By Joseph Trevithick
The U.S. military had first expressed interest in arming helicopters in the 1940s, but it was not until the Korean War that they saw just how useful the helicopter itself could be in combat. After having proven itself as transport and ambulance, the Army looked to find new roles for the helicopter. Using the helicopter as a scout was seen as having particular promise. As early as 1956, the U.S. Army had plans for a “Sky Cavalry.” In June of that year, Colonel Jay D. Vanderpool, then the Chief of the Aviation School’s Combat Development Office (CDO), was also tasked with investigating the idea of the armed helicopter, to give guns to this new breed of scouts and cavalry troopers among other things.
Guns for the “Sky Cav” were part of a larger effort organized by Colonel Vanderpool, which had both humble and inauspicious beginnings. His superior, Commandant of the U.S. Army’s Aviation School at Fort Rucker, Brigadier General Carl I. Hutton, believed in the work, but was fearful that his own superiors might not. The U.S. Army had already published a report in the early 1950s determining that helicopters were unsuitable weapons platforms. General Hutton therefore required Vanderpool to work in secrecy, with no official funding for the project. Vanderpool had to largely source the helicopters, weapons, and other equipment for his experiments himself, or with the help of those assembled in his new test unit.
Even so, in July 1956, within a month of the project being started, Vanderpool’s experimental unit, quickly nicknamed “Vanderpool’s Fools,” tested their first weapon system on a Bell H-13 helicopter from the Aviation School’s fleet. The composition of this first armament kit were two .50 caliber AN/M2 aircraft machine guns and four 8cm Oerlikon rockets. One machine gun and two rockets were mounted on an improvised frame on the landing skids on each side of the helicopter. The machine guns were mounted on the top of the framing and were fixed in position. Ammunition was fed to the guns from an open square metal box attached to the frame. The rockets were mounted under the frame.
The old AN/M2 machine guns, relics of the Second World War and a time before the creation of a separate U.S. Air Force, as well as the rockets, had been sourced from weapons the Aviation School had in storage from various other projects. The decision to use .50 caliber weapons rather than .30 caliber weapons also available was based solely on a desire to see what the H-13 helicopter could handle. While the first few firing runs went fine, the vibrations from the guns eventually shattered the helicopter cockpit canopy. It was found that the canopy was installed on the helicopter directly in contact with the canopy frame’s rivets. Soft-mounting the canopy solved the problem.
Vanderpool’s unit continued its work, using the same basic setup, but substituting the .50 caliber weapons for four .30 caliber AN/M2 machine guns, two on each side. These machine guns too were remnants from U.S. Army Air Forces stocks. Ammunition was fed to the guns from circular drums, one per gun, fitted to the outer edges of the framing. Each drum had two-hundred and fifty rounds of .30 caliber ammunition.
In March 1957, Vanderpool’s unit officially became the Sky Cavalry Platoon (Provisional) and began conducting demonstrations with its ever growing fleet of modified helicopters, including a large number of the light H-13 types. A much smaller number of similar H-23 helicopters were also acquired. As the U.S. Army transformed to its Pentomic force structure in 1958 and dropped the term “cavalry” from the lexicon, the Sky Cavalry Platoon was redesignated as the 7292nd Aerial Combat Reconnaissance Company (Experimental). Despite the name change, the focus of the unit’s experiments remained on developing armament for helicopters.
By the end of 1958, CDO had published a report for other interested units with step-by-step instructions on how to fabricate the numerous armament kits it had developed, especially the mounting hardware. It was hoped that this would help other units conduct their own experiments, as interest in arming helicopters was beginning to gain traction in the Army as a whole. No official nomenclature exists for these systems. In September 1968, University of Southern California’s Division for Aerospace Safety and Management published a research report entitled the “Evolution of the Armed Helicopter and Future Design Requirements” as part of a course requirement for their Fort Rucker Branch. In it, the author, Major C.O. Griminger, provides a simple and useful listing of the CDO’s systems using arbitrary letters, A through V, which will be used here. Kits A, B, C, E, F, G, H, I, and J involved systems featuring guns, as well as other weapons in some cases, for scout type helicopters. Kits A through H and J were developed by Vanderpool’s unit. Kit I was the only commercial armament kit to be tested. The other kits involved various other types of weapons, such as just rockets or missiles, and other types of helicopters.
