High-Low System
Encyclopedia
The High-Low system also referred to as the "High-Low Pressure system", the "High-Low Propulsion System", and the "High-Low projection system", is a design of cannon and antitank launcher using a smaller high-pressure chamber for storing the propellant. When the propellant is ignited, the higher pressure gases are bled out through vents (or ports) at reduced pressure to a much larger low pressure chamber to push the projectile forward. With the High-Low System a weapon can be designed with reduced or negligible recoil. The High-Low System also allows the weight of the weapon and its ammunition to be significantly reduced. Manufacturing cost and production time are drastically lower than standard cannon or other small-arm weapon systems firing a projectile of the same size and weight. It has a far more efficient use of the propellant, unlike earlier recoilless weapons
, where most of the propellant is expended to the rear of the weapon to counter the recoil of the projectile being fired.
, Nazi Germany
researched and developed low-cost antitank weapons. Large antitank cannon firing high velocity projectiles were the best option, but expensive to produce as well as requiring a well trained crew. They also lacked mobility on the battlefield once emplaced. Antitank rocket launchers and recoilless rifle
s, while much lighter and simpler to manufacture, gave the gunner's position away and were not as accurate as antitank cannon. Recoilless rifles used a huge amount of propellant to fire the projectile, with estimates ranging from only one-fifth to one-ninth of the propellant gases being used to push the projectile forward.With the simple one man Panzerfaust
the ratio was even higher. The German military asked for an antitank weapon with performance in-between that of the standard high velocity cannon and the cheaper rocket and recoilless infantry antitank weapons. They also stipulated that any solution had to be more efficient in the use of propellant as German war industry had reached maximum production capacity of propellant for cannon.
In 1944, the German firm Rheinmetall
-Borsig came up with a completely new concept for propelling a projectile from cannon, which, while not recoilless, greatly reduced recoil and drastically reduced the manufacturing cost. This concept was called the Hoch-und-Niederdruck System which roughly translates to "High-Low Pressure System". With this system, only the very back of the cannon's breech had to be reinforced against the high firing pressures.
Rheinmetall designed an antitank cannon using their "High-Low Pressure System" that fired a standard general purpose HE
8.1-cm mortar bomb which had been modified to function as an antitank round with a shaped charge
.Today, shaped charges are commonly called HEAT
warheads or HEAT projectiles. HEAT is an acronym for "High Explosive, Anti-Tank". Normally, a mortar bomb cannot be fired from a cannon, because their thin walls can not endure the high stress forces upon firing from a cannon. The 8.1-cm round was mounted on a rod which was fixed to a round steel plate with eight holes in it. A shear pin held the round to the rod. The round and the plate were fitted at the mouth of a cut-down cannon shell casing which contained two propellant bags. On firing, the pressure would build up in the shell casing, which along with the reinforced breech, acted as the "High Pressure Chamber" and bled out the steel plate holes at half the pressure to the thinner walled cannon barrel which acted as the "Low Pressure Chamber". Unlike standard cannon, in which the propellant "kicks" the projectile out the barrel with an almost instant acceleration to maximum muzzle velocity, the Rheinmetall concept "shoved" the projectile out the barrel at a constantly increasing muzzle velocity. There was recoil, but nowhere near the recoil of the 5-cm and 7.5-cm antitank cannons being used at that time by the German forces, which required a heavy constructed carriage, and both a heavy and complex hydraulic recoil mechanism as well as a muzzle brake to contain the massive recoil on firing. The Rheinmetall solution required only a lightweight recoil unit and muzzle brake. The only major drawback was its maximum range of 750 meters (in direct fire against tanks), but this was offset by an armor penetration of 140 mm and no telltale back-blast. The Germans ordered the Rheinmetall gun into production, designating it as the 8-cm Panzer Abwehr Werfer 600 (PAW 600)
.In the last months of the war, the German Army re-designated it the Panzerwurfkanone 8H63 (PWK 8H63). Only about 250 were produced before the war's end. None were reported to have seen combat.
Limbo
antisubmarine weapon, which launched depth charge-like projectiles. The Limbo was a development of the World War II Squid
, which, while effective, was limited by a set range of 275 meters. The Limbo, by opening and closing vents that varied the pressure of the gases on firing, allowed for a range that could be varied between 336 meters to almost 1000 meters.
