
diode no-fire test. The laser diode was operated continuously (CW) for 5 minutes while performing
this test. As before, a 200 l_m diameter optical fiber was used to couple the laser diode to the
detonator and the detonators were conditioned to 74°C. Data analysis yielded a .001/95% no-fire
level of 98 mW. Again, this value includes one connector between the diode and detonator. Note
that there is only a 15% decrease in the no-fire level between pulse widths of 20 ms and 5
minutes. This is attributable to the small spot size achieved by coupling the fiber directly to the
powder, which minimizes the effective thermal mass.
As with all thermally ignited devices, function time of a laser diode initiated detonator varies
inversely with the applied power. Twenty additional detonators were fabricated to assess the
effect of laser diode power on detonator function time. Two detonators each were fired at levels
from 300 mW to 1.2 watts, in increments of 100 mW. The tests were conducted at ambient
temperature using a 20 ms laser diode pulse and a 200 I_m diameter optical fiber. A graph of
function time versus diode power is shown in Figure 1. The line drawn through the data points
in Figure 1 is a least squares fit of the form y=a+b/x 2. As before, note that the data is based on
a single SMA connector between the laser diode and detonator.
The final performance characteristic of laser diode initited detonators to be discussed is output.
Standard EBAC test detonators include a one grain HNS-IA output charge housed in a .005 inch
thick stainless steel cup and pressed at 32,000 psi. In tests at ambient temperature with steel
witness blocks, the average dent produced by the laser initiated detonators was .012 inches.
FINITE DIFFERENCE MODELING
Because of their axial symmetry, typical laser diode initiated detonators are excellent candidates
for finite difference modeling techniques. In reference 4, the author presented a laser diode
ignition model based on a one-dimensional finite difference solution of the governing time-
dependent heat conduction equation. An improved two-dimensional model has since been
developed and used to predict the performance of EBAC laser diode initiated detonators. The
general heat conduction equation is shown in equation (1) below. Note that a cylindrical
coordinate system has been chosen and that a heat generation term has been included.
25O
@aT 1 @T 1 @2T
Oz2 _ a_ za 0_2
+ 02____T+ __q _ 1 @T
az 2 k a at
(i)
where: T = temperature
t = time
z = axial coordinate
r = radial coordinate
= angular coordinate
q = heat generation rate
k = thermal conductivity
a = thermal diffusivity
For an axially symmetric system, as is the case with typical laser diode initiated detonators,
equation (1) reduces to:
_._._T + 1 82" + ____T + (:It_ 1 aT (2)
az2 r ar az 2 k a 8t
Optical absorption of the incident laser radiation by the donor charge is handled by the heat
generation term, which may be written as follows:
q= c* "p 0
where: (_ = absorptivity of first fire mix
p = incident power density
As first order approximations, it may be assumed that thermal conductivity and heat capacity are
constant and do not vary with temperature. Also, as a first order approximation, the model does
not include a self-heating term for decomposition of the explosive material, but instead assumes
a constant autoignition temperature.
251
Equation (2) may be solved using conventional finite difference techniques. Due to the large
number of nodes needed to reasonably model a laser initiated detonator, a computer is needed
to carry out the detailed calculations.
The finite difference model has been used to model the laser diode ignition characteristics of a
fine particle HMX/Carbon Black blend. Figure 2 shows the predicted function time as a function
of laser diode power assuming a 100 I_m diameter spot size. Calculated results are shown for the
two-dimensional model described by equation (2) and also for a one-dimensional (axial only)
model. Also noted in Figure 2 for reference, are a pair of experimental data points for fine particle
HMX/carbon black (Ref. 1).
10
7
I,,,,
• 5
134
_3
l v , p , i
• 2-D MODEL
• 1 -D MODEL
[] EXPER_MENIAL DATA
, i i _ i i i I i I i I b i i i , i
I00 200 300 400 500 600 700 800 900 1 000
POWER (roW)
Figure 2
Comparison of I-D and 2-D Finite Difference Calculations
Note that the two-dimensional model predictions are in excellent agreement with the experimental
data points. In addition, note that the function times predicted by the one-dimensional and two-
dimensional models converge as the diode power increases. At relatively high power levels, the
one-dimensional results are also in reasonably good agreement with the experimental data,
however, at lower power levels the one-dimensional model underestimates the diode power
required for ignition because radial heat losses are not accounted for.
