Overview:
While there is great progress being made in energetic materials research to the end of increased destructive efficacy, fuel-air explosive devices have many unique properties that make them quite effective in many specialized applications. The fuel’s ability to flow around and inside structures, as well as the fact that the combustion draws massive amounts of oxygen from the surrounding air, makes them especially effective against field fortifications and structures.
However, one of the most significant properties of the device is that it produces a longer sustained blast wave when compared to conventional explosives of similar weight. This is partially due to a higher energy/mass ratio, as the device contains nearly 100% fuel, but also due to the significant rarefaction caused by the intake of atmospheric oxygen. Fuel-air explosive are unique in this regard in that they rely on blast as their primary mechanism of destructive action, as compared to the modern trend towards metal fragment emission and shaped-charge jets. While such processes can be highly effective against hardened targets, they lack the general efficacy of a large high temperature fireball and a tremendously high impulse.
Most FAE (also called FAX) consist of three main parts: the fuel, a dispersion charge and an ignition charge. Due to the turbulent conditions in which air-dropped air-burst munitions typically detonate, there hasn’t been much investigation in the effect that the shape of the fuel cloud dispersion can have on the blast power of the device. Although, it is worth mentioning that the State Key Laboratory of Explosion Science and Technology in China has done some research in this area in their paper “Influences of the Cloud Shape of Fuel-Air Mixtures on the Overpressure Field”. In this paper, they evaluate the blast wave power by measuring overpressure, which is one of the main parameters used to evaluate the risk level of a shockwave. In this paper, the authors focus primarily on the radial dimensions of the fuel cloud and not so much on the actual shape of the cloud.
Drawing on the wavy nature of the shockwave produced by these devices, I posit that a fuel dispersion device can be used to produce clouds such that there is a series of sequential fuel ignitions that causes constructive interference of each blast wave. This could be used to produce a sum shock wave that directs energy more effectively towards the given target, increasing the devices efficacy.
In the image above, we can see the three stages involved in FAE use: deployment, dispersion and detonation. The focus of this investigation will be the second stage, and how the dispersion pattern of the fuel can be such that the constructive interference of the pressure wave(s) maximizes destructive potential in the target area.
However, there are some engineering considerations that must be made due to the fact that a synchronization of blast waves also suggests an interference of the rarefaction, which may cause issues with the oxygen intake of the combustion. This may be alleviated using multiple delayed dispersion charges so that there is an adequate separation of the explosions such that the position of the charges still result in constructive interference of the blast waves. The actual geometry of the fuel cloud that results from these dispersion charges needs to investigated to discover how constructive effects can arise. This may require a movement away from dispersion charges (which indiscriminately blasts fuel into all directions somewhat uniformly) to a dispersion device which can control where the fuel is dispersed, how much is dispersed and the rate at which the dispersion occurs. Combined with the possibility of multiple devices that begin the fuel dispersion at different times, one can cause constructive effects to come about.
One possibility of cloud shape is a series of concentric rings so that the cylindrical wavefronts of each blast coalesce.
Experimental Methods:
The first steps for this project will likely be examining how to measure the overpressure field of the device at a mini scale (to avoid a racket with the department of homeland security). This mini-scale experiment will likely involve the use of an analogue fuel (most likely an alkane), although the use of more novel materials, such as nanofuels, will likely be involved in modernizing the design further. This presupposes the design of the dispersal nozzles, making considerations regarding nozzle geometry, size and arrangement to optimize the fuel density for combustion once ignited. Using this apparatus, these design elements can be adjusted to result in the desired fuel cloud shape and the effects that these changes have on the overpressure field (and the presence or absence of blast wave interference) can be recorded. This setup won’t account for the environmental factors involved in the actual usage of the device, but these considerations can be addressed in the later stages of the project.
One possibility for this apparatus involves the use of a small seismic array, recording both relevant acoustic and seismic data. Additionally, high-speed infrared imagining may be used to measure the temperature and pressure generated by the combustion.