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Major F-20 versions and their designations:

  • Engine change only (ECO): F404 engine replacing two J85 engines of the F-5E. Avionics, wings and nose identical with the F-5E CPO6000 configuration. Modifications were required to the F-5E fuselage aft of the cockpit to accommodate the F404. The depth of the center and aft fuselage sections was increased. The aft fuel cell was modified to accommodate the larger engine and expanded inlet ducts, but the F-5G had the same total fuel volume the F-5E. The F-5E inlet design could accommodate the F404 engine by being stretched vertically but keeping the same horizontal profile, meaning no changes to the wing/fuselage junction were needed. Therefore the F-5G also retained the F-5E wing. However the attachment structure of the vertical stabilizer had to be changed due to the aft fuselage recontouring. Designations:

    • Northrop in-house: F-5/404 (May 1975-June 1978); F-5G (June 1978 to August 1980); F-5G-1 (August 1980 to May 1982); Tigershark (no F- number; May 1982 to November 1982). Aircraft flown as F-20 GG1001 thereafter (decision taken not to market this model after Taiwan program cancelled by Reagan administration in January 1982).
    • Northrop project office PROCAT: GG (August 1980-on. Individual airframes would receive the designations GG1001, GG1002, GG1003, etc).
    • Northrop Engineering Release: 40- (April 1980 on; engineering drawings and specifications for F-5G specific parts received number 40-13905-1, 40-55904-1, etc)
    • Air Force official: None. Although referred to in correspondence with the Air Force as the F-5G from 1978 to 1982, such a designation was ever officially promulgated by the USAF.
  • Integrated digital avionics configuration. The new avionics would allow Northrop to market an aircraft that in fact was better than the F-16A in many ways. Airframe as ECO configuration, except to accommodate the avionics a new fuselage forward of frame 138 was required. This allowed the avionics bays to be rearranged, and provided a more bulbuos nose with a much larger radome for the new radar. The opportunity was taken to provide the Tigershark with a new 'panoramic canopy' providing better all-around visibility than the F-5E.

    • Northrop in-house: F-5G-2 (August 1980 to May 1982); Tigershark (no F- number; May 1982 to November 1982); F-20A Tigershark (November 1982 on).
    • Northrop project office PROCAT: GI (August 1980-on. Individual airframes would receive the designations GI1001, GI1002, GI1003, etc).
    • Northrop Engineering Release: 50- (August 1980 on; engineering drawings and specifications for F-5G-2 specific parts received number 50-64920-1, 50-62914-1, etc)
    • Air Force official: F-20A (17 November 1982 on); F-20A Tigershark (30 March 1983 on).
As a result of development and marketing the F-20A configuration evolved (Extended Range Radar, Maneuvering Flaps, etc) but the designations remained the same (e.g. F-20A Tigershark, GI1003, 50-74900-1, etc etc)

Configurations and Designations over Time

The F-5 series of aircraft had their origin in 1953 with a design called by Northrop the N-102 Fang. By 1957 this had evolved into the N-156. The N-156 became the T-38 in a trainer version, and the F-5A in a fighter version. The F-5E was an improved version of the F-5A with an Emerson Electric fire-control radar and uprated engines. Further improvements were made to the F-5E models built for Saudi Arabia and Switzerland in the mid-1970's, including a Litton inertial navigation system and other avionics that allowed the fighter to carry Maverick and other advanced weapons. This configuration was the baseline for any further improvements on the F-5.

Northrop had an Advanced Design Group, which, like all such groups, spent its time not only imagining new aircraft, but dreaming up and evaluating all possible modifications of existing designs.

Early work on major improvements to the F-5E were referred to within the company as F-5X (such designations were unofficial and not allocated by the US government). This included an F-5E equipped with AIM-7 Sparrow capability, as desired by Taiwan. This would require a new radar, and deletion of one M39 gun to make room for the extra radar avionics. But carriage of the big Sparrow missiles so degraded the performance of the basic F-5E that it was clear a new-technology engine would be required.

Advanced design had already looked at such possibilities over the years. One possibility was replacing the two J85 engines of the F-5E with a single F404 engine from the F-18 (the F-5/404 concept of May 1975). This had been studied as soon as the F-18 contract had been awarded. It certainly provided the needed performance improvement, and engine reliability had improved so much between the 1950's and 1970's that a fighter with a single F404 would (theoretically) have the same safety as a twin-engine aircraft with J85's. Preliminary aerodynamic studies showed the F-5E wings and forward fuselage could be kept unchanged. The aft fuselage would turn from the flat lifting body of the F-5E to a cylindrical barrel section to accommodate the F404 and its intakes, but the aerodynamic characteristics would be maintained by building a kind of shelf between the barrel fuselage and the F-5E wings.

