Recoilless Weapons for Modern Warfare
In past decades, recoilless weapons were considered lethal and formidable antitank assets. Recoilless antitank grenade launchers (ATGLs) for squads have seen tremendous improvements. But, as tank armor has improved, most of the crewed systems for use by AT teams operating above squad level have seen fewer upgrades. Today, many of those crewed AT weapons are considered obsolescent. However, upgrades are available; and new weapons are now fielded. With more changes, crewed recoilless weapons can be effective.
One advantage of recoilless systems is their utility. They offer a variety of munitions, including HEAT, HE, flechette, and others, to service most battlefield targets. Some recoilless launchers (aka, recoilless guns or rifles, or mounted grenade launchers) are light enough for easy transport in light vehicles, with ability against infantry while outranging most infantry weapons. A well-proliferated example is the SPG-9/9M, with HE range beyond 4,000 m.
Recoil from a fired vehicle mounted gun system causes excessive motion of the vehicle, at times, creating the possibility of toppling the vehicle or causing extreme discomfort to the gun crew. As stated by Newton's third law of motion, i.e., to every action there is always opposed an equal reaction, the momentum manifest within a gun system during the weapon launch is equal and opposite to sum of the momentum which is imparted to the projectile launched from the gun system, including the propellant gases that are subsequently ejected from the gun system. Minimizing the recoil increases the utilization of these gun systems.
Several methods are known to reduce the total forward momentum imparted during the launch of a projectile from a gun system. Momentum, as a vector quantity, does not dissipate as kinetic energy does, which is a scaler quantity. For a traditional gas gun, the launch momentum equals momentum imparted to the projectile, and the propellant gas that follows the projectile out of the muzzle. For a given projectile momentum, the total launch momentum may be reduced by redirecting the forward moving propellant gas to lower its forward speed, or reverse it. Alternatively, some other inertia may be ejected out of the gun in the opposite direction from the projectile to achieve some degree of momentum cancellation.
All guns are subject to recoil, thus the problem has exited ever since the invention of the gun. The first known concept of a recoilless gun was sketched by Leonardo da Vinci (1452-1519) in which a gun fired two projectiles; one forward and one rearward, to balance the momentum. During World War I, Commander Cleland Davis, United States Navy, reduced the two projectile gun to practice. The Davis gun fired an ordinance projectile at the target, and a dummy projectile of Vaseline and lead dust, having an equal mass to the ordinance projectile, was fired in the opposite direction. The original Davis gun used a cannon that was open at both ends and loaded in the middle, and was apparently intended to target high altitude Zeppelins. Problems with the Davis gun include hazards to friendly forces from the rearward fired projectile and subsequent muzzle blast, added system weight from a second barrel and additional charge needed to obtain equal fire power of the projectile, and containing pressure in two directions. As such, the Davis gun has logistical burdens, munitions handling problems of heavy ammunition, a high system weight, and double length gun barrels that may limit mobility of a fighting vehicle.
Other developments in eliminating recoil from gun systems resulted in recoilless guns. Recoilless guns incorporated a nozzle in the breech to eject propellant gases out of the rear of the gun, permitting part of the propellant gas to flow backward and counter-balance the momentum of the fired projectile and any propellant that was propelled forward. A gun system that incorporated diversion of propellant gases through a nozzle located at the breech was developed by the Russians in 1936 using a design from a patent filed in 1917 by the Russian mathematician Riabouchinski.
During World War II, United States Army Colonel Rene R. Studler, at the Research and Development Service of US Army Ordnance, developed a lightweight recoilless gun resulting in a shoulder fired weapon that could propel a three pound (1.36 Kg) explosive shell with a muzzle velocity of 1200 feet per second (366 m/s). The recoilless concept is used in the U.S. Army M40AD 106 -mm recoilless rifle that allows propellant gases to escape through a perforated chamber case between the projectile and breech, and the German LG 42 105 mm recoilless Howitzer that uses a bursting disk located at the breech to contain the propellant through ignition as opposed to a perforated chamber. Problems with recoilless rifles include poor ignition characteristics of propellant within the open chamber system, ejection of unburned propellant through the nozzle, erosion problems in large caliber direct-fire tank cannons from created pressures and temperatures (limiting recoilless guns to relatively low pressure applications), loss of substantial amounts of chemical energy from the propellant, added cost and weight from a perforated cartridge to contain the propellant while it burns, revealing back-blasts that also hazard firing crews, and limited nozzle design for recoilless rifles dictated by interior ballistic pressure.
