Sensitive Evolution of Insensitive Munitions

Published Indrastra Global 24 Dec 2019

Everyone was stunned by the explosion and the shrapnel that hit all about the island…. Both twin agent units [full of flame-retardant foam] were knocked out. Hoses flopped about wildly, geysering spumes of foam and salt water. Men were on fire, the wounded moved feebly, the dead were still.

                                                Michael Joe Carlin, USS Enterprise, January 1969

Background- In the 1950s the US Navy felt the need to develop rocket propellants with a high specific impulse for use in its new weapons, the new propellants had some characteristics of the high explosives (TNT-Trinitrotoluene or 2-methyl-1,3,5-trinitrobenzene) being used in the warheads at that time. Also, with the introduction of faster aircraft, the missiles/rockets carried by them were subjected to aerodynamic heating and at times led to an unintended explosion of the missile warhead. To overcome these issues pressed plastic bonded explosives PBX were developed for use in the warheads. This composition comprised of HMX 8 (in use in nuclear weapons) with a thermoplastic binder. The PBX came with fitment issues and could not be utilised for large munitions. After that, PBXN-101 was developed which had polyester thermosetting plastic binder and HMX, and it was introduced in the warheads by 1956. PBXN-101 was castable, but it was sensitive to shock, impact and heat. The castable PBX main charges of the underwater weapons were having a few characteristics of the propellants used in the rocket motors. Both MK-46 and MK-48 torpedo warheads had PBXN-103 composition. The underwater explosive characteristics of these warheads were 50% more than that of TNT, RDX, Aluminium-based explosive fillings. The interesting point is that as developments proceeded, both the main charge and the rocket propellant compositions had some common characteristics probably because many members of the development teams were common to both projects.  

The main focus of the US Navy’s explosive development program up to the late 1960s was to develop explosives with high energy density to impart maximum damage to the target. Since safety during manufacture, storage and transport were catered for, accidents were considered as isolated cases, and therefore there was no direction to develop ‘less’ sensitive munitions.

Accidents Involving Munitions- A supercarrier of the US Navy, the USS Forrestal was positioned off Vietnam and poised to carry out bombing runs on 29 July 1967 when a Zuni rocket on an F-4B Phantom got launched due to an electrical malfunction and struck an external fuel tank of an A-4 Skyhawk. This resulted in a large-scale fuel fire on the deck exploding two bombs in quick succession, this, in turn, led to rupturing of fuel tanks of other aircraft and explosions of bombs in the vicinity. The fire killed one hundred thirty-four sailors, 161 were injured, and more than twenty aircraft were lost. USS Forrestal took two years to be repaired at the cost of $72 million. The Zuni 5-inch Folding-Fin Aircraft Rocket (FFAR), is a 5.0 in (127.0 mm) unguided rocket developed by the Hunter-Douglas Division of Bridgeport Brass Company. The propellant used in the Zuni rocket motor was a double-base solid propellant comprising of nitro-glycerine and nitrocellulose. The Zuni rocket had a Composition B warhead (60 per cent RDX and 40 per cent TNT mixed with wax) that was prone to cooking off when exposed to roughly 350 degrees of heat.       

In another incident involving the ammunition cargo ship SS BADGER STATE on 26 December 1969, bomb pallets broke loose during a storm in North Pacific, and a bomb exploded. This led to the sinking of the ship with 5336 long tons of munitions. The warheads of the munitions had been filled with explosive compositions such as TNT/ammonium nitrate/aluminium, TNT/RDX/aluminium /wax and TNT/aluminium.

The US Navy and Coast Guard suffered sixteen in-bore premature projectile explosions from 1965 to 1973, leading to deaths of 59 personnel and injuries to many others apart from extensive material damages. Further, there were four flight deck accidents during 1966 and 1988, leading to deaths of two hundred and twenty sailors and injuries to over seven hundred. The aircraft lost or severely damaged were ninety-six in number. It cost the US Navy $1.3 billion in material damages alone.

Insensitive Munition Development- These incidents, among others brought to home the stark realities of dangerous living in confined spaces on warships. The magazines and the living spaces of officers and sailors are in close vicinity. The requirement to sail for long periods at distances far away from home ports required maximising explosives and ammunition quantities on board. Accidental fires and initiation of explosives could lead to a severe loss of men, material, and the Warship itself.             

