Since the March release of the 2020 defense budget, the Pentagon’s decision to purchase new F-15X fighters to replace geriatric F-15Cs has occupied a disproportionate share of defense coverage, analysis, and congressional attention. Confusion reigns, as initial explanations by the Pentagon, convinced almost no one and were often met with befuddlement because the Defense Department did not adequately explain the purchase to Congress. Why, seemingly out of the blue, the Pentagon was asking for more fourth-generation aircraft when then Air Force Secretary Heather Wilson had previously stated that achieving the proper mix of fighter aircraft meant buying stealthy fifth-generation planes, “not new fourth-generation aircraft.”

In the absence of a convincingly argued case for the F-15C replacement decision, the public debate devolved into a fight between Boeing’s F-15X, the modernized fourth-generation fighter, and Lockheed’s F-35A, the stealthy fifth-generation alternative. Framing the F-15X purchase as an either/or proposition vis a vis the F-35 makes for a compelling story but does not accurately capture the complexities of the case for replacing aging F-15Cs. Policymakers and observers have focused too often on comparing the capability and physical characteristics of each aircraft. In reality, Pentagon officials made the decision primarily based on mission requirements, short-term readiness concerns, and the long-term costs of operating the fighters.

Even so, because the comparison between F-15X and F-35A is shot through with uncertainties, the decision to purchase F-15Xs should be viewed as neither a slam-dunk nor a harebrained idea. However, the brouhaha over F-15X and F-35A distracts from far more important debates the nation should be having about the future of American airpower. The most valuable commodity in politics is time, not money—and the F-15C replacement debate represents a massive opportunity cost for an Air Force with nowhere near the capacity and capability to carry out its missions according to the National Defense Strategy.

“Out of the Blue”

The Pentagon proposes to buy 80 F-15Xs over the next five years, likely expanding to an eventual buy of 144 or more to “refresh” the F-15C/D fleet—and potentially the F-15E fleet down the road. The Air Force did not request these aircraft. Rather, the Pentagon’s independent cost-estimation shop—the Cost Assessment and Program Evaluation Office (CAPE)—ran its own analyses and eventually brought the Air Force around to its position, whether through convincing or by fiat. Either way, former Secretary of Defense Jim Mattis signed off on the decision.

Sen. Tammy Duckworth (D-IL) spoke for several lawmakers when she admitted that she was "a little bit confused" about the F-15X purchase, given years of watching the Air Force continuously pound the drums for fifth-generation aircraft. Rep. Donald Norcross (D-NJ) similarly mentioned that it made no sense for F-15X to be a last-minute decision since the Air Force knew about F-15C retirement for years. HASC Ranking Member Mac Thornberry (R-TX) spoke for many when he stated that the Air Force must convince lawmakers that the F-15C replacement strategy makes sense. The botched rollout by the Pentagon is all too familiar of late, as the building continues to fail to explain its actions and build relationships with Congress.

As the weeks have worn on, the Pentagon’s done a better job of detailing the case for purchasing F-15X, but its explanations have not yet convinced everyone that this is the right path to take. As Chairman of the Joint Chiefs of Staff Gen. Joe Dunford has admitted, the F-15X decision involved “four or five independent variables,” each of which demands analysis and inevitably engenders debate.

What is not up for debate, however, is that America’s aging F-15C fleet needs replacement, as its average aircraft age hits 35 years. When the iron laws of aircraft maintenance meet the cold calculus of compound interest, the bills begin to add up. Not only do older aircraft suffer from higher costs per flying hour, but those costs also increase faster over time. No matter whether the Air Force picks the F-15X or F-35A, the decision to replace F-15Cs pays for itself in less than ten years.

Costs of Replacing the F-15Cs

The Air Force has long known that its 235 F-15C/D Eagle air superiority aircraft face the end of their service lives in the mid-2020s. Most of these aircraft serve in Air National Guard units conducting the homeland air defense mission, intercepting Russian military aircraft and civilian aircraft that stray where they shouldn’t. And while several commentators have argued the F-15 purchase doesn’t “align with the National Defense Strategy," and its emphasis on the great power competition with Russia and China, that strategy itself tasks the Air Force with continued homeland air defense duties. As Air Force Chief of Staff Gen. David Goldfein notes, the F-15s are not going to make it to the 2030s, and “We have got to refresh the F-15C fleet because I can’t afford to not have that capacity to do the jobs and the missions right.”

