L-165 Volumetric Explosives Part 1: Fuel/Air Explosives

July 2010
Dr Ernst-Christian Koch (Energetic Materials)

Fuel-air mixtures are powerful volumetric explosives (FAE) that were developed at the end of World War II. FAE may virtually use any particulate combustible such as cornstarch powder, metal powders, liquid hydrocarbons or boron hydrides and any type of combustible gases.

Dispersions or mixtures of these fuels with air thermally ignite at defined stoichiometry, temperature and pressure. Deflagration of such a system may transition to detonation by a number of mechanisms effecting coherent energy release e.g. (SWACER). However, transition to detonation of a fuel/air mixture requires supercritical size of the cloud appropriate stoichiometry, temperature and pressure of the system. Generally high temperatures and pressures further DDT and lead to reduction of critical size.

Likewise fuel/air mixtures may be directly initiated. The energy required to directly initiate charges correlates with stoichiometry, temperature and pressure.

Combustible metal powders mixed with vapour-phase FAE affect the detonation properties. Generally small particles increase the stable regime for detonation whereas coarse metal particles increase the critical size of the cloud. In addition fine particles may affect a delayed second detonation spike due to metal/air reaction. This delay correlates with the surface area of the metal particles and thus is small for large surface area and large for small surface area materials.

Detonation of fuels which are liquids at ambient temperature requires sufficient evaporation to create a gaseous phase that can mix with the air. Thus, small droplets with large surface area are more susceptible to initiation than big droplets. The ignition sensitivity however may be altered by addition of very sensitive fuels such as alkyl nitrates.

Metal/air dispersions show similar stoichiometry and surface area dependencies as do show liquid fuels. However, peak pressures obtained with metal/air explosives are 3 – 4 times higher than with hydrocarbon fuels.

Finely dispersed high explosives and mixtures of high explosives and oxidizers do detonate as well when dispersed in the air and yield similar velocity and pressure levels as aforementioned liquid fuel/air explosive mixtures. However, large surface area materials seem to effect rapid cooling of the gas phase and thus incidentally prevent initiation in some cases.

The blast effects from FAE are characterized by slower pressure decay as compared to conventional high explosives. This together with the volumetric character of the detonation causes the much higher destructive force typically encountered with FAE in comparison to common HE.