Profile – North American F-100 Super Sabre

Back in February, 1949, engineers at NAA began to investigate the possibility of pushing the F-86 Sabre up to a maximum speed of Mach 1.

Their first study, basically increasing the aircraft’s wing sweep from 35° to 45°, did not give much improvement. Modification of the fuselage to reduce drag helped somewhat but it was still unable to cope with the huge increases in drag found when nearing the speed of sound. Even if the airframe had been suitable, the Sabre’s engine did not have sufficient power to achieve the desired result.

Engine manufacturers were quick to respond to the need. Potential powerplants were offered from the Allison Division of General Motors (an upgraded J35 turbojet) and General Electric (an advanced J47 turbojet with afterburner). NAA began a new series of drawings and studies around the latter. Features of the company’s Advanced F-86D design, with a 45° wing sweep angle and an area ruled fuselage and an Advanced F-86E proposal, with a redesigned slimmer fuselage and a nose inlet, were combined in a third proposal and given the new designation of Sabre 45 because of the wing sweep angle. The new aircraft was actually built around a Pratt & Whitney J57-P-1 turbojet rated at 15,000lb (6,803kg) thrust with after burner. Performance calculations for the Sabre 45 showed an anticipated maximum speed of Mach 1. 3 at 35,000 feet (10,670m), a combat radius of 580 nautical miles (1,073km) and a combat gross weight of 23,750Ib (10,771kg).

The company asked the USAF to fund the building of two prototypes, one for aerodynamic testing and the other for armament development. The nature of the times influenced the Air Force’s decision. East and West were in the middle of the Cold War, co-existing in a state of fluctuating tensions and there ‘!”as already heavy fighting in Korea. Following the recommendations of the Aircraft Weapons Board, the Air Force Council recommended that the plane should be bought in quantity before its flight testing programme was completed. The idea, known as ‘concurrency’, was to have a deliberately calculated overlap of research and development with production. It was argued that by building the prototypes with production tooling at the beginning, the aircraft would become operational at an earlier date. Faults found during flight testing could thus be fixed quickly during production. The risk was that faults found late in testing could result in expensive modification programmes after large numbers of aircraft had been built or were in the latter stages of assembly. The Council’s decision based on this concept was to prove costly. On November 1, 1951, the USAF authorised the building of two Sabre 45 prototypes and the beginning of an engineering design for production tooling for about 94 aircraft.

On December 7, 1951, the Sabre 45 received its official designation as the F-100A, first of the century series of American fighters. This designation also led to the eventual introduction of its slang name: ‘The Hun’.

Requirements for Speed

By June 1952 there had been various modifications made to the design. Reshaping of the fuselage was carried out to further reduce drag, the horizontal slab tailplane was moved to below the plane of the wing and an extended clamshell canopy replaced the sliding type for better streamlining. The hydraulic control systems were separated, a caution in case of battle damage and to save 60Ib (27. 2kg) weight the tailskid was removed.

Meanwhile, wind tunnel tests and studies at NASA’s forerunner, the National Advisory Committee for Aeronautics (NACA), brought forth suggestions to further reduce drag and increase thrust. To improve intake flow and deliver higher energy air into the engine, the inlet lip was thinned to a very fine edge and the nose itself was extended 9in (22. 9cm) more. A change which was too late for the two prototypes was reduction of the thickness/chord (t/c) ratio of the vertical fin and horizontal slab tailplane to 3½%. This was introduced on production aircraft.

The aerofoil of the Super Sabre’s 45° wing had been thinned down to a tic ratio of 8. 2% from that of the F-86 Sabre’s 10% and the addition of a glove aerofoil to its leading edge reduced it even further to 7%. The leading edge had five segment automatic air load slats to increase lift as conventional flaps had been eliminated because the ailerons were located inboard to prevent the chances of wing twist through aileron deflection.

Other features included small diameter twin nosewheels and a large door style airbrake faired into the belly of the fuselage in front of the main landing gear well. Air intake ducting bifurcated passed the cockpit and landing gear and then went up over the wing to the engine. A totally new innovation was the specification of an expensive new metal-titanium. It was light, about 40% less in weight than stainless steel, strong and tolerant of loads at high temperatures making it an ideal choice for the rear fuselage section around the engine afterburner duct. The use of afterburning, the burning of additional fuel in the engine’s exhaust gases to increase their velocity through the adjustable tail nozzle and so dramatically boost thrust, was in itself relatively new and George Welch remarked that it was like receiving “a kick from a well fed mule”.