The F-15s in question were supposed to have been replaced already by the stealthy F-22 Raptor. Then Secretary of Defense Robert Gates truncated that program in 2008 at 187 aircraft, far short of original plans for 750, and left the F-15s without a replacement. In the wake of that decision, the Air Force has squeezed extra life out of the Eagles, but airframes only last so long. How to deal with the retirement of F-15Cs has been a topic of conversation in the Pentagon for years, as it weighed the costs and readiness considerations of potential replacements.

Two factors comprise long-term aircraft operating efficiency: cost per flying hour and escalation rate. There are many ways to calculate cost per flying hour, but the metric can generally be thought of as measuring how maintenance-intensive an aircraft is combined with how much fuel it burns in flight. Second, the escalation rate measures how fast an aircraft’s cost per flying hour grows over time. In short, some aircraft age more gracefully than others. Cost per flying hour generally follows a “bathtub” model. New aircraft are initially expensive to operate as maintainers go through growing pains. Costs then decrease and stabilize for a period, but as aircraft age, they present new problems that drive costs upward, as any car owner knows.

As a baseline, the 30 year-old F15C fleet currently costs $42,000 per hour to operate, and that rate is growing quickly, which means that either the F-35A or the F-15X is cheaper to operate after about a decade. CAPE pegs new F-15X cost per flying hour at $27,000. Given extensive experience with the modern version of the F-15 airframe, those costs may be slightly higher or lower, but they are relatively well known. F-15Xs would enter service in the "mature phase" of the bathtub. In comparison, according to recent testimony by CAPE director Bob Daigle, the F-35A currently costs $44,000 per flying hour. However, the F-35A remains in the "immature phase" of the bathtub, and the Pentagon and Lockheed Martin plan to drive the cost per flying hour down to $34,000 per hour by 2024 and are shooting for $25,000 per hour by 2025, according to the head of the program. Achieving the ambitious goals for F-35A operating costs will not be easy. As the program head noted, “the 25 by 25 wasn’t generated by the program office nor by CAPE. It was a stretch goal given to us by our leadership.” And while the F-35 program has made steady progress on driving costs down, the F-35 still faces significant problems with spare parts availability, the Automated Logistics Information System, and depot-level maintenance.

If F-35 operating costs flatline at $34,000 in 2024, procuring F-15Xs instead of F-35A would save $1 billion by 2030 and about $3 billion by 2040. However, if the Air Force and Lockheed were to drive F-35A costs down to F-15X levels, there is almost no difference between the long-term business case for the two aircraft. In short, whether one assumes a worse or a better case for operating costs for the F-35, the “savings” in buying the F-15X rather than the F-35 is relatively minimal over the long haul in the grand scheme of the U.S. defense budget.

Apart from steady-state operating costs, other long-term factors undoubtedly played into the F-15X decision. While officials noted more frequently over time that they considered industrial base health into the decision, this claim should be met with skepticism given the nebulous requirements of the developmental sixth-generation fighter program. Further, U.S. allies and partners operate significant fleets of both F-35s and F-15s, rendering the argument for stimulating either production line relatively moot.

Near-Term Considerations

In addition to the relatively minimal differences in long-term operating costs, analysis shows that differences in initial costs don’t move the needle all between the options. The five-year upfront costs of both the F-15X and F-35A are remarkably similar. According to Air Force budget documents, the F-15X will cost an estimated $80 million per plane. F-35As currently cost $89 million a copy, and the Air Force will likely meet its near-term goal of $80 million a copy. Other upfront costs are also similar.

If such cost comparisons are largely a wash, the achievable pace of replacement matters a great deal to the Air Force, which places foremost weight on the readiness implications of aircraft replacement. According to budget documents, F-15X deliveries would not match the possible numbers produced by the F-35 line. But the delivery of aircraft represents just one step in the process of standing up operational squadrons. One key fact the Pentagon only belatedly clarified is the exact timeline for the conversion of existing F-15C squadrons to either F-15X or F-35A squadrons.

Reading between the lines—given the nearly identical procurement costs and the uncertainty about long-term operating costs—Pentagon officials placed an extremely high value on how quickly on the transition timeline for F-15C replacement options. Officials, including Gen. Goldfein and Secretary Wilson, noted the transition from F-15C to F-15X would likely be very smooth. As Goldfein sums it up: “it allows you to use the same hangars, same construction, same base, same operating equipment which is 90 percent common, same maintainers, same operators and no time and minimal cost to make a transition.”  In a May hearing, Air Force military procurement chief Lt. Gen. Arnie Bunch testified, “the time to transition from an F-15C to an F-15X we estimate at three to six months, while the time to transition from an F-15C to an F-35 could be anywhere from 18 to 36 months and would require MILCON and other attributes that are not in the budget.”

