Both aircraft were designed in response to the A-X air force requirement for a Close Air Support (CAS) aircraft that could operate from forward positions on a battlefield. This meant an aircraft that would need to be able to operate from relatively rough fields, carry heavy armor, and be able to loiter over target areas. It was purpose-built modern replacement for the A-1 Skyraider which could perform the ground attack mission over Vietnam more effectively than faster supersonic jets. While turboprops were considered in some designs for the A-X project, turbofans were selected for the flyoff prototypes. The A-9 used the Lycoming F-102 engine, while the A-10 used the GE TF34 as used in the contemporary S-3 Viking and proposed for use in the E-3 Sentry.1 The YA-9 saw its first flight May 20, 1972.
Apart from aircraft themselves, the A-X program was also developing a powerful cannon for the winning plane to carry. Eventually, this would result in the GAU-8 Avenger, but for trials, the YA-9 and YA-10 were both equipped with the smaller M61 Vulcan Gatling gun. By comparison, the A-10 engine location at the tail is less conventional, but offers greater survivability in the case of a hit on the engine area. The double tail of the A-10 also hides the engine infrared and noise signature, and is redundant in case one of the tails is shot away.
It is alleged that the YA-9 more strongly impressed the Soviets than the US Air Force, and the Su-25 'Frogfoot' was largely based on spy footage of this aircraft. Both aircraft put their engines below the wing root. However, actual resemblance is very superficial as the Su-25 does not use high-bypass engines, and work on what later became the Su-25 began in 1968, four years before YA-9's first flight.
A fly-off of the two prototypes took place between October 10 and December 9 1972, with the YA-10 declared the winner on January 18 1973. The two YA-9 prototypes were subsequently relegated to NASA for continued flight testing before being quickly retired. One is preserved at the March Field Air Museum at March Air Reserve Base. When retired, the YA-9s' custom-built engines were removed and were later mated to a C-8 Buffalo airframe as part of the NASA-Boeing joint QSRA study into a quiet short-haul commercial aircraft.
length: 53 ft 6 in (16.3 m)
span: 57 ft 0 in (17.4 m)
height: 17 ft 10 in (5.4 m)
area: 580 ft² (53.88 m²)
empty weight: 23,076 lb (10,467 kg)
loaded weight: 28,575 lb (12,961 kg)
max takeoff weight: 41,795 lb (18,958 kg)
engine: Lycoming YF102-LD-100
type of engine: turbofans
number of engines: 2
thrust: 7,500 lbf (33.4 kN)
max speed: 523 mph (837 km/h)
thrust/weight ratio: 0.33