Energetic Materials and Explosives

Propellant, primary and secondary: not all explosives do the same job

"Explosive" is used loosely to cover three functionally different roles, and telling them apart is worth doing before anything else, because a question naming a specific compound is usually testing which role it plays.

A propellant burns rather than truly detonates, a controlled, sustained combustion that generates expanding gas to push a bullet, shell or missile stage. This is the role covered under Missile Systems and Their Classification for solid and liquid rocket fuel.

A primary explosive is extremely sensitive, detonating from a small shock, spark or flame, and is used only in tiny quantities as a detonator, to reliably start the much larger reaction in the main charge. Its entire purpose is sensitivity, not power.

A secondary explosive, which is what CL-20 and HMX are, is comparatively insensitive and stable enough to handle, transport and store safely, but releases enormous energy once properly detonated by a primary explosive's initial shock. This is the actual payload in a warhead or shell, and it is why energy density, not sensitivity, is the metric that matters for it, exactly the property covered next.

What makes an explosive "high energy"

An explosive's usefulness in a weapon is not simply about how big a bang it makes. What matters is energy density, how much destructive energy is packed into a given mass and volume. A higher energy density means a warhead can be made smaller and lighter while delivering the same effect, or the same size warhead can deliver a substantially larger effect. That single trade, size and weight against power, is the reason militaries fund entire research laboratories devoted to inventing marginally more powerful molecules.

CL-20 (hexanitrohexaazaisowurtzitane) is the clearest illustration. It is a nitramine explosive roughly 15 times as powerful as HMX, which was itself the previous benchmark high-energy explosive. India's own High Energy Materials Research Laboratory (HEMRL), in Pune, has synthesised CL-20, and it is now manufactured domestically. The practical payoff of a compound like CL-20 is exactly the trade described above: a missile's warhead can be made smaller for the same effect, which frees up mass for fuel, guidance electronics, or range.

The institution behind India's explosives programme

HEMRL, a DRDO laboratory based in Pune, is the institution most of this material traces back to. It does not simply synthesise explosive compounds in isolation; it supports the propellant and warhead needs of some of India's most prominent weapons programmes, including the Pinaka rocket system and the Pralay missile, both of which draw on HEMRL's work in solid propellants and high-energy formulations.

This is worth knowing because it connects a subject that looks like pure chemistry to a subject that looks like pure strategic hardware: Pinaka, developed by the Armament Research and Development Establishment with HEMRL's support, is a multi-barrel rocket system with the Mk-II variant reaching a range of roughly 60 km. Pralay is an indigenously developed, solid-propellant, quasi-ballistic missile, precise enough to be classed separately from the pure ballistic missiles covered elsewhere on this site, and also draws on HEMRL's propellant expertise.

Where the propellant meets the gun: ATAGS

The Advanced Towed Artillery Gun System (ATAGS), a fully indigenous 155mm/52 calibre howitzer developed by DRDO's Armament Research and Development Establishment with Bharat Forge and Tata Advanced Systems, is a useful concrete example of how the propellant chemistry above turns into fielded range. ATAGS achieves a firing range of over 40 kilometres, using a Bi-Modular Charge System that lets a single gun be loaded with different propellant charge combinations depending on the range required, rather than needing a fixed, single-purpose charge for every shot.

The gun also carries an integrated fire control system, including a Muzzle Velocity Radar that measures the actual speed of each shell as it leaves the barrel, feeding that real measurement back into the ballistics computer rather than relying on an assumed, textbook velocity. That is the same underlying idea as Doppler radar covered elsewhere: measuring the real behaviour of a fast-moving object directly, rather than estimating it.

Quick revision points

• Three roles, easily confused: propellant (sustained burn to push a projectile), primary explosive (highly sensitive, used only as a small detonator), secondary explosive (stable to handle, releases enormous energy once triggered by a primary, the actual payload). CL-20 and HMX are secondary explosives.
• What matters for a weapon is energy density, not raw explosive power, because higher energy density lets a warhead be made smaller and lighter for the same effect, freeing mass for fuel, guidance or range.
• CL-20: a nitramine explosive roughly 15 times as powerful as HMX, synthesised by India's High Energy Materials Research Laboratory (HEMRL), Pune, and now manufactured domestically.
• HEMRL supports the propellant and warhead work behind major Indian weapons programmes, including Pinaka (multi-barrel rocket system, Mk-II range roughly 60 km) and Pralay (solid-propellant, quasi-ballistic missile).
• ATAGS: fully indigenous 155mm/52 calibre howitzer, DRDO with Bharat Forge and Tata Advanced Systems, range over 40 km using a Bi-Modular Charge System for variable-range loading, with a Muzzle Velocity Radar feeding real (not assumed) shell speed into its fire control computer.
• This chapter is the chemistry underneath the hardware covered in Missile Systems and Their Classification: propellant chemistry there, warhead chemistry here.