By Dr. J. Saji & KPS Murthy
Explosives are a broad class of materials with very high stored chemical energy that can be released in a concise duration of time of the order of a few microseconds when triggered employing mechanical, thermal or shock waves stimuli like impact, heat, friction or spark. An explosive releases a large amount of heat and gaseous products on the trigger due to a self-sustaining exothermic decomposition reaction accompanied by a shock and loud noise.
The temperature generated is in the range of 30000-50000C, and the gases produced expand 12000-15000 times than the original volume. The reaction of a high explosive is called ‘Detonation’ wherein the reaction front (detonation wave) propagates in the order of 5000-10000 m/sec.
Explosive technology development is primarily focused on enhancing the energy produced per unit and reducing the sensitivity of the munition. The energy content of the explosive dictates the munition’s lethality, and the sensitivity of the explosive determines its vulnerability. Explosives are used for constructive and destructive purposes in military and civil applications. Explosives are used in various munitions like bombs, artillery shells, torpedoes, grenades, rocket and missile warheads.
They are also used as blasting agents in mining, quarrying and construction activities etc.
Explosives are categorised by their sensitivity. Sensitive materials that can be initiated by a relatively small amount of heat or pressure are classified as primary explosives, and relatively insensitive materials are secondary explosives. Although many materials have explosive properties, very few have found practical applications due to stringent storage stability, safety, and compatibility requirements.
Primary explosives are energetic materials that possess exceptionally high initiation sensitivity to stimuli like impact, friction, electrostatic discharge, heat, and shock and very quickly detonate after initiation. A large amount of energy released upon initiation in heat or a shock wave is used to initiate the less sensitive secondary explosives. Primary explosives are ubiquitous in military munitions and commercial applications as the key ingredient in initiators such as detonators, blasting caps, and pyrotechnic percussion primer formulations.
The most common primary explosives used by the military are lead azide, which is used often in detonators and blasting caps and lead styphnate, which is commonly found in percussion primers. Since the lead content of these materials is unfavourable due to toxicological and environmental concerns, there has been a significant interest to develop lead-free new primary explosives like DBX, KDNP, NHN etc., in recent years.
Secondary explosives are relatively insensitive to mechanical shock and flame and are safer for handling, storage, and transport. Typically they have higher performance in terms of detonation velocities and detonation pressures. They explode with greater violence and offer higher fragmentation or shattering effects (brisance).
Trinitrotoluene (TNT) is the most commonly known secondary explosive today, and its use has historically been prevalent in military and civilian sectors. Since World War I, it has found widespread use and is a common ingredient in explosive mortar and artillery shells and bombs. Subsequently, RDX and HMX were synthesised and manufactured on a massive scale that was widely used post World War II.
In the last three decades, many HEMs high-performance explosives like CL-20, thermally stable explosives like TATB and FOX-7, melt-cast explosives like TNAZ, insensitive explosives like NTO and energetic binders like GAP, poly-NIMMO, and poly-GLYN for cast cure formulations have been developed for enhancing the lethality.
Despite the higher performance of CL-20, it has failed to find many applications in explosive formulations due to the costs associated with the preparation, which involves palladium catalysis. Efforts are in progress to invent metal-free syntheses of CL-20, but no successful alternative has been demonstrated to date. The explosive compositions are optimised to achieve the specified performance criteria depending on the warhead applications.
Fragmentation Warheads generate high-velocity fragments that can pierce through soft and hard targets, including metal structures. Explosive compositions with high brisance are widely used for this purpose and employed in almost all the anti-aircraft and ballistic missile defence warheads like Akash, Astra, QRSAM etc.
Blast is often associated with explosives by the general populace. The blast warheads are commonly used to create havoc in enemy areas. A blast warhead is designed to create damage by fast propagating Omni-directional shock waves. To achieve such performance, specialised metal loaded explosive compositions are developed to enhance the duration of blast wave interaction with the targets.
High Energy Materials Research Laboratory (HEMRL) of DRDO, situated at Pune, has the technological expertise built over the last five decades in this class of compositions. The products developed by HEMRL have been used in area neutralisation rockets like Pinaka, missile warheads of Prithvi, Brahmos, etc. and various aircraft bombs.
The explosive compositions produced by the laboratory have higher performance with TNT equivalence ranging from 1.3 to 1.6. A typical less sensitive composition of this category was developed by HEMRL for penetration cum blast warheads for short-range ballistic missiles, air-launched bombs, runway denial bombs etc. These munitions or bombs can penetrate through hard concrete followed by explosions leading to upheaval, rendering the runways and bridges unsuitable.
Another class of warheads known as shaped charges with metal lined conical cavities are developed, wherein a hypervelocity metallic jet is generated by high-performance explosives. The jet can penetrate high strength steel used as the armour of main battle tanks within a millisecond.