But Northrop had for years advocated the safety features of two engines. A twin engine aircraft was not just safer in flight operations, but also in sustaining battle damage in combat and being able to return to base. So a faction within Advanced Design looked at re-engining the F-5E with two improved engines. Garret built a modern turbofan engine for business jets in the correct thrust class, the TFE-731. For a fighter a version of this engine with afterburners would have to be developed, but this was not seen as a major risk area.

A sharp debate within Northrop developed. Northrop had a traditional relationship with engine manufacturer General Electric, going back to the development of the radical Turbodyne engines for the B-49 flying wing in the 1940's. Northrop sincerely believed that GE engines were better engineered, more reliable, and had lower life-cycle costs than the equivalent engines from Pratt and Whitney. Garrett had no experience in building engines for high-performance fighter aircraft, which were abused like no other engines in the air.

Furthermore, Northrop was expecting big sales of the F-18L land-based version of the Navy F/A-18. This was nearly an entirely different aircraft, with Northrop having the lead design responsibility, and was more akin to the F-17 that was rejected by the USAF than the Navy F-18. The F-18L would be powered by two F404's. This meant that Northrop engineering, logistics, and contracting would already have developed intimate relationships and understandings with General Electric concerning the F404. Furthermore, an F404-powered version of the F-5E would allow Northrop to market a high/low F-18L/F-5X fighter mix using a common engine. This would mirror the F-15/F-16 high/low fighter mix using the Pratt and Whitney F100 engine.

All of these arguments, but in particular the 'special relationship' between Northrop and General Electric at the highest corporate levels, led to the decision in June 1978 to use the F404 engine in the Taiwan aircraft and other future F-5 developments.

At this time the Carter administration had promulgated a fighter export policy that sought to reduce military spending by developing countries and dictatorships. This policy prohibited American companies from marketing or selling front-line American fighters to foreign countries. In particular the F-14, F-15, F-16, and F-18 were not to be marketed or sold to countries outside American's primary allies (NATO, Australia, New Zealand, Japan; and after the Camp David accords, Egypt and Israel). Any fighters exported could only be modifications of existing fighters and 'not as good as' American front-line fighters.

In order to make clear that the F-5/F404 was merely a 'modification' of the F-5E Tiger II, it was given the official F-5G designation by the US government. Northrop had developed a radome for the F-5E that improved the aerodynamics compared to the F-5G, and this had the same shape as a shark's head. So the appearance combined with the name of the F-5E suggested the name Tigershark for the aircraft.

Carter's strict export policy would seemingly prohibit Northrop from marketing radically-improved avionics. So Northrop conceived a two-phase program, whereby Taiwan and other customers would first build 'Engine Change Only' (ECO) aircraft. These would use the F404 engine but still be equipped with F-5E avionics. However under an 'F-5G Phased Improvement Program' improved avionics would be developed and phased in later when the policy (or administration) changed.

By the time the first contracts were let for the new avionics systems, the Carter administration was indeed out of office and the Tigershark was now in direct competition with the F-16A front-line fighter. So the avionics version of the aircraft was already superior in most ways to the F-16A when it started development.

The F-5G-1 was the designation given for the 'Engine Change Only' version of the aircraft. This would be the first put into production, and would retain the entire old F-5E fuselage and avionics.

The F-5G-2 was the designation given for the version with the engine change and completely new digital avionics. To accommodate these avionics a new fuselage forward of frame 138 was required. This allowed the avionics bays to be rearranged, and provide a more bulbous nose with a much larger radome for the new radar. The opportunity was given to provide the F-5E with a new 'panoramic canopy' and a modern zero-zero ejection seat.

Engineering designations given to Northrop aircraft had begun anew with the N-156F series in 1957. The numbers applied to all internal engineering drawings consisted of a first number indicating the aircraft model on which the part had first appeared. The second five-digit sequence indicated the type and location of the part within the airframe. A final one to three digit dash number indicated versions of the same basic part. Under this scheme, 2-13002-1 was the T-38 canopy. The same canopy was used on the F-5B and F-5F two-place aircraft, so that part was used on those aircraft with the same designation. Parts new for the F-5E received 14- designations, such as 14-13002-1 for the F-5E canopy. F-5F parts received 18- designations. F-5G-1 parts had 40- designations, and F-5G-2 50- designations.

Once the F-5G was in direct competition with the F-16A, the F-5G designation became a hindrance to marketing efforts. The F-5G-1 version had been dropped, and Northrop concentrated on marketing the better-than-an-F-16A F-5G-2 version. But the F-5G designation, used to imply to the Carter administration that this was just a modification of an existing aircraft, provided the same connotation to potential customers. Northrop needed a new fighter designation to ensure customers that the aircraft was all-new. Furthermore any number assigned would be 'higher' than F-16, also providing the connotation that the aircraft was newer and more advanced than the competition. So Northrop requested, and obtained, the official designation of F-20 for the aircraft.