52
Another obvious limitation of the one-dimensional model is its inability to accurately predict spot
size effects. The laser diode ignition threshold for HMX/carbon black has been found to scale as
the diameter raised to the 1.4 power (Ref. 5). A one-dimensional model, however, inherently
assumes that initiation is a function of power density. In other words, the one-dimensional model
will predict that the ignition threshold varies as the diameter squared. The two-dimensional model
is in much closer agreement with experimental data, predicting that ignition threshold scales with
the diameter raised to the 1.6 power. The effect of varying the laser diode pulse duration has also
been studied using the finite difference model. Figure 3 shows the predicted ignition threshold as
a function of spot size for laser diode pulse widths of 10 and 20 ms.
1000
90O
800
7oo
600
500
)..
z 400
300
20O
1oo
0 i I , L i
0 50 450 500
• 10 ms LASER PULSE [
• 20 ms LASER PULSE _l t
i I i L i I i i i I i I I I
1 O0 1,50 200 250 300 350 400
RBER DIAMETER(mlorons)
Figure 3
Effect of Laser Diode Pulse Width on Ignition Threshold
Due to the larger thermal mass associated with a _arger spot size, the penalty of a higher ignition
threshold is reduced by using a longer pulse duration.
Finally, the finite difference model has been used to predict the effect of temperature on ignition
threshold. Figure 4 shows the predicted ignition threshold as a function of spot size for
temperatures of -54°C, 22°C, and 74°C. A 20 ms laser diode pulse was used as the basis for
these calculations.
" 53L.
1000
9OO
800
_s 7oo
500
400
300
2O0
1 oo
[ _ i _ i p T r
• +74C
• _22C
• -54c
b _ h , I a
50 450 500
i h J , I _ i , I i I a
1 O0 1 50 200 250 300 350 400
RgF.RDIAMETER(mlo_n_
Figure 4
Effect of Temperature on Ignition Threshold
EXPERIMENTAL DESIGN TECHNIQUES
The finite difference modeling discussed above addressed only the ignition characteristics of laser
diode initiated detonators. Design parameters affecting the ignition characteristics can be
optimized with the aid of the heat transfer model, however, testing is obviously required in order
to verify the model predictions.
In addition, the design of laser diode initiated deflagration-to-detonation transition (DDT)
detonators also requires optimization of those parameters affecting the transition from burning to
high order detonation.
At EBAC, advanced experimental design techniques have been used to investigate the effects
of parameters such as transition charge diameter, loading density, specific surface area, and
confinement disc thickness on the run distance to detonation and function time of laser initiated
detonators. Although the tests were conducted using a pulsed Nd:YAG laser source, the run
distance to detonation data is applicable to laser diode initiated detonators, as well. Although
EBAC considers the specific parametervalues and test results to be companyconfidential
information,a summaryof the resultsis presentedhere.
For the transitioncharge designedexperiment,a total of twenty-threetestswere conducted.
Threelevelswere consideredfor eachof the four parametersstudies,exceptconfinementdisc
thickness, for which there were only two levels. From the results, an optimized design
configuration was established via statistical analysis. In addition, a trend analyses was performed
to evaluate the effect of each parameter on the run distance to detonation. Figure 5 shows the
combined effect of charge density and specific surface area on run distance to detonation.
REFERENCES
. Ewick, D. W., et al., "Ignition of HMX Using Low Energy Laser Diodes", Proceedings of
the Fourteenth Symposium on Explosives and Pyrotechnics, San Francisco, California,
February 13-15, 1990, 14 pp.
2, Dinegar, R. T., All-Secondary Explosive Hot-Wire Devices, LA-7897-MS, Los Alamos
National Laboratory, Los Alamos, New Mexico (October 1979), 7 pp.
. Neyer, B. T., More Efficient Sensitivity Testinq, MLM-3609, EG&G Mound Applied
Technologies, Miamisburg, Ohio (October 1989), 31 pp.
. Ewick, D. W., "Finite Difference Modeling of Laser Diode Ignited Components",
Proceedings of the Fifteenth International Pyrotechnics Seminar, Boulder, Colorado, July
9-13, 1990, pp. 277-296.
1 07:
i 0
1 04-
1 03-
1 02-
i 0
D
E
N
S i
I
T
Y
0
0 98-
0 97-
0.93
0.6 0.7
1.85
0.8
MARGIN
THICKNESS = 1.4, DIAMETER = .67
1 95
1.9
1 85 1.95
19
35 1.95
1 9
0.9
2
2.05
2
I ,
I I
1.0 i.i
i i i I , I , : _,
205 2\
2.1
1
2 15
2.1 //,
I
1.2 1.3 1.4 1.5 1.6 1.7
SPECIFIC AREA
Figure 5
Combined Effect of Density and Specific Surface Area


Laser diode initiated detonators for space applications

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