Perhaps the most important factor in the effectiveness of chemical weapons is the efficiency of dissemination. A variety of technologies can be used to weaponize toxic chemical agents. Munitions include bombs, submunitions, projectiles, warheads, and spray tanks. Techniques of filling and storage of munitions are important.
The principal method of disseminating chemical agents has been the use of explosives. These usually have taken the form of central bursters expelling the agent laterally. Efficiency is not particularly high in that a good deal of the agent is lost by incineration in the initial blast and by being forced onto the ground. Particle size will vary, since explosive dissemination produces a bimodal distribution of liquid droplets of an uncontrollable size but usually having fine and coarse modes. The efficacy of explosives and pyrotechnics for dissemination is limited by the flammable nature of some agents. For flammable aerosols, sometimes the cloud is totally or partially ignited (flashing) in the dissemination process. For example, explosively disseminated VX ignited roughly one third of the time it was employed. The phenomenon was never fully understood or controlled despite extensive study. A solution would represent a major technological advance.
Aerodynamic dissemination technology allows non-explosive delivery from a line source. Although this method provides a theoretical capability of controlling the size of the particle, the altitude of dissemination must be controlled and the wind direction and velocity known. Accurate weather observations can enable the attacker to predict wind direction and velocity in the target area.
An important factor in the effectiveness of chemical weapons is the efficiency of dissemination as it is tailored to the types of agent. The majority of the most potent of chemical agents are not very volatile. Indeed, the most volatile of the G-agents is GB (sarin), which has a volatility near that of water. All are nonvolatile liquids or solids at room temperature. VX is an oily liquid.
In some respects, long-range strategic weapons pose a lesser problem than short-range tactical weapons that are fired over, or in the vicinity of, one’s own forces. The agent must be dispersed within the boundary layer (<200-300 ft above the ground) and yet high enough to allow effective dispersal of the agent. This poses design problems because the ground/target detection device must be substantially more sensitive than for conventional munitions. The increased sensitivity also results in increased susceptibility to false firing due to noise, mutual interference, and electronic counter-measures (ECM).
Casualties due to premature initiation of the warhead are unacceptable in tactical weapons. Accordingly, an additional function such as a simple electrical or mechanical timer may be used to arm the height-of-burst sensor. A more recent attempt to control aerosol particle size on target has been the use of aerodynamic dissemination and sprays as line sources. By modification of the rheological properties of the liquid, its breakup when subjected to aerodynamic stress can theoretically be controlled and an idealized particle distribution achieved. In practice, the task is more difficult, but it represents an area where a technological advance could result in major munition performance improvements. The altitude of dissemination must be controllable and the wind direction and velocity known for a disseminated liquid of a predetermined particle size to predictably reach the ground and reliably hit a target.
Thermal dissemination, wherein pyrotechnics are used to aerosolize the agent has been used particularly to generate fine, inhalable clouds of incapacitants. Most of the more complex agent molecules, however, are sensitive to high temperatures and can deteriorate if exposure is too lengthy. Solids are a notoriously difficult problem for dissemination, since they tend to agglomerate even when pre-ground to desired sizes.
Dispersion considers the relative placement of the chemical agent munition upon or adjacent to a target immediately before dissemination so that the material is most efficiently used. For example, the artillery rockets of the 1950’s and early 1960’s employed a multitude of submunitions so that a large number of small agent clouds would form directly on the target with minimal dependence on meteorology. Another variation of this is multiple “free” aerial sprays such as those achieved by the BLU 80/B Bigeye weapon and the multiple launch rocket system. While somewhat wind dependent, this technique is considerably more efficient in terms of agent quantities.
Testing requirements for munitions seek to measure the efficacy of dissemination. This has been done historically on instrumented grids with samples of the disseminated material taken at known positions. The positions are assigned area values and these are integrated to determine total dosage and dose isopleths. While the technique was constantly improved, it still was crude by most standards and required numerous tests to provide useful information. Instrumental methods such as versions of light detection and ranging (LIDAR) may well be better suited to more accurate measures but without the signature of the chemical grids.
Modeling dissemination patterns for agent laydown can be an effective way to predict dispersal without physical testing. Little testing would be required given good, verified models. The problem, however, is model verification.
In World War I, canisters of chlorine were simply opened to allow the gas to drift across enemy lines. Although this produced limited results, it is indicative of the simplicity of potential means of dispersion. Although central bursters have limitations, countries usually use this method in the early stages of CW development, although it does not have to be the first one. There is sufficient open literature describing the pros and cons of various types of dissemination to dictate the consideration of all of them by a proliferant. Most countries could develop the toxic agents and adapt their standard munitions to carry the agents. It is much more difficult, however, to achieve success in effective dispersion and dissemination. Weather observation and forecasting are essential to increase the probability of effective CW dissemination and reduce the risk of injuring friendly forces.
Most countries have the capability to develop chemical weapons. Those with a well-developed military infrastructure could readily adapt existing munitions for chemical warfare. During the Iran-Iraq War, Iraq delivered mustard and tabun with artillery shells, aerial bombs, missiles, and rockets. Virtually any country or subnational group with significant resources has sufficient capability to attain the minimum capability that would be needed to meet terrorist aims. Any nation with substantial foreign military sales or indigenous capability in conventional weapons will have (or have ready access to) both the design know-how and components required to implement at least a moderate capability.
- The Militarily Critical Technologies List Part II: Weapons of Mass Destruction Technologies (ADA 330102), “Chemical Weapons Technology” – U.S. Department of Defense, Office of the Under Secretary of Defense for Acquisition and Technology, February 1998