The person who was awarded a patent in 2017 for describing a system of drones that could be used to drop liquids on wildfires wrote a paper earlier this year with two other authors that claims to have determined how many drones would be needed for suppressing a small section of a fire.
The patent, #WO2017208272A1, awarded to Marco Ghio, is quite vague and does not supply any technical details. It says that instead of applying fire retardant or water in a conventional manner, a “rain” concept would be used:
Dropping small quantities of firefighting liquid or drizzling it over the fire, and its subsequent spreading on a large area instead of in a concentrated manner. This method, both theoretically and experimentally, is acknowledged as being particularly effective, whereas, on a practical level, it is effectively used in domestic and/or industrial firefighting systems.
In the United States fire retardant dropped from an approved air tanker is applied at coverage levels ranging from 1 to 9 gallons per 100 square feet, depending on the situation. It is not clear what coverage level “rain” would produce.
The patent specifies that drones would transport the liquid in removable containers. Upon returning empty to the mobile base the containers would be autonomously replaced with full containers, along with a charged battery if needed.
The drones and the other equipment would be transported in standard metal shipping containers which would be strategically positioned. The system would include “a control unit for the coordination of missions, the flight paths to be followed, and the selection of the ideal drop points optimized according to the environmental conditions.”
Details about how all of this would be accomplished are not specified.
The patent and the research paper written by Mr. Ghio, Elena Ausonio, and Patrizia Bagnerini assumes that the cargo capacity of the drones would be 5 to 50 liters (1 to 13 gallons), much less than currently carried by helicopters (up to 3,000 gallons) and fixed wing aircraft (up to 17,500 gallons) that routinely fight wildfires.
Their analysis (below) takes into account wind speed, flame length, the length of fire line to be suppressed, and the dead fuel moisture. It indicates that about 75 linear meters (246 feet) of the fire’s edge could be extinguished with 120 drones each carrying 20 liters (5 gallons) or 80 drones carrying 30 liters (8 gallons). The vegetation is assumed to be grass or brush, but not timber. The example below assumes that the wind speed is 20 km/hour (12 mph) and the dead fuel moisture is 18 percent. A moisture content of 18 percent for 1-hour and 10-hour time lag fuels is quite high for a very active wildfire. It should not be very difficult to suppress a fire under those fuel conditions.
In my opinion the most difficult part of using drones to assist firefighters would be applying the retardant or water at the exact location where it can be useful. That is difficult enough when you have good communication with ground personnel, adequate aerial supervision, and experienced highly qualified air crews in helicopters or air tankers.
I don’t think the principle of “rain” in the application of retardant or water from dozens or hundreds of drones is a thing, at least when you’re talking about drones that can just carry a few gallons of water and must have the batteries replaced every 20 minutes. The suppressant still has to be delivered in a timely manner in a quantity and at the location where it can be useful. Maybe when drones are carrying 50 to 100 gallons of water, and the technology improves for placing the retardant on target, it might be useful in very remote areas when the fire is very small, less than 1/10 of an acre, and the wind speed does not exceed 5 mph.
Rain Industries is working on an Unmanned Aerial System (UAS) that could carry up to 400 pounds of cargo, or 50 gallons of water.
Drone Amplified, the developer of the IGNIS prescribed fire system currently being used for aerial ignition, and Parallel Flight Technologies, have received a $650,000 grant from the US Department of Agriculture to support further development of a large-scale Unmanned Aerial System (UAS) for prescribed fire. Parallel says their hybrid gas/electric UAS can carry 100 pounds for up to two hours, numbers that are much larger than battery operated drones. When paired with the upgraded aerial ignition payload under development which will hold and dispense 3,500 incendiary spheres, it will have eight times the payload carrying capacity of drones being used today, and ten times the flight duration.
The paper was published by MDPI, which is food for thought.