When Bachmann and Sheehan were developing their method of manufacturing RDX, Sheehan isolated another explosive compound during the purification process. This turned out to be an even more explosive, denser molecule, cyclotetramethylene tetranitramine, now known as HMX. About 8% to 10% of the yield from the Bachmann RDX production method is HMX.
In the place of RDX’s six-sided molecule is an eight-sided molecule with the same decorating nitro groups attached. Instead of RDX’s three NO3 groups, it has four, as tetryl does.
The extra nitrogen atoms and the extra nitro group give HMX a detonation velocity of 9,100 meters per second (compare to 8,750 for RDX, or 7,570 for tetryl).
In Germany and Russia, HMX is called octogen (recall that RDX is called hexogen there). The name HMX is an obvious derivative of its close relation RDX, and there are many guesses as to what the acronym means (High Melting eXplosive, High-velocity Military eXplosive, etc.), but no actual records are available. It may have simply been a code word.
Initially, HMX was simply extracted as a by-product during the manufacture of RDX by the Bachmann method.
In 1961, Canadian chemist Jean-Paul Picard patented a method of making HMX directly from hexamethylenetetramine (US2983725). Previous methods only had yields of 50%. The new method had a yield of 85%, with purity over 90%. A drawback of the Picard method is that it is a multi-stage process, with ageing intervals, which makes it slow.
In 1964, the Indian chemists H. K. Acharya and R. T. Limaye developed a single stage process, bringing the cost down considerably.
HMX is more stable than RDX. It ignites at a higher temperature (335° Celsius instead of 260°), and it has the chemical stability of TNT or picric acid. It has a higher detonation velocity, and in Tauzl lead-block expansion tests, it expands a cavity in a lead block by 48 cubic centimeters per gram to RDX's 45.
HMX is used where its high power outweighs its expense (about $100 per kilogram). In missile warheads, for example, a smaller charge of a more powerful explosive allows the missile to travel faster, or have a longer range. It is also used in shaped charges to penetrate armor and defenses, where a less brisant explosive would fail.
A mixture of 70% to 75% HMX with TNT goes by the name Octol. It advantage is that it is castable, and yet retains the high brisance of HMX. Another mix, called OKFOL, is 95% HMX, desensitized with 5% wax, which also makes it formable, although the detonation velocity falls to 8,670 meters per second.