HEMRL has established the technology of explosives for high precision-shaped charges for third-generation state-of-the-art anti-tank NAG, Helina, Dhruvastra etc. Explosive composition and liner configuration have been optimised to achieve a jet velocity of 10 to 11 km/s. This explosive technology has found extension in the area of contact and stand-off torpedo warheads which use shaped charges as well as blast simultaneously.
Explosive Reactive Armour (ERA) is an add-on protection system used against anti-tank threats such as shaped charges. In ERA, the explosive sheet sandwiched between two metal plates initiates and propels the metal sheets to interact with the incoming metallic jet and reduces its penetration capability. HEMRL has developed ERA mounted on T-72 to counter shaped charge threats. The lab has recently developed and demonstrated the new generation ERA with protection capability against shaped charge warheads, KE projectiles and tandem Shaped charge threats.
The establishment of the synthesis process, followed by the manufacture of HMX at a pilot scale in HEMRL, was a major technological breakthrough, as this benchmark explosive of today was not available to the country due to various restrictions. HMX is a critical component of explosive compositions for generating high power anti-aircraft and anti-tank missile warheads.
Recently a significant milestone has been achieved, with the synthesis of the most powerful explosive of today, CL-20, without using Palladium catalyst, with the association of academia. This cage compound surpasses the power potential of HMX. It can increase the lethal power of the warhead to the extent of 10-15% and thereby will act as a force multiplier.
Today’s thrust area of research in the field of high explosives is aimed at reducing the vulnerability of warheads to accidental stimuli. Secondary explosives possessing a lower sensitivity are highly crucial for developing insensitive munitions (IM). These new classes of explosive materials shall exhibit higher thermal stability, lower sensitivity to bullet/fragment impact and minimise the unintended (sympathetic) detonation by exploding of a nearby munition or excessive heat generated by fuel fires in the vicinity.
These characteristics can be achieved by employing inert polymeric binders like hydroxyl-terminated polybutadiene (HTPB), Nitrile butadiene rubber, etc. as ingredients of explosive compositions in place of trinitrotoluene (TNT). IM compliant systems improve the safety and survivability of personnel and assets and maximise the storage capabilities and improve flexibility in logistics.
Moreover, safety during processing, handling and operation provide a high confidence level to production teams and soldiers during operations. Although such compositions were considered a prime requirement of ship/ submarine borne torpedoes, their application will proliferate to missile, rocket and bomb warheads with the increasing emphasis on low sensitive or insensitive munitions.
HEMRL has taken the lead in this direction and developed RDX and HMX based low vulnerable cast cure explosive compositions and productionised for warhead fillings of Heavy Weight Torpedo Varunastra, Sea Mine MIGM, RGB-60 and RL-140 naval rockets with TNT eqv. of 1.2 to 1.8. These warheads were found to qualify in IM tests for thermal stimuli during cook-off tests and mechanical stimuli during bullet/fragment impact.
To bridge the technological gap in the field of low sensitive and insensitive munitions, HEMRL has undertaken a program to develop HE compositions qualifying the highly insensitive detonating substances criteria. To meet the requirements, intrinsically less sensitive explosives like NTO, RS-RDX, FOX-7 and TATB are synthesised and productionised through Transfer of Technology to Indian industry.
Thermobaric explosives (TBE) are one of the recent advances in the field of high explosives which have captured the interest of scientists and users due to the changing nature of modern-day warfare and the need for maximising the lethality of warheads. The fuel-rich TBE compositions can create high blast impulse and generate thermal impulse with a duration varying from a few hundred to a few seconds. These explosive formulations are optimised to partition the energy release in multiple stages (anaerobic and aerobic), and the total energy output is significantly higher than conventional high explosive compositions.
The enhanced blast and thermal output makes TBE detonation more effective against soft targets, especially in enclosed spaces like tunnels, bunkers, field fortifications, underground structures, buildings, machine-gun posts, etc. In the recent past, TB explosive formulations were developed and demonstrated for 1000 lbs and 120 kg bombs, 81mm infantry Mortar, 105mm IFG and 120mm ammunition for MBT Arjun.
These explosives generated higher lethality with TNT eqv in the range of 1.5 to 1.7. The ongoing research efforts in this area focus on the use of new reactive fuels, innovative designs like surround (layered) charges and energetic binders to achieve TNT equivalence of more than 2.
HEMRL, Pune made a modest beginning in the late ’60s with a character of life assessment of explosive components of missile warheads. The laboratory started the development of lethal explosives during the ’70s. Over the period, excellent infrastructure for testing & evaluation is created, and sufficient production sources have been established.
During the last two decades, the laboratory has pursued high performance and lower sensitivity explosive formulations to meet the latest safety and storage standards. The laboratory has achieved a high level of expertise and self-sufficiency in the area of scientific facilities required for cutting edge research, leading to self-reliance in the area of explosive development and production.
