In a corner of the Mojave Desert in California, sits a naval air base called China Lake. It is the size of that key metaphor for the sizes of large areas — the state of Rhode Island. Located in a desert many miles from the dense population areas, it is the perfect place to develop and test the latest high explosive compounds.
China Lake is where CL-20 was developed.
More commonly called CL-20 for some reason (the CL is for China Lake, of course), the cage-like molecule hexanitrohexaazaisowurzitane is more powerful than HMX (9,660 meters per second detonation velocity, to HMX's 9,100), and denser, fully twice the density of water. Developed in 1986 by Arnold Neilsen, it is one of the densest organic molecules known, and has the highest energy content.
Two things give CL-20 its advantage over HMX, the previous record holder for most powerful explosive. First, it has a higher oxygen balance, so more of the carbon atoms are oxidized. In addition, the cage structure has several strained bonds. Bonds that normally would end up at 120 degrees are pushed and pulled into different angles. Like bending a bow to shoot an arrow, these strained bonds store extra energy in the molecule, above that delivered when the molecule rearranges into the reaction products. These strained bonds also make the compound denser, increasing its performance.
CL-20 is a little more sensitive than HMX (closer to PETN, one of the least stable of the modern military explosives), but it is most commonly bonded to rubbery polymers in a polymer bonded explosive, which helps to manage the sensitivity. In mixtures with HMX and TNT, it improves the performance of the other explosives, and gains some of their insensitivity.
Because of the complexity of the molecule, a view from another angle is interesting:
One of the ways to reduce sensitivity to friction and impact or shock is to co-crystallize the molecule with another, less sensitive, high explosive. In a co-crystal, molecules of one substance combine with those of another in integer combinations, such as 1:1 or 2:1. In a co-crystal of CL-20 and HMX, there are two CL-20 molecules to each HMX molecule, in a nice regular crystal structure. This reduces the detonation velocity to 9,470 meters per second, what would be expected as an average of the two explosives. However, the surprise was that the sensitivity was the same as plain HMX. The co-crystallization seems to have made the CL-20 more stable, and the HMX becomes the limiting factor. The increased density of the result makes the mixture 20% more powerful than HMX.
As a high-energy molecule, it finds uses besides as an explosive. It has been developed as an efficient solid rocket fuel, with the advantage that (because of its oxygen balance) it produces less smoke than the mixtures it replaces. Its high density also means more propellant can be packed into the missile, giving it more range and speed. As unmanned aerial vehicles get smaller, there is a premium put on the size of the arms they carry.
The current cost (in 2015) is over a thousand dollars per kilogram. This is expected to drop tenfold, into the HMX price range, when production scales up.