All Department of Defense (DoD) military services use a 2.75-inch rocket system currently produced by a conventional batch method that is labor intensive and generates large amounts of waste propellant, nitroglycerin, and process water. The objective of this project was to demonstrate a lower cost manufacturing process that would reduce the amount of waste and pollution generated in the manufacture of the Mk 90 double-base propellant grain used in the Mk 66 2.75-inch rocket system. The new process explored the use of a continuous shear roll mill and twin screw mixer/extruder to reduce the propellant scrap, nitroglycerin emissions, and labor while increasing safety by utilizing remote control technology.
This project explored the use of novel processing techniques to reduce the amount of waste and emissions generated in the conventional manufacturing of 2.75-inch rocket motor grains. The new process used a shear roll mill to manufacture solventless double-base propellant pellets to provide a feedstock to a twin screw mixer/extruder. The new manufacturing process was to be evaluated relative to the conventional batch process for acceptable product performance, minimum environmental impact, and low manufacturing cost. The merits of the new continuous process were to be assessed by characterizing and comparing the product manufactured and the amount of waste and toxic air pollutants potentially generated by the twin screw process to the conventional batch process, in addition to trying to validate the cost reduction of the new technology. Standard propellant characterization tests were to be used to compare the twin screw and conventional batch products if possible. Similarly, the propellant grains manufactured by each process were to be tested to determine ballistic performance. A detailed mass balance on both processes was to document the waste and air emission reduction. A shear roll mill process model was developed by the U.S. Army Armament Research, Development, and Engineering Center to conduct an independent environmental and cost audit.
The shear roll mill process optimization study was completed, and the process model and software were successfully developed for use in the scaled up process. However, the shear roll mill process team was not able to collect the data needed to empirically validate the waste reduction and labor savings objectives. Because of technical issues with the twin-screw extruder, no acceptable grain was extruded and only a small amount of propellant was actually processed, which meant that no statistically significant data could be collected and, thus, the waste reduction and labor savings objectives also could not be validated empirically.
Continuous twin screw processing is a reduced cost technology for the manufacture of a variety of energetic materials. These lower manufacturing costs are realized as a result of reduced environmental costs, improved operating efficiency, flexible manufacturing capabilities, and enhanced operator safety. Additional work is needed to fully demonstrate and implement this technology for manufacturing Mk 90 double-base propellant grain used in the Mk 66 2.75-inch rocket system. (Project Completed – 2006)