In the early 1970s, McDonnell Douglas proposed a carrier-based version of the F-15 for the US Navy. Known as the F-15N, it promised extraordinary speed, a lighter airframe, and more advanced avionics. But adapting the Eagle for carrier operations meant solving one major problem: how to give it the fleet-defence capability of the F-14 Tomcat.
Recently, we looked at the F-18L, Northrop’s attempt to create a land-based version of the Hornet. Ultimately a failed project, it was nonetheless one that made sense; several prospective customers were considering the new Hornet for Air Force use, and the design itself was based on a long line of land-based fighters developed by Northrop, from the F-5A through to the YF-17 Cobra.
The F-18 wasn’t alone; several years earlier, a more ambitious cross-service proposal had been made, a carrier-spec F-15 designated the F-15N. The Eagle was the exemplary counterpoint to the Hornet; it had been built from the ground up with painstaking precision to be an unmatched air superiority fighter. Converting it for carrier use was more ambitious than the reverse, but if anyone could do it, it would have been McDonnell Douglas.
BACKGROUND
In the late 1960s, the US Navy launched the VFX program to procure a next-generation platform for fleet defence.
Among the many submissions was Grumman’s proposal, the Model 303. Technologically advanced, it would use design elements already validated in third-gen fighters – like variable-geometry wings – while also integrating more ambitious digital tech, setting it apart as a next-gen design. As a result, the Grumman proposal – which was now taking shape as the F-14 – quickly became attractive to the Navy.
Among the advanced technology it would carry, two systems stood out as particularly impressive: the first was the radar set, the 10-kilowatt AWG-9. Originally designed for the F-111B, it was intended to be one of the most powerful systems of its kind ever installed in a fighter, tuned to track and intercept targets at incredible ranges.
Unlike the latest Air Force proposals, the AWG-9 wouldn’t rely on solid-state internals. Instead, it would be hybrid, with high-powered analogue components, aided by certain digital systems that would quantise the received signal. This logic had several long-term trade-offs, but it had already been validated as workable.
In the late 60s, the Air Force had decided to equip the new F-4E with the solid-state APQ-120. That same year, in 1967, the Navy had decided on the opposite approach for its new F-4J: the hybrid AWG-10. While the 120 was far more precise, thanks to its ability to quantise data, the Navy’s choice was more contextually appropriate. Not only was it incredibly powerful, but it also used well-understood technology. It would be a trade-off of sorts: the radar would be more finicky, given it was largely analogue, and it wouldn’t be a long-term solution, but in the short term it would offer unparalleled long-range coverage with new look-down capabilities.
Similarly, the AWG-9 adopted a comparable, yet more modern approach: it relied on older system logic than what the Air Force was promoting, but it used systems that were nearly guaranteed to work straight out of the box.
The other piece of tech was the AIM-54 Phoenix, a long-range, radar-guided missile intended to complement the AWG-9. It promised long standoff ranges – far longer than the Sparrow – and the future ability to operate in either semi- or fully active launch modes: something almost unheard of in the early 70s.
By 1971, Grumman’s 303 was the most impressive, yet perhaps the most viable, of the major VFX proposals.
Among other designs that ultimately wouldn’t make the cut were the General Dynamics 316, similar in some respects to the 303, and the Vought 507, which likewise bore many similarities.
Another original design, the MD-225, had been put forward by McDonnell Douglas but was ultimately dropped. Yet McDonnell remained keen on a Navy contract as the Phantom became less future-proofed.
At the time, the company already had a winning design on their hands: their concept for the F-X competition had culminated in the F-15A. By 1971, the Eagle had arguably become the most anticipated military aircraft in development: it would fill the symbolic role of frontline air defence for the US and feature a variety of radically new technologies. The airframe itself had been designed around new principles and promised to be stable, faster, and more agile, all in one package. The internals would be packed with solid-state digital tech, including the new APG-63 radar. Unlike the AWG-9, this new system – thanks to its digital architecture – would be able to easily burn through certain types of jamming, operate more reliably at altitude without clutter interference below, and be updated with software patches, all without relying on easily fatigued vacuum tubes. The trade-off was that it wouldn’t be as powerful or support the Phoenix missile.
