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Beyond the Century Series: The US Air Force's Missing and Mystery 100s

In a system of sequential numbering that began with the Curtiss P-1 Hawk in 1924, the Air Force designated each successive fighter design with the next number in order. When they reached 100 with the North American Super Sabre, the fighters designated F-100 through F-106 became famous as the Century Series and, once the Air Force reached reached the F-111, the numbering of production fighters started over with the uniform numbering system in use today (with one significant exception). But there were a handful of aircraft that received 1xx designations that either never made production or even made it off the drawing board, along with some which weren’t US Air Force fighters at all.


Republic XF-103


An artist’s impression of the proposed XF-103 interceptor. (US Air Force)
An artist’s impression of the proposed XF-103 interceptor. (US Air Force)
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In the early days of the Cold War, before the advent of intercontinental ballistic missiles, the US feared that hordes of long-range Soviet bombers might attack America, most likely by flying over the North Pole. In order to meet the threat, the Air Force sought a high-speed interceptor that could reach the bomber fleets long before the bombers would reach the United States. Two of the proposals received by the Air Force did eventually became successful interceptors (the Convair F-102 and Lockheed F-104), but one of Convair’s entries was a truly radical aircraft powered by a combination turbojet-ramjet designed to reach speeds of Mach 4. The turbojet, with afterburner, would be used for takeoff, then once airborne, air would be fed directly to the afterburner to make it a pure ramjet. After reaching the target, the XF-103 would return to base on turbojet power alone.

The mockup of the XF-103. The placement of the large air intake behind the nose wheel led to serious concerns about foreign object debris getting sucked into the engine. (US Air Force)
The mockup of the XF-103. The placement of the large air intake behind the nose wheel led to serious concerns about foreign object debris getting sucked into the engine. (US Air Force)
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To handle all of this engine and fuel, along with weapons and a powerful radar, the XF-103 ended up being a very, very large aircraft. With all the titanium and stainless steel used to ward off the heat of supersonic flight, the interceptor weighed in at a whopping 20 tons. In an effort to reduce drag, a traditional cockpit was replaced with windows in the bullet-shaped nose, and the pilot was given a retractable periscope to look forward. To down Soviet bombers, the XF-103 was to be armed with six Aim 4 Falcon air-to-air missiles or four Falcons and two nuclear-tipped missiles, along with 36 Mighty Mouse FEAR rockets in an internal bomb bay. Nine years of development led to a single massive mockup before the project was canceled due to concerns over underpowered turbojets and a potential lack of performance.


North American F-107


(US Air Force)
(US Air Force)
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Not all the Air Force 100s outside of the Century Series were vaporware. North American became famous early in the jet era with their remarkable F-86 Sabre, and followed that with the larger and more powerful F-100 Super Sabre, the aircraft that kicked off the Century Series and was the first US fighter capable of supersonic speeds in level flight. While the F-100 was an eventual success, the Air Force still needed a tactical fighter-bomber. So North American took the Super Sabre and developed it into just such an aircraft, one that might be referred to as a Super-super Sabre or, less cheekily, the Ultra Sabre.

The third prototype North American XF-107 (55-5120) at Edwards AFB in 1959 (US Air Force)
The third prototype North American XF-107 (55-5120) at Edwards AFB in 1959 (US Air Force)
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The first thing the F-107 needed was a radar, so the nose air intake of the F-100 was moved to the top of the F-107, just behind the canopy. This large intake fed a Pratt & Whitney YJ75 turbojet that provided 24,000 pounds of thrust and a top speed of Mach 2+. While there were concerns that the F-107 had no rearward cockpit visibility, fighter doctrine of the time indicated that all future combat would take place with long range missiles rather than close-in dogfighting, so such a lack of view was unimportant. More important to the pilot, though, would be the thought of ejecting with a giant, knife-edge intake, nicknamed the “Man-eater,” behind him. However, North American assured pilots that they would be shot well clear of the aircraft. Fortunately, nobody ever had to find out.

