Tactical Fighter Experimental TFX
The General Dynamics (GD) F-111 is unarguably the most controversial fighter-attack aircraft ever developed. For both the Air Force and the Navy, the requirements were often in flux during development, but essentially both services' performance needs were not aerodynamically compatible in a single aircraft. The TFX program failure has haunted joint programming and people's opinions of joint programs ever since. It showed that a secretary of defense, however "strong," cannot always get the services to do what they strongly oppose. It suffered from a nearly impossible multi-role/multi-service requirement specification, and a protracted development cycle in which numerous serious technical problems had to be identified and corrected.
In 1961, Secretary of Defense Robert S. McNamara initiated the Tactical Fighter Experimental (TFX) program for the US Navy and Air Force. McNamara believed that Navy and Air Force requirements for a new tactical fighter could best be met by development of a common aircraft. McNamara defined the basic mission requirements when the Air Force and Navy could not agree, and in October 1961, a request for proposals (RFP) was issued to industry. McNamara made his fateful choice of the F-111 as the bi-service, limited-war fighter for the 1960's from a menu of specific operational requirements defined by the Air Force (and indeed by the Tactical Air Command) alone, and which reflected TAC's interests rather than those of the Secretary of Defense or the President. As a result, the fighter that emerged did not have capabilities suited for the Kennedy Administration's limited war strategy, but instead was designed for a nuclear mission McNamara meant to de-emphasize.
All the Secretaries of Defense since Robert S. McNamara (1961-68), who launched the TFX (F-111) aircraft development, the first cross-service major system program, have pushed joint programs. The practical problem, however, is that a secretary can order a program merger, but cannot mandate performance or degree of interservice cooperation. DOD has more management by negotiation than many critics appreciate. Choices among rival service systems are very difficult for the Secretary of Defense: consolidating programs represents a major challenge.
Secretary McNamara, who closely supervised the F-111 aircraft joint program from the start of his term, contended with considerable opposition and deep conflict in biservice requirements all through weapon development. One month after he left office, the decision was made to cancel the Navy version. The Air Force continued to develop the F-111 and the Navy went on to develop the F-14.
In 1957 the US Navy requested industry responses for the design of a low-altitude strike fighter. John Stack of NASA Langely briefed senior Navy managers that a proposed British low-altitude strike fighter, the NA-39, would be much more advanced than the Navy aircraft. He also suggested the application of variable sweep to leapfrog the capabilities of the NA-39. Following briefings by Langley personnel to the Navy, the mission specifications for the new Navy fighter were expanded to include multimission capability with a requirement that variable-sweep applications be studied. The request for proposals went to industry in early December 1959 and set the stage for what would ultimately become the Tactical Fighter Experimental (TFX) Program.
In the late 1950s the Tactical Air Command (TAC) of the USAF expressed a future need for a replacement for the F-100, F-101, and F-105 fighter-bombers which were currently in service. With this goal in mind, on March 27, 1958, the Air Force issued General Operational Requirement (GOR) Number 169, calling for Weapon System 649C, which was a Mach 2+, 60,000 foot altitude, all-weather fighter capable of vertical and short takeoff and landing. The Air Force wanted this aircraft to be ready for operational deployment by 1964.
Air Force Tactical Air Command (TAC) Requirements Division at Langley Air Force Base (adjacent to the NASA Langley Research Center) was attempting to define a replacement for the F-105 fighter-bomber aircraft. TAC was interested in an aircraft that could carry nuclear weapons internally, fly transatlantic routes without refueling, operate from semiprepared fields in Europe, have a top speed of Mach number of 2.5 at high altitudes, and fly at high subsonic speeds at low altitudes. The aircraft would perform a "low-low-high" mission, wherein it would cruise into the vicinity of the target at low altitudes and subsonic speeds, perform a low-altitude dash to the target at high subsonic speeds, and perform a high-altitude, long-range cruise back to base at subsonic speeds. The Mach number of 2.5 capability would be used for high-altitude engagements against enemy fighters.
Initial analysis by industry of the request indicated that a fixed-sweep aircraft capable of meeting the requirements would weigh in excess of 100,000 lb (too heavy for unprepared fields) and demand the attributes of low sweep for transatlantic flight, but high sweep for the high-speed requirements. TAC was therefore in a stalemate without a viable design approach to its requirements.
