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

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If a lighted match is dropped on a bed of forest litter, the fire catches first in the topmost leaves or needles and then
spreads slowly outward over the surface and down into the fuel bed. Soon the fire has grown to an appreciable size and
resembles a rough circle. all aflame. Eventually, the originally ignited fuels burn out, and the fire looks more like a doughnut
with ashes in the center and unburned fuel on the outside. Since the heat ouput of the fire depends on the amount of
fuel burning at any time and since the area on fire is continually increasing the heat output increases over time until
the burned out central area becomes so large that the fire no longer behaves as a single unit Because the rate of spreadt
flame height. and other fire characteristics are largely determined by the fire's heat output, the fire will accelerate its rate
of spread and develop continually higher flames from the time of ignition until the central area burns out. Then the rate
of spread and flame height stabilize and the fire acts as a continuous line of fire rather than as a solid area or a doughnut.
What happens when two lines of fire come together? Each has its own column of hot gasses above it rising up through
a coolet. denser atmosphere. As the two fires approach, the columns of hot gas mingle, resulting in a marked increase in
convective activity above the space between the fires. The single convective chimney above the two fires causes the flames
to tilt towards each other, and to accelerate their spread. These effects are measurable when the fires are a distance equal
to 10 flame lengths apart. They bectme pronounced when the fires are 3 flame lengths apart. and the intervening distance
is bridged in a rush.
How can we achieve the maximum burning rate over any given area, or, to put it anotherway, how can we get the
maximum amount of fuel burning at one time? If fires are started too far apart, much areawill have already burned out
by the lime they meet. If fires are spaced too closely, they will join before they have reached their maximum "normal"
heat output. Ignition spacing is optimum when all fires are exactly 3 flame heights apart at the exact moment they reach
maximum normal flame height. Three variables are involved: rate of spread,veoght of available fuel, and the size of the
area to be burned. Figures 41 and 42 can be used to determine a nominal ignition spacingand a correction factor for area.
The two values, when multiplied, give the ignition spacing required to achieve maximum burning rate on any sized target area.
Actually, it is seldom logistically possible to actually set incendiaries at the optimum spacing. The number of incendiaries
varies as the reciprocal of the square of the spacing (it takes four times the number of sets to cut the spacing in half).
Figure 43 shows the increase in burning rate that can be achieved when the incendiary spacing actually used varies from
that required by the formula. Table 10 shows the calculations for three incendiary operations it Vietnam. Even on Operation
Pink Rose 11, when incendiaries were spaced at nearly six times the optimum distance, the overall burning rate was
increased by 20 percent.
Table 10
Fire Name Hot Tip Pink Rose It U. Minh (Maximum day)
Available fuel weight (lb./ft.2) 0.34 0.095 0.30
Shrub burning index 7 6 28
Target area (acres) 5,200 10,000 Not applicable
Nominal spacing (Fig. 41) 14 1.5 24
Area correction (Fig. 42) 8.5 10.0 Not applicable
Required spacing (ft.) 119 15 Not applicable
Actual spacing (ft.) 125 85 a& (single Ignition)
Actual/Required 1.05 5.7 O
Intensity Factor (Fig. 43) 4.7 1.2 1.0
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