The McDonnell proposal was intended to be a low-cost alternative to the more expensive variable-geometry design that Grumman had pinned its hopes on.
DEVELOPMENT
According to the limited information available, the concept was first proposed to the Navy in July 1971.
Similar to General Dynamics’ efforts with the F-16 at the same time, the so-called F-15N was proposed either as a high-low pairing with the Tomcat or as a cost-effective alternative—though the specific details are unclear.
From the outset, it was clearly a radical departure from the Tomcat proposal; Grumman’s proposal was a long-range, high-speed interceptor designed to hunt bombers; the F-15 was not.
Even as nuclear bombers took a back seat to ICBMs, the role of fleet defence remained. Over the open ocean, with enemy access to long-range anti-ship missiles, standoff weapons needed extensive coverage. The Tomcat promised to do just that. With the Eagle, the outcome was unclear. What McDonnell’s proposal did offer were advantages in other areas, which could – in theory – benefit the Navy.
ADVANTAGES
Aside from cost, the 15N would also feature the inherent advantages of the Eagle concept over the Tomcat. This includes not only more modern, future-ready internal systems but also a higher speed, reaching Mach 2.4. Additionally, it would be considerably lighter, with an empty weight of approximately 28,000 pounds compared to the Tomcat’s estimated 40,000 pounds.
Yet even beyond this, many smaller details also fell in favour of the Eagle. It would use the same F100 engines planned for the Air Force variant, which were expected to deliver excellent fuel efficiency and an incredible thrust-to-weight ratioThrust-to-Weight Ratio (T/W): A dimensionless parameter that measures the relative power of an aircraft by comparing the total thrust generated by its... More.
This was arguably one of the main advantages of the 15N proposal on paper. At that time, Grumman sought a new engine, the F401, for its Tomcat. However, part of the VFX contract required reusing not only the AWG-9 but also other leftover F-111B components, including the TF-30 engine. Although they offered significant power, they were older models. Due to the Tomcat’s engine placement, this was critical; a cutoff or failure could lead to dangerous asymmetric thrust.
Additionally, smaller enhancements increased the Eagle’s appeal: the F-15’s construction was designed to be more durable and mechanically simpler for ground crews. The airframe was also safer. Although it wouldn’t match the F-14’s quick turn rate at low speeds, it excelled as a rate fighter at higher speeds, maintaining control during extreme manoeuvres and quickly regaining speed thanks to a superior thrust-to-weight ratioThrust-to-Weight Ratio (T/W): A dimensionless parameter that measures the relative power of an aircraft by comparing the total thrust generated by its... More.
ADDITIONS TO THE F-15N
Beyond technological advantages, modifying the F-15 for carrier ops was also seen as relatively straightforward. A proper hook with reinforced mounting would be added, along with stronger landing gear, a new front gear strut and catapult lock, and a redesigned wing that would fold upward for carrier storage and, according to some claims, possibly be clipped to add a wingtip hardpoint. However, given how risky it is to mount weight too far out on the F-15’s wings, this was probably unrealistic.
This would all be done at minimal cost and with only a small increase in weight —the mods were reportedly coming in at just over 2,000 pounds extra. Despite this, Navy interest was initially minimal. The concept was nonetheless pushed forward with further upgrades.
To improve take-off and landing performance, a slotted aileron, blown trailing flaps, and full-length leading-edge flaps were added. The refuelling receptacle would be removed in favour of a probe, and the internal avionics would be revamped; the Tactical Electronic Warfare System would be removed and replaced by an assortment of Navy-spec equipment.
This second overhaul – with the specific designation of Model 199-A-11 – was still light, adding only 3000 pounds to the air force spec design.
