Ignition in solid energetic materials due to electrical discharge

Fatal mishaps have been blamed on accidental ignition of solid rocket propellant by electrostatic discharge (ESD). Despite study of this problem since the first incident in 1985 involving a Pershing II motor stage, our understanding of ESD ignition remains limited. To determine the physics of electrically-induced ignition, the temperature, density, and size of electrical arc channels have been quantified during electrical discharge experiments on a composite solid propellant. This material consists of ammonium perchlorate (AP), aluminum, and hydroxyterminated polybutadiene (HTPB). To simulate the discharge in ESD scenarios involving insulated motors, a charged cable pulser provided electrical discharges with durations from 50 to 400 ns. The lowest ignition energy for these experiments was 160 $\pm$ 1.4 mJ. (Ignition was defined as establishment of a sustained reaction determined experimentally by total consumption of the sample.) Measurements from pressure transducers placed 3.18 mm from the arc channel showed no reaction occurred in the propellant during the discharge. In addition, infrared detectors measured time-to-reaction following the discharge; the shortest induction time was 5.7 $\pm$ 0.2 ms. These results indicate a relatively slow thermally-induced reaction rather than prompt ignition from a shock wave. A high-speed framing camera (2 $\times$ 10$\sp7$ frames/sec) observed expansion of the arc channel as electrical energy was deposited. A 61.8 $\pm$ 21.4 $\mu$m radius channel was observed for an energy deposition of 850 mJ over 400 ns. An analytical model was adapted to predict arc channel expansion for various discharge profiles. Spectra taken at intervals during the discharge indicate the plasma reaches an equilibrium temperature of 13,000 K. Post-test dissection of inert samples revealed the arc channel forms in the binder material around the crystalline constituents. Plasma density was estimated assuming the plasma mass corresponds to the mass of binder trapped by initial formation of the discharge channel: the value is 150 $\pm$ 66 kg/m$\sp3$. The trapped aluminum was excluded from this calculation because insufficient energy exists to evaporate it. An ignition model based on energy transport, via radiation and thermal conduction, from the plasma to the surrounding energetic constituents is proposed. A one-dimensional thermal-chemical kinetics code, XCHEM, was used to demonstrate the importance of radiation in the ignition model.