The most well known use of the High-Low System was by the U.S. Army with the introduction of the M79 grenade launcher
shortly before the Vietnam War
. The M79 fired a 40-mm shell which contained a standard fragmentation grenade with a modified fuze. The cartridge casing contained a heavy cup shape "High Pressure Chamber" in the bottom. On firing, the propellant builds up pressure until it breaks through the copper cover, venting out to the "Low Pressure Chamber". The U.S. Army referred to their high-low system as the High-Low Propulsion System. Along with a heavy rubber pad on the M79 butt stock, recoil forces were kept in bounds for the infantryman using the weapon.
The M79 was later replaced by the M203
which mounts under the barrel of an assault rifle. Later, the U.S. Army developed a higher velocity 40-mm round
using their High-Low Propulsion system for use by heavier machine gun-type grenade launchers used on vehicles and helicopters. Today, besides the U.S. military, the 40-mm grenade family is extremely popular and in use by armies worldwide and variants of it are in production by countries other than the U.S., with one reputable reference publication in 1994 needing almost a dozen pages to list all the variants and nations producing 40-mm grenade ammunition based on the U.S. Army's development of the 1960s.
introduced a 40-mm grenade launcher that used the High-Low Principle, but with a twist on the original design. The GP-25 40-mm grenade launcher fits under the assault rifle and fires a caseless projectile that is muzzled loaded. Instead of having a case, the high-pressure chamber is located on the rear of the projectile with ten vent holes, in which the launcher barrel acts as the low-pressure chamber. The ignition of the propellant gases also causes the drive band to engage the launcher grooves, similar to the American Civil War
Parrot muzzle loading rifled cannon.
While little documentation exists, in the 1950s the Soviet Army
developed 73-mm cannon for wheeled armored reconnaissance vehicles that fired a munition very similar in operation to the original World War II German concept. However, it was never introduced into service, and instead the Russians developed a low velocity 73mm cannon
that fired a rocket projectile which was ejected by a small charge in the normal fashion.
firm FFV in their development of the 1960s-era Miniman
one man infantry antitank weapon. The Miniman was simpler and cheaper than anything imagined by designers in World War II. Inside what looked like a rocket launcher tube, is a HEAT projectile attached by a break away bolt to an alloy aluminum tube with ports drilled in in it and which acts like a kind of high-pressure chamber. The launch tube in which it is mounted acts at the high-pressure chamber. When the propellant is ignited in the aluminum tube, gases escape through the ports and are allowed to build up in the launch tube to the point of almost causing a recoil. The break away bolt then snaps, allowing the projectile to move forward. Unlike other High-Low Systems, gases are allowed to escape to the rear of the launch tube, achieving a totally recoilless effect.
Recoilless rifle
A recoilless rifle or recoilless gun is a lightweight weapon that fires a heavier projectile than would be practical to fire from a recoiling weapon of comparable size. Technically, only devices that use a rifled barrel are recoilless rifles. Smoothbore variants are recoilless guns...
, where most of the propellant is expended to the rear of the weapon to counter the recoil of the projectile being fired.
Origin
In the final years of World War IIWorld War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, Nazi Germany
Nazi Germany
Nazi Germany , also known as the Third Reich , but officially called German Reich from 1933 to 1943 and Greater German Reich from 26 June 1943 onward, is the name commonly used to refer to the state of Germany from 1933 to 1945, when it was a totalitarian dictatorship ruled by...
researched and developed low-cost antitank weapons. Large antitank cannon firing high velocity projectiles were the best option, but expensive to produce as well as requiring a well trained crew. They also lacked mobility on the battlefield once emplaced. Antitank rocket launchers and recoilless rifle
Recoilless rifle
A recoilless rifle or recoilless gun is a lightweight weapon that fires a heavier projectile than would be practical to fire from a recoiling weapon of comparable size. Technically, only devices that use a rifled barrel are recoilless rifles. Smoothbore variants are recoilless guns...
s, while much lighter and simpler to manufacture, gave the gunner's position away and were not as accurate as antitank cannon. Recoilless rifles used a huge amount of propellant to fire the projectile, with estimates ranging from only one-fifth to one-ninth of the propellant gases being used to push the projectile forward.With the simple one man Panzerfaust
Panzerfaust
The Panzerfaust was an inexpensive, recoilless German anti-tank weapon of World War II. It consisted of a small, disposable preloaded launch tube firing a high explosive anti-tank warhead, operated by a single soldier...
the ratio was even higher. The German military asked for an antitank weapon with performance in-between that of the standard high velocity cannon and the cheaper rocket and recoilless infantry antitank weapons. They also stipulated that any solution had to be more efficient in the use of propellant as German war industry had reached maximum production capacity of propellant for cannon.
