Testing the limits of MAV defence

Two innovative MAV defence concepts using acid and nets are set to change the nature of homeland security in the US. Daniel Jensen, Chris Edlund, Brad Wheeler and Erinn Scott-Peck of the Department of Engineering Mechanics, US Air Force Academy, explain how.


In today's terrorist-saturated war zones, protecting a forward operating location from attack by a remote-controlled or autonomous micro air vehicle (MAV) carrying biological, chemical or conventional weapons has become a focus for commanders at all levels of the military.

The possibility of these attacks becoming a reality has also increased the relevance for homeland security applications within the US.

Cadets at the US Air Force Academy have completed a project that focused on designing, building and testing an unmanned system that can detect, track and defend against an airborne terrorist attack.

The solution space was limited to an air-to-air concept in which the enemy aircraft is destroyed by a 'friendly' MAV deployed from the base being attacked.

Possible solutions include using a deployable net attached to an MAV, an MAV that uses aerosol glue or acid to destroy its enemy's control surfaces, proximity explosives, and MAV-borne electronic deterrents.

The functional prototypes described demonstrate two of the most promising neutralisation capabilities.

The objectives for this project were focused on innovative, cost-effective solutions to protect US military bases and high-value civilian targets, such as major outdoor sporting events, from both conventional (explosives) and non-conventional (biological or chemical) MAV-borne threats.

Concept generation

"New unmanned system can detect, track and defend against an airborne terrorist attack."

This research included the use of a suite of concept generation techniques to produce ideas for prototyping, resulting in the SprayMAV and NetMAV concepts.

Using this concept generation suite, more than 80 unique neutralisation concepts were created.

The concepts were narrowed down using engineering decision-making techniques, and considerations such as budget and time for development.

Many of the concepts were either too big, too expensive or used technology that is not currently available. A modified Pugh method was employed to create the final ranking for the concepts.

The SprayMAV and NetMAV concepts were determined to be cost-effective designs that met all customer needs and team objectives.

The SprayMAV system

One of the primary concepts for neutralising an MAV threat was spraying acid, or some other corrosive substance, from one MAV onto another, thus degrading the aggressor's flight-control surfaces and causing it to crash. The benefits of this system include its low weight, allowing it to be carried by a wide range of MAVs, the low cost of building the system (since all components are commercial off-the-shelf) and finally, its durability.

Additionally, this system could be adapted to spray a range of liquids for varying purposes, making it versatile. However, the idea is not without its challenges. For example, the amount of liquid that can be carried by an MAV is severely limited, based on both weight and volume constraints.

Also, varying flight and weather conditions will change the way in which the acid falls and spreads out after being sprayed. Finally, there is not one perfect aqueous solution involving an acid that can sufficiently degrade every material. Each acid is capable of degrading certain materials, but no single acid can sufficiently degrade all materials within the allotted time frame. A prototype of the spray system has been developed that is compatible with various remote-controlled aircraft.

An 8in×0.75in PVC pipe was used to contain the fluid in the system. A windshield wiper fluid pump powered by a 9V battery was used to spray the acid, while the other side of the tube was capped with a combination of foam and hard plastic. This combination was lifted slightly to break the seal when the prototype was spraying, thus eliminating the issue of creating a vacuum when the liquid was sprayed. The long, thin shape of the prototype was chosen so that it could fit inside the aircraft or be attached on the exterior while creating only minimal drag. Keeping the weight of this prototype low was critical since both the total weight and the centre of gravity affect how the planes fly. This effect is enhanced as the planes are quite small.

"Assuming a 30mph flight speed, a five-second spray will cover over 200ft, providing abundant coverage."

The operations concept for the SprayMAV is generally straightforward. A spray system will be mounted onto an existing MAV platform and the system's capabilities (volumetric capacity, spray intensity, and so on) will vary depending on the host platform's payload capabilities.

Once a threat is detected by the modified bird-detection airport radar, the MAV will be launched towards the adversary. A flight path will be set so that the defending MAV will fly above the attacking MAV with a separation of 10ft-20ft. Once the MAVs are within approximately 30ft of each other, the spray system will be activated. It will continue to spray until the MAVs have flown approximately 30ft past each other.

This buffer zone compensates for imprecision in the bird-detection radar and varying weather conditions that could change the direction in which the acid flows after being sprayed.