Recoilless guns also have been developed using front orifices developed by the Frigidaire Division of the General Motors Corporation in collaboration with the Armour Research Foundation in the early 1950's. The front orifices allow the ignition process to occur in a closed chamber, increasing efficiency. The gun initially behaves as a conventional weapon. However, shortly after the projectile begins to travel down the bore of the gun barrel, orifices integrated within the gun barrel that lead to a rearward facing contraction expansion nozzle are uncovered by the projectile obturator, allowing propellant gases to be vented from the gun and achieve forward thrust for recoil cancellation.
Problems with a front orifice recoilless rifle include adequate ducting of escaping rearward muzzle gases at a substantial distance along the bore from the rear of the weapon, reduced pressure behind the launching projectile after the orifice is enabled and throughout the remaining duration of the ballistic cycle, limited chamber pressure, increased ammunition weight, and the initial imbalance in recoil loads requires a flexible, i.e., heavier and more complicated, mount to accommodate the initial rearward motion of the gun system prior to the uncovering of the orifices and their recoil mitigation effect. As such, recoilless and front orifice rifles present a logistical burden and munitions handling of heavy ammunition relative to that achievable with a traditional closed breach gun system, limited internal ballistic pressure, and an inability to operate in a closed-breech mode when firing low impulse rounds.
Muzzle brakes may be used to reduce firing impulses. French Colonel Chevalier Truielle de Beaulieu, in 1842, recorded the first known diversion of propellant gas using a crude muzzle brake to reduce the combined launch momentum. Muzzle brakes deflect the gases flowing out of the muzzle thus redirecting a substantial portion of the gas momentum. The efficiency of muzzle brakes generally ranges between 30 and 40 percent, with exceptional muzzle brakes achieving efficiencies as high as 70 percent. In this context, the muzzle brake efficiently is defined as the percentage reduction in the kinetic energy imparted to the recoiling gun system mass. During launch, the projectile is propelled by the high pressure propellant gases and when the rear of the projectile exits the main gun barrel it enters the muzzle brake. The muzzle brake allows the propellant gases to escape from behind the fired projectile.
Through a combination of further gas expansion, and redirection of the gas flow, the net forward momentum of the propellant gases may be dramatically reduced, or even reversed by the muzzle brake. Problems with muzzle brakes include crew hazards from the excessive over-pressure of the blast, i.e., air disturbances or propellant gas moving at high velocity, loud noise and heat, creation of debris in the air obfuscating targets, added weight and cumbersome barrel redirection, limitations to end barrel use, and that high velocity gases remain to follow the projectile straight out of the muzzle brake unimpeded. As such, muzzle brakes present reduction of the recoil energy of a gun system by more than half, a health hazard, reduced vision in front of the gun, added weight to the end of the cannon and reduced projectile exit velocity (by reducing the pressure at the base of the projectile prior to exit from the gun system).
Heat imparted to the bore of a gun during firing may result in unacceptable temperature increase of the of the gun, and will accelerate the wear and erosion of the bore. The former consideration may limit the sustained rate of fire, while the latter may limit the life of the gun. The heat transfer to the gun is governed by complex partial differential relationships between the temperature, density, and velocity of the gas, and its interaction with the bore surface. The heat transfer coefficient as proportional to the velocity of the gases washing over the bore surface and the density of the gases. The net heat transfer integrates the rate of heat transfer over the duration of the exposure, therefore heat transfer also increases in kind with duration. Experimental evidence has shown substantial increase of bore erosion with bore temperature. In simple terms, the hotter the bore, the less resistant the material is to removal by wear and erosion.
Recoilless weapons are limited by certain design and operational considerations. The bore size limits size of rounds and their ability to penetrate modern tank armor. However, many offer sufficient penetration to defeat other armored and unarmored vehicles (which outnumber tanks on the battlefield). Those systems are best used to augment other forces in a fight.
A key value is in their general lethality. With a higher rate of fire than most antitank guided missile (ATGM) launchers at 5 or more rounds per minute, they are useful as a fire support asset that can augment fires of other weapons against various targets. Most lack the range of ATGMs. All lack the precision. But these multi-role systems can digest various rounds to defeat vehicles, then kill exiting personnel with large HE blast munitions. In a close fight or ambush, many of these weapons can kill any vehicle other than a main battle tank (MBT) from any aspect. Some can also damage or kill MBTs from the side or rear. As noted for infantry antitank grenade launchers (ATDLs), crewed weapons include new ones, and upgraded munitions with tandem HEAT warheads which can kill all tanks from the side or rear.
