Vietnam War: Forest Fire as a Military Weapon - Paperless Archives

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The advantages of multiple ignition are shown even more clearly in Figure 4A which shows the type of fire that can
be expected for any combination of Nominal Incendiary Spacing and Intensity Factor. Operation Hot Tip, which spaced
incendiaries at nearly optimum distances, came much closer to achieving a crown fire (in fact, 17 percent of the area'did
crown out)than did the single ignitions of the U Minh forest, even though U Minh burned under weather conditions 4 times
as facorable for fire spread.
Incendiary Devices
The proper spacing of incendiary sets is much more critical to the success of forest burning operations than is the
the type of device used to produce ignition. But there are certain optimum criteria to be considered in selecting(or tdesigning)
incendiary ordnance for use over forests. These criteria differ in several respects from those used to optimize the
results of attacks on buildings or personnel. For use on structures, an incendiary needs sufficient weight and strength to
penetrate the roof (and upper floors of multi-story structures), and a suffit-iently high heat output to ignite materials within
a room even though the incendiary filling does not come into direct contact with easily ignitable material. This requires
a relatively high weight, high strength casing with a high heat output filling. For anti-personnel pumoses the weapon should
maximize radiant heat output. This requires an incendiary that produces a large volume of flame with a rapid combustion
rate and high flame temperature.
Initiation of forest fire,, is a different problem. In the outdoors, most of the convective and radiative heat from large,
intense, incendiaries i6 lost to the surroundings, and only flames in contact with the fuel produce ignition. An old and
proven ru!e of thumb is that fire will not spread in a litter fuel if it cannot be ignited by a well-placed kitchen match.
Small scattered flames are therefore much more effective than large radiant heat sources in starting fires in forest litter. As
a practical matter, ignition of forest litter is most readily accomplished by small, cluster-type incendiary weapons designed
for direct flame contact with the fuel.
Table I I lists the thermal properties of common incendiary fillings. This table shows why napalm, developed as an
anti-personnel incendiary, has been notoriously unsuccessful in starting forest fires. Not only does napalm have the lowest
burning rate per square foot of coverage of any common filling, but the material boils off from the covered surface at
temperatures below that at which it ignites.Thus, napalm produces a large volume of flame, but the flaming starts at some
distance above the fuel surface. Even PT-|, the best of the petroleum-based fillings from the standpoint of forest ignition,
has a significant fraction of volatiles whose distillation temperatures are below ignition temperature.
The ideal incendiary device for forest burning operations would be a cluster of a great many light-weight, unarmoured,
low-volatile devices. The experimental CBU53-54B series should be a great improvement over existing munitions for use
over forests
Table I I
Heat Value Ignition To Distillation TO Burning Rate Flame Tr
Agent 1000 Btu./lb. OF. Initial (OF.) End Point lb./ft. 'Min. OF.
I
Napalm B 19.0 825 95 390 .05- .2 3140
JP - 4 18.6 470 325 450 .1 - .4 2640
PT- I 13.7 510 370 680 .3 - .7 2400
Magnesium 11.8 1000 1100 - .9 -1.4 2400
White Phosphorus 10.6 34 280 .2 - .4 -
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