Reports of the Pentagon’s pressure on the Air Force to procure new F-15 airframes for the first time in 18 years immediately sparked a backlash among Lockheed Martin’s fan base. They contend diverting funding away from the F-35 Joint Strike Fighter to purchase an older design doesn’t make any real strategic sense. Now, as more details regarding the Air Force’s F-15X procurement plans surface, the distinctions between the F-35 and a new F-15X are beginning to materialize–with one glaring point seemingly in the F-15X’s favor.

Last year, the Pentagon began devoting a considerable amount of resources into closing the capability gap presented by Chinese and Russian hypersonic missile platforms. The Air Force, in particular, is hoping to begin fielding air-launched hypersonic missiles by the mid-2020s at the very latest. These missiles would undoubtedly be designed to be carried by both fourth- and fifth-generation fighter platforms, as both are expected to continue serving side-by-side for decades to come. The problem is, these large missiles likely won’t fit inside the F-35’s somewhat modest internal weapons bays.

Hypersonic missiles may be why the Air Force wants 80 new F-15X fighters
Internal weapons bays of a U.S. Navy F-35C (Navy)

They could likely, however, be mounted on the F-35’s wings to be carried externally, though doing so would compromise the aircraft’s stealth profile. At that point, the F-35 would still offer a greater degree of situational awareness through its data fusion capabilities over an F-15X, but it would fall short of the dated platform in terms of total ordnance it can carry and top speed.

With stealth out of the picture, the F-35 does still boast a number of strengths the F-15X would lack, but it would also come at a larger operational expense. Although Lockheed Martin and the Air Force have recently begun arguing that the F-35 doesn’t require as much support equipment to maintain, and therefore isn’t as expensive per hour as some might suggest, the jet is still more expensive to run than Boeing claims its F-15X will be.

Hypersonic missiles may be why the Air Force wants 80 new F-15X fighters
(Photo by Christopher Furlong/Getty Images)

In fact, Boeing claims its new F-15X fighters, leveraging years worth of continued research and development for export to American allies like Qatar and Saudi Arabia, will offer a significant drop in operating costs over America’s existing fleet of F-15s–each more than 30 years old. That drop is claimed to be so significant (despite not providing hard numbers) that Boeing says the Air Force will recoup the expense of purchasing the new airframes through reduced maintenance alone.

Add to that a proposed lifespan of a whopping 20,000 hours, and the offer of a fixed-cost at $80 million per aircraft, it becomes clear that Boeing is chasing this contract using a very different methodology than Lockheed Martin did with the F-35. Where Lockheed Martin convinced the taxpayer to provide what amounts to a blank check to get the F-35 in the air, Boeing has promised to eat any additional production costs that surpass that price point. Some of the earliest F-35Bs delivered to the United States Marine Corps, on the other hand, may only have operational lifespans of just 2,100 or so hours without significant repairs that will have to be footed by the taxpayer, thanks to structural cracks that have surfaced during durability testing.

Hypersonic missiles may be why the Air Force wants 80 new F-15X fighters
A United States Air Force F-15 fighter jet based at RAF Lakenheath speeds through the Dinas Pass, known in the aviation world as the Mach Loop, on February 16, 2018 in Dolgellau, Wales, United Kingdom. (Photo by Christopher Furlong/Getty Images)

If the F-35 can’t carry hypersonic missiles without trading in its stealth capabilities, those in the F-15X camp can argue it would benefit the branch to have 80 new versions of the fastest fighter in the American stable that can. According to Pentagon and Lockheed Martin officials, F-15X purchases will not affect the F-35 procurement schedule whatsoever, and these jets do fill traditionally different roles. However, many in the media and defense community continue to pit these two jets against one another as though they must be somehow mutually exclusive endeavors.

While there is little doubt the F-35 is a more capable platform inside contested airspace, only about half of the F-35s that have been delivered to the United States and its partners are actually operational. It will still be many years before these fighters exist in sufficient numbers to replace their fourth-gen counterparts, so no one disputes the fact that F-15s and F-35s will be sharing airspace for decades to come. But if the Pentagon was looking for a good reason to purchase new “old” jets, being able to carry the missiles they’re paying Lockheed Martin a billion dollars to invent is a pretty good start.

Ultimately, you won’t find many who would argue the F-15 is superior to the F-35, but until the F-35 is as broadly capable and resilient as Lockheed Martin keeps saying it will be, the United States will look for ways to fill holes in its air strategy in a budget-friendly way. If the F-15X is as cost-effective as Boeing claims, purchasing them may not be what America wishes it had to do, but it may be what it needs to do.