RPG Defense System
The Key Problem
To be effective in real-world combat scenarios an active RPG defense system must be capable of aiming and launching countermeasures within 100 milliseconds (100ms) from the time of threat detection. According to sources with combat experience, the RPG is most typically a “100-yard weapon”. Adversaries know that to be effective, the RPG needs to be fired at close range. The RPG-7’s maximum flight velocity is around 300 meters per second, so assuming roughly 100 yards to target, it could take as little as 300 milliseconds from launch to impact. Given that any (aimed) active defensive system must detect the inbound threat, classify it, process a response, and aim and launch the countermeasure before the threat makes contact; a realistic system design goal would require the aiming mechanism to be capable of addressing a threat from any direction in 100 milliseconds or less. There is presently no traditional “gun and gimbal” turret system that can aim this rapidly. The basic physics of the 100ms aiming requirement would seem to rule out any traditional gun-pointing system regardless of development efforts. For fast-moving threats, a fundamentally new approach to aiming and launching active countermeasures is needed.
A Fundamentally New Approach
For RPG defense, a continuously-rotating countermeasure aiming and launching system provides a much faster response than a traditional “gun and gimbal” turret system. Many active RPG countermeasures are self-propelled munitions. They do not require robust gun barrels for launch. They may be launched from short tubes that are little more than storage capsules. Self-propelled munitions also do not produce significant recoil. These factors enable the concept of a novel, fast aiming-and-launch system in the form of a rotating drum. A drum-shaped countermeasure launch container houses multiple munitions, each in a short launch tube, in an outwardly-facing circular array. The circular array lies on a plane perpendicular to the drum’s axis. Given that the launch tube is only about as long as the munition itself, this circular array could be realized in a drum-shaped launcher whose overall diameter is slightly larger than twice the length of an individual countermeasure round. Rotating the drum about its axis creates a situation in which any point, or multiple points, on a planar azimuth of rotation are continuously brought into the line of fire of the munitions in the array. Any point within a full 360 degrees can be targeted in the time that it takes to rotate the nearest-available munition (array element) to that point. In one example, 8 countermeasure munitions are arranged in the outward-facing 360 degree circular array with munitions spaced equally every 45 degrees. In this example the drum must rotate (at most) 45 degrees in order to bring the nearest munition to any point around a 360-degree azimuth. Depending upon the rotational speed of the drum, the time required to bring munitions to bear could be on the order of milliseconds. The preceding description provides a rotational turret capable of firing 8 countermeasures. However a drum-shaped launcher suggests the possibility of superposing (stacking) multiple arrays of countermeasures. So a single drum-type launcher is capable of firing 16 or more defensive munitions prior to reloading.
A continuously rotating drum-type RPG countermeasure launcher is capable of aiming to any point on the compass in 100 milliseconds or less. For example, the above-described drum launcher, rotating at a speed of 100 RPM, could align a countermeasure munition to a firing point at any angle on a 360 degree azimuth in about 75 milliseconds or less:
A typical household ceiling fan rotates at about 300 RPM. A drum-type RPG countermeasure launcher that rotates at the speed of a ceiling fan would theoretically address any point on an azimuth in 25 milliseconds or less. This is the amount of time it takes an RPG to cover about 7 meters.
A 25 millisecond interval is also roughly equivalent to the time it takes for a Mach 2+ missile like the FIM92A Stinger to travel 18 meters. This suggests the possibility that a rotating drum-type countermeasure system may be developed to defend against supersonic missiles, MANPADS, anti-tank missiles and IR, wire-guided or other high-velocity battlefield missiles. Finally, the 25 millisecond interval is also roughly equivalent to the time it takes for a bullet to travel 20 meters. This suggests the possibility that a rotating drum-type countermeasure system may be developed to defend against small arms fire such as snipers or automatic weapons. Of course, these would require rapid means of detecting incoming threats, but such means are within the development scope of technologies such as LIDAR and IR detectors.
Inherent Advantage for Detecting Threats
A rotating countermeasure launcher provides an inherent advantage for threat detection systems. A laser triangulation sensor could simply be affixed to the top of the rotating launcher. This would create a LIDAR system (when coupled to the rotation of the drum) which continuously sweeps the threat area for signs of an incoming threat. The use of LIDAR technology for threat detection would presumably benefit from the massive worldwide R&D activity that is currently focused on LIDAR for use in autonomous vehicles.
An IR detector(s) could also be affixed to the rotating drum. Both laser and IR sensor technologies operate at speeds which would easily achieve the mission response time objectives, taking several thousand samples per second. This scenario is further aided by the fact that any real threat will have signal characteristics that are relatively easy to discriminate from the surrounding environment. That is, anything moving at less than about 400mph and/or not toward the sensor can be ignored. This relieves the system of processing just about everything except incoming rockets and bullets. The system could not be falsely triggered by throwing a rock or firing slingshots or arrows toward it.
This unitized approach also has practical benefits in terms of reduced controls and component needs, reduced cost and complexity, and enhanced serviceability. A unitized system would be highly portable, for bolt-on deployment with a variety of platforms.
Inherent Advantage for Defeating Threats
A rotating countermeasure launcher would offer a higher probability of successfully defeating RPGs and other missile threats, than any other system. Because it stores a large number of countermeasure munitions and delivers them very rapidly, the Rotating RPG Defensive System would be capable of launching several countermeasures into the pathway of an incoming threat. Most active RPG countermeasures are zone kill munitions. They offer the probability of defeating an RPG if they are in the right place at the right time. Precise aiming and delivery on the 100ms timescale is highly difficult, but launching multiple countermeasures into the path of an approaching threat would increase the probability of success radically. This advantage could be decisive in enabling the development of defensive systems for use against high-velocity threats such as anti-tank missiles and MANPADS, which would likely require a more robust defensive obstacle than an RPG. In that scenario the inbound threat missile could be confronted by a closely sequenced series of outbound countermeasure munitions, making it highly unlikely that the missile would survive the encounter.
Cost / Energy Budget
A continuously-rotating drum launcher would be cheap, lightweight, and easy to produce. The proposed drum, containing a plurality of launch tubes and a central rotational axis, could be made of relatively inexpensive molded composite materials. This would save weight, particularly when compared to solutions such as slat armor. A used drum could be quickly changed out for a fresh one. The drum could engage a splined hub and be driven by cheap servomotor via a toothed rubber belt. The motor power requirements would be modest due to the lack of a requirement for this system to perform any abrupt movements. The motor would only be required to maintain drum rotation. Coordinating the angular position of the launch tube array(s) could be achieved through the use of a common rotary encoder. The encoder would also provide rotational speed data to the system so that slight rotational speed variations (inherent in a low cost system) would be incorporated into the closed-loop firing algorithm.
Shooting Back Quickly
Offensive munitions could be deployed in addition to and along with defensive munitions in the Rotating Defensive System. This would provide an offensive weapon system that could direct fire almost instantaneously toward an adversary. It may be envisioned that the Rotating RPG Defensive System could immediately and simultaneously launch defensive RPG countermeasures as well as offensive munitions (bullets) in the direction of an inbound RPG attack. This offensive response would also occur on a millisecond timescale, proceeding from the detection of the incoming threat.
A Rotating RPG Defensive System is the only system that can engage multiple threats simultaneously. Because countermeasures may be fired simultaneously from any of the available munition tubes in the drum, the Rotating RPG Defensive System is capable of delivering munitions onto multiple target vectors simultaneously. This is not physically possible with a gun and gimbal system.