$0.00

Insensitive Munitions: Where Are We Now?

Figure 1:  USS Forrestal Aircraft Carrier Fire Caused by Weapon Cookoff.

Figure 1: USS Forrestal Aircraft Carrier Fire Caused by Weapon Cookoff.

Author(s): 

Anyone involved in weapons systems development during the past 30 years should be familiar with the seemingly contradictory term “Insensitive Munitions” (IMs).  According to MIL-STD-2105D, Hazard Assessment Tests for Non-Nuclear Munitions, IMs are:

“Munitions which reliably fulfill (specified) performance, readiness, and operational requirements on demand but which minimize the probability of inadvertent initiation and severity of subsequent collateral damage to the weapon platforms, logistic systems, and personnel when subjected to unplanned stimuli.”

In other words, IMs generally describe those munitions that will not react to unintentional stimuli, such as fast or slow heating or bullet or fragment impact, in such a way as to cause catastrophic collateral damage that impairs warfighting capability.  Originating in the 1970s as a response to devastating shipboard munitions fires (such as that shown in Figure 1), the IM initiative has grown from a Navy firefighting and cookoff program to a Department of Defese (DoD)-wide effort to reduce the sensitivity of all munitions, from small-caliber rounds to large-diameter strategic missile systems.  In the past 20 years, the IM cause has taken root in other parts of the world as well, most notably in the NATO Allied Nations, driven by interoperability requirements for NATO weapons.  The establishment and continued successful operation of the NATO Munitions Safety Information Analysis Center[1] (MSIAC) at NATO Headquarters in Brussels have significantly impacted the spread of IM knowledge and technical capability among NATO member nations’ armed forces.

Many substantial improvements in munitions safety have been achieved through the combined efforts of the international weapons and energetics research communities.  New plastic bonded explosives (PBX) and melt-cast formulations have reduced or eliminated the inadvertent detonation threat for many bombs and missile warheads.  Design concepts that mitigate violent responses, such as venting systems and composite cases, have been successfully employed on rocket motors, warheads, guns, and ammunition systems.  And solid propellants for gun and rocket propulsion systems have been formulated to reduce sensitivity to thermal decomposition and shock.  As a result, the weapons portfolios of today’s military are indeed much safer than those of past decades.  MSIAC’s IM State of the Art resource captures many of the latest technology developments, as well as tracks trends in IM design maturity and performance for various munition groups.  This resource illustrates the significant and increasing number of reduced vulnerability systems that have entered service or are in development in the United States and other MSIAC member nations.[2]

Despite the undeniable advances made in IM system design, several significant challenges remain for researchers to overcome.  The Office of the Secretary of Defense (OSD) directs a robust 6.2/6.3 technology development and demonstration program, the Joint Insensitive Munitions Technology Program (JIMTP), aimed at providing solutions to these challenges.  The JIMTP is “a joint, focused science & technology program with the goal of developing and demonstrating enabling technologies so that future weapon systems can become IM compliant.”[3] JIMTP efforts are focused in five major areas, known as Munition Area Technology Groups (MATGs), including high-performance and minimum-smoke rocket propulsion, blast fragmentation and anti-armor warheads, and large-caliber gun propulsion.  Research and development (R&D) tasks, based on Program Executive Office (PEO)-identified technology gaps, cover development of new energetic and component technologies, as well as the integration of new IM technologies into system-level demonstrations.

One IM requirement that continues to challenge technologists—and generate a significant amount of debate within the international munitions safety community—is the mitigation of the response to shaped charge jet (SCJ) impact.  This threat, typical of a variety of rocket-propelled grenades (RPGs) and top-attack bomblets, is recognized as one of the most difficult IM challenges due to the high-velocity SCJ generating intense shocks in energetic materials.[4] The violence of the SCJ attack is typically scaled using a parameter called V2d, where V is the velocity of the jet and d is the jet diameter (Figure 2).

Figure 2:  Illustration Of Shaped Charge Jet Diameter (d) and Velocity (V).5

Figure 2:  Illustration Of Shaped Charge Jet Diameter (d) and Velocity (V).5  

In the NATO Standard Agreement covering SCJ attack testing, STANAG 4526 Edition 2:  Shaped Charge Jet – Munitions Test Procedures, four representative V2d values are defined to represent different sizes of shaped charges (Table 1).[4]

  Threat   Representative
V2d (mm3/μs2)
Table 1: Representative V2d Values for Different Shaped Charges.
  Top Attack Bomblet   200
  SCJ with Characteristics of 50-mm Rockeye   360
  Rocket-Propelled Grenade   430
  Anti-Tank Guided Missile   800
 

 

Issues have arisen within the munitions safety community regarding the accuracy of the V2d values in the STANAG and whether they represent the correct aggression levels for the threats identified for particular munition systems.  Test parameters and measurement techniques can have significant effects on V2d values, and SCJ impact test results.  Additionally, in some of the MSIAC member nations, the standard 50-mm Rockeye called out in the test procedure is unavailable, precluding its use in the SCJ impact testing.  Because of these and other inconsistencies and variations in testing performed by test centers within the NATO member nations, the need exists for review and modification of the relevant agreements and test requirements.

To address this situation, MSIAC held a workshop on SCJ assessment in May of this year.  At the IM and Energetic Materials Technology Symposium in October 2013, MSIAC expressed the goals of the SCJ Assessment Workshop:[6]

Development of an assessment methodology for SCJ attack, with improved understanding of reaction mechanisms

  • Recommendations for improving STANAG 4526 SCJ all-up-round test based on a sound scientific understanding
  • Identified capability gaps with recommendations on improvements
  • Exploitation of scientific understanding, small-scale tests, and modeling to support IM assessment
  • Inroads to IM assessments based on whole body of evidence approach vs.  single all-up-round test results to improve confidence in assessment.

Findings and recommendations resulting from the MSIAC workshop will be reported once the proceedings and final reports are made available.

So, where are we now with IM? Our munitions are less vulnerable to attack than ever before because of the technologies, design approaches, and implementation policies delivered by the munitions safety community.  Although challenges remain, resources are in place to address current and evolving threats and to provide designers and program managers with viable system-level approaches to achieving compliance with IM requirements.