All of the stencils. A detail of the XF-107 on display at the USAF Museum seems to indicate that North American printed the pilot’s manual on the side of the aircraft. (Tim Shaffer)
All of the stencils. A detail of the XF-107 on display at the USAF Museum seems to indicate that North American printed the pilot’s manual on the side of the aircraft. (Tim Shaffer)
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The F-107 competed against another Century Series aircraft, the Republic F-105 Thunderchief, and the Thud emerged the victor. Of the three F-107s were built, one was damaged in an accident and subsequently destroyed, one is in the collection of the Pima Air & Space Museum, while the third is on display at the National Museum of the United States Air Force in Dayton, Ohio.


North American F-108 Rapier


Artist’s impression of the North American XF-108 Rapier (Bzuk)
Artist’s impression of the North American XF-108 Rapier (Bzuk)
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At a time when speed was ultimate aim of bomber design, North American tried to outdo everybody with the development of the XB-70 Valkyrie, a massive six-engine bomber capable of speeds in excess of Mach 3. But just like the bombers of WWII, the the Valkyrie would need an escort on its long-range missions to attack (presumably) the Soviet Union, so North American undertook development of a smaller two-engine escort fighter with similar performance, in essence a baby Valkyrie.

An artist’s impression shows one of the delta wing configurations considered for the Rapier. Another was a classic delta wing. (Author unknown)
An artist’s impression shows one of the delta wing configurations considered for the Rapier. Another was a classic delta wing. (Author unknown)
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With its delta wing and long nose, the Rapier did in fact resemble the Valkyrie, though with a single tail, cranked delta plan, and no forward canards. The engines were the same General Electric J93 afterburning turbojets that propelled the XB-70, and the pair would have pushed the Rapier to a planned top speed of Mach 3. The cutting edge of the Rapier would be provided by three Hughes GAR-9A (later Falcon) air-to-air missiles housed in an internal rotary weapons bay.

Wooden mockup of the F-108 Rapier (US Air Force)
Wooden mockup of the F-108 Rapier (US Air Force)
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Though the XB-70 progressed to the completion of two prototypes, the loss of one in a crash, along with the development of intercontinental ballistic missiles, meant that the role for the Rapier had essentially evaporated. There was talk of converting it into a long-range reconnaissance asset, but with a proposed cost of $35 billion to develop the fleet it was simply too much money, and the Rapier never progressed past a wooden mockup. All of the effort put into the F-108 was not in vain, however. Much of the design work and many of the components from the Rapier project found their way into the North American A-5 Vigilante, a supersonic carrier-based reconnaissance aircraft built for the US Navy which bears a striking resemblance to its Rapier forbear.


Bell XF-109 (D-188A)


(National Archives)
(National Archives)
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By the middle of the 1950s, it had only been eight years since Chuck Yeager broke the sound barrier in the Bell X-1, and supersonic flight had become the norm rather than the exception. However, getting big fighters laden with fuel and weapons into the air required lengthy and improved runways, and the Air Force and Navy wanted a supersonic fighter that could land and take off vertically, either from short rough fields or the deck of a carrier (the Navy had found its first true supersonic fighter in the Douglas F4D Skyray in 1951).

The mockup of the XF-109 on display at Bell Aircraft (Bell)
The mockup of the XF-109 on display at Bell Aircraft (Bell)
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At the request of the Navy and Air Force, the Bell company began work on a radical new fighter with no less than eight General Electric J85 turbojet engines. Two engines were mounted on each wingtip in a rotating nacelle, while two more were mounted in the fuselage behind the cockpit to augment vertical lift. The remaining two engines were placed in the rear. The wingtip and aft engines were fitted with afterburners that provided 3,850 pounds of thrust, and the wingtip engine nacelles could also be rotated to 10 degrees beyond vertical to allow the XF-109 to move backwards in hover. All six vertical engines were needed to lift the nearly 24,000-pound fighter off the ground. Bell projected a top speed of Mach 2.3, and armament would have been provided by four 20mm cannons and a mix of rockets and bombs. Though the number XF-109 was never officially assigned (it was referred to by Bell’s internal designation D-188A), the project failed to advance beyond the construction of a mockup.