The first two kits, A and B, were largely similar to those that Vanderpool’s men had originally experimented with. These were tested on H-13C helicopters. Kit B also improved on the initial design by adding a recoil absorbing mount designed by the Edgewood Arsenal for the four .30 caliber machine guns. Kit C consisted of just the guns from Kit A mounted on an H-13E helicopter. Kit E, mounted on an H-13E helicopter, replaced the rockets used on Kit A with eight 1.5” NAKA folding-fin rockets. The destructive power of the 1.5” rockets was determined to be too limited and their use was eventually discontinued.
Kit F, another kit for H-13 types, consisted of six .50 caliber AN/M2 machine guns, mounted on framing on top of the landing skids. Similar in overall design to earlier kits using these weapons for the H-13 series, Kit F’s weapons were initially fed from open ammunition boxes as on Kit A. The guns themselves were mounted with the inboard gun furthest forward and outboard weapon furthest back, to allow straight ammunition feeding. Kit F’s firepower was notably impressive, but was determined to be “overkill” and sub-variants were developed with only four or two guns fitted. These modified kits featured improved ammunition feeds using flexible ammunition chutes and purpose built magazines. This allowed the weapons in the four-gun arrangement to be fitted side-by-side. Modified versions of Kit F also featured various types of rockets.
Kit F was the only system to be tested on the H-23, as the ACR Company had very few of them to work with and decided to focus on the H-13 family. A variant of the four-gun configuration of Kit F, Kit G was developed for the H-13E, but with .30 caliber weapons instead of the .50 caliber types. In addition, two six-tube 2.75” rocket launchers were added. The machine guns fed from the same magazines used in Kit B.
Kit G overloaded the H-13E helicopter and proved to be the impetus in acquiring the more powerful H-13H helicopter. The first kit to be used with that type of helicopter, Kit H, consisted of two AN/M2 .30 caliber machine guns, again feeding from the magazines used in Kit B, a configuration that was becoming an integral part of armament kits for the H-13 type. Six individual launch tubes for the 2” T214 rocket were added to the machine guns to complete the system. The improved power of the H-13H made Kit H one of the best performing.
Kit J was the result of requests made to the Aviation School by other commands for advice on fabricating weapon systems of their own for H-13 helicopters. Vanderpool’s men had made good use of the School’s stock of old aircraft machine guns, but these weapons were rare in regular Army units. Kit J was a simple system consisting of two .30 caliber M37 machine guns, mounted one on either side of the aircraft, in a similar fashion as the other kits. The M37 machine gun, a derivative of the famous M1919 Browning machine gun family and the standard co-axial tank machine gun at the time, was far easier for regular Army units to acquire.
The sole commercial armament kit to be tested, Kit I, was the result of a conversation in 1956 between Colonel Vanderpool, Lieutenant Colonel F.C. Goodwin, and General Electric engineer Thurlow T. Mayhood in Burlington, Vermont. During the meeting, to talk about General Electric’s interest in developing a helicopter armament kit, a drawing for an armament kit made on a paper napkin was given to Mayhood. Mayhood and his colleague Jack Harding agreed to develop the kit. The kit was to be the first in a long line of helicopter armament kits developed by the General Electric Company.
As delivered the kit consisted of an entirely self contained pod with eight tubes for the 89mm T290 fixed fin aerial rocket centrally located between two of the Army’s new 7.62mm M60 machine guns. By the time the pod reached the ACR Company they had already expended their entire stock of T290 rockets and there was no intention to acquire more. The pod was then modified to fire more readily available 2.75” folding-fin aerial rockets. The more powerful rocket motors meant that the pod had to be modified and in the end only the center four tubes could be used.
The M60 gun was the product of a number of prototypes developed by the US Army, some of which had incorporated design features from weapons captured from the Germans during the Second World War. The M60 was intended primarily to be a successor to the Browning M1919 family. It was also hoped that it would obviate the need for submachine guns in infantry units and potentially replace other weapons as well. Unlike Browning .30 and .50 caliber machine guns used in many of the other tests, the M60 was not able to be converted to feed ammunition from both the right and left. Inside General Electric’s pod, the left gun had to be inverted to feed from the magazine, located behind each weapon.
