World War 2 (1939-1945) thrust the idea of dedicated strike aircraft for the Close Air Support (CAS) role into the forefront of modern warfare. CAS provided ground forces with heavy fire support from the air, these aircraft and airmen charged with precision attacking of enemy soldiers and vehicles near allied forces. Such on-call firepower often spelled the difference between victory and defeat in the many offensives of the war.
During the war, fighter platforms were either modified for the role (becoming fighter-bombers) or specialized attack planes were developed. For the American Army in the war, overhead help was delivered by the likes of the Republic P-47 Thunderbolt, Lockheed P-38 Lightning, and others. This served the service well and helped in supplying heavy firepower against determined enemy ground forces during the marches on Rome, Berlin, and Tokyo.
In September of 1947, the United States Army Air Force (USAAF) evolved to become the United States Air Force (USAF) which took away direct CAS from the Army. Additionally, the Army was given restrictions on the type of fixed-wing aircraft it was allowed to acquire so as not to interfere with aircraft already in service with the USAF. This joint arrangement was first put to use during the Korean War (1950-1953) where many World War 2-era platforms were still in play and jets began their emergence. The CAS requirement was as alive as ever and fulfilled by one newer entry - the prop-powered, post-World War 2-era Douglas Skyraider.
This large aircraft proved a capable heavy-hitter that could haul large war loads while also providing loitering capabilities lacking in faster, thirstier jet aircraft. Jet aircraft certainly fulfilled an important role themselves - and were no doubt the way of the future - but their powerplants held little useful range (particularly for loitering) and their approach speeds were largely unsuitable for more precise attacking - and "second runs" on a target being highly dangerous actions.
As the American commitment grew in Southeast Asia (the Vietnam War of 1955-1975), many types of USAF aircraft were pressed into service in the CAS role including several World War 2-era stalwarts. However, these airframes were either largely unsuitable against a more prepared foe or simply past their prime usage years leaving the USAF to force its high-speed jets into the fighter-bomber role. The Army eventually adopted the Rockwell OV-10 Bronco (born from the Light Armed Reconnaissance Aircraft (LARA) program) but it still required a heavier hitter with better survivability as North Vietnamese Anti-Aircraft Artillery (AAA) became increasingly effective and surface-to-air missiles and enemy jets only worsened the issue. It was only a matter of time before enemy armor became a sufficient threat to forward operators on the ground as well.
No longer satisfied on USAF reliance for CAS, the Army moved on its own program - the Advanced Aerial Fire Support System (AAFSS) initiative - to help fill the requirement. To bypass the fixed-wing restriction, the aircraft was to be developed along the lines of a "compound helicopter" and offer speeds nearing 255 mph with a serviceable war load. This endeavor eventually became the abandoned Lockheed AH-56 Cheyenne attack helicopter, first flying in 1967, though the program was not to be and only ten helicopters were completed before the end.
Back in 1966 it was decided by the USAF to pursue the Attack-Experimental (A-X) program which was developed alongside the Fighter-Experimental (F-X), the latter becoming the famous McDonnell Douglas (now Boeing) F-15 Eagle air superiority fighter. The A-X itself was intended to fulfill the standing CAS role and called for a low-cost aircraft with inherent low-level maneuverability, good survivability for pilot and airframe alike, strong rough-field operational capability, a heavy ordnance load, a minimum speed of 400 miles per hour, and to have low maintenance requirements. The wish list was essentially a successor for the Skyraider which was performing admirably under the conditions presented in the Vietnam War.
Because of the low altitude operational nature of this new aircraft, cockpit and critical systems armoring would be a key factor to its survivability. Additionally, jet engines were be a requirement for the speeds envisioned of the machine and newer high-bypass engines becoming available offered the necessary range that the older low-bypass installations lacked. Much thought would have to be given to the internal makeup of the aircraft to support the USAF requirements in terms of positioning fuel stores, avionics, armament, systems redundancy, etc... Several American aircraft makers including Northrop were approached to develop separate design studies for the CAS platform.