The fact that transitioning squadrons from the F-15C to the F-35A could take from one to three years effectively erases Lockheed’s ability to deliver 80 aircraft slightly more quickly. Of course, 18 to 36 months is a vast range. Using the lower bound of 18 months, the delay to transition from F-15Cs to F-35As might be matched by Boeing’s slower F-15X delivery timeline. At the higher bound of 36 months, the transition could be delayed by years, worsening the existing readiness problem created by aging F-15Cs and tilting the near-term analysis of operating costs in favor of the F-15X.

Capabilities and Missions

Coverage, commentary, and congressional questioning thus far has disproportionately focused on questions of capability—whether fourth-generation aircraft like F-15s are survivable in the high-end fight, the 2018 National Defense Strategy focuses on. Barrels of ink have been spilled over the past few months arguing about the degree to which future air warfare will require a mix of fourth- and fifth-generation aircraft, whether the F-15X’s payload capacity in comparison with the F-35’s makes it more attractive, and whether the ability to launch hypersonic attack weapons factored into the decision. In some cases, Air Force officials have actively engaged in these debates themselves, muddling the service’s own case.

The Pentagon and the Air Force did not give capability considerations as much weight in the analysis of F-15C replacement as the commentary would suggest. The gulf in capability between the F-15X and the F-35A is fairly obvious. As General Goldfein said, “The F-15 will never be the F-35,” though it’s not as if the F-15X is worthless, even in a contested environment. Still, numerous American Enterprise Institute analysts in recent years have extolled the virtues of the F-35A and called for drastically increasing the production rate both to meet mission objectives and to improve the health of the tactical fighter fleet. The transformational capability improvements of the Joint Strike Fighter are increasingly well-documented.

However, the mission of homeland air defense, which the F-15C replacement aircraft will perform, does not require a ton of eye-watering capability. As one defense official explained, this is a “mission set for which we do need the capabilities” of a tactical aircraft, “but for which we don’t need a penetrating aircraft that’s more expensive.” And, even if there were a major conflict with Russia or China, it’s not the case that F-35s involved in homeland defense could be just added to the front-line force. In such a scenario, it’s unlikely the defense secretary or the joint chiefs would recommend (and the president accept) the idea of stripping fighter squadrons from homeland defense to send abroad. In this respect, arguments over the comparative capability of the F-35A and F-15X somewhat miss the point.

Forest, Meet Trees: The Future of the Air Force

One way or another, the question of F-15C replacement will almost certainly be answered this year. Indeed, the pressure to purchase F-35As instead of F-15Cs appears overwhelming; the vast majority of interactions about this issue between Pentagon officials and Congress have evinced a high degree of skepticism about the decision. Ninety-nine House lawmakers signed a letterdemanding higher F-35 production, and dozens of members from both chambers have signed similar bipartisan letters. However, the Senate and House Armed Services Committees, along with the House Appropriations Committee, have all supported the F-15X buy, though each committee (and particularly HASC) has written scathing report language about the Air Force’s shoddy work on the program’s acquisition strategy and information flow to Congress.

The Air Force clearly and badly needs more F-35s. As General Goldfein recently bemoaned, “We should have 1,100 F-35s today. We have less than 300.” Meeting operational challenges in high-threat environments created by Russia and China will require a combination of F-35s, F-22s, and the F-22’s eventual air superiority replacement slated for fielding in the early 2030s. But the F-35A and F-15X should not be pitted against one another. The Air Force faces massive shortfalls in both the health and capability of its fighter fleet. Alongside the need for high-end capability, the Air Force simply needs more tactical aircraft immediately begin building a healthy force. As Air Combat Command chief Gen. Holmes mentioned, buying too few fighter aircraft per year means the Air Force will continue to pay ever-increasing amounts for smaller numbers of operational aircraft: “buying 48 F-35s a year will merely create a force, 30 years in the future, that averages 30 years of age per airplane.” General Goldfein has explained the exact same phenomenon: “[The F-15X buy] helps us to get at our target, which is 72 aircraft a year, which is what we need to be able to drive aircraft age from its current 28 average years to 15—which is what we think we can manage, by about a 2040 time frame. So the F-15C is about capacity; we're not taking a dime out of the F-35, nor would we, to buy F-15s, but we've got to fulfill this capacity shortfall with the F-15C.”

In the end, the debate over choosing to buy F-15Xs or more F-35s is important but pales in comparison to much more consequential public debates that Congress and the Pentagon should be having. For instance, how will the Air Force achieve its 386-squadron goal for overall combat capability under the flat defense budgets proposed by the Trump administration? Can the Air Force accelerate the development of a new sixth-generation aircraft capable of operating in the highest-threat areas of the future to maintain American air dominance? Why hasn