The entire General Electric pod was suspended though a simple mounting arrangement consisting of metal brackets designed to clamp to the landing skids and lock into the pod. It was intended that the system could be mounted on any light helicopter using a similar tubular landing skid arrangement. Sales literature shows artists impression of the pod mounted on a Bell H-13 type, a Hiller UH-12 type, and a member of the Kaman K-240/K-600 family. In addition, the pod was suitably sized to allow two pods to be mounted to a single helicopter. In the end, the ACR Company only received one pod. The pod was mounted offset to the right side on an H-13 type.
As the ACR Company continued its work, the basic Kit J design also followed General Sutton to Germany when he took command of Seventh Army. Seventh Army developed a similar configuration, called the Seventh Army Fire Suppression Kit. Unlike Kit J, the Seventh Army’s kit was mounted on the helicopter fuselage on a frame between the landing skids, with the ammunition housed in the center of the entire assembly, feeding to the two M37Cs on either end. The M37C variant had specifically been designed for remote firing applications and was fitted with a solenoid trigger. The kit could be elevated four degrees to compensate for the aircraft’s forward motion. Three levers installed in the cockpit controlled this elevation, charging the weapons, and engaging the safeties. The Seventh Army’s kits represented the first design to receive some level of standardization and each kit was assigned a Federal Stock Number. This meant the armament kits could be ordered through the logistics chain for mounting on H-13H and -13G helicopters.
Kit J and the Seventh Army’s kit, both of which used a weapon system in active Army usage, were clearly the most viable of the early designs for light helicopters. Taking this to heart, Chief Warrant Officers Lawrence C. Hammond and Clarence J. Carter, members of the ACR Company, developed an improved armament kit derived from those designs. This new armament kit subsequently received the nomenclature KX-13.
The initial KX-13 kit was still using the .30 caliber AN/M2 machine guns, but laid the ground work for the first standardized helicopter armament subsystem. The first improvement to the design came in the form of a cushioned mount for the weapons, with the modified version becoming known as the KX-13-A1. A remote mechanism to charge the weapons, similar to that developed for the Seventh Army kit, was installed, along with a modification to the ammunition magazines and feeds, and the system designated as the KX-13-A1-1. The final design, the KX-13-A1-2, was fabricated out of aluminum to reduce weight. This system was tested with both the M37C and M60 machine guns, and served as the basis for Springfield Armory’s XM1 and XM2 armament subsystems.
Springfield Armory’s XM1 and XM2 armament subsystems, developed in the early 1960s, featured two M37C or M60 machine guns respectively. With the development of the remote firing M60C variant, this weapon was substituted for standard infantry M60s previously used on the XM2 system. The XM1 had magazines designed to hold six-hundred and fifty rounds of .30 caliber ammunition, while the magazines on the XM2 held two-hundred and seventy-five rounds of 7.62mm ammunition. The fully loaded .30 caliber magazines weighed a total of two-hundred and eighteen pounds. When loaded with 7.62mm ammunition, they weighed two-hundred and eleven pounds.
A product improved XM1, using flexible ammunition chutes and with minor changes to the mounting hardware was designated the XM1E1. Flexible feed chuting was also used on the XM2. The XM1E1 took an average of twelve minutes to reload on the ground in between missions. The guns could be swapped out if necessary in approximately four minutes, and the whole system took twelve hours to install on H-13 type helicopters. Both the XM1 and XM2 systems could also be installed on H-23 types.
The XM2 was issued to aviation units that saw combat in Vietnam. The armament subsystems were authorized to many unit types utilizing the H-13 and H-23 type helicopters, including what had become termed “Air Cavalry” units. The Army’s new doctrine of “airmobility,” which included the development and subsequent fielding of an entire airmobile division, the 1st Cavalry Division (Airmobile), had such units at its core. The new “Aeroscouts” were happy to use their authorized weapon systems and made modifications of their own. Still, the systems significantly reduced the aircraft’s speed and maneuverability. Desiring more flexibility and less weight, one or both machine guns were often removed during their service in Vietnam, replaced by an infantry-type M60 held by the helicopter’s observer. These weapons were also mounted to the cockpit canopy via bungee cord, as was being done on other helicopter types, or were mounted on a variation of the Army’s swing-arm mount for transport helicopters developed at the Army’s Sagami Depot on Okinawa in Japan. Individual weapons were also often carried, which also provided another means of defense for the crew if the helicopter was downed. Grenades and improvised bombs filled out the arsenal of the Aeroscout.