Illustration for article titled Beyond the Century Series: The US Air Forces Missing and Mystery 100s
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Though the XF-109 never made it into the air, German manufacturer Entwicklungsring Süd (EWR) did create a very similar jet fighter to the Bell XF-109 in the VJ 101. The VJ 101 had four jets in rotating wingtip pods, and two behind the cockpit mounted vertically. Unlike the XF-109, the VJ 101 didn’t have rear-mounted jet engines. EWR made two aircraft, labeled X-1 and X-2, and test flights transitioned from hover to forward flight, and even broke the sound barrier. However, like the XF-109, the project was not adopted.


McDonnell Douglas F-110A Spectre


(US Air Force)
(US Air Force)
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The use of the number F-110 was short-lived, and it was the original (unofficial) designation of the F-4 Phantom II in Air Force service. Since the Phantom II was originally designated F4J in Navy service, the Air Force would have used their own designation, but with the United States Tri-Service aircraft designation system implemented in 1962, the Phantom became the F-4 in Air Force, Navy and US Marine Corps service. Since F-110 was never used officially, the Air Force was free to use it for a different, clandestine project later.

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Project Constant Peg


MiG-21 Fished E, known to the USAF as the YF-110 (US Air Force)
MiG-21 Fished E, known to the USAF as the YF-110 (US Air Force)
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With the F-110 overlapping with the reset of aircraft numbering, the numbers 111 and above were free to use for different projects. From the beginning of the Cold War through 1990, the United States Air Force 4477th Test and Evaluation Squadron, based at the Tonopah Test Range in Nevada, flew captured Soviet- or Chinese-built fighters to determine their capabilities, test them in flight against US fighters, and develop tactics to counter them. In order to mask the true identities of these aircraft in Air Force inventories, they were given pre-production YF designations rather than their original Soviet designations.

A MiG 23MS (“YF-113C”) belonging to the 4477th Test and Evaluation Squadron leads a pair of US A-10 attack planes painted in European green livery (US Air Force)
A MiG 23MS (“YF-113C”) belonging to the 4477th Test and Evaluation Squadron leads a pair of US A-10 attack planes painted in European green livery (US Air Force)
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YF-110B: Soviet MiG-21F-13
YF-110C: Chinese Chengdu J-7B (MiG-21F-13 variant)
YF-110D: Soviet MiG-21MF
YF-112: Soviet Su-22
YF-113B: Soviet MiG-23BN
YF-113E: Soviet MiG-23MS
YF-114C: Soviet MiG-17F
YF-114D: Soviet MiG-17PF
YF-115: Skipped?
YF-116: Soviet MiG-25
YF-118: Soviet MiG-29


Lockheed F-117 Nighthawk


(US Air Force)
(US Air Force)
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With the numbers 110-118 taken up by Soviet aircraft, one number is notably absent from the list: 117. That number was given to the super secret F-117 Nighthawk, which came to symbolize the air war during the first Gulf War. While the Nighthawk is certainly well known, and a total of 65 were built and saw combat in Central America and the Middle East, many wonder why this tactical bomber received a fighter designation. Like the Soviet aircraft of Constant Peg, the Nighthawk was originally called the YF-117 to disguise its true mission, and over time the number stuck. The F designation stuck too, perhaps as a canard to mislead prying eyes, or as a means to attract top Air Force pilots who didn’t want to fly an attack jet or bomber.


For more stories about aviation, aviation history, and aviators, visit Wingspan. For more aircraft oddities, visit Planes You’ve (Probably) Never Heard Of.

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