1.  Field of the Invention
The present invention relates to fire extinguishing systems and, more particularly, to an environmentally-friendly fire extinguishing bomb for aerial deployment, and programmed explosive release of its contents. 
2.  Description of the Background
Large-scale forest fires are prevalent throughout the Midwest and Western states, and significant sums are spent on firefighting equipment.  The conventional approach is aerial firefighting using fixed-wing aircraft and helicopters to drop chemicals such as water, foams, gels, or other specially formulated fire retardants.  These chemicals are dropped from large air tankers with tanks that can be filled on the ground at an air tanker base.  It has been reported that “The U.S. Forest Service and Bureau of Land Management own, tease, or contract for nearly 1,000 aircraft each fire season, with annual expenditures in excess of US$250 million in recent years.  Borate salts were used in the past to fight wildfires but were found to sterilize the soil, kill animals, and are now prohibited.  Newer retardants use ammonium sulfate or ammonium polyphosphate with a thickener.  These are less toxic but still not environmentally friendly.  Brand names of tire retardants for aerial application include Fire-Trol™ Phos-Chek™. 
In addition to toxicity, there are serious questions about the effectiveness of airtankers. 
The state of Victoria, Australia tested the effectiveness of a fleet of DC-10 Air Tankers during their wildfire season in 2009-2010, and concluded that these aircraft would not be effective in suppressing bushfires, especially in areas where the forest meets communities of relatively high populations.  This was partly because the drop cloud released by the DC-10 is not uniform, but has thick and thin sections which leave areas on the ground with insufficient coverage.  In addition, one drop impacted an Eucalyptus forest with such force that it broke off a number of trees with diameters of 4 to 10 inches.  While the researchers did not have adequate equipment to accurately determine the drop height, it was thought that the aircraft was unintentionally flying too low and the retardant was still moving forward, rather than straight down, when it impacted the forest.  Optimal dispersion without damage occurs when the drop is made straight down at 100-200 feet above the tree line, and this is difficult in an airplane.  The government was also concerned that such drops have the potential to cause serious injury should the load fall on a person. 
Rather than flying low and at slower speeds, an aircraft can drop an explosive payload from a higher altitude, and the concept of a fire extinguishing bomb for extinguishing forest fires is well known.  For example, U.S. Pat.  No.  4,344,489 to Bonaparte issued Aug. 17, 1982 shows a forest fire extinguishing projectile filled with an inert gas under pressure which is dropped into a fire and, upon impact, automatically disperses the gas. 
U.S. Pat.  No.  2,703,527 to Hansen issued Mar. 8, 1955 shows a similar fire-extinguishing bomb filled with fluid. 
U.S. Pat.  No.  6,318,473 to Bartley et al.  issued Nov. 20, 2001 shows a fire extinguishing system including a sealed and explodable container with an explosive trigger for opening the sealed and explodable container to release the fire extinguishing agents.  In use, the sealed and explodable container is placed at a base of the fire either by air dropping the container to the ground or by placing the container in the path of the fire, whereupon the container is opened with either an explosive device or by impact. 
U.S. Pat.  No.  4,964,469 to Smith issued Oct. 23, 1990 shows a device which, upon impact, will broadcast a dry material such as fire-suppressing chemicals by explosive force.  The device includes an explosive charge within a frangible rigid-wall container, a dry powder payload, and a fuse cord that ignites upon impact. 
U.S. Pat.  No.  4,285,403 to Poland provides an explosive fire extinguisher that is designed to be dropped from an aircraft into fires such as forest fires.  The device may be shock triggered on impact. 
U.S. Pat.  No.  7,261,165 to Black issued Aug. 28, 2007 shows a fire-extinguishing bomb with fire-smothering chemical and explosive charge is located inside a housing that is detonated when the housing unit impacts the ground. 
U.S. Pat.  No.  7,089,862 to Vasquez issued Aug. 15, 2006 shows a water pod that ruptures when dropped from an aircraft.  The water pod may have a barometric activated explosive that is activated at a predetermined altitude. 
Most of these prior art attempts rely on ground impact to release the fire retardant upon impact.  Only the '862 patent to Vasquez suggests a water bomb with a timed-detonation or barometric activated explosive that is activated at a predetermined delay or altitude, but no design details are given, it is no easy task to design a barometric-activated fire-extinguishing bomb. 
Consequently, there remains a need for an altitude-activated fire-extinguishing bomb that can be pre-programmed to explode at anywhere between 2-200 feet above the tree line, detonating with extreme accuracy and reliability at a preset altitude, and thereby disperse a fire suppressant or fire retardant fire retardant uniformly over a consistent area. 
There is a further need for an altitude-activated fire-extinguishing bomb as above that is substantially biodegradable such that after detonation it presents no environmental problem, or one that can be readily recovered by a GPS locator and reused, thereby increasing economy and reducing environmental concerns.