F-15N | the forgotten 'Sea Eagle'

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 the 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. This video is brought to you by our supporters, and we can't do this without you. Become a YouTube member for early access or join us at our members and merch site at peacemaker.net.au. You can also catch us on various social media platforms, links in the description. In the late 1960s, the U.S. Navy had launched the VFX program. The goal had been to procure a next-gen design with a crucial focus on fleet air defense. The culmination had been the Grumman 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 envisioned for the Navy's F-111, it would be the most powerful radar ever put in a fighter, tuned to track and intercept targets at incredible ranges. Unlike the latest proposals, the AWG-9 wouldn't rely entirely on solid-state internals. Instead, it would be hybrid, with high-powered analog components, aided by certain digital systems that would quantize the received signal. This logic had several long-term tradeoffs, 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 quantize 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 tradeoff of sorts. The radar would be more finicky, given it was largely analog, and it wouldn't be a long-term solution, but in the short term it would offer unparalleled long-range coverage with new lookdown 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 concept was well under development, having secured the VFX contract two years prior. Other designs that hadn't made the cut included the General Dynamics Model 44 and the Vought 507, both aesthetically familiar to the 303. McDonnell Douglas' proposal had been the 225. Like its competitors, it was intended as a highly capable but technologically achievable product. Following VFX selection in 1969, the design was dropped. Yet, McDonnell remained keen on a Navy contract tied to a fourth-generation product. By 1970, such a proposal was taking shape. At the time, the company already had a winning design on their hands. Their concept for the FX 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 had been designed around new principles and promised to be safer, more stable, faster and more agile than the Phantom, all in a single crew configuration. 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. McDonnell Douglas noted that despite using new design principles and extensive digital components, the F-15 was still projected to cost less per unit than Grumman's design. This was, as the story goes, the pivotal factor that led to a revised McDonnell submission to the Navy. Technically, it wouldn't actually be a VFX competitor. Instead, McDonnell hoped that a gap would open for an Eagle derivative to fill. 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. 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 backseat to ICBMs, the role of fleet defense remained. Over the open ocean, with enemy access to long-range anti-ship missiles, standoff weapons needed extensive coverage. Given its operational range, the Tomcat was clearly cut out for the role. What McDonnell could offer were advantages in other areas. The F-15 would be the fastest, at Mach 2.5, with internal systems that were both more advanced and less prone to failure. 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 favor of the Eagle. It would use the same F-100 engines planned for the Air Force variant, which were expected to deliver excellent fuel efficiency and an incredible thrust-to-weight ratio. This was arguably one of the main advantages of the 15N proposal on paper. While Grumman had hoped for a new engine, designated the F-401, part of their contractual agreement was the reuse of the TF-30, an older power plant intended for the F-111. Although they offered significant power, they were older models. Due to the Tomcat's engine placement, this was critical. A cut-off 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, with better handling across most speed envelopes and more predictable flight characteristics when near or in a stall. Beyond technological advantages, the carrier conversion itself was perceived as relatively straightforward, as detailed in the first 15N proposal in 1971. 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 with less than 2,000 pounds of extra weight. Despite this, Navy interest was initially minimal. The concept was nonetheless pushed forward with further upgrades. To improve takeoff and landing performance, blown trailing edge and full 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 a specific designation of Model 199A11, was still light, adding only 3,000 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. 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 counter-proposal to the Air Force for the F-14FG-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 3, the team simulated an Eagle configuration, in which the nosecone was enlarged and outfitted with both the AWG-9 and Phoenix missiles. 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-15NPHX. Taking the initially upgraded concept, Concept Model 199A-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. The model has fast packs, or CFTs, on board, which weren't a thing until late 1974, indicating it may have been a later mock-up. However, we can envision the Navy concept resembling something like this. 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. Such a fly-off never took place. Interest subsided, and before long the 15N was shelved. While similar proposals followed, none were ever seriously pursued, and the idea was forgotten. A Navy study group applied lessons learned from this experience, which contributed to the eventual selection of the F-A-18 several years afterwards. 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 hard points 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 have 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.