Based on data from the bird-detection radar, there is only approximately three minutes between contact with the enemy and the time it is over its intended target, so any corrosive substance used must act within this time frame. Tests of acid effectiveness were conducted using aqua regia, a mixture of hydrochloric acid and nitric acid, as well as concentrated solutions of nitric acid and sulphuric acid. These were tested on specimens of Styrofoam, balsa wood and carbon fibre / Kevlar composite weaves. These materials were chosen based on their common use in the construction of commercial off-the-shelf MAVs. Although some degradation effects were noted, overall these results were not deemed sufficient within the time frame allowed. Finally, the team tested acetone on Styrofoam. This solution was extremely effective and required less than 10ml of the liquid to completely degrade a representative airfoil. Because approximately 80% of the commercial off-the-shelf, remote-controlled planes are made of some type of Styrofoam, acetone can be said to have reasonable applicability to the problem.

Another promising chemical to consider is perchloric acid. Research indicates that this may have quick degradation performance across a number of common materials used for MAV airfoils. However, testing perchloric acid is a very closely controlled and dangerous process. As a result, these tests were not performed.

The system can spray a continuous stream for 15 seconds. This proved to be sufficient time for several 'passes' against the target. Assuming a 30mph flight speed, a five-second spray will cover over 200ft, providing abundant coverage. The time required to disable the enemy MAV depends on the type of fluid used. In testing, acetone took approximately 30 seconds to eat through the entire airfoil.

Net gains

"Each acid is capable of degrading certain materials, but no single acid can sufficiently degrade all materials."

The NetMAV concept involves packaging a net onto the friendly MAV, and deploying it just prior to passing the enemy MAV. The enemy MAV is caught in the net and, as a result, brought to the ground.

The net used during testing was custom made and had a trapezoidal shape, with the short section that is attached to the aircraft being 6ft long. The bottom of the net is three times this length in order to facilitate a tri-fold initial folding strategy. The top length of 6ft was chosen to conform to the wingspan of the aircraft used to demonstrate the concept.

The net's square holes have a diagonal of approximately 25in. The hole size was determined based on a study of commercial off-the-shelf, remote-controlled aircraft size and payload capacity. It ensures that there is little possibility that an enemy MAV will pass through the net without being caught. Once the net has been tri-folded into a 6ft × 20ft size, it is folded using an accordion-type pattern into its stowed state. In this stowed state it has a diameter of roughly 3in.

The stowed net is then attached to the aircraft under the wings, approximately at the centre of gravity. This keeps the basic aerodynamic stability variables such as static margin intact. In-flight deployment uses a single servo under the fuselage of the aircraft to release the servo arms from the webbing that is used to hold the net. This causes the net to unfold from the wings and be dragged below the aircraft. In order to keep the 18ft edge of the net spread out after in-flight deployment, the 18ft section is divided into three sections, each 6ft in length. A very light, stiff rod (carbon fibre is the preferred material) is attached to each 6ft section. The straightening of the three 6ft sections from their tri-folded pattern is facilitated by coil springs attached between each of the three rods.

The net and deployment system weighed approximately 1lb. This was close to the payload capacity of the aircraft, but proved light enough to generate positive test results. Somewhat surprisingly, the aircraft maintained reasonable flight controllability with the net fully deployed. Using only visual flight controls, a successful capture of the enemy MAV with the NetMAV system was accomplished.

In conclusion

Commercial off-the-shelf MAVs could potentially be used by the enemy to deliver explosive, biological or chemical agents. In simulations, protecting against this threat has proven difficult. Although successful tests have been carried out using advanced weapons systems against this threat, these sophisticated weapons may not always be available when and where an enemy attacks. In light of this, a simple and reliable anti-MAV system is sought. Cadets at the US Air Force Academy have developed two potential concepts for defeating this threat. One concept uses a friendly MAV to spray acid onto the enemy MAV during flight. Initial prototypes show that spraying acetone in small quantities will significantly degrade the styrofoam wings of an enemy MAV. The other concept uses a net deployed from the friendly MAV to capture the enemy MAV. This concept also proved to be effective in initial testing.

This article was first published in our sister publication Defence & Security Systems International.

Acknowledgments

This work is partially supported by a grant from the Air Force Research Labs (AFRL/RW, Eglin Air Force Base and AFRL/RB, Wright Patterson Air Force Base). Particular thanks go to Colonel Mike Hatfield, Dr Greg Reich, Dr Mikel Miller, Major Aaron Norris and Dr Greg Parker at AFRL. In addition, we acknowledge the support of the Department of Engineering Mechanics at the US Air Force Academy. Any opinions, findings or recommendations are those of the authors and do not necessarily reflect the views of the sponsors or the US Air Force.