John Stack approached the TAC planners in 1959 with the benefits of variable sweep to enable an aircraft to meet the requirements. The extended ferry range that is provided by variable sweep was of prime importance to TAC, since estimates indicated that transat-lantic range might be possible. Together with the commander of TAC, Stack laid out a realistic set of aircraft performance requirements that included the desired low-altitude dash capability at high subsonic speeds. [Unfortunately, as the requirements went through the TAC system for approval, the final specifications called for a 210-n-mi, sea level dash at a speed that had increased from a Mach number of 0.9 to a Mach number of 1.2. Upon learning of the supersonic low-altitude speed requirement, NASA Langley quickly informed the Air Force that this capability was impossible to meet for the range specified. Nonetheless, TAC was committed to the unrealistic specification. (In flight tests of the F-111A in 1969, the actual low-altitude supersonic dash performance of the aircraft was only 30 n mi.)].
General Operational Requirement (GOR) Number 169 lasted only a year. GOR 169 was cancelled on March 29, 1959, the Air Force recognizing that a V/STOL fighter capable of such performance was simply not feasible with the current technology. On February 5, 1960, the Air Force rewrote its requirements and issued System Development Requirement (SDR) No. 17, incorporating most of the provisions of GOR-169 but eliminating the VTOL requirement. It allowed the subsequent development of specific requirements for a new weapon system--WS-324A.
The general requirements of SDR-17 were brought together into Specific Operational Requirement number 183 (SOR-183), issued on June 14, 1960. It called for an attack aircraft capable of achieving a Mach 2.5 performance at high altitude and a low-level dash capability of Mach 1.2. It was to have a short and rough airfield performance, and was to be capable of operating out of airfields as short as 3000 feet in length. The low-level radius was to be 800 miles, including 400 miles right down on the deck at Mach 1.2 speeds. In addition, it was to have an un-refueled ferry range capable of crossing the Atlantic Ocean. It was to have a 1000-pound internal payload plus a lifting payload between 15,000 and 30,000 pounds. Based on NASA Langley research, the Air Force considered that a variable sweep wing and a turbofan engine would be needed to satisfy these diverse requirements.
At the same time, the Navy had a requirement for a two-seat carrier-based fleet air defense (FAD) fighter that would replace the McDonnell F-4 Phantom and the Vought F-8 Crusader. This aircraft was to have the ability to loiter on patrol for much longer times with substantially larger and more capable air-to-air missiles, and was to be able to meet and counter threats to the carrier group at much larger ranges.
Originally, the Navy had planned to meet this FAD requirement with the Douglas F6D-1 Missileer. The F6D-1 was a subsonic aircraft that looked a lot like a scaled-up F3D Skyknight. It was to be powered by two 10,000 lb.s.t. Pratt & Whitney TF30-P-2 turbofans, and was to carry a three-man crew (pilot, co-pilot, and weapons system operator). The Missileer was to be capable of remaining on patrol for up to six hours, tracking targets at long range using its powerful Hughes pulsed-Doppler track-while-scan radar and attacking threats with its six long-range Bendix XAAM-10 Eagle air-to-air missiles. The Eagle was a massive long-range air-to-air missile with a maximum speed of Mach 4. It was equipped with an advanced pulse-Doppler active radar homer. The warhead of the Eagle could be either conventional or nuclear.
The F6D aircraft was considered by the Navy to be too costly and too specialized, and was thought to be too slow to be capable of defending itself once its missiles had been launched. Consequently, the F6D and its Eagle missiles were both cancelled in December of 1960 in the waning days of the Eisenhower administration. This still left the FAD requirement unfulfilled.
The Air Force and Navy requirements were at first sight completely different. However, on February 16, 1961 the new Secretary of Defense, Robert McNamara, directed that the Services study the development of a single aircraft that would satisfy both the requirements of the Air Force's SOR 183 mission and the requirements of the Navy's FAD mission. In addition, McNamara wanted the aircraft to be capable of being used by the Army and the Marine Corps as a close-support aircraft. It was hoped that this strategy would reduce procurement costs substantially. The project came to be known as the Tactical Fighter Experimental, or TFX for short.
It did not take long for the services to convince Secretary McNamara that the close air support mission requirement could not be satisfied by the TFX, and the Marine Corps and the Army were dropped from the program at an early stage. However, Secretary McNamara stuck doggedly to his idea of maximum commonality between USAF and Navy versions of the TFX, and in June 1961, he instructed the Air Force and the Navy to work closely together to combine their requirements before issuing a joint RFP, although both the USAF and the Navy thought that this idea was completely unrealistic.