Another consideration was range. On internal fuel, both the Tomcat and F-4J had a wider combat radius; the Tomcat over 400 nautical miles and the Phantom just over 300.
However, from the Navy’s perspective, regardless of hypothetical performance improvements, the F-15 proposal still had one glaring drawback: its inability to field the Phoenix.
The Navy correctly understood that if it was to be a fleet defender, the intended Eagle loadout of Sparrows and Sidewinders wouldn’t cut it. What was needed was an upgrade.
UPGRADING THE F-15N
In July 1972, the F-15 would make its maiden flight, during which it met or exceeded almost all performance predictions.
Perhaps as a result of this recent success, it was reported that by 1973, the US government was properly reviewing the F-15N. Interestingly, during this period, Grumman made a counterproposal to the Air Force for the F-14 FG-014 in mid-1972, but it did not develop further.
Since the F-15N was first proposed in 1971, a number of changes had been made. In the lead-up to the 1973 review, the Navy conducted its own feasibility study, independent of McDonnell Douglas. Under Navy Fighter Study Group III, the team simulated an F-15 configuration in which the nose cone was enlarged, and both the AWG-9 and Phoenix were directly fitted. The study found that the modified airframe would weigh 10,000 pounds more than the 15A, effectively nullifying one of its key advantages. Mock carrier landings also showed that the added weight resulted in landing angles exceeding 12 degrees.
While details are scarce, McDonnell responded during this period with a revised proposal, designated the F-15N-PHX, bringing the design closer to what Grumman was offering in the Tomcat.
Taking the initially upgraded concept – concept model 199-A-11 – McDonnell and Hughes prototyped pairing the Phoenix with an uprated APG-63. It would operate at less power than the 10-kilowatt AWG-9 and more than the conventional 5-kilowatt APG-63. Coming in at around 7 kilowatts, it would be designated the APG-64, featuring specific software patches for the Phoenix, a custom track-while-scan mode, and a specialised command link for the missile. The central computer was modified to be compatible with all of this, as was the radar antenna.
Inside, the cockpit would be slightly altered, and new hardware would be installed near each pylon to operate the liquid-cooling system required for the Phoenix. The end result would be an aircraft capable of carrying up to six AIM-54s at once, though whether it could have landed again with this full loadout was another question. A scale mock-up from around this period shows what this would look like. However, given that the model has FAST tanks equipped – which weren’t really a thing until October 1974 – these could have been images of a later project. Nevertheless, they indicate the general look of the end product.
By 1973, the Senate Armed Services Committee, through its Tactical Air Power subcommittee, formally reviewed the design on March 30th. This review was prompted partly by various issues encountered during the development of the F-14As. During the session, Missouri senator and Navy veteran Thomas Eagleton suggested that the F-15 should have a fly-off against the Tomcat. However, such a fly-off never took place. In the following months, the lack of serious interest led to the shelving of the F-15N, and the project was largely forgotten. Later, a Navy study group applied lessons learned from this experience, which contributed to the eventual selection of the F/A-18 several years afterwards.
CONCLUSION
Despite its drawbacks, the F-14A was probably the right aircraft for the job. Its use of older hybrid tech and design principles allowed it to get off the drawing board more quickly. Once in the air, despite various mechanical issues, it could indeed fill the role the Navy wanted it to. Many of the most serious issues would be ironed out in the F-14B, which was closer to Grumman’s original vision for the platform.
How a Navy-spec F-15 would have fared is a complete mystery. While a good proposal on paper, it’s unclear whether the price and complexity wouldn’t have ballooned if it had gone into production. After all, the frames would have needed serious structural overhauls for carrier ops, the panels might have needed anti-corrosion coating, the hardpoints would have needed new plumbing for liquid cooling, and unless equipped with FAST packs or tanks, the combat radius would have been limited. However, on the plus side, it would probably still have ended up cheaper, easier to work on, and with more advanced digital systems available from the outset, all at the fingertips of a single pilot.
