1. Introduction
Unmanned systems have become one of the fastest growing weapons platforms in the American armed services. These systems have significantly benefited from the inclusion of MEMS devices. So called “smart” weapons range in size from the 21,000 lb GBU-43 MOAB to the 35 lb APKWS 70mm guided rocket. These “smart” weapons, when combined with unmanned systems, have proved to be extremely lethal combinations, and it would follow that combining “smart” technology with unmanned systems in small arms would be equally as valuable.
American small arms ammunition for military applications has been of the same basic design since the introduction of the M1906 ammunition. The projectile for this ammunition comprises a lead slug encased by a copper jacket. While improvements have been made to increase the aerodynamic efficiency of the projectile, and various sub-caliber hardened penetrators have increased performance against hard targets, MEMS devices and systems have yet to be incorporated into small arms ammunition1.
Small arms gun design has likewise remained dedicated to the concept of a shoulder fired weapons manually operated by individual people. To maximize performance as part of a weapon integrated with an unmanned system, gun design must be optimized with respect operation from these non-traditional unmanned, “robotized” sytems.
1.1 MEMS Ammunition Issues
The nature of traditional small arms has hampered the use of MEMS devices in the projectiles. A traditional small arms projectile experiences an extremely hostile environment, comprising of extreme accelerations, and temperatures. The current state of the art in durable MEMS systems is insufficient to overcome the difficulties inherent in producing MEMS capable small arms ammunition.
The primary impediment to the development of MEMS devices and systems is the high accelerations that traditional forms of small arms ammunition projectiles are subject to during the firing process. There are two acceleration components in traditional small arms ammunition; lateral and radial. When fired from military standard rifles, the projectiles typically accelerate to speeds of 2,500 ft/sec to 3,100 ft/sec in distances from 14.5 in to 45 in. Traditional small arms ammunition is stabilized in flight by imparting a spin to the projectile as it passes through the barrel. Spin rates for military weapons vary from 1 turn in 7 inches to 1 turn in 15 inches. The lateral accelerations are from 40kG to 100 kG while the radial accelerations vary from 80 kG to 120 kG1. Table 1 indicates the accelerations produced by the three most commonly used military small arms ammunition types. These accelerations are much greater than Commercial Off The Shelf (COTS) devices are capable of surviving, which tend to be in the hundreds of Gs, while most shock-durable laboratory grade devices from entities such as Sandia National Laboratories cannot survive past 50kG of acceleration2,3.
There are several ongoing research programs at multiple facilities devoted to the development of increasingly durable MEMS. These programs have studied the uses of less common materials such as silicon carbide and metallic alloys, and while they have advanced the accelerations that MEMS can survive, progress has ultimately been slow and costly without yet producing a device that can survive the small arms environment.
The propellant of traditional small arms ammunition is a nitrocellulose based powder than burns at temperatures of several thousand Celcius degrees. The base of the projectile can reach temperatures beyond 500 oC, while the stabilization grooves can reach friction based temperatures of 300 oC. These temperatures are beyond the designed temperature range of MEMS devices designed for the space environment2.
It is apparent from comparing the small arms projectile environment against the current durability limits that MEMS systems and devices cannot be included into existing forms of ammunition. If MEMS technology is to be incorporated into ammunition, a new type of ammunition must be designed.
1.2 Current Small Arms and Unmanned Systems
The operation and design of current small arms is not optimized for unmanned systems operation. Military small arms are fed either by belt or by magazine. Each time a round is fired, the gun must extract and eject the empty case from the chamber, strip the round from the feed mechanism, load it into the chamber, reset and then release the trigger mechanism. Each of these actions is purely mechanical, and each offers its own unique failure mechanism. Additionally, if a malfunction should occur, the round cannot be bypassed, and an individual operator must manually reset the system. These guns also weigh 30 lbs to 50 lbs when fully loaded, and cannot be carried by the small unmanned systems. The empty casing from fired rounds and the recoil of very powerful rounds of ammunition can potentially damage the unmanned system to which the gun is mounted.
1.3 Design Requirements
In order to maximize the usefulness and lethality of an unmanned system equipped with small arms, several novel advancements must be made. The gun system should fire caseless ammunition electronically, without requiring moving parts during the firing process. This gun system should have minimal weight, but still have manageable recoil. The ammunition must produce accelerations and thermal loads that are survivable by existing MEMS designs, and both gun and ammunition must be designed for one another.
2. Gun and Ammunition Design
2.1 Ammunition Design
The primary concern with the design of the ammunition was the reduction of the radial and lateral accelerations. Aside from spin stabilization, another common form of stabilization for projectiles is drag stabilization. In drag stabilization, the projectile must posses a significantly greater length to diameter ratio than is found in conventional small arms ammunition, implying a “long” projectile for a given caliber, or bullet diameter. Drag stabilization can be found in several types of ammunition, such as the M829 120 mm Armor Piercing Fin Stabilized Discarding Sabot (APFSDS) round designed for tank guns. A common form of drag stabilized small arms projectiles is the so-called “Foster slug”. It is designed to be fired from shotgun barrels, and the concentration of mass in the nose of the projectile aids in stability. An additional benefit of drag stabilization is the reduction in bullet to bore friction due to the elimination of rifling grooves. This increases the kinetic energy of a projectile for a given propellant load. Finally, drag stabilized projectiles with large length to diameter ratios have a high Ballistic Coefficient (BC), resulting in minimal speed loss to air resistance as the projectile is in flight.