McNamara and Director of Defense Research and Engineering Harold Brown also directed that the TFX Air Force and Navy variants would be over 80% common in the airframe, engines, subsystems and avionics in order to save what DOD estimated to be a billion dollars in development cost for a joint service aircraft compared to two service-specific development programs. The commonality was to be both in terms of structural weight and parts count. However, conflicting requirements between AF and Navy missions made a “common” TFX or F-111 a nearly impossible task for the contractor and government to accomplish. The mission analysts at the Air Force and Navy were very aware of this, as was the contractor, but DDRE, SECDEF and the civilian leadership at the Service Secretary level believed that a common system would save development cost.
Both the USAF and the Navy agreed that the use of variable-geometry wings would be a good idea. However, on just almost everything else, they differed substantially. The Navy favored side-by-side seating for its FAD fighter, whereas the Air Force preferred tandem seating. The Navy wanted an aircraft equipped with a long-range search and intercept radar having a dish 48 inches in diameter, whereas the Air Force needed an aircraft equipped with a terrain-following radar optimized for low-altitude operations. The Navy wanted an aircraft that was optimized for long loiter times at medium to high altitudes at subsonic speeds, whereas the Air Force insisted on an aircraft capable of low-altitude operations and supersonic dash performance. Undaunted, Secretary McNamara pressed forward with the project and directed that the Air Force would be the lead service for the development of a common TFX aircraft.
By August of 1961, the Secretary of the Navy reported to Secretary McNamara that the compromise TFX design could not meet the Navy requirements. The Air Force wanted an aircraft weighing 75,000 pounds gross, while the Navy wanted the gross weight to be kept below 50,000 pounds. In addition, carrier operational requirements necessitated that the overall length be kept below 56 feet so that it could fit aboard existing carrier elevators. McNamara ordered the Navy to accept a design sized to accommodate a 36 inch radar rather than the 48 inch radar it really wanted and to accept a gross takeoff weight of 55,000 pounds.
The design of a weapon system to meet the common performance requirements and over 80% empty weight commonality while maintaining cost, weight and schedule, was clearly shown to be exceedingly difficult in the four rounds of proposals received from industry in late 1961 through late 1962. Most of the industry, including one of the finalists, Boeing, conceded that it simply could not be done and proposed a design which was about 40% common based on structural (empty) weight. General Dynamics and sub-contractor Grumman proposed what they believed to be an innovative design that, although close to weight limits for the Navy version, could exceed the 80% commonality requirement.
An example of a trade-off which was not permitted by the DOD leadership was the analysis of the impact of dash speed at sea level versus sea level dash distance and the resultant impact on survivability against enemy defenses. The political climate from SECDEF, and from Congress, tied the hands of the Program Director.
From the systems architecting and conceptual design standpoint, the question should have been – very early in the conceptual design process- “what is the survivability benefit of Mach 1.2 versus Mach 0.7 or 0.8, or whatever Mach number is just below the ‘drag rise’ for the F-111 aerodynamic configuration?” The 200 NMi dash could have been met easily if the dash Mach number was reduced to subsonic, rather than Mach 1.2.
This dash-range/mission-range trade off analysis was shown in the original TFX proposal by GD in 1962, but was never pursued further because the SPO believed that the Tactical Air Command absolutely required Mach 1.2. GD did not do a survivability analysis because in 1962, the threat models were not accurate enough to perform a credible analysis. Operations research analysts at Wright-Patterson questioned whether the increase in survivability from enemy defenses would be significantly increased by a Mach 1.2 dash compared to Mach 0.8. The analysts concluded that the survivability increase was marginal, but the impact on aircraft weight to penetrate for 200 NMi at sea level flying at Mach 1.2 was substantial.
It was well known (in the 1950’s) from aerodynamic theory and test that Mach 1.2 could be within the “transonic drag rise” which occurs when an aerodynamic body accelerates from about Mach 0.7 to about Mach 1.4. This Mach range (dependent on body cross-sectional area distribution and ratio of body length to diameter) is where the shock waves are nearly perpendicular to the surface and the resultant pressure distribution on the body creates the most drag.
