The prospect of high speed, high altitude bombers captivated war planners on both sides of the Cold War. Advancements in jet technology and supersonic flight made such desires reality and many projects emerged from this period of military aviation history. The counter to such a bomber threat - prior to the arrival of effective missile defense networks - was the high-speed interceptor charged with racing to meet the foe head-on. This deterrent was as old as military flight itself - the dedicated interceptor bred for the sole purpose of intercepting inbound enemy threats.
In 1956, the USAF adopted the Convair F-102 "Delta Dagger" for the interceptor role and, in 1959, its improved successor followed as the Convair F-106 "Delta Dart". Over 1,000 of the former and 340 of the latter were realized during their respective production runs. The Delta Dart was of particular note for its Mach 2.3 speed and 29,000 feet per minute climb rate. To continue along this same design approach, a new high-performance, long-range interceptor initiative was being fleshed out by the USAF at the same time the F-102 was coming online and the F-106 was in the works.
The USAF specification of October 1955 envisioned a sleek interceptor capable of Mach 1.5+ speeds with a service ceiling up to 60,000 feet and an operational range out to 1,000 miles. The type would be charged with interception of current generation Soviet bombers threatening North American and European airspace. For survivability and the required speeds, the airframe would incorporate two engines in a side-by-side arrangement. The mission workload was to be spread about a crew of two. As with other interceptors of the period, the aircraft would carry an advanced Fire Control System (FCS) to aid in the delivery of ordnance - in this case bomber-killing missiles.
The specification was greeted by eight competing concerns though only Lockheed, North American, and Northrop were invited to further designs. Of the three, the North American design fit the USAF requirement best and its submission was accepted for development as the "XF-108" (the "Rapier" name was not given to the aircraft until May of 1959). However, all things came to an end when the program was terminated on May 9th, 1956 due to the usual suspects - budget issues and politics. Once the wrangling had ended, the program was resurrected in April of 1957 and two prototypes were ordered from North American Aviation.
The North American product (Model NA-257) began a lengthy design process which moved forward as requirements changed and technology advanced. Hughes was charged with development of the FCS to manage the missiles and General Electric supplied the engines. During this time, North American was also committed to the USAF's XB-70 "Valkyrie" Mach 3 supersonic bomber and, thusly, the XF-108 shared the XB-70s GE engine. The same ejection capsule of the XB-70 was also instituted in the XF-108. The XF-108's design lines also mimicked that of the XB-70 to an extent - the delta wing planform, canard foreplanes, the squared-off underside, etc...
Factors began to work against the XF-108 program - the Soviet commitment to effective missile defense networks (which rendered high-altitude supersonic bombers obsolete) and priority given to InterContinental Ballistic Missiles (ICBMs) for nuclear warhead delivery (nixing the strategic bomber approach) forced USAF interest in a new, expensive interceptor to wane. Early on, the XF-108 was to feature canard foreplanes for pitch control, a base delta wing planform, and no fewer than three vertical stabilizers (one on the fuselage and the other two at each of wing trailing edges). The finalized form was something of a rewriting of the aircraft - showcasing the continual evolution of this interceptor - which saw the canards and two of the three vertical stabilizers (upper sections only) removed while the delta wings became a "double-delta" arrangement (the main wing leading edge swept at 65-degrees and the wingtips with a 45-degree sweep. A full-scale representation of the XF-108 was presented for USAF review in January of 1959. Thirty-one developmental "YF-107" aircraft previously on order were by this time reduced to just 20 aircraft.
The XF-108 program was to produce nothing more than the aforementioned mockup for, on September 23rd, 1959, the USAF cancelled the interceptor. Some of the work put into the XF-108 was reconstituted for the more successful A-5 "Vigilante" reconnaissance-strike bomber by North American which was adopted by the United States Navy (USN) in 1961. The A-5 shared some of the design form of the XF-108 (retaining its fuselage and systems) and saw production reach 156 total units. Vigilante aircraft recorded combat service in the Vietnam War (1955-1975).
Performance specifications of the XF-108 included a maximum speed of Mach 3 and an operational range of 1,150 miles. Weight (gross) was 102,000 lb. 4 x 20mm cannons was to be standard armament while support for 2.75" rockets was to be included as was capability for carrying up to 4,000 lb of stores into action.
The program cost reached $142 million USD. During its early design phase, the XF-108 was also considered for the bomber escort role - escorting the B-70 (XB-70) Valkyrie into enemy airspace. This role was dropped as the XF-108 would have lacked the required range to see the B-70 all the way to and from its target area.
(OPERATORS list includes past, present, and future operators when applicable)
Ability to intercept inbound aerial threats by way of high-performance, typically speed and rate-of-climb.
✓X-Plane (Developmental, Prototype, Technology Demonstrator)
Aircraft developed for the role of prototyping, technology demonstration, or research / data collection.
89.2 ft (27.20 m)
57.4 ft (17.50 m)
22.0 ft (6.70 m)
50,927 lb (23,100 kg)
102,537 lb (46,510 kg)
+51,610 lb (+23,410 kg)
(Showcased structural values pertain to the base North American XF-108 Rapier production variant)
2 x General Electric YJ93-GE-3AR turbojet engine developing 29,300 lb of thrust each with afterburner.
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