During 1967, the enemy in Vietnam began using armored formations more frequently which posed a new risk to lightly armed and armored infantry and vehicle elements - pushing for a broader CAS concept. Battlefield experience in other global conflicts by other participants (mainly Israel) had shown the value of the 30mm cannon as an effective measure against tank armor from the more vulnerable sides, top, and rear of the vehicle. Tests were conducted with the existing American 20mm M61 Vulcan but this smaller-caliber weapon was ruled out and a proposal made for a new Gatling-style rotary cannon of 30mm caliber with a 4,000 rounds-per-minute rate-of-fire. There also stood the threat of Soviet armor crossing into Western Europe at this point in history which pressed the tank-killing quality of the new aircraft further. The cannon request was issued to several players with the key competitors becoming General Electric and Philco-Ford.
Despite the CAS aircraft proposal being sent to some twelve total companies, few took interest in the specific design. These key contenders became Cessna, General Dynamics, Lockheed, Northrop, and Republic (Fairchild-Hiller) and, of these companies, only Northrop and Republic were selected for their interesting submissions. Each aircraft was granted the formal designations of "YA-9" and "YA-10" respectively in March of 1971. General Electric eventually beat out Philco-Ford for the 30mm cannon contract and this product became the GAU-8 Gatling gun system. For the interim (while testing continued on the gun), the YA-9 and YA-10 would each carry a 20mm M61 in its place.
Northrop's YA-9 approach was largely conventional - its design arrangement somewhat reminiscent of an upsized Bell P-59 Airacomet - America's first jet-powered fighter emerging during World War 2. Straight shoulder-mounted wings allowed for the necessary number of underwing weapon stations and additional control at lower altitudes. The wings would also housing the needed fuel stores required for operational ranges and loitering times while also distancing the flammable substance from ignition sources. The cockpit was set aft of a nose cone assembly and under a bubble-style canopy offering excellent vision out-of-the-cockpit. The engines were set to either side of the fuselage while under the wings and exhausted well before the tail unit. The empennage consisted of a single vertical tail fin with upward-canted horizontal planes found along either side of the fin. A low-clearance undercarriage was fitted, the main legs under the aircraft's center mass and a nose leg installed under the cockpit.
The Northrop submission was a far departure from the Republic YA-10 - the competing design featuring engines held in individual, externally-mounted nacelles fitted between a twin tail rudder assembly. The wings were also straight as in the YA-9 but low-mounted and the cockpit also held under a bubble-style canopy for excellent vision. Republic's design was certainly a new kind of airplane for the period and offered benefits that the Northrop engineering team missed out on: the placement of the engines helped to shield their signature some from ground-based, probing radar and their well-spaced nature protected the loss of the entire aircraft - losing one engine did not doom the entire airplane. Additionally, high-mounted engines kept the intake openings from ingesting debris when attacking at low altitudes - a quality not found in the low-mounted engines of the YA-9. With the YA-9's fuel stores also residing in the wings, this complicated the armoring of the aircraft which was still not instituted in the design. The YA-10 held its ordnance externally as the YA-9 did and featured straight wing mainplanes with multiple hardpoints underneath. Its long landing gear legs provided better ground crew access to the weapons stations as well as rough-field operational service.
Performance between the two aircraft were on equal footing so it would be in the finer details of each aircraft design that would eventually see the YA-10 emerge as the winner. YA-9 carried 2 x Lycoming YF102-LD-100 turbofan engines developing 7,500 lb of thrust each while YA-10 was given 2 x General Electric TF34 engines. First flight of the YA-9 prototype - of which two were completed - came on May 30th, 1972 and the official "fly-off" between the two designs arrive on October 10th of that year. The competition continued into early December by which time the YA-10 was the winning bid announced the following month.
With their usefulness as CAS prototypes all but over, the Northrop aircraft were passed on to NASA for flight testing work. These airframes then spent years sitting underutilized at the Dryden Flight Research Center before being passed on to museum ownership. The first prototype was sent to Castle Rock AFB and the second to March Field. The former was then relocated to Edwards AFB with the closing of Castle Rock AFB (currently awaiting restoration (2015)) while the latter went on to full display at the March Field Air Museum, March Air Reserve Base in California.
The YA-10 became the storied A-10 Thunderbolt II and has since survived the Cold War military drawdown. Recent talk has emerged of retiring or selling off the A-10 fleet though there have been very vocal motions to keep the system flying for the foreseeable future. The A-10 became a proven tank-killer with its use in the Persian Gulf War where its 30mm cannon decimated Iraqi tank ranks. Its bomb-hauling capability and missile and rocket support only broadened the line's tactical value for warplanners and commanders. As such, the fate of the A-10 has yet to be written.