The cross-sectional area distribution of the F-111 was poor for transonic drag because the Navy required that the two crew members would sit side-by-side rather than in tandem. In order to maintain “at least 80% commonality between AF and Navy variants” – as required by the TFX Request for Proposal, General Dynamics designed the TFX with the Navy cockpit arrangement. General Dynamics knew the drag at Mach 1.2 would be high but their analysis, based on data from a small scale model in the Cornell Aeronautical Lab transonic wind tunnel, indicated that the 200 NMi dash requirement could be met.
NASA Langley had tested an F-111 model in their larger transonic wind tunnel and the drag was indicated to be considerably higher. Using the NASA data, the 200 NMi dash would be less than 100 NMi. Eventually the NASA data was shown to be more nearly correct, and when all the flight test data was in, the predicted dash distance was only 39 NMi. In 1970, this was pointed out by the US Senate Permanent Subcommittee on Investigations of the Committee on Government Operations, (chaired by Senator John L McClellan (D) of Arkansas), as a serious deficiency.
In 1978, at the 67th Wright Memorial Lecture, David S Lewis, Chairman and CEO of General Dynamics said: “The F-111 is truly a remarkable aircraft but unfortunately is very heavy, expensive and has poor reliability. Had more thorough tradeoffs been made at the outset, it is almost certain that a decision would have been made that a sea level dash speed of Mach 0.8 or 0.9 would have had an acceptably high probability of survival. The airplane would have been smaller, simpler and much cheaper. The USAF could have afforded many more and the effectiveness of the overall inventory would have been much higher for the dollars expended”.
On September 29, 1961, a Request For Proposals was issued to Boeing, General Dynamics, Lockheed, Northrop, Grumman, McDonnell, Douglas, North American, and Republic. The Air Force's version of the TFX was to be designated F-111A, with the Navy's version being designated F-111B. In the spirit of commonality, the Air Force and Navy versions did not carry separate designation schemes.
Nine responses were received in early December of 1961. Only Northrop turned down the invitation to submit. In their first evaluation of the proposals on January 19, 1962, the Air Force Selection Board and a Navy representative endorsed the Boeing proposal, but the Air Force Council rejected the Boeing bid as requiring much more work. In late January of 1962, both the Air Force and Navy agreed that none of the proposals were really acceptable, but that two of them --- the Boeing and General Dynamics proposals --- warranted further study. A letter contract was issued to each company requesting more design data.
In the spring of 1962, Boeing and General Dynamics submitted second proposals. In May of 1962, both the Air Force and Navy Secretaries rejected the two contractor's second proposals for lack of sufficient data. A third submission took place in late June. At this time, the Air Force endorsed the Boeing proposal, but the Navy was unhappy with their version and refused to commit themselves. A frustrated Secretary McNamara ordered a final competition for later that year on the basis of a point system for categories based on performance, cost, and commonality.
Boeing and General Dynamics resubmitted their fourth round of proposals in September of 1962. The Air Force Council, the Air Force Logistics Command, and the Bureau of Naval Weapons (the Navy organization which had replaced the Bureau of Aeronautics in 1959) all indicated that they preferred the Boeing design.
The Air Force and Navy military operators favored Boeing because two structurally different planes would not be compromised as much as a common design to meet disparate service performance requirements – in effect, two different airplanes, as originally desired by AF and Navy. But to SECDEF McNamara, commonality in the GD/Grumman design validated his fundamental premise that a joint-service fighter was feasible, and would be most likely to realize the cost savings inherent in the TFX joint development approach, so he unilaterally over-turned the source selection board recommendation for Boeing.
On November 24, 1962 the Defense Department announced that the General Dynamics design had been selected. The reason given for the selection of the General Dynamics proposal was its promised greater degree of commonality and its more realistic approach to program cost.
A political storm broke out, with Senator Henry Jackson [aka "The Senator from Boeing"] leading the fray in Congress in loudly denouncing the choice in no uncertain terms. A series of congressional investigations led by Senator McClellan from Arkansas was initiated in 1963 (with follow-up hearings in 1970), and the TFX stayed in the headlines for many months. Nevertheless, the decision of the Secretary stood, and the contract remained with General Dynamics. GD received a 479 million dollar letter contract in December 1962 for F-111 RDT&E, including 23 test aircraft (18 Air Force and 5 Navy).
Of the 1,726 total aircraft buy that had originally been planned in 1962, only 562 production models of seven different variants were completed when production ended in 1976.
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