Serious thought began to be given to design and development of munitions that were safe not only during initiation but also those that would be insensitive to sympathetic detonation, shrapnel penetration, bullets, accidental drops and fires. The warships also had increased usage of aluminium and foam, which were prone to catching fire quickly; the above waterline magazines were more susceptible to enemy action. Thus, began the era of Insensitive munition development spearheaded by the US Navy which rechristened its Explosives Advanced Development EAD program as the Insensitive Munitions Advanced Development IMAD program. The IMAD included the complete round of munition, unlike the EAD program which covered only the high explosive components.

At this juncture, it would be appropriate to look at some definitions taken from the MIL-STD 2105 D.

Explosive- An explosive is a solid or liquid energetic substance (or a mixture of substances) which is in itself capable, by chemical reaction, of producing gas at such temperature, pressure, and speed as to cause damage to the surroundings. Included are pyrotechnic substances even when they do not evolve gases. The term explosive includes all solid and liquid energetic materials variously known as high explosives and propellants together with the igniter, primer, initiation, and pyrotechnics (e.g., illuminant, smoke, delay, decoy, flare, and incendiary) compositions.

Explosive device– Is an item that contains explosive material(s) and is configured to provide quantities of gas, heat, or light by a rapid chemical reaction initiated by an energy source usually electrical or mechanical.

Munition– is an assembled ordnance item that contains explosive material(s) and is configured to accomplish its intended mission.

Munition subsystem– An element of an explosive system that contains explosive material(s) and that, in itself, may constitute a system.

Insensitive munitions (IM)– Munitions which reliably fulfil (specified) performance, readiness, and operational requirements on demand but which minimise the probability of inadvertent initiation and severity of subsequent collateral damage to the weapon platforms, logistic systems, and personnel when subjected to unplanned stimuli.

Explosive Response descriptors:

Type I (Detonation reaction)-. The most violent type of munition reaction where the energetic material is consumed in a supersonic decomposition.

Type II (Partial detonation reaction)- It is the second most violent type of munition reaction where some of the energetic material is consumed in a supersonic decomposition.

Type III (Explosion reaction)– It is the third most violent type of munition reaction with the sub-sonic decomposition of energetic material and extensive fragmentation.

Type IV (Deflagration reaction)- It is the fourth most violent type of munition reaction with ignition and burning of confined energetic materials which leads to a less violent pressure release. 

Type V (Burning reaction)- It is the fifth most violent type of munition reaction where the energetic material ignites and burns non-propulsively. 

Naval Munitions 

Warheads-There is an almost unending requirement to develop new and more powerful explosives to fill in the warhead of a weapon; however, it is complicated because of the different end uses of the weapons which require different characteristics of the explosive material. A generic explosive compound cannot, therefore, meet the stipulated safety or Insensitive criterion in such cases. For instance, an explosive is required to generate very high fragmentation velocity for the anti-aircraft role, whereas for underwater target a large volume generation is needed and for the guns, a different kind is required for armour piercing role. The behaviour of a munition not only depends upon the explosive fillings in it but also upon the design and material of the casing in which it is filled which in turn is a function of the role for which the munition is designed. Thus, an explosive may be IM compliant in a particular munition, say a large calibre shell, but may not meet specific IM requirements in a different munition like a bomb. All these munitions must meet the IM criterion as well as safeties in production, transportation and storage. As an example, a general-purpose explosive PBXN-109 was developed for both fragmentation and blast effects, and it was qualified for filling in the MK 83 GP bomb; however, it could not fulfil the sympathetic detonation criterion. The explosives like PBXN-103, PBXN-105 and PBXW-115 developed for underwater weapons while more than doubling the lethality volume could not meet all of the stipulated IM criteria. The explosives for use in anti-air weapon warheads gave good results in fragment impact tests and fast cook-off tests, but newer weapons of the likely adversaries produce higher fragment velocities under which the anti-air warhead explosives would fail. Similarly, IM explosives are required to be developed for metal accelerating characteristics needed for the shaped charges for armour piercing role. 

The above discussion brings to fore the fact that the development of IM is a complex issue which does not have simplistic solutions like the reduction of the sensitivity of the explosive filling of the warhead. Improvements in IM compliance is a continuous process and better and better solutions would have to be found be it for the main charge, the booster, the initiator, the casing or the fundamental design of the munition in totality. However, keeping the national security aspects, the weapons have to continue with the existing fillings until the qualified Insensitive High Explosives (IHEs) are available.

