Dual Pulse Rocket Motor (Photo Credit: U.S. Army)

A team at the U.S. Army Research, Development and Engineering Center, Aviation and Missile, has been working to develop a new dual pulse rocket motor incorporating insensitive munitions capabilities. A dual pulse rocket motor contains propellant that is divided into multiple sections by an internal barrier, which will perforate as the second igniter fires to allow the propellant to exhaust through the same nozzle as the first propellant.

The project was born in FY13 out of a program known as the Accelerated Improved Interceptor Initiative, or AI3 Interceptor, as a potential upgrade to the baseline rocket motor. The AI3 Interceptor program was halted, causing the JIMTP funded team to change their focus and downscale. Because the team was successfully demonstrating insensitive munitions technologies that hadn't been demonstrated before, the program was encouraged to continue.

The rocket motor had to meet insensitive munitions requirements. Insensitive munitions testing is a set of hazards assessment testing for munitions. Six tests will potentially exploit vulnerabilities in a munition: testing for bullet impact, fragment impact, slow cook off, fast cook off, sympathetic reaction and shaped charge jet impact. Scores for these tests are labeled as Type reactions and range from Type I - Type VI. Each one measures a different level of reaction, ranging from a detonation reaction being Type I, to no reaction being a Type VI.

Because the JIMTP is developmental, the scores given to this rocket motor were unofficial. "Through a significant amount of design work and subscale testing, our final design culminated in Type IV reactions in fragment impact, slow cook off, and fast cook off testing. Due to program funding limitations we did not conduct the other three insensitive munitions tests. So, the Type IV reactions were great results, especially in fragment impact," said Nathan Mathis, a chemist with WDI's Energetic Materials function.

The team scored a Type IV, but because the insensitive munitions test is a systems level test there may be certain mitigations that will help the Type IV become a Type V score. Official insensitive munitions testing is focused on the final system and the manner in which it is deployed.

For a missile, this would go beyond the rocket motor to include all of the other missile components.