This resulted in a lot of speculation over the years as to what happened to the F-19 designation. At the time speculation about development of the still-secret stealth fighter was rife, and it was assumed that the F-19 designation had been given to this aircraft. Articles in reference books authoritatively used the designation, and model kits of the 'F-19 Stealth Fighter' were released. Andreas Parsch , using the Freedom of Information Act, finally uncovered the real story:

The F-19 designation was never assigned. The official explanation by DOD was "to avoid confusion with MiG-19", which was generally regarded as very implausible (because so far no numbers had been skipped to avoid clashes with foreign designators). Therefore, it was much speculated whether F-19 was really skipped, and if so, for what real reason. One viable theory was that F-19 was originally allocated to (or at least reserved for) the F-117A Nighthawk, but eventually not usedů The other main line of reasoning was that Northrop specifically requested the F-20 designator for its then new Tigershark (originally designated F-5G) to make it look as "the first of a new fighter generation" (i.e., the "20" series).

The truth is in fact a combination of the second idea and the official line. The designation "F-19A" was indeed officially skipped at Northrop's request. They wanted to redesignate the F-5G as F-20A, because they preferred an even number. The Soviet competitors in the export fighter market of the early 1980s all used odd numbers, and Northrop wanted to stand out from these. So the official "confusion with MiG-19"-story is in fact more or less close to the truth, although the phrase is a bit misleading. Nobody would "confuse" an "F-19A" with a MiG-19, especially because the latter was obsolete anyway at that time. To say it again, Northrop didn't want to avoid "confusion" with MiG-19 in particular, but to use an even number to stand out from all the Soviet odd ones. The F-20A designator was approved despite official recommendation by the USAF Standards Branch (at that time responsible for nomenclature assignments) to follow the regulations and use "F-19A".

The facts are documented by several letters exchanged between various USAF/DOD offices during the process of requesting and assigning the F-20A designator to Northrop. On 28 October 1982, HQ Aeronautical Systems Divison, USAF (apparently handling the F-5G program for the Air Force) wrote a letter to the USAF Standards Branch to request a new model number for the F-5G on behalf of Northrop Corporation. To quote the relevant part (remarks in [brackets] are by me):

1. In mid 1981, Northrop Corporation's Intermediate Export Fighter candidate was designated the F-5G. Since that time, the F-5G has incorporated substantial changes in structure, engine and aircraft systems. Northrop Corporation believes these changes would be best reflected by a model designation change from F-5G to F-20A, "Tigershark". Northrop's reason for specifically requesting the model 20 designation is that being an even number series [sic], it would be unique in the foreign market which typically sees odd numbered threat designators (MIG 19; MIG 21; MIG 23).

On 1 November 1982, this request was in turn forwarded by the Standards Branch to USAF HQ in the Pentagon for approval. However, it was clearly stated that the designation should be "F-19A" instead, to follow existing regulations:

1. The attached request [see quote above] is forwarded for your consideration and approval of a new Mission-Design-Series (MDS) designator.

2. MDS designator F-5G was approved for Northrop's Intermediate Export Fighter candidate in May 1981. Based on the information contained in the subject letter, the aircraft has been changed sufficiently from the original F-5G configuration to warrant assignment of a new MDS as requested.

3. Our records indicate that -19 is the next number available for assignment in the "F" series and to comply with AFR 82-1 paragraph 3f we feel that F-19A should be assigned to this aircraft.

4. The popular name "Tigershark" has not been approved at this time and should not be listed in DODL 4120.15 [DOD's official listing of approved aerospace vehicle designations] until an MDS has been assigned to this aircraft. We will take action to obtain approval of the popular name when an MDS has been established.

5. Our recommendations for entry into DODL 4120.15 are as follows:

a. [MDS] F-19A
b. [Manufacturer] Northrop
c. [Popular Name] unassigned
[...]

(The copy of the letter has a hand-written note at the bottom, saying "Dissapproved [sic] See F-20A folder".)

On 17 November 1982, HQ USAF approved the F-20A designation (but not yet the Tigershark name), apparently ignoring the Standards Branch's advice, and introduced the misleading "confusion with MiG-19" phrase:
1. Redesignation of the Northrop Corporation's intermediate export fighter from F-5G to F-20A is approved. Substantial changes in design and capability warrant a different basic design number. Northrop's concern for potential confusion with the MIG-19 in their foreign markets is a sound basis for bypassing that number.

2. The assignment of the popular name "Tigershark" is being worked through public affairs channels. [...]

3. Please advise ASD and Northrop of the F-20A designation and the status of the popular name request.

As a side note, the name Tigershark for the F-20A was eventually approved on 30 March 1983. It had originally been requested on 4 September 1981 for the F-5G, but was then rejected "due to a proliferation of popular names for the F-5 aircraft series and the speculative nature of the F-5G venture" [USAF quote].

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