Propellants- Propellants are also explosives but not in the same category as the main charges of the warhead. The propellants constitute the filling of the rocket motor of the missile or a rocket. The propellants are required to impart maximum energy possible to the rocket motor while remaining safe to an external stimulus. These are also required to be compliant with IM standards.

Insensitive Munitions

At this juncture, it would be appropriate to look at the advantages accrued by incorporating IM in the weapons. It would be apparent that IM provide much better safety than hitherto through reductions in the probability of accidental initiation as well as reductions in response if and when the IM does get accidentally initiated. The most significant benefit lies in the promise of the IM to reduce the cost of human lives due to accidents involving munitions both in times of peace and war. However, a cost-benefit analysis can never estimate the cost of human life since it is an intangible which can at best be semi-quantified. The loss of a combatant during the war, even though an acceptable consequence, could lead to a tactical loss whereas during peacetime it would affect the morale of the community. IM promises to reduce both such risks arising due to accidents and human errors.

On the operational side, IM lead to denser concentrations of munitions and higher resilience and fighting back capabilities in case of an attack on the same. Reduced loss of lives and material due to enemy action or accidents. Less collateral damage to man and material during handling and transportation accidents. Easier logistics, through reduced life-costs, making more efficient use of storage facilities, more flexible movement and handling.

Taking a holistic view of the requirements of the IM for all services the US Congress on 12 December 2001, codified it into law. The law requires that “The Secretary of Defense shall ensure, to the extent practicable, that munitions under development or procurement are safe throughout development and fielding when subjected to unplanned stimuli”. 

Insensitive Munitions Threats and Response Requirements that have to be met by a munition have been outlined in JSP 520 Part 2, Vol 11 (V4.2 15 July) and those specific to Serial Potential Threats IM Response Requirement include:

Fast Heating (Magazine, Store, Aircraft or Vehicle fuel fire) -No response more severe than Type V (Burning)

Slow Heating (Fire in Adjacent Magazine, Store or Vehicle) -No response more severe than Type V (Burning)

Bullet Impact (Small Arms Attack) -No response more severe than Type V (Burning)

Fragment Impact (Fragmenting Munition Attack) -No response more severe than Type V (Burning)

Sympathetic Reaction (Most severe reaction of the same munition in magazine, store, aircraft or vehicle) -No propagation of reaction more severe than Type III (Explosion)

Shaped Charge Jet Impact (Shaped Charge Weapon Attack) -No response more severe than Type III (Explosion)

Qualification of Explosives- Certification of explosives/energetic materials and munitions as safe and suitable for use is known as a qualification. All energetic materials have to be ‘Qualified’ for use during development, improvement and other test and trial programs, further, for their incorporation into Operational Weapon Systems they need to be “Final (Type) Qualified”. Qualification process requires that the energetic material should undergo pre-laid down tests including those required for certification as IM, after that, the energetic material is tested for safety in the munition it is intended to be used. The Final (Type) Qualification is specific to the end-use/ application of the energetic material and is not a general clearance for use in all munitions. Extensive documentation regarding tests conducted, results obtained and waivers granted if any, forms the backbone of the Qualification process. To fine-tune the Qualification process further, the US requires that “Insensitive Munitions Threat Hazard Assessment” (IMTHA) be provided to the developer. The IMTHA is a guideline to the developer regarding likely hazards along with their frequency of occurrence that a particular type of munition may encounter during its service use. The IMTHA should include benign components as well as components having energetic materials, and it should focus on the differences in the test conditions and likely conditions it may encounter during its service life. IMTHA should be as detailed as feasible, and include, for example, the likely damage caused by the munition on the enemy warship as well as likely damage caused due to causative initiation of munition and fuel storage on board the enemy ship. Thus, a well-defined IMTHA would enable the design and development team to maximise achieving of IM status for a specific munition during its service life. NATO publication AOP-39′ Guidance on The Assessment and Development of Insensitive Munitions (IM)’, provides guidelines on design considerations and techniques that should be applied to a munition to reduce its vulnerability to the threats and make it IM compliant to NATO standards.