A number of features have improved precision of these weapons. A variety of electro-optical sights can be mounted on these weapons. SPG-9M and SPG-29 Mounted can use the Russian 2Ts35 laser- rangefinder sight or a widely marketed lightweight ballistic computer sight such as the Simrad IS2000. Adaptable night sights include II sights like Simrad KN250F, and various compact thermal sights. A challenge to all grenades is their relatively slow velocity, which reduces hit probability (Ph) against moving vehicles. Some have high velocity (600 m/s for Italian Folgore, 700 for SPG-9) and flat trajectories to increase Ph vs movers.
A few manufacturers are looking for improved accuracy munitions. The ultimate solution is to add ATGM launch capability to recoilless launchers. Israeli IAI has offered to produce a version of the LAHAT for use in the 90 mm M40 recoilless rifle. Even the best recoilless round cannot kill a modern tank from the frontal aspect. Any ATGM exiting a bore of < 150 mm has a low probability to defeat MBT front armor. But a top- attack tandem warhead missile, e.g., a 90 mm version of LAHAT for the M40 launcher offers probable major damage (or catastrophic kill) against an MBT turret or hull. Other vehicles would expect a catastrophic kill. Israeli IAI has offered development of the ATGM variant. In a beyond line-of-sight (BLOS) environment, ATGM capability dramatically increases effectiveness of this type of weapon.
Another limitation for recoilless weapons is their detectability. Most have high smoke and noise signatures in the backblast. Most are fairly tall and must be hand-loaded from a standing or kneeling position, which exposes the team to counter-fires. Older weapons are heavy enough to require breaking down and loading into vehicles for moves, limiting their ability to shoot and move quickly. One example of poor mobility is the old Russian B-10 (at 85 kg towed carriage, 72 without). With an anti-armor range of 400- 1,000 m, the first shot had better be accurate and lethal, or the crew is in serious trouble. Thus many users only employ these older weapons in combined arms defenses and ambushes, to augment other fires.
Some old launchers have been modified to reduce weight, plus break down into components for dismounted moves. The Chinese Type 65 is a lighter weight version of B-10, at 28 kg, and uses improved ammunition. The Serbian M79 variant also weighs less than 30 kg. With improved sights, it has an anti- armor range of 670-1,000 m. Better range aids survivability.
Several newer recoilless weapons have been designed for reduced weight, lower operating profile, and reduced move and setup times. Examples include RPG-29 Mounted (next page) and the Serbian M90. The best of the modern lightweight crewed launchers may be the Chinese 120-mm PF-98. Although it operationally resembles RPG-29, it is actually an amalgam of features from a variety of modern systems. Like the RPG-29, it comes in shoulder-launch version or tripod-mount crewed version. The launcher appears to be directly derived from the Montenegro/Serbian 120-mm M90. However, they followed the Russian design by adding a lightweight tripod, and a canistered grenade which attaches to the launcher to extend its length. The ammunition is another amalgam, with warheads which could be derived from recent Carl Gustaf rounds, and a rocket motor that resembles a scaled up RPG-29 motor. The sight is a modern EO/LRF ballistic computer sight. The result is a state-of-the-art launcher with 800/2,000 m range, 800+ mm penetration, light weight (< 18 kg loaded), and competitive precision.
Rapid mobility, as noted earlier, is a critical factor for survivability and utility of these weapons. Since most legacy systems cannot be easily adapted for mobility, an alternative use is to mount them on vehicles. They offer good lethality to protect vehicles; and the vehicles facilitate launch-and-move operations, without downtime for disassembly. Examples include fire support versions of BTR-50 and Czech OT-21 APCs, and various weapons on the BTR-152 armored transporter. They have also been fitted on a motorcycle and on boats.
A good weapon for ground and vehicle mounts is the Russian SPG-9, which has been seen pintle-mounted on a UAZ-469 TUV. The launcher is well proliferated, and seen several upgrades. On the SPG-9M upgrade, 2Ts35 or other more modern LRF ballistic sights are available. AT ranges are 1,300 m for improved HEAT, and 1,000 for tandem HEAT. The best of these weapons for vehicle mount is the US M40 106 mm recoilless rifle. The Bofors Retrofit Kit updates it into a modern and effective fire support weapon. Sights include the CLASS laser sight, and others. Munitions include flechette, HEP-T, and tandem HEAT. Addition of the LAHAT ATGM (above) would greatly expand its lethality. With these and other expected upgrades, and with new designs in production, planners can expect to see recoilless weapons employed against U.S. forces for many more years.
|Join the GlobalSecurity.org mailing list|