Airborne fire suppressant foam delivery apparatus

An airborne foam delivery apparatus is adapted to be carried as a slung bucket beneath a helicopter to deliver a high volume of fire suppressant foam for wildland firefighting. The delivery apparatus comprises a liquid tank for holding a foamable liquid. A support frame extends downward from the liquid tank to support the liquid tank above the ground during ground operations. Three nozzles are directed generally downwardly from beneath the liquid tank within a protective apron. A liquid valve is positioned between the liquid tank and the each nozzle. Each liquid valve is remotely operable to allow passage of the foamable liquid through the corresponding nozzle. A pressure regulator is connected to receive compressed air from gas containers mounted within the liquid tank, and to controllably discharge the compressed air into the liquid tank to maintain a positive regulated pressure within the liquid tank. The positive pressure expels the foamable liquid through tile nozzles when tile liquid valves are operated. An aerating screen is positioned beneath each nozzle to aerate and foam the foamable liquid as it emerges from the nozzle.

TECHNICAL FIELD 
This invention relates to an airborne foam delivery apparatus for applying 
fire suppressant foam to wildland fires. 
BACKGROUND OF THE INVENTION 
Wildland fires are a threat in many areas of the world. Effectively 
controlling and containing these fires is essential to the preservation of 
national parks, forests, and private property. Significant efforts are 
therefore directed to developing and improving wildland firefighting 
techniques. 
Wildlands are characterized by difficult and often inaccessible terrain. 
Fighting fires in such rugged terrain is difficult. In many situations, 
the only realistic access to burning areas is by air. Therefore, wildland 
firefighting efforts quite often include the application of water from 
fixed wing airplanes and helicopters. Water, as it is converted into 
steam, has tremendous capacity to absorb and carry away heat. It has the 
further advantage of being environmentally safe. In addition, adequate and 
convenient reservoirs of water can often be found near burning fires. 
Helicopters equipped with "slung buckets" can quickly load from such 
reservoirs without landing. The close proximity of a reservoir also 
minimizes round trip flying time. 
Despite the advantages of water in fighting wildland fires, water alone can 
be inefficient-especially when applied from the air. Much of the water 
evaporates before reaching the ground. Water which does reach the ground 
beads up and rolls off fuels before absorbing its full heat capacity. It 
has been estimated that water is only 5% to 10% efficient. 
Many fire retarding or suppressing chemicals are available which are more 
efficient than water. However, they are often difficult to use against 
wildland fires. One problem with such chemicals is that they are 
environmentally toxic. They are also difficult to transport to remote 
locations in large quantities. In many cases, fire retardant chemicals 
require specialized and bulky application equipment. Both the chemicals 
and the application equipment are expensive. 
Foam is one type of fire suppressant which is particularly efficient in 
terms of both effectiveness and cost. Foam fire suppressant can be created 
by different methods. However, most methods start with adding a foam 
concentrate to water in concentrations ranging from 0.1% to 1.0%. 
PHOS-CHEK.TM. fire suppressant foam concentrate is one popular foam 
concentrate, available from Monsanto Chemical Company of St. Louis, Mo. 
One reason for its popularity in fighting wildland fires is its relative 
biodegradability and its approval by necessary government agencies. 
Foam varies from being wet and runny to being as stiff as lather. The 
stiffness depends on bubble size and expansion ratio. Expansion ratio is 
the increase in volume as the water becomes foam. Applying foamable liquid 
under pressure through a standard nozzle results in a relatively low 
expansion ratio, and a somewhat runny foam. Special air-aspirated nozzles 
can result in much higher expansion ratios, producing much drier foam. 
Foam fire suppressant can be applied from standard fire engines and other 
vehicles. Foam suppresses fires by surrounding fuel with a thick layer of 
water, which does not bead up and roll off. This allows the water to 
absorb its full capacity of heat, and also allows more water to be 
absorbed into the fuel. Furthermore, a foam blanket creates a vapor 
barrier between fuel and oncoming fire. It reflects oncoming radiant heat, 
insulates fuel, continuously releases water from the foam's bubble 
structure, and helps smother the fuel. A further advantage is that 
firefighters can easily see where foam has been applied. 
Until recently, experts were skeptical of the utility of applying foam to 
wildland fires from the air. However, this skepticism has now been 
overcome, and foam concentrate is sometimes added to water applied from 
aircraft. However, because of the large volume of water which must be 
quickly dumped to effectively fight wildland fires, applying the water 
under pressure through nozzles has been impractical. Rather, the water is 
dumped from conventional slung buckets or fixed tanks. Foam is generated 
by air turbulence produced by free fall of the water. 
