Journal of Scientific & Industrial Research

Vol. 64, March 2005, pp 175-180

Studies on ignition of TPE based RDX propellants by laser impulse

R R Sanghavi*, S G Sundaram, M W Kulkarni, S N Asthana & B M Bohra

High Energy Materials Research Laboratory, Sutarwadi, Pune 411 021

Received 19 July 2004; accepted 27 January 2005

Laser light exhibits specific characteristics in the energy transfer to high-energy materials (HEMs) leading to

subsequent decomposition process. A set-up was established for laser ignition. Induction time is one of the critical

parameters of the HEMs used for various applications. During this study, it was experimentally determined for TPE based

RDX-propellants, which are potential candidates for low vulnerability ammunition. Laser beam from a solid state pulsed

Nd-YAG operating at 1.06 µm wavelength was used as energy source. An interesting observation was, increase in ignition

delay on incorporation of glycidyl azide prepolymer and decrease in ignition delay on addition of ballistic modifier.

Keywords: Induction time, Threshold energy, Energy fluence, TPE based RDX propellants

IPC Code: B 23 K 26/10

Introduction

Lasers as an ignition source are significantly important for large calibre ballistic systems. Using lasers,

primers could be completely eliminated, resulting in a simplified ignition train. It can also overcome the severe

ignition problems associated with Low Vulnerable Ammunition (LOVA) propellants because of the presence of

inert binder and thermally stable cyclotrimethylene trinitramine (RDX) in the formulation. By proper positioning

of the laser, a more uniform flame-spreading event is anticipated, thereby reducing detrimental pressure gradient

within the gun chamber. Ignition is the first step in propellant combustion. It is a transient phenomenon leading

to steady-state combustion. As the propellant sample is subjected to ignition stimuli, there is an increase in the

surface temperature and the build-up of a thermal profile. This leads to decomposition of the propellant with

evolution of gaseous reaction products, which react exothermically, resulting in increase in the gas-phase

temperature and consequently the reaction rates. Laser ignition1 offered several advantages as source of radiative

heating of solid propellants. Ritchie et al2 have experimentally verified a model for CO2 laser induced ignition of

nitramine based propellants wherein ignition delay time data for a nitramine based composite propellant (XM39)

has been generated. They have predicted a gas-phase ignition model, which relies on chemical kinetic scheme.

Kuo et al3 examined the pre-ignition dynamics of RDX mixed with glycidyl azide polymer (GAP) binder

plasticized with trimethylol ethane trinitrate (TMETN) and Butane triol trinitrate (BTTN) using a CO2 laser.

They also measured ignition delay times and carried out species measurements above the propellant at ambient

pressures. Nitramine based propellants4,5

get readily ignited with oxidizer rich igniter materials.

Present study is an attempt to establish methods for laser-induced ignition of LOVA propellant samples.

Induction time and energy fluence for different thermoplastic elastomer (TPE) based RDX (LOVA) propellants

were determined experimentally. Effect of incorporation of oxygen rich oxidizer, ammonium perchlorate (AP) at

the cost of RDX, and the energetic plasticiser glycidyl azide polymer (GAP) at the cost of inert plasticizer

triacetin (TA), was also evaluated. As the effect of ballistic modifiers on ignition transient is not well

established, compositions containing 2 parts of ballistic modifiers like iron oxide, copper chromite, basic lead

salicylate + cuprous oxide + carbon black, sodium borohydride on aluminium oxide were also evaluated.

Experimental Set-up A typical experimental set up assembled for this study comprised: i) Ignition block; ii) Sample capsule; iii)

Laser unit; and iv) Delay measurement unit.

J SCI IND RES VOL 64 MARCH 2005

176

Ignition Block

The ignition block is made up of mild steel (Fig. 1). The test fixtures are designed to allow laser beam to fall

on test sample and to record the flash emitted due to sample ignition. As the sample ignition leads to generation

of pressure and evolution of gases, the block is fabricated in such a way that it effectively withstands the

pressure without damaging the optical components used in the experiments including the main laser system.