Despite their useful service, the H-13 and H-23 helicopter types were rapidly becoming outdated. Their age and vulnerability showed in Vietnam and there was a clear need for a replacement. The Army had long been interested in doing just this. In 1958, three potential replacements were tested, but none of the types were selected. In 1960, the Army had opened a new competition in conjunction with the Navy, eventually referred to as the Light Observation Helicopter (LOH) competition. At the behest of the Army, Hughes’ Model 369 was added to the competition, for which Bell and Fairchild Hiller had already submitted the D-250 and FH-1100 respectively. Armament was a key part of the competition, with each competitor required to submit their own designs for separate subsystems, one consisting of two M60C machine guns and another with a single 40mm M75 automatic grenade launcher, an entirely new type of weapon the US Army had developed. The winner of the competition would also supply the armament subsystems, to be designated XM7 and XM8. Only one system had to be mounted at a time. Fairchild-Hiller’s was the lone entry capable of mounting both simultaneously, one on each side. Hughes’ entry mounted the weapons on the left side of the aircraft.
By the time Hughes’ entry, designated YOH-6A, was declared the winner in 1965, the Army had eliminated the XM7 requirement and had modified the XM8 requirement for an improved automatic grenade launcher, the M129. The XM7 requirement had been dropped because of the development by General Electric of a family of modern Gatling-type multiple-barrel rotary guns. The Army requirement subsequently required Hughes to develop a system to incorporate the newly developed electrically driven variant of the rifle caliber member of the family, the XM134 (later standardized as the M134). The U.S. Army had also explored a gas-powered variant, the XM133, which was not standardized. Known popularly as the “Minigun,” as it was smaller than the earlier 20mm and .60 caliber designs, the M134 had an extremely high rate of fire, tested as high as six-thousand rounds per minute. In most applications the weapon was generally set to a rate of either two or four thousand rounds per minute. Two M60s could only combine for between one thousand and one thousand one hundred rounds per minute.
Hughes utilized much of the same mounting hardware that it had developed for its XM7 and XM8 designs for the new subsystem, subsequently designated as the XM27. The original design for the XM27 called for a large aerodynamic “pod,” which completely encased the weapon. A similar arrangement was designed for the initial XM8. In the end, the pod was deemed to be too complicated and it was reduced to a simpler aerodynamic fairing. The result system was designated as the XM27E1, which was in turn standardized as the M27. The finalized M27 design allowed the weapon to be elevated ten degrees and depressed twenty-four, and provided two thousand rounds of ammunition in a magazine mounted in the main cabin of the helicopter. The pod was also eliminated entirely on the revised design for the XM8, which did not receive a new designation.
Hughes also attempted to sell the Army on a weapon they were developing themselves, aptly named the “Heligun.” Hughes hoped that the benefits of this 7.62mm twin-barreled rotary breach machine gun, mainly its lower weight, would be appealing over General Electric’s Minigun. As in the case of the Minigun, the Heligun was in large part a scaled down version of a 20mm weapon, the Mk 11 that Hughes had previously sold to the US Navy. The weapon’s weight, twenty-five pounds less than the Minigun, meant it was also smaller. The Heligun’s diminutive size led at least one Hughes sales team to carry a demonstration weapon around in a violin case. The weapon was also self-powered, unlike the Minigun, and could reach similar rates of fire. In July 1965, the Army tested a Heligun mounted on an OH-6A helicopter using the same basic hardware used for previous Hughes armament subsystems. The tests were conducted at the U.S. Marine Corps base at Twenty-Nine Palms, California. The Marine Corps had been investigating the weapon as part of the Light Armed Reconnaissance Aircraft (LARA) program and the U.S. Navy had already assigned the designation EX 17 Mod 0 to the gun. The LARA program would eventually result in the adoption of the OV-10A aircraft by the Navy and the Marine Corps. Though tests of the weapon by the Army and the Marine Corps showed the weapon to be a viable design, both services decided not to adopt it. The Army stuck with the M134 Minigun for their helicopters and OV-10As, when so fitted, were armed with M60C machine guns.