In 1944, the German firm Rheinmetall
Rheinmetall
Rheinmetall AG is a German automotive and defence company with factories in Düsseldorf, Kassel and Unterlüß. The company has a long tradition of making guns and artillery pieces...
-Borsig came up with a completely new concept for propelling a projectile from cannon, which, while not recoilless, greatly reduced recoil and drastically reduced the manufacturing cost. This concept was called the Hoch-und-Niederdruck System which roughly translates to "High-Low Pressure System". With this system, only the very back of the cannon's breech had to be reinforced against the high firing pressures.
Rheinmetall designed an antitank cannon using their "High-Low Pressure System" that fired a standard general purpose HE
Explosive material
An explosive material, also called an explosive, is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure...
8.1-cm mortar bomb which had been modified to function as an antitank round with a shaped charge
Shaped charge
A shaped charge is an explosive charge shaped to focus the effect of the explosive's energy. Various types are used to cut and form metal, to initiate nuclear weapons, to penetrate armor, and in the oil and gas industry...
.Today, shaped charges are commonly called HEAT
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
warheads or HEAT projectiles. HEAT is an acronym for "High Explosive, Anti-Tank". Normally, a mortar bomb cannot be fired from a cannon, because their thin walls can not endure the high stress forces upon firing from a cannon. The 8.1-cm round was mounted on a rod which was fixed to a round steel plate with eight holes in it. A shear pin held the round to the rod. The round and the plate were fitted at the mouth of a cut-down cannon shell casing which contained two propellant bags. On firing, the pressure would build up in the shell casing, which along with the reinforced breech, acted as the "High Pressure Chamber" and bled out the steel plate holes at half the pressure to the thinner walled cannon barrel which acted as the "Low Pressure Chamber". Unlike standard cannon, in which the propellant "kicks" the projectile out the barrel with an almost instant acceleration to maximum muzzle velocity, the Rheinmetall concept "shoved" the projectile out the barrel at a constantly increasing muzzle velocity. There was recoil, but nowhere near the recoil of the 5-cm and 7.5-cm antitank cannons being used at that time by the German forces, which required a heavy constructed carriage, and both a heavy and complex hydraulic recoil mechanism as well as a muzzle brake to contain the massive recoil on firing. The Rheinmetall solution required only a lightweight recoil unit and muzzle brake. The only major drawback was its maximum range of 750 meters (in direct fire against tanks), but this was offset by an armor penetration of 140 mm and no telltale back-blast. The Germans ordered the Rheinmetall gun into production, designating it as the 8-cm Panzer Abwehr Werfer 600 (PAW 600)
8 cm PAW 600
The PAW 600 was a lightweight anti-tank gun that used the high-low pressure system to fire hollow charge warheads. It was used operationally by Wehrmacht in 1945 in small numbers.-Background:...
.In the last months of the war, the German Army re-designated it the Panzerwurfkanone 8H63 (PWK 8H63). Only about 250 were produced before the war's end. None were reported to have seen combat.
Further development
The Allies captured and examined the PWK, but initially showed little interest in the new system developed by the Germans. The first example of a type of High-Low System developed after World War II was the BritishUnited Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
Limbo
Limbo (weapon)
Limbo, or Anti Submarine Mortar Mark 10 , was the final British development of a forward-throwing anti-submarine weapon originally designed during the Second World War. Limbo, a three-barreled mortar similar to the earlier Squid that it superseded, was developed by the Admiralty Underwater Weapons...
antisubmarine weapon, which launched depth charge-like projectiles. The Limbo was a development of the World War II Squid
Squid (weapon)
Squid was a British World War II ship-mounted anti-submarine weapon. It consisted of a three-barrelled mortar which launched depth charges. It replaced the Hedgehog system, and was in turn replaced by the Limbo system....
, which, while effective, was limited by a set range of 275 meters. The Limbo, by opening and closing vents that varied the pressure of the gases on firing, allowed for a range that could be varied between 336 meters to almost 1000 meters.
M79 40-mm grenade launcher
M79 grenade launcher
The M79 grenade launcher is a single-shot, shoulder-fired, break-action grenade launcher that fires a 40x46mm grenade which used what the US Army called the High-Low Propulsion System to keep recoil forces low, and first appeared during the Vietnam War...
shortly before the Vietnam War
Vietnam War
The Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...