Provision is made for test sample capsule replacement after each firing without affecting the alignment. In the

main steel chamber, the laser entrance aperture is fitted on one side and on the other side socket is located, in

which the sample capsule is push fitted. The socket can be moved along the axis so that the sample can be

brought to the focus of the laser beam.

Sample Capsule

The sample was loaded into a brass holder. An inside diam (10 mm) for the sample holder is such that the

laser beam (diam, 7 mm) spot will be wholly covered by the sample. The sample (1 g) was pressed in the

capsule by tapping and applying mild pressure to get a height of 5 mm with a loading density of approx 1.8

g/cm3. For holding the sample in capsule, the other open end of the capsule is sealed off, using transparent APC

solution. The inner diam of the spout region was 2 mm and length 10 mm. The length of the spout region allows

the sample capsule to be fixed snugly to the socket.

Laser Unit

A solid-state laser unit (Nd-YAG of λ = 1.06 µm) operating in a pulsed mode (12 pulses/sec) was used.

Initially, the energy of each pulse, at the spot (where the laser beam is incident on the sample) was calibrated

using an energy metre. Each pulse had energy of 315 mJ and width of 77 µs.

Delay Measuring Unit

The delay-measuring unit (Fig. 2) uses two BPX-25 photo diodes, has response time of the order of 5-10 ns

Fig. 1 Ignition block

SANGHAVI et al.: STUDIES ON IGNITION OF TPE BASED RDX PROPELLANTS BY LASER IMPULSE

177

and gives an output of –5V. There is a built-in comparator, which prevents the unit from functioning due to

ambient light. As soon as a small fluctuation is sensed by the photocell, the comparator gives a negative output

of –5V. The start and stop terminals are connected to an “APLAB” electronic counter, which has least count

measurement of 0.1 µs. Method

Fig. 2 Pulse generation unit for delay measurement

Fig. 3 Experimental set up for LASER ignition of propellant

J SCI IND RES VOL 64 MARCH 2005

178

The laser beam from unit (Fig. 3) is taken out to an open space with the help of reflecting mirrors (100%). A

plain glass plate (2 mm thick) was kept at an angle in the path of the beam at a distance of 50 cm from the

ignition block. This enables to derive from the main beam a partially reflected beam (4%), which is made to

incident in the photocell to serve as a trigger for the start of the counter. The energy of the transmitted beam

from the glass plate at the point of

incidence on the sample surface was monitored using an energy meter. The average energy of the pulse was

computed from 5 to 6 readings. A scatter (5%), observed for each sample, can be attributed to experimental error

in making a sample capsule and sample surface reflection. Number of pulses per second is set in the console of

the laser unit. The beam diam is determined by using a fascimile impression of the beam on a carbon paper. The

flash on ignition of the sample is sensed by the second photocell, which is kept along the axis in front of the

sample (25 cm apart from the ignition block). To avoid spurious sensing of the initial reflected laser beam from

surface of the sample, a filter is mounted in front of the stop photocell, to cut off 1.06 µm wavelength. This

ensured that the stop signal is derived only from the flash of the ignition of the sample (Fig. 4).

The working of start of the delay unit is verified by using the laser beam without placing the sample in its

path. Stop functioning is verified by allowing incidence of a spurious beam on the stop photocell. Once the

working of start and stop is ensured, the sample mounted in the ignition block is subjected to laser beam. The

counter and its related accessories are kept at a sufficiently safe distance from the testing place. After every

firing, glass plate is cleaned or if necessary a new glass plate is used. The filter in front of the stop photocell is

also adequately cleaned every time. For each sample, 5 to 6 experiments are conducted and the average

induction time is arrived at. After test evaluating a sample, the other subsequent sample capsule is fixed to the

socket without disturbing the alignment of the ignition block with the laser beam. After 3-4 shots, the energy of

beam at the sample interface position is monitored. The selected propellant had basic composition of RDX

(80%) and EVA (16%) plasticized with inert plasticizer TA (4%).