As the OH-6A began to arrive in Vietnam in the late 1960s, Aeroscouts encountered many of the same problems that they had previously experienced with regards to armament subsystems for their previous steeds. As with earlier subsystems, the M27 subsystems were often removed entirely. Whether replaced or not, an observer seated in the main cabin with an infantry-type M60, often attached to the helicopter via a bungee cord, was often employed. Other experimental field modifications were also tried, including at least one instance of a pintle-mounted M134 in the rear cabin. Individual weapons and grenades also remained popular.
The utility of mounted weapons on scout helicopters was a matter of debate. A 1966 report by the Army Concept Team in Vietnam (ACTIV) had determined that both the existing OH-13 types and the new OH-6A were incapable of carrying the ordnance load deemed essential for operations in Vietnam. When ACTIV published their report on the OH-6A in 1968 it included the conclusion that the authorized M27 armament subsystem had proven to be both beneficial and detrimental in various operational scenarios. The matter remained undecided even as the decision was made to replace the OH-6A with the OH-58A helicopter, which was also capable of mounting Hughes’ M27 armament subsystem. Observers again supplemented or replaced the M27. The more spacious cabin of the OH-58A allowed two such individuals with weapons to ride in the main cabin if the subsystem was not fitted. The introduction of the first helicopters designed from the ground up as gunships further complicated matters.
In the period following Vietnam, OH-58As rarely carried their authorized M27 armament subsystems, except during exercises or annual gunnery qualifications. Only during the latter instance would they load and fire live ammunition. The remaining M27 subsystems were further modified to remove the aerodynamic fairing, leaving just the exposed gun and mount. These subsystems were designated M27E1. Improved OH-58Cs, introduced in the early 1970s, were not authorized to carry the M27E1 at all. Intended to act as advance scouts for attack helicopters in both cavalry and aviation units, first the AH-1 series and then the AH-64A, OH-58Cs were not expected to require any armament.
While it might have seemed like the era of guns for the Aeroscout was coming to an end, various events kept such development alive. The first of these were tests, associated with the Modem Army Selected Systems Test, Evaluation and Review (MASSTER), which involved a subsystem developed by Emerson Electric, initially sold commercially as the Mini-TAT, in reference to their Tactical Armament Turret (TAT) line. Emerson had designed the Mini-TAT to be able to be fitted to almost any type of helicopter. The subsystem featured a small turret armed with a Minigun that was mounted so that it would fold down from a stowed position and have a full three-hundred and sixty degrees of rotation below the helicopter.
In addition to MASSTER, the Canadian military was investigating the system for its UH-1N fleet and in 1978 even loaned some systems to the U.S. Air Force for them to use as part of the Joint Countering Attack Helicopters (J-CATCH) program. J-CATCH was intended to develop tactics to counter a perceived increased threat from enemy helicopters such as the Mi-24 Hind. The U.S. and the Canadians both decided against the system, which Emerson continued to sell as the Flexible Turret System (FTS). As a result of the sort of concerns that prompted the J-CATCH program, an air-to-air variant of the FIM-92 Stinger missile family, referred to as the Air-to-Air Stinger (ATAS), became the only authorized armament for the OH-58C.
Much of the impetus came in the end from special operations requirement for light armed helicopters for a follow-on attempt to rescue American hostages in Tehran, who were being held by revolutionaries following their overthrow of the Iranian monarchy. Development of a Light Combat Helicopter (LCH) resulted in two designs, one based on the OH-6A and one based on the OH-58C, both of which were armed with the M27E1 subsystem and a seven-tube 2.75-inch rocket pod on the opposite side of the helicopter. The JOH-6A LCH went on to become the AH-6C, the first of the “Little Birds.” The remaining JOH-58C LCHs were then appropriated for use by the 9th Infantry Division at Fort Lewis, Washington.
The 9th Infantry Division was at that time the test unit for the Army’s High Technology Test Bed (HTTB) / High Technology Light Division (HTLD) experiments. A cornerstone of the new Division was to be a Cavalry Brigade (Air Attack). As part of the tests, the Division established a Light Air Cavalry Troop (LACT): Troop C, 3rd Squadron, 5th Cavalry. The LACT was equipped with the JOH-58C LCHs, where the aircraft were themselves subjected to numerous modifications. In the end, no two aircraft were the same. C/3-5th Cavalry tested an array of weapons on the JOH-68Cs, including the M27E1 armament subsystem and the M18A1 gun pod, also fitted with an M134 Minigun.