The first YF-100A (52-5754) was completed ahead of schedule and ferried by road to Edwards AFB from where Welch took it up on April 24, 1953 and exceeded Mach 1 during its first flight. The second prototype (52-5755) had its maiden flight six months later on October 14 and just over a fortnight later, on the 29th, the first production F-100A (52-5756), which featured the new thin horizontal tail and cut down vertical fin, took to the air for the first time. That very same day Pete Everest flew the ·first prototype at an average speed of 755. 15mph (1,215. 04km/hr) over the Salton Sea course, California, to set a new world speed record.

At the same time the production line at NAA’s factory at Inglewood, California, was full of new F-100As and in a few weeks the 479th Fighter-Day Wing at George AFB, California would start converting. That October in 1953 the future looked rosy for the Super Sabre but in just 12 months the picture would have changed dramatically . . .

Disaster

On October 12, 1954, the crew of the Boeing B-47 Stratojet flying at 25,000ft (7,620m) over Palmdale, California, were attracted by a contrail up at about 45,000ft (13,000m) and watched as the speck in front of it rolled and commenced a long fast dive towards the valley floor. The plane passed well to their starboard side still hurtling down . . . then suddenly, unbelievably, it exploded into orange flame. Horrified, they radioed what they had witnessed but at the same time they saw a parachute.

The man on the end of the parachute was George Welch, the same George Welch who, on December 7, 1941 had been a lieutenant in the 47th Pursuit Squadron stationed in Hawaii. He had been one of the few pilots able to takeoff during the bombing of Pearl Harbour and that day he had shot down four Japanese aircraft. The same George Welch had flown the prototype Super Sabre faster than sound on its first flight back in May, 1953 and today he had been called upon to test the ninth F-100A (52-5764) in a maximum ‘G’ pull up from a supersonic dive. It had ruptured at the cockpit area and broken into fragments, some striking the test pilot. He had been fatally injured and died shortly after his parachute reached the ground.

Before Welch’s tragic death some Air Force and NAA test pilots including Pete Everest had already experienced directional control and stability problems with production F-100As and expressed their concern. Worse was to follow. By November 10 there had been six major accidents, two of them in the two preceding days with two fatalities. The Air Force grounded all Super Sabres pending the results of an investigation. A total of 70 had been delivered and a further 96 were undergoing testing or were on the production line.

Recording instruments recovered from the scattered wreckage of aircraft 52-5764 showed that when Welch pulled out of his dive the plane yawed violently and swiftly beyond capability of the vertical tail fin to correct and then swerved viciously out of control. The Super Sabre, now an out of line projectile travelling at supersonic speed, was blown apart by the force.

To rectify the problem was relatively simple – return to the taller tailfin of the two prototypes. The area of the vertical fin was increased by 27% and at the same time wingspan was increased 26ins (66. 04cm) to 38. 78ft (11. 8m) resulting in the wing area being enlarged from 376sq ft (34. 94m sq) to 385. 21sq ft (35. 8m sq). Modifications were also carried out on the aileron and stabiliser powered controls. These changes, which caused major disruption to NAA’s production programme, were carried out on all aircraft before the grounding order was lifted in February 1955. Those already delivered were modified at George or Sheppard AFBs. ‘Concurrency’ was expensive and proving to be no real shortcut. It was not until the 184th aircraft that changes arising from the accident could be built into normal assembly.

Production

The flow of new F-100s began again early in 1955 and the last of 203 accepted by the USAF was delivered in July. As an air superiority fighter, the ‘A’ model’s length of service in first line squadrons was short and it began to be phased out in 1958. Mainly, they went to the Air National Guard (ANG) and into storage except for an initial batch of 15 which was sent to Taiwan. More were sent later and, in the long run, of 118 Super Sabres delivered to the Nationalist Chinese over 58% were F-100As. Further changes during its production life included an improved cockpit introduced on the 104th aircraft and, with the 168th, introduction of the more powerful Pratt & Whitney J57-P-39 turbojet which superseded the J57-P-7.

NAA’s F-100B began as an advanced derivative of the ‘A’ model with a more powerful engine but it was destined for other things. As it was developed it began to change dramatically from its predecessor through the introduction of an all moving ‘slab’ vertical tail, spoiler ailerons and a large, fully variable dorsal air inlet to feed its new Pratt & Whitney J75 powerplant. It turned into a ‘next generation’ aircraft and went into comparison trials against Republic’s F-105 Thunderchief.