Fig. 4 Photograph of ignition of sample

SANGHAVI et al.: STUDIES ON IGNITION OF TPE BASED RDX PROPELLANTS BY LASER IMPULSE

179

Results and Discussion Knowing the induction time, the threshold energy for each composition is calculated as:

ETH = Ti RE

where ETH = Threshold energy, Ti = Induction time, R = Repetition pulse rate per second of the laser system (12

in this case), and E= Energy per pulse (315 mJ in this case).

The energy fluence for each sample is calculated from laser beam diam (7 mm in this case) as:

Energy fluence = ETH/πr2,

where “r” is the radius of the laser beam.

RDX based formulations (Table 1) gave: Induction time, 27 sec; Threshold energy, 102 J; and Energy

fluence, 265 J/cm2. Although, GAP is an energetic plasticizer, its incorporation as replacement of TA resulted in

marginal increase in induction time (33.7 sec) as well as threshold energy (127 J) and energy fluence (330

J/cm2). Incorporation of oxygen rich AP at the cost of oxygen deficient RDX also lead to increase in values of

these parameters (40.8 sec, 154.3 J, 400.9 J/cm2). Ballistic modifiers brought down the induction time

tremendously to the level of 1-2 sec with a drastic reduction in threshold energy (4.8-5.8 J) and energy fluence

(13.7-14.7 J/cm2) except in case of NaBH4 on Al2O3.

Results show that the GAP does not contribute towards preignition phenomenon despite its energetic nature

and burning rate enhancement capability. AP based system requires more energy flux for ignition. This study

presents that the ballistic modifiers catalyse not only the combustion process but also the reactions occurring

during ignition transient. The effect was more pronounced with basic lead salicylate + cuprous oxide + carbon

black.

Several zones are reported3

of preignition dynamic events. The first zone (inert heating zone) starts from the

onset of laser heating to the first gas evolution from the propellant surface. The second zone exists between the

first gasification and first light emission boundaries. The third zone represents the active preignition reaction

region with the go/no-go boundary as its upper limit. Beyond the go/no-go ignition boundary, the fourth zone

usually called the self-sustained ignition zone, the propellant flame cannot exist due to dynamic extinction

following deradiation. The preignition dynamic phenomena encompass the three zones before reaching the

fourth zone. For Bh x 4 samples in a helium atmosphere3 subjected to part test gaseous species samples

Table 1 Induction time, threshold energy and energy fluence of various compositions

Sl No Composition Induction time

sec

Threshold

energy

J

Energy fluence

J/cm2

1

2

3

4

5

6

7

8

9

10

11

12

80% RDX + 16% EVA + 4% TA

80% RDX + 16% EVA + 4% GAP

60% RDX + 20% AP + 16% EVA + 4% TA

60% RDX+ 20% AP+ 16% EVA+4% GAP

60% RDX + 20% AP + 16% EVA + 4% TA + 2 parts Fe2O3

60% RDX+20% AP+16% EVA+4% GAP + 2 parts Fe2O3

60% RDX + 20% AP + 16% EVA + 4% TA + 2 parts Copper chromite

60% RDX+20% AP+16% EVA + 4% GAP + 2 parts Copper chromite

60% RDX + 20% AP + 16% EVA + 4% TA + 2 parts BLS + Cu2O +

C-black

60% RDX+20% AP+16% EVA + 4% GAP + 2 parts BLS + Cu2O + C-

black

60% RDX + 20% AP + 16% EVA + 4% TA + 2 parts NaBH4 on Al2O3

60% RDX+20% AP+16% EVA+4% GAP +2 parts NaBH4 on Al2O3

27.0

33.7

29.8

40.8

1.3

1.5

1.4

1.6

1.3

1.5

5.3

6.3

102.2

127.2

112.6

154.3

5.0

5.7

5.3

5.9

4.8

5.8

20.1

23.9

265.5

330.5

292.6

400.9

13.1

15.3

13.7

15.4

12.4

14.7

52.3

62.1

J SCI IND RES VOL 64 MARCH 2005

180

collection and analysis, the concentration of NO, N2O, C2, N2 as well as HCN decreased and that of CO2 and CO