The foam created in this manner is relatively runny, having a low expansion 
ratio. However, the surfactant qualities of the foam concentrate, even 
without thick foam, are enough of an advantage over water alone to justify 
the addition of foam concentrate. Nevertheless. it would be desirable to 
provide thicker foam from airborne applicators. 
The most significant obstacle to creating high foam expansion ratios from 
airborne applicators is the extremely high volume of water which must be 
applied, at a very high flow rate. The need for a high flow rate is 
brought about by the speed of the aircraft. In a fixed wing airplane, 
minimum ground speeds are determined by the very nature of the aircraft. 
When a helicopter is used to apply water, downdraft or "rotor wash" from 
the helicopter rotors mandates minimum ground speeds. Helicopter rotor 
wash is a serious problem for helicopters in wildland fire fighting. It 
can quickly defeat the effects of applied fire suppressants. The ground 
turbulence from rotor wash causes the fire to expand, and can endanger 
nearby fire crews. To minimize rotor wash problems, water applicators are 
usually suspended a considerable length beneath the helicopter, such as by 
a distance of fifty feet or more. In addition, minimum ground speeds are 
maintained to reduce the rotor wash impact on any particular ground area. 
The minimum ground speeds required of aircraft when fighting wildland fires 
mandates a very high application rate of fire suppressant. The flow from a 
standard firehose nozzle is simply not adequate. However, equipment to 
produce adequately high application rates of high-expansion foam has not 
been available for airborne operation. Accordingly, most firefighting 
efforts continue to rely on standard slung buckets, which depend on 
gravity to quickly jettison their entire water loads. Foam concentrate is 
added to the water to act as a surfactant and to provide a minimal amount 
of foaming through turbulence with the air. 
Airborne firefighting effectiveness could be significantly improved if it 
were possible to apply high-expansion foam from aircraft at application 
rates approaching those achieved with standard slung buckets. To this 
date, however, there exists no practical high-expansion foam applicator 
which will achieve these rates. The invention described below fills the 
need for such an airborne foam applicator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
This disclosure of the invention is submitted in furtherance of the 
constitutional purposes of the U.S. Patent Laws "to promote the progress 
of science and useful arts." U.S. Constitution, Article 1, Section 8. 
FIGS. 1-4 show an airborne foam applicator or foam delivery apparatus in 
accordance with a preferred embodiment of the invention, generally 
designated by the reference numeral 10. Foam applicator 10 is designed for 
operation beneath a helicopter to deliver a high volume of high-expansion 
fire suppressant foam. 
Foam applicator 10 comprises a sealed or sealable liquid tank 12 for 
holding a foamable liquid such as a water and foam concentrate solution. 
Liquid tank 12 is generally cylindrical in shape, having a diameter of 
approximately 6 feet and an interior capacity of approximately 1000 
gallons. It is constructed of stainless steel to reduce interaction 
between internal tank surfaces and contained foamable liquid. Liquid tank 
12 is designed to be capable of withstanding positive internal pressures 
of at least 100 pounds per square inch. 
Liquid tank 12 includes mounting features which allow it to be connected to 
a helicopter sling, in a fashion similar to a helicopter slung bucket, for 
supporting the liquid tank from beneath the helicopter. Specifically, 
liquid tank 12 includes a support frame 14 which extends downward from 
liquid tank 12 to support it above the ground. Support frame 14 includes a 
circular upper mounting ring 16 which encircles the upper portion of 
liquid tank 12. Support frame 14 also includes a circular lower support 
ring 18. Both rings 16 and 18 have a diameter slightly larger than the 
outer diameter of liquid tank 12. Four support frame legs 20 are spaced 
about foam applicator 10, and extend vertically between upper ring 16 and 
lower ring 18. 
Support frame 14 serves several functions. For instance, upper ring 16 is 
connectable to a helicopter sling, allowing foam applicator 10 to be 
conveniently connected for operation beneath a helicopter. Support frame 
14 also serves as a pedestal or stand for supporting liquid tank 12 above 
the ground when it is not in use or when it is being refilled. In 
addition, the support frame acts as a bumper or protective guard to reduce 
damage from collision of foam applicator 10 with other objects. A 
plurality of mounting brackets 22 extend between legs 20 and liquid tank 
12 to position liquid tank 12 within support frame 14. 