Establishing drag stabilization as the preferred technique eliminates radial accelerations. To decrease the lateral acceleration while still retaining a high final projectile speed requires that the bullet accelerate over a much greater distance than the 14 in to 45 in of existing gun barrels.
This requirement can only be met by having the projectile accelerate outside of the barrel, where the projectile literally becomes a bullet-sized rocket. This approach works well with the drag stabilization requirement. A bullet containing its own propellant must have a large length to diameter ratio to store the propellant, and by locating low-density propellant towards the base of the projectile and locating high-density heavy metals and devices towards the nose, the projectile becomes very well suited to drag stabilization. This approach also allows for a modular form of bullet construction, where a given body/fuel chamber can be mated to a particular device chamber/nose cone to be optimized for a given mission. Additionally, the device chamber/nose cone can be thermally isolated from the propellant chamber. The longer acceleration and lower momentum of the projectile as it leaves the barrel also greatly reduces the recoil forces that the projectile exerts on the gun.
By using a rocket based form of ammunition, lateral accelerations can easily be reduced to the 50-100G range, while a sufficiently long burn time can ensure that the projectile still reaches lethal velocities. It is also possible to thermally isolate the propellant chamber from any devices. This combination of low accelerations and thermal loads greatly increase the number of MEMS systems that can be included into this ammunition. Commercial COTS MEMS can be utilized without having to use more expensive purpose-built MEMS systems. A cut-away drawing of the rocket bullet ammunition is shown in Figure 1.
2.2 Rocket Bullet Gun Design
Traditional small arms guns use either high pressure gases pushing the projectile or recoil forces to cycle. Rocket based ammunition, as described in the earlier section, will not have sufficient recoil forces to operate the guns. Rocket based ammunition uses high velocity gases to propel the projectile, but does not generate high in-barrel pressures, unlike traditional small arms ammunition, where in-barrel pressures can typically exceed 55 kpsi. These two factors require that rocket based ammunition will require an alternative form of gun operation.
Rocket propellant is typically ignited by a electronic fuse, wherein a layer of explosive material is deposited over a narrow conductive wire. Current passed through the conductive wire heats the explosive material until it auto-ignites, which initiates the rocket propellant. The electronic nature of this fuse allows rocket based ammunition to be fired without moving parts.
The barrel of a rocket bullet gun is not a pressure containing system as is found in conventional small arms. The sole purpose of the barrel is to guide the projectile during its initial portion of flight. As such, it does not require the intricate machine necessary for the production of traditional small arms barrels, and can be made of lightweight, inexpensive polymers. Rocket bullet projectiles can also be stacked on top of one another, fired electronically, and multiple barrels can be formed out of a single piece of polymer. This would allow for the production of an inexpensive gun with high capacity, low recoil, electronic ignition, and no moving parts. Further, if there were a failure of a particular rocket bullet to ignite, it would be possible to bypass that round or that barrel, and to continue firing despite a malfunction, and without requiring a individual operator to manually correct the malfunction. A drawing of this type of rocket bullet gun is shown in Figure 2.
3. System Integration
3.1 Ammunition
Current rocket bullet ammunition is based on a 16 mm caliber projectile. This diameter allows for compatibility with commercially available 16 mm solid fuel rocket propellant, and withcommercially 12 gauge barrels. The fuel chamber/body of the projectile is aluminum, with female threaded ends. This allows for the attachment of a variety of modular device chambers, nose cones, and nozzles. A picture of an assembled 16 mm caliber rocket bullet is shown in Figure 3. Initial tests were conducted with aluminum nozzles. It was found that the aluminum nozzles were significantly damaged during firing with a corresponding loss of accuracy. This has led the use of steel nozzles, which are not found to degrade in flight.
3.2 Unmanned System
To integrate the rocket bullet with an unmanned system, several SDR, Inc. robot kits were acquired. The primary test bed is a 6 wheel drive robot with an articulating arm. Two Thompson/Center 12 gauge barrels were acquired and mounted to the arm through M1913 specification rail systems and clamp mounts. An optics rail was also mounted to the arm, and can accept any M1913-specfic mounts. A picture of this system is shown in Figure 4.
All 16 mm caliber rocket bullets were fired electronically, and all test firings were conducted at the Ruston Gun Club. After the inclusion of steel nozzles into the design, it was possible to reliably hit a man-sized target at 100 yd. Figure 5 shows the armed robot firing a rocket bullet into the 100 yd. berm at the Ruston Gun Club.
The robot is controlled through a miniature server built into the chassis. Through this system, it is possible to operate the robot through wireless Internet connections and to electronically fire the rocket bullets through a system of relays.
