The design and testing of an impact ignited deflagration to detonation experiment

Abstract

Certain elements of the mechanisms behind the deflagration-to-detonation transition (DDT) of high explosives (HE) in weak confinement are still poorly understood. At Los Alamos National Laboratory, we have developed a piston-impact-ignited DDT experiment to promptly initiate the one-dimensional compressive burning of a column of weakly confined HE in order to explore and observe how the Type I DDT process in weak confinement varies from what has been well described when strong metal confinement is used. The experiment consists of a smokeless-powder-propellant gun and an HE-sample holder. The smokeless-powder gun is used to accelerate a right-circular-cylindrical projectile to impact and ignite a column of weakly confined HE held in the test-sample holder. The gun itself houses two diagnostics: a custom single/dual-channel optical encoder used to continuously track the position of the projectile during the informative majority of the test, and a piezoelectric pressure sensor used to observe the pressure of the propelling gas in the bore of the gun. Multiple other diagnostics can be successfully fielded on each test including high-speed video, streak imaging, and an in-house four-channel pyrometer. To study the DDT process, we completed a test series of more than 25 successful impact-ignited tests involving confined HE using this new experiment. The majority of the impact velocities observed in this series ranged from ~120-160 meters per second, usually using a projectile mass of ~19 grams. Results from the test series showed interesting behavior pertaining to Type 1 DDT of both pressed-pellet and hand-tamped-powder PETN samples. Among the most notable results from the test series was the recording of a Type 1 DDT of a column of PETN powder. The sample was hand-tamped to 75% theoretical maximum density (TMD) in an annealed-polycarbonate-confinement tube and impacted by a 19.8g projectile traveling 154 m/s. Two high-speed video cameras (Phantom v2512 color @ 271,000 images per second (ips), and Phantom v710 monochromatic @ 990,000 ips) captured all stages of the experiment including: impact, generation of compaction wave, initiation of a 1D compressive burn, and the transition from deflagration to detonation.