increased when the incident energy flux was increased above 400 J/cm2. A luminous flame was observed above

the energy flux. The intensity of the flame increased with the energy flux. These observations imply that the

intermediate species NO, N2O, C2N2 and HCN were consumed in the flame, producing CO2 and CO as well as

other product species H2O, H2 and N2. Ritchie et al2 proposed that RDX decomposition follows the two general

pathways:

RDX (g) 3CH2O (g) + 3N2O (g) [Ea = 36 k cal/gmol]

RDX (g) 3HCN (g) +3/2 NO(g)+3/2 NO2 (g)+3/2 H2O(g) [Ea = 45 kcal/gmol]

The decomposition path for RDX that produces HONO and HCN (with rapid removal of HONO to form NO,

NO2 and H2O) becomes more dominant at temperatures above 600 K.

Cellulose acetate butyrate (CAB) decomposes as follows:

CAB: C15H22O8 6CH2O (g) + 3C2H2 (g) + CH4 (g) + 2CO (g)

EVA, an oxygenated alkyl polymer, is also expected to undergo similar decomposition process.

Primary flame zone is generally accepted as a reasonable source of heat feedback to the surface as follows:

NO2(g) + 5/7 CH2O(g) NO(g) + 3/7 CO (g) + 2/7 CO2(g) + 5/7 H2O (g) (Ea = 19 kcal/gmol)

The amount of heat released by this reaction can significantly increase the gas-phase temperature adjacent to

the propellant surface, leading to conductive heat feedback. At ignition point, there is a slight increase in the gas

phase composition of CH2O because of the increasing rate of decomposition of RDX.

Conclusions Ignition of TPE based RDX-propellants is possible using a laser source of pulsed energy. Induction time is

influenced by the addition of AP and energetic plasticizer. The ballistic modifiers appear to have a remarkable

influence on the induction time. Experimentally measured ignition delay time data for a nitramine-based

propellant have been compared with the predictions made with a gas-phase ignition model that relies on a

simplified chemical kinetic scheme. Increase in heat flux was accompanied with the lowering of the ignition

delay time. This supports the assumption that RDX decomposition is primarily temperature controlled. More

detailed information can be obtained by undertaking studies on radiative absorption properties of the

experimented mixtures using upgraded experimental setup.

Acknowledgements The authors acknowledge the support of Prof A W Joshi, Head, Physics Department, Dr S I Patil, Reader and

Shri Ravi Bhate, SRF, University of Pune, Pune.

References 1 Ohlemiller T J & Summerfield M, AIAA J, 6 (1968) 878-886.

2 Ritchie S J, Thynell S T & Kuo K K, Modelling and experiments of laser-induced ignition of nitramine propellants, J Propulsion

Power, 13 (1997) 367-374.

3 Kuo K K, Kim J U, Fetheroff B L & Torikai T, Preignition dynamics of RDX-based energetic materials under CO2 laser heating.

Combustion Flame, 95 (1993) 351-363.

4 Roller C, Strauss B, Downs D S, & Varney M, Proc 22nd JANNAF Combust Meet (CPIA Publication, 432, II) 1985, 377-390.

5 Harris L E, The low pressure combustion of nitramine propellant and the ignition of low vulnerability tank and artillery ammunition.

Proc 25nd JANNAF Combust Meet (CPIA Publication, 498, II) 1988, 135-141.

____________________ *Author for correspondence

Fax: 25869316