Perhaps the most dramatic modification of all involved the fitting of a 20mm M197 rotary-barreled cannon in the nose of one of the aircraft. The gun itself had to be mounted in the co-pilot’s position, feeding from an ammunition magazine in the main cabin. A Stability and Control Augmentation System (SCAS) was fitted to help manage the weapon’s recoil. During tests it became clear that the SCAS was not sufficient, and the helicopter had a tendency to pitch severely nose downward when the weapon was fired, even with additional pilot intervention. The inability to maintain any sort of accuracy was compounded by the fact that large amounts of gun exhaust gases also filled the cockpit during firing runs, and the experiment was terminated.
By the end of the 1980s, the Army was becoming less and less interested in what the modified 9th Infantry Division had to offer. In addition, the program manager for the Advanced Helicopter Improvement Program (AHIP), working on what would become the OH-58D, had felt threatened by the Division’s usage of the JOH-58Cs. As a result, attempts to leverage AHIP components to improve the JOH-58Cs were consistently denied.
However, armament for these improved helicopters suddenly became important in 1987, as the war between Iran and Iraq expanded to threaten international shipping in the Persian Gulf. As part of Operation Prime Chance, a number of OH-58Ds with early armament subsystems (often referred to informally as AH-58Ds) were deployed aboard U.S. Navy ships to deter Iranian boats from attacking shipping and laying mines. Included in the armament package was a gun pod developed by Global Helicopter Technologies and sold commercially as the CFD-5000. Capable of being fitted with various different guns, the U.S. chose to fit a modified variant of the .50 caliber M2, designated the M296. This sub-variant had an adjustable rate of fire, from five hundred to eight-hundred and fifty rounds per minute, and no bolt latch, allowing for single-shot operation. The weapon was fed from a semi-external ammunition magazine loaded with five-hundred rounds of ammunition on the side of the fuselage.
After Operation Prime Chance, the Universal Weapons Pylon mounts and the CFD-5000 became standard options for the OH-58D helicopter. Neither subsystem received a U.S. Army designation. Though the gun pod could be fitted to either side of the aircraft, it was only authorized for mounting on the left-side of the aircraft. Throughout the 1990s and into the 2000s, the gun pod and the associated M296 machine gun remained the only gun armament option for the OH-58D.
In 2007, 7th Squadron, 17th Cavalry submitted an Operational Needs Statement (ONS) for a replacement for the M296 machine gun, which had been found to be unreliable. As a result, the Kiowa Product Office team and the U.S. Army Training and Doctrine Command Capabilities Manager at Fort Rucker, Alabama were tasked with finding a solution. In 2004, the Army Missile Command's Maintenance Operations Procedures and Prototype facility had begun looking at ways to repurpose stocks M3P variant of the .50 caliber M2 machine gun developed by Fabrique Nationale (FN). The M3P had been a standard armament on the Army’s Avenger air defense system, but had been determined to be of significantly less utility than the system’s Stinger missiles against the majority of the threats. As a result the guns, with rates of fire between nine-hundred and fifty and one-thousand one-hundred rounds per minute, had been removed. The M3P offered a good and cost-effective replacement for the M296. By 2009, units equipped with OH-58D were turning in their gun pods and M296 machine guns for M3Ps and a new, simplified recoil-absorbing mount. This new configuration had become the new standard.
The U.S. Army is currently looking to field the improved OH-58F and develop follow-on replacements as part of programs like the Armed Aerial Scout. As part of that development, the U.S. Army may well look forward similarly to developments being made in lightweight heavy caliber machine guns, such as the XM806, and previous lightweight cannon developments such as the XM301 as armament options. Progress made by private industry, such as with General Dynamics Armament and Technical Products GAU-19/B, a rotary-barreled .50 caliber weapon derived from General Electric’s .50 caliber design, will not go unnoticed either. Guns for scout and other helicopter types continue to provide cheap and functional alternative to missiles and guns will no doubt continue to be an important part of the Aeroscout arsenal.
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