Meanwhile, Air Force requests for the Super Sabre to have fighter bomber capabilities and increased fuel capacity led to the F-100C. As early as October, 1952, NAA had been asked to study the possibility of modifying the aircraft’s wings to house integral fuel tanks. After an affirmative reply, it was later asked if this new ‘wet’ wing could have its structure strengthened in order to carry additional external stores. On March 12, 1954, the fourth production F-100A (52-5759) began to be modified into the prototype F-100C but, because of the difficulty in changing the wings of this fully completed machine, it remained a ‘dry’ wing prototype.

The first production F-100C rolled off the assembly line the following October 19 only to run into the grounding order. This delayed its testing until the large tail and long span wings were incorporated.

The F-100C’s ‘wet’ wing structure could hold a wide variety of equipment including external fuel tanks, chemical tanks, bombs, up to a dozen 5in (12. 7cm) high velocity air rockets (HVAR) and nuclear weapons on six underwing points. Included too was a detachable wing mounted inflight refuelling probe. Maximum fuel capacity was 451 US gallons (1,707 litres). The powerplant for most ‘C’ models was Pratt & Whitney’s J57-P-21, an upgraded version of its J57-P-7, which boosted speed by approximately 35 knots (65km/hr) to a maximum of 795 knots (1,475km/h) at 35,000ft (10,670m). An increased rate of climb by about 10% meant that the plane could reach that altitude from sea level in 3. 5 minutes.

To ensure that F-100Cs entered service rapidly, NAA made use of its newly acquired factory at Columbus, Ohio, as well as building them at Inglewood, California. The latter produced 451 aircraft and after tooling up Columbus added another 25. Before the last ‘C’ models were accepted by the USAF in July, 1956, a new absolute speed record of 822. 09mph (1,323. 03km/h) was achieved in an F-100C flown by Col Horace A Hanes USAF on August 20, 1955.

Like that of the F-100A, the front line service life of the F-100C was relatively short lived and the first were handed over to the ANG in the middle of 1959. This began a gradual phase out which lasted through to March, 1970. Unfortunately ‘C’ model Super Sabres were not regarded as fully fledged fighter bombers which was what the Air Force was actually after.

With NAA’s F-100D, the first of which flew on January 24, 1956, the USAF finally got what it wanted. This model featured a completely new wing which switched the two piece ailerons to the outboard position and employed broad slotted flaps inboard which created a new ‘kinked’ training ledge, a recognition feature which easily distinguished it from earlier Super Sabres. Sophisticated equipment included a Minneapolis-Honeywell autopilot and the newly developed Low Altitude Bombing System (LABS) for automatically tossing a nuclear bomb from low level in a steep zooming climb which gave the attacking aircraft a theoretical chance to escape the impending explosion. The wings also housed extra integral fuel tanks and there was a ‘buddy’ inflight refuelling system which could also be used for the probe and drogue technique developed by Flight Refuelling in Britain. Other changes included an even higher vertical tail surface and this fin also carried a larger fuel jettison fairing which later also housed the AN/APR-26(V) tail radar warning antenna. There were six underwing hard points for loads, like the ‘C’ model but these now featured forced ejection equipment instead of relying on gravity release.

The first F-100D built at the Columbus factory had its maiden flight on June 12, 1959, five months after its Californian counterpart and the following September first deliveries began the 405th Fighter Bomber Wing, Tactical Air Command (TAC), at Langley AFB, Virginia. A total of 940 F-100Ds were built at Inglewood and Columbus produced a further 334. As well as serving in the USAF, approximately 100 F-100Ds were supplied to the air forces of France and Denmark, where they enjoyed long and productive service careers.

F-100D Super Sabres were involved in several modification programmes. These included provision for Bullpup air-to-surface missiles, adapted to 65 aircraft; rewiring to incorporate Side-winder air-to-air missiles; and the last 148 production aircraft had strengthening built in for two Astrodyne rockets of 150,000lb (667KN) thrust for Zero-length launch capacity (ZEL).