. The M79 fired a 40-mm shell which contained a standard fragmentation grenade with a modified fuze. The cartridge casing contained a heavy cup shape "High Pressure Chamber" in the bottom. On firing, the propellant builds up pressure until it breaks through the copper cover, venting out to the "Low Pressure Chamber". The U.S. Army referred to their high-low system as the High-Low Propulsion System. Along with a heavy rubber pad on the M79 butt stock, recoil forces were kept in bounds for the infantryman using the weapon.
The M79 was later replaced by the M203
M203 grenade launcher
The M203 is a single shot 40 mm grenade launcher designed to attach to a rifle. It uses the same rounds as the older M79 break-action grenade launcher, which utilize the High-Low Propulsion System to keep recoil forces low. Though versatile, and compatible with many rifle models, the M203 was...
which mounts under the barrel of an assault rifle. Later, the U.S. Army developed a higher velocity 40-mm round
40 mm grenade
The 40mm grenade is a military grenade caliber for grenade launchers in service with many armed forces. There are two main types in service: the 40×46mm, which is a low-velocity round used in hand-held grenade launchers; and the high-velocity 40×53mm, used in mounted and crew-served weapons. The...
using their High-Low Propulsion system for use by heavier machine gun-type grenade launchers used on vehicles and helicopters. Today, besides the U.S. military, the 40-mm grenade family is extremely popular and in use by armies worldwide and variants of it are in production by countries other than the U.S., with one reputable reference publication in 1994 needing almost a dozen pages to list all the variants and nations producing 40-mm grenade ammunition based on the U.S. Army's development of the 1960s.
Russian developments
Shortly after the Vietnam War ended, the Soviet UnionSoviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
introduced a 40-mm grenade launcher that used the High-Low Principle, but with a twist on the original design. The GP-25 40-mm grenade launcher fits under the assault rifle and fires a caseless projectile that is muzzled loaded. Instead of having a case, the high-pressure chamber is located on the rear of the projectile with ten vent holes, in which the launcher barrel acts as the low-pressure chamber. The ignition of the propellant gases also causes the drive band to engage the launcher grooves, similar to the American Civil War
American Civil War
The American Civil War was a civil war fought in the United States of America. In response to the election of Abraham Lincoln as President of the United States, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ; the other 25...
Parrot muzzle loading rifled cannon.
While little documentation exists, in the 1950s the Soviet Army
Soviet Army
The Soviet Army is the name given to the main part of the Armed Forces of the Soviet Union between 1946 and 1992. Previously, it had been known as the Red Army. Informally, Армия referred to all the MOD armed forces, except, in some cases, the Soviet Navy.This article covers the Soviet Ground...
developed 73-mm cannon for wheeled armored reconnaissance vehicles that fired a munition very similar in operation to the original World War II German concept. However, it was never introduced into service, and instead the Russians developed a low velocity 73mm cannon
2A28 Grom
The 2A28 Grom is the main armament of the BMP-1 infantry fighting vehicle. It is a 73 mm low pressure smoothbore semi-automatic gun with wedge breechblock.-Description:...
that fired a rocket projectile which was ejected by a small charge in the normal fashion.
Swedish use
Besides the previously mentioned family of popular 40-mm grenades, the only other major use of a High-Low System was by the SwedishSweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
firm FFV in their development of the 1960s-era Miniman
Miniman antitank weapon
The Miniman is a disposable single-shot 74-mm unguided anti-tank smooth bore recoilless weapon, designed in Sweden by Försvarets Fabriksverk and became operational in 1968. The Miniman is delivered with the HEAT projectile pre-loaded launch tube...
one man infantry antitank weapon. The Miniman was simpler and cheaper than anything imagined by designers in World War II. Inside what looked like a rocket launcher tube, is a HEAT projectile attached by a break away bolt to an alloy aluminum tube with ports drilled in in it and which acts like a kind of high-pressure chamber. The launch tube in which it is mounted acts at the high-pressure chamber. When the propellant is ignited in the aluminum tube, gases escape through the ports and are allowed to build up in the launch tube to the point of almost causing a recoil. The break away bolt then snaps, allowing the projectile to move forward. Unlike other High-Low Systems, gases are allowed to escape to the rear of the launch tube, achieving a totally recoilless effect.