Design Aspects

Warhead Case Design-Generally, warhead casings for use in the Navy are designed for penetration before the explosion or fragmentation. The casings are thick, and the designed thickness for their role also acts as a protection for the explosive filling against impinging shrapnel or fragments. Developers are exploring new materials for the casing as well as the casing cum liner combination for shock attenuation and achieving the IM characteristics. There are issues with thermal expansion in warheads because they are sealed & degradation in explosives or liner can cause voids to expand or lead to cracks in the filling, to address this issue there are designs with stress risers which are being attempted. The stress riser would enable the casing to open up before the explosive content is initiated. Warheads with dual casings are also being developed wherein the sensitive and powerful explosive composition is contained in the inner casing and the less sensitive explosive forms the outer layer. Fuses and boosters also form an essential part of the warhead, there have been cases when warheads have been initiated by the fuse or the booster or in combination due to an undesired external stimulus. The fuse functions on satisfactory completion of a set of designed conditions and activate a primary explosive initiator which in turn initiates the high explosive charge or the booster. The booster charge enables detonation of the main charge. In addition to the safeties in arming the fuse, the initiator is kept misaligned from the booster charge till all conditions for main charge detonation are met. The Fuse – Booster subsystem is also a part of the IM development program. The IM developments are focussing upon reducing the probability of inadvertent or accidental initiation of the fuse-booster chain as also safe ignition of the new insensitive explosives. Booster casing materials, multi boosters, embedded cones in the main charge, slapper detonators, and shaped charge initiation methods are currently being researched.

Propellant case designs-Unlike the warhead casing and its explosive filling, the rocket motor casing is much thinner and thus more susceptible to impact from fragments, however, the propellant is milder composition and leads to massive scale fire instead of detonation. Various designs of rocket motor casings using different materials and case venting techniques are being developed which can prevent shock, impact and cook-off. 

Current Research

Insensitive High Explosives IHE entered the Navy with the PBX-101 composition, and it has been modified or upgraded regularly depending upon the role of the warhead. IMX-101 has been approved for use in place of TNT filled charges in the US Military. Triaminotrinitrobenzene (TATB) has also emerged as a leading IM composition in addition to fire & shock-resistant plastic/polymer-bonded explosives, TEX, nitroguanidine, & FOX-7. TATB does not detonate when burned in a fire or impacted with fragments. It is also noteworthy that nuclear devices also make use of IHEs like PBX-9502 and LX-17-0. The new IHEs are now classified as “extremely insensitive detonating substances” (EIDS). However, these explosives are very difficult to initiate by the standard fuse-booster chain, and therefore current research is focussing upon new types of main charge initiation systems. 

Ordnance Systems Inc. (OSI) of BAE Systems has developed Ordnance Systems Explosive-Common Ammunition Newfill (OSX-CAN) utilising 2, 4-dinitroanisole (DNAN) and 3-nitro-1, 2, 4-triazole-5-one (NTO) along with other ingredients. It has performed satisfactorily in the fragment attack, slow- and fast-heating, sympathetic detonation, and shaped charge attack tests for IM.

Propellants-A large number of propellants are being developed. These include among others: Minimum Smoke Propellant based on ammonium nitrate (AN) oxidiser in a ballistically modified smokeless casting powder; Minimum Smoke Propellants based on hydroxylammonium nitrate (HAN) replacing AP.227; Minimum Smoke Propellant based on a crosslinked nitrocellulose binder system plasticised with nitrate esters in a castable double base (CDB) propellant; Reduced Smoke Propellant based on replacing HTPB with hydroxy-terminated polyether (HTPE) binder.

Disposal of IM-The newer IMs are tough to dispose of using traditional techniques of Explosives Ordnance Disposal (EOD). IMs which can resist shaped charge jets need new methods for destroying the unexploded ordnance. The problem with traditional methods of EOD using plastic explosives leaves explosive residues or un-burned IHE in large quantities. New methods using high power lasers, thermite torches, and heavy shaped charges are being explored for EOD of IMs. 


An attempt has been made above to provide the reader with a glimpse into the complex evolutionary world of Insensitive Munitions. US Navy has been spearheading the IM movement which has now found its place in all the major arsenals of the world. The calibrated approach adopted by the militaries to replace the existing high explosive fillings only after IM qualified fillings are available has ensured the battle-readiness of the forces at all times. IMs would keep developing in the future and sooner than later all the munitions would be filled with insensitive compositions. In India, it is understood that the DRDO laboratories are undertaking pioneering work and the Indian Navy has started incorporating IHEs in some of its weapons.

(Acknowledgement: The author is indebted to the extensive research and writings of Raymond L Beauregard, Dr R.E. Bowen, Ernest L. Baker, Anthony R. Di Stasio, Nancy Gray, Patrick Brousseau, Andrew Victor, and Marcin Nita among others)