Blast Resistant Design of Explosive Containment Chamber

Hypervelocity projectile rail guns require instantaneous electrical switching of extremely high energies. To achieve the necessary switching speed, cord explosives are used to drive strips or slats out of aluminum switch plates creating precise gaps which allow the electrical energy to arc. This paper describes the dynamic design of aluminum containment vessels for the explosive switches. The switching mechanism for the rail gun consisted of four 30" x 84" switch plates for main current diversion and two final interrupt switches to dissipate any current remaining after the test. Configuration of the electrical bus bars attached to the switches could not be interrupted which required separate half cylindrical vessels. Large magnetic fields produced by nearby inductors required that the vessels be fabricated from aluminum. Use of this material for blast loads produced special problems with fabrication and structural response which necessitated close examination of the localized stresses, especially in areas which were welded. The containment vessels were designed for a test frequency of two per week for a 5 year life. Due to the testing frequency, the vessels had to be easily removed for cleaning between shots. Cord explosive used to open the switches consisted of 8 lines of l00 gr./ft PETN. The blast pressures and fragments produced by detonation of the cord were required to be contained to protect personnel and equipment. In addition to the blast loads from the explosives, the electrical energy passing across the switch created additional blast pressures. It was found that the energy per unit length of the switch arcs approximated that of common lightning bolts. Pressure prediction formulas for this strength of energy dissipation were used to predict blast loads. The paper describes design requirements, load prediction techniques, structural analysis methods, material response, and operational considerations. Introduction Design of structures and equipment to contain the effects of explosions presents some very unique challenges to the designer, especially those involving repeated application of the load. Designs for accidental explosions are generally able to employ plastic response to absorb the load and permit at least moderate damage. Elements which are required to remain operational under repeated loads however, must remain elastic under the design load with a high degree Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE AUG 1994 2. REPORT TYPE 3. DATES COVERED 00-00-1994 to 00-00-1994 4. TITLE AND SUBTITLE Blast Resistant Design of Explosive Containment Chamber 5a. CONTRACT NUMBER