A protective shroud or apron 30 extends downward from the circular 
periphery of liquid tank 12 to enclose a central nozzle area 31 directly 
beneath liquid tank 12. Apron 30 has an upper screened section 32 and a 
lower solid section 34. Upper screened section 32 is formed of a screen 
mesh to allow substantial quantities of air to pass through protective 
apron 30 and into the central nozzle area. Lower solid section 34 is 
fabricated from a material which blocks air. Protective apron 30 serves 
both as a physical guard to the enclosed nozzles, and as an air block or 
wind deflector as will be more fully described below. 
Three aerating nozzle assemblies 40 are positioned within central nozzle 
area 31. Nozzle assemblies 40 are best observed in FIG. 4, being directed 
generally downwardly from beneath liquid tank 12. The three nozzle 
assemblies are equally spaced from each other about an imaginary circle 
beneath liquid tank 12. Each of nozzle assemblies 40 includes a turret 
pipe 42 which is in fluid communication with the interior of liquid tank 
12 and which is directed generally downwardly from the bottom of liquid 
tank 12. The turret pipes are also directed slightly outward and away from 
each other, an approximately 12 degree angle from vertical. Each turret 
pipe 42 has an approximately 21/2-inch internal diameter. 
Each turret pipe 42 extends from liquid tank 12 to a liquid valve 44, 
specifically an air-actuated butterfly valve. In the preferred embodiment, 
butterfly valve 44 is a 21/2-inch "type 75" valve manufactured by 
Asahi/America of Malden, Mass. It is equipped with a pneumatic actuator 
and an electric solenoid valve 96 for remote actuation. This valve is 
capable of being fully opened in half a second. While other types of 
valves could be used, such as diaphragm and gate valves, they would not 
provide the quick response of the butterfly valve described above. The 
solenoid valve 96 accepts a 24-volt electrical signal from a helicopter 
through a control line 97 to release foamable liquid from liquid tank 12. 
A nozzle 46 extends beyond each valve 44. Each nozzle 46 has a two inch 
orifice with a 90-degree full cone projection, allowing a flow of about 
945 gallons per minute at 100 pounds per square inch water pressure. At 
this flow rate, water or foamable liquid emerges from nozzles 46 at a 
velocity of approximately 96.5 feet per second. A liquid velocity of 60 
feet per minute is a preferable minimum for obtaining optimum foam 
expansion ratios. Nozzles such as used in the preferred embodiment are 
available from Bete Fog Nozzle of Greenfield, Mass. 
One liquid valve 44 thus corresponds to each nozzle, with the liquid valve 
being operably interposed between the liquid tank and the corresponding 
nozzle. Liquid valve 44 is remotely operable between a closed position and 
an open position to allow passage of the foamable liquid through the 
corresponding nozzle. 
An aerating screen 48 is positioned beneath each nozzle 46 to aerate and 
foam the liquid emerging from the nozzle. The distance from screen 48 to 
nozzle 46 is important in achieving high foam expansion, and must be 
determined experimentally for different nozzle types and sizes. In the 
preferred embodiment, maximum expansion is obtained at a screen-to-nozzle 
spacing of about 14 inches. Each aerating screen 48 is concave or 
spherically shaped, having a radius of curvature approximately equal to 
the distance from the nozzle to the aerating screen. Individual aerating 
screens are formed in quadrants of stainless steel mesh having a wire 
spacing of from thirty to sixty wires per inch. The wire spacing in the 
preferred embodiment is forty wires per inch. Each screen extends over an 
arc of at least ninety degrees to fully encompass the cone projection of 
the corresponding nozzle. The screens are mounted to the nozzle assembly 
by sets of mounting struts. 
Liquid tank 12 has a top access hatch 50 which is about 18 inches in width 
to allow entry of a person into the tank for inspection. Access hatch 50 
incorporates a pressure safety head 52 with a rupturable disk to provide 
secondary protection against excessive internal pressures within tank 12. 
Access hatch 50 is sealable against a rim 54 with appropriate fastening 
mechanisms (not shown). 
Liquid tank 12 also has a fill port or tank coupling 55 positioned near the 
bottom of tank 12 to allow filling or refilling the tank with foamable 
liquid. Locating the fill port on or near the bottom of the liquid tank 
reduces agitation and aeration within the tank during filling. 