To the pilots who flew it, particularly in Vietnam, the Super Sabre was a strong machine with a reliable weapons system and ‘honest flight characteristics’ but it was something of a brute to land. Consequently, it was an aircraft which suffered from an accident rate higher than most, so to NAA and the Air Force alike there was an obvious need for a two seat trainer in which pilots converting to the type could learn its handling characteristics. The manufacturers submitted a proposal to the USAF on September 2, 1954, which offered to modify, cost free, a standard single seat F-10OC into such a trainer. Aircraft 54-1966 was loaned to NAA for the purpose and it became the prototype TF-100C. Its fuselage was lengthened 3ft (. 914m) to accommodate tandem dual control cockpits which were enclosed by a one piece clamshell canopy. In its new form 54-1966 flew for the first time on August 3, 1956, but it crashed on April 9 the following year during a demonstration spin test.

Meanwhile, an improved version of the F-100D, designated F-1O0E was not built and therefore the new training aircraft were allocated the designation F-100F. With NAA pilot Mace Gage at its controls, the first production aircraft took to the air on March 7, 1957. This model utilises the same wing as the F-100D and it could carry a load of 6,000lb (2,772kg). Its armament was reduced from the usual four to two Philco-Cord M-39 20mm cannon with 350 rounds of ammunition. F-1OOFs were built exclusively at the Inglewood plant and 339 of them were delivered. The last of a total of 2,294 Super Sabres was delivered in October, 1959.

Combat

It was during the war in Vietnam that the Super Sabres, or ‘Hun’, proved themselves, historically a case of being available to be in the right place at the right time. They began flying sorties in Vietnam early in 1965 but prior to then, they had seen limited action in operations from Thailand in 1962 over northwest Laos. On February 8, 1965, F-100s participated in Operation Flaming Dart which was the first USAF attack on North Vietnam. This was followed by ‘Rolling Thunder’.

The Super Sabre was used briefly for MiGCAP and RESCAP duties but in terms of air-to-air combat it could not match NVAF MiG-17s and MiG-21s. Because of this and because of the aircraft’s ability to carry heavy loads of ordnance, it was eventually withdrawn from escort missions over the north and used primarily for close air support in the south.

A number of modifications improved the Hun’s combat effectiveness and life. These included a total wing centre section modification programme completed by 1969 involving 682 F-100s; installation of a Motorola SST-181 X-band radar transmitter for night and poor weather bombing; and for ‘Wild Weasel 1 ‘ operations, AN/APR-25 and IR-133 radar receivers for detecting S-band signals emitted by enemy SA-2 fire control radar and AAA radar, an AN/APR-26 crystal receiver to detect missile guidance launch signals, and a KA-60 panoramic camera. Beginning in 1966 Wild Weasel F-100s began using AGM-45 Shrike missiles which enabled them to attack missile control radar sites from a relatively safe distance.

From 1957-1970 when they were being superseded by McDonnell Douglas F-4 Phantoms and General Dynamics F-111s, Super Sabres equipped 16 Wings of the USAF and in Vietnam such was the intensity of operations that by 1969 the 3rd, 31st, 35th and 37th Fighter Bomber Wings alone flew more missions than the 15,586 F-51 Mustangs in World War 2. They were mostly F-100Ds and Fs although a relatively small number of F-100Cs were also deployed in South Vietnam.

By 1972, the last front line units at Cannon AFB, New Mexico, and RAF Lakenheath, UK, had converted from Super Sabres but the Huns continued on in ANG squadrons up until 1980. Denmark’s three squadrons of F-100Ds and Fs were not fully replaced by General Dynamics F-16 Fighting Falcons until 1982 while French F-100Ds were phased out by 1980. The largest user of second-hand Huns was the Turkish Air Force which acquired some 310 F-100Cs, Ds and Fs. At the time of writing only a few remained in service in the training role.

During its career, NAA explored the potential of the Hun as an interceptor, fighter-bomber, trainer and fast reconnaissance aircraft. As well, the USAF assigned model letters to four proposed sub-types. The F-100J was an all weather type offered to Japan through the Foreign Military Sales (FMS) programme: the F-100L was a variant replacing the Pratt & Whitney J57-P-21A afterburning turbojet with a J57-P-55; the F-100N was a simplified ‘D’ model with less electronic equipment; and the F-100S was an attempt by NAA to create a production line in France for 200 planes utilising F-100F airframes and Rolls- Royce RB168-25R Spey afterburning turbofans.

The F-100 design pioneered new ground into the little understood world of supersonic flight and despite the difficulties it had to overcome and an initial accident rate higher than many, it was strong and tough and came to be regarded with honest respect by the men who flew it.