The North American XF-108 Rapier is a high-speed, long-range interceptor aircraft project developed by North American Aviation to protect the United States from Soviet supersonic bombers. To limit development costs, the program shares engine development with the XB-70 Valkyrie strategic bomber program and uses a large number of elements from previous interceptor projects. By 1959 only a mock-up had been built, before the project was canceled due to lack of funds and the Soviets adopted ballistic missiles as the primary means of nuclear attack. If it had flown, the F-108 would have been the heaviest fighter aircraft of its day.
Development
In the early 1950s, the USAF offered a very high performance long-range interceptor. On July 20, 1955, official development of what would become known as the Long-Range Interceptor, Experimental (LRI-X) was approved, intended to replace the F-102 Delta Dagger and F-106 Delta Dart2. The specification was filed on October 6, 1955, calling for an interceptor capable of flying at 18,000 m (60,000 ft) at a speed of Mach 1.7 (1,806 km/h) and with a range of 1,600 km. It must have a crew of two men and at least two engines2. A further consideration calls for the installation of a fire control system, allowing the interception of a bomber at 110 km (60 NM) and the destruction of three targets during the same mission.
Of the eight companies interested, contracts for preliminary studies were sent to North American Aviation, Lockheed and Northrop Corporation on October 11, 1955, five days after the specification was issued. On paper, North American's proposal, named "NA-236", is the most promising. The NA-236 shares similarities with what would become the XF-108, although the most obvious differences are the addition of two mid-span pinnules for the elevators, as well as a canard plane. Political and budgetary difficulties lead to the cancellation of the program on May 9, 1956.
Following considerable confusion, the program was revived on April 11, 1957 when North American received a contract for two prototypes. The project received the designation F-108, and is also known as "Weapon System 202A" (Weapon System 202A, WS-202A). The designation used by North American is "NA-257", although it is basically identical to the NA-236. At this time, the Air Defense Command (ADC) is planning an order for 480 aircraft.
The resulting model underwent considerable evolution, both due to its state-of-the-art technology and the continuous revision of USAF requirements. The first revisions relate to an aircraft with a prominent canard plan, a wingspan of 6.04 m (19 ft 10 in) and a sail sag of 53.5°. In this configuration, the aircraft must have a maximum takeoff weight of 45,088 kg (99,400 lb) and an operating ceiling of 22,110 m (75,550 ft). In addition to its role as an interceptor aircraft, the F-108 is also offered by North American as an escort fighter for its B-70 Valkyrie supersonic bomber prototype. Commonalities between the B-70 and the F-108 include the ejection capsule and General Electric YJ93 engines. Another role envisioned for the F-108 is to serve as a “gap-filler” in the DEW (Distant Early Warning) line system; due to its high speed, the F-108 must be able to sweep more than 720,000 m2 per hour.
From September 1958, substantial engineering and design changes were made; however, Strategic Air Command (SAC) lost interest in the escort fighter idea. To accompany the B-70 all the way to the target and back, the F-108, according to its initial characteristics, should have a marginal range at best. On December 30, 1958, the order for YF-108A pre-production aircraft was reduced from 31 to 20 test aircraft, and the first flight was postponed from February to April 1961. The final model of XF-108, of which a mock-up was built in full size, was presented to Air Force officers between January 17 and 20, 1959. The project was given the name "Rapier" on May 15, 1959, following a request for a suggestion made by the Air Defense Command to airmen.
Cancellation
Even as the XF-108 program progresses well, signs that will ultimately lead to its cancellation are beginning to appear. The unconfirmed bomber threats of Soviet long-range aviation, the overwhelming shift in favor of offensive and defensive nuclear missiles in the late 1950s and early 1960s, and rising costs, contribute to the termination of the XF-1088 contract. The cancellation was announced on September 23, 1959. North American continued to refine the design throughout 1960, hoping the program could be revived. Despite the extra money invested and time spent on the Rapier, the work is not entirely in vain; the A-5 Vigilante supersonic reconnaissance aircraft developed for the Navy retains the Rapier fuselage/armament and systems. In many ways, the Vigilante can be seen as the successful application of Rapier design principles to a supersonic model.
Hughes Aircraft wishes to continue the development of the advanced fire control system and the GAR-98 missile. Development of the F-108's radar and missiles was continued by the USAF, and the system was eventually used in the YF-12 program. The final configuration for the rear cockpit on the YF-12A resembles that of the F-108 since it features the same displays and controls required for the Hughes ASG-18 fire control system.
Technical description
In its initial configuration, the F-108 is equipped with a very large "angled" delta wing. Small fixed vertical stabilizers are placed under the wing, mounted at mid-span, and the large fin is fully movable, complemented by two ventral stabilizers that deploy when the landing gear retracts. Although some early versions of the model featured a distinct empennage and later a canard plan at the front, these were dropped in the final design. The large fuselage and wing have two and five fuel tanks respectively, which give the aircraft an estimated range of 2,000 km (1,100 NM). The maximum speed is estimated at 3,190 km/h (Mach 3) at an altitude of 24,900 m (81,800 ft)5. The aircraft is powered by two General Electric J93 turbojet engines, also used on North American's XB-70 Valkyrie bomber, located in the fuselage.
The XF-108 is initially to be fitted with the Hughes AN/ASG-18 radar, the first US pulsed Doppler radar set. He must have an ability to look down and shoot, but he can only track one target at a time. The radar is coupled with an infrared search and tracking (IRST) system placed on the leading edges of the wings. The radar is used to guide the Hughes GAR-9 air-to-air missile (subsequently redesignated AIM-47 Falcon), three of which are to be carried in a rotating launcher placed in the weapons bay, inside of the plane8. The GAR-9 is a very large, long-range weapon that has its own radar set for terminal homing. It must fly at Mach 6, with a range of 180 km.
Builder North American Aviation
Role Prototype interceptor aircraft
Status Program canceled in 1959
Investment $141.9 million
Number built A model
Crew :2 members
Motorization
Engine General Electric YJ93-GE-3AR
Number 2
Type Turbofan engines with afterburner
Unit thrust 93 kN dry 130.3 kN with afterburner
Dimensions
Wingspan 17.5 m
Length 27.2 m
Height 6.7 m
Wing area 173.4 m2
Masses
Empty 23,098 kg
Maximum 46,508 kg
Performance
Maximum speed 3,190 km/h (Mach 3+)
Ceiling 24,400 m
Range of action 2,033 km
Endurance 4,004 km
Wing loading 183.4 kg/m2
Thrust to weight ratio 0.56
Armament
Internal 4 x 20mm cannons
External 3 Hughes GAR-9A air-to-air missiles in a rotating bomb bay
1,814 kg (4,000 lb) of bombs