Three high-pressure gas containers or canisters 60 are securely mounted 
within liquid tank 12 for containing compressed gas, and preferably for 
containing compressed air. Gas canisters 60 have high pressure outputs 
which are manifolded together through a high-pressure supply line 62. 
High-pressure supply line 62 extends through the wall of liquid tank 12 to 
a pneumatic control system 64. A low-pressure supply line 66 extends back 
into liquid tank 12 from pneumatic control system 64 and upward to the top 
of liquid tank 12. A high-pressure gas coupling 68 is in fluid 
communication with pneumatic control system 64 to allow refilling or 
recharging gas canisters 60 with compressed air from an external source or 
compressor. 
Pneumatic control system 64 is shown schematically in FIG. 5, along with 
associated pneumatic and hydraulic components. As mentioned above, gas 
canisters 60 are manifolded together through high-pressure supply line 62. 
Supply line 62 is connected to a plurality of parallel pressure regulators 
82, so that the pressure regulators receive compressed air from gas 
canisters 60. Supply line 62 is also connected through a check valve 84 to 
high-pressure gas coupling 68. A pressure relief valve 86 is connected to 
supply line 62. 
At least three and possibly four pressure regulators are required to meet 
the high air flow requirements of the system. Pressure regulators 82 have 
pressure outputs which are manifolded together and connected for fluid 
communication with the interior of liquid tank 12. Regulators 82 are 
operable to controllably discharge compressed air from canisters 60 into 
liquid tank 12 and to maintain a positive regulated pressure within liquid 
tank 12. The regulators are set to produce a regulated output of about 100 
pounds per square inch. In the preferred embodiment, they must be capable, 
together, of supplying about 3000 cubic feet of air per minute at 100 
pounds per square inch. 
The pressure-regulated air is connected through a check valve 88 to a two 
position valve 90. In a first position, valve 90 allows fluid 
communication between the regulated outputs of regulators 82 and 
low-pressure supply line 66. In a second position (not shown), valve 90 
allows fluid communication between low-pressure supply line 66 and the 
ambient atmosphere, or between the interior and exterior of tank 12. 
A low-pressure relief valve 92 is also connected to the outputs of pressure 
regulators 82, and the pressure-regulated air is supplied through a check 
valve 94 to drive butterfly valves 44, which include close-coupled 
solenoid actuators 96. A 24-volt valve control line 97 is connected to 
actuate solenoids 96. 
The various control components are contained within apron 30 to be 
protected from collision with external objects. They are accessed by doors 
(not shown), with appropriate interlocks (not shown) to protect operators. 
While still providing a protective function, upper screened section 32 
allows entry of air into central nozzle area 31 to permit aeration of the 
foamable liquid. Nozzles 46 are preferably positioned just below upper 
screened section 32, within lower solid section 34. Lower solid section 34 
protects the nozzles from wind to prevent the wind from interfering with 
the interaction between the nozzles and aerating screens 48 as applicator 
10 is carried by a helicopter. 
In operation, foam applicator 10 is prepared for operation by filling 
liquid tank 12 through fill port 55 with foamable liquid, and by charging 
gas canisters 60 with compressed air. During filling, two-position valve 
90 is set in its second position to isolate the pneumatic control system 
from the liquid tank interior and to allow air to escape from liquid tank 
12. 
Filling liquid tank 12 with foamable liquid requires connecting a water 
supply line (not shown) to fill port 55 and pumping water into the tank. 
Foam concentrate is added to the water either prior to its entry into tank 
12 or by dumping it into tank 12 from access hatch 50. 
Concurrently with filling tank 12 with foamable water, an external 
compressor is connected to high-pressure gas coupling 68 to charge 
canisters 60 with compressed air. Minimum fill times are desired. 
resulting in a need for one or more high-horsepower compressors. 
The embodiment described above is designed for a working liquid capacity of 
about 900 gallons. The gas canisters are sized to displace all 900 gallons 
of liquid under a pressure of 100 pounds per square inch. To meet this 
requirement, each canister has a capacity of 300 cubic feet at 1800 pounds 
per square inch. A minimum desirable fill time is five minutes. A more 
preferable fill time is three minutes. Charging each of these cylinders to 
approximately 1800 pounds per square inch within three minutes requires an 
approximately 300 standard cubic feet per minute external compressor or 
several smaller compressors. 
Locating high-pressure canisters 60 within liquid tank 12 results in at 
least two significant advantages. First, the canisters are protected from 
collisions with external objects. Second, the heat generated within the 
canisters by rapidly charging them with compressed air is dissipated by 
the water within liquid tank 12. This warms the water, resulting in more 
efficient formation and expansion of foam. 
Once the applicator has been filled and charged, two-position valve 90 is 
returned to its first position. With the valve in this position, the 
pressure regulators are connected to maintain a positive regulated 
pressure within the liquid tank of approximately 100 pounds per square 
inch. 
Foam applicator 10 is then borne by a helicopter to a target location for 
application of foamed fire suppressant over a wildland fire. Applicator 10 
is preferably suspended by cables, designated in FIG. 1 by the reference 
numerals 11, at least 50 feet below the helicopter so that the foam 
applicator can be operated close to the ground while minimizing downdraft 
from rotor wash. When over the target, a helicopter pilot actuates a 
remote release actuator which is connected to supply a 24-volt signal 
through control line 97 to solenoids 96. This opens butterfly valves 44. 
The positive pressure within the liquid tank, regulated to 100 pounds per 
square inch, expels the foamable liquid through nozzles 46 when butterfly 
valves 44 are operated. The pressure regulators maintain the positive 
regulated pressure within the liquid tank during expulsion of the foamable 
liquid, to maintain a constant liquid velocity to aerating screens 48. The 
foamable liquid is expelled under pressure through nozzles 46 and through 
aerating screens 48. 
The desired maximum time for expulsion of all liquid from the tank is about 
30 seconds, corresponding to an expulsion rate of about 1800 gallons per 
minute. In practice, the particular selection and arrangement of 
components and regulated pressures as described above are responsible for 
achieving an expulsion time of less than 19 seconds, corresponding to an 
expulsion rate of greater than 2800 gallons per minute and a velocity of 
about 96 feet per second through the nozzle. This rate has not previously 
been achieved in a device which is practical for airborne operation. 
The use of plural nozzles is one feature of the invention which allows such 
a high expulsion rate. While it would be possible to use a single nozzle, 
with a larger orifice, this would present practical difficulties. For 
instance, a larger nozzle would be disproportionately heavier than the 
combination of three smaller nozzles. The corresponding aerating screen 
would not work as efficiently with a large nozzle to achieve high 
expansion rates. Air actuated valves would also be a problem--a 5-inch 
air-actuated butterfly valve requires four seconds to open, rather than 
the 0.5 seconds of the specified 21/2-inch valve. 
Avoiding the use of liquid pumps is another feature which makes the 
invention practical. An approximately 500 horsepower pump would be 
required to achieve flow rates of 2800 gallons per minute through the 
specified nozzles. Such a pump would be as heavy as the entire apparatus 
described above. 
The use of regulated air pressure within a pressurized liquid tank is a 
further feature which contributes to the practicality of the invention. 
This feature is in contrast to much smaller devices (such as hand-held 
fire extinguishers) which charge the entire internal volume of a liquid 
vessel to a pressure which is high enough to expel all liquid from the 
vessel. This type of charging would require that liquid tank 12 be capable 
of withstanding extremely high internal pressures--an impractical 
requirement in a 1000-gallon tank. Furthermore, it would result in very 
high initial discharge rates, tapering off to very low discharge rates as 
the tank was emptied. In contrast, the embodiment of the invention 
described above achieves a constant discharge rate as the tank is emptied, 
and requires that the tank be designed for maximum internal pressures of 
only about 100 pounds per square inch. 
The particular nozzle and aerating screen arrangement described above 
results in very high expansion of foam-much better than is achieved by 
simply opening dump gates and relying on turbulence generated by the free 
fall of water through tile atmosphere. The degree of foam expansion is 
also much higher than would be achieved by use of nozzles alone. Using 
foam concentrate in conventional slung buckets results in expansion ratios 
of 12:1. In contrast, tests have indicated that the nozzle and aerating 
screen described above, when operating at a water pressure of about 100 
pounds per square inch, are capable of producing foam expansion ratios of 
40:1. These results are achieved when using PHOS-CHEK.TM. WD 881 foam 
concentrate in a 1% solution. 
In compliance with the statute, tile invention has been described in 
language more or less specific as to structural features. It is to be 
understood, however, that the invention is not limited to the specific 
features described, since the means herein disclosed comprise preferred 
forms of putting the invention into effect. The invention is, therefore, 
claimed in any of its forms or modifications within the proper scope of 
the appended claims appropriately interpreted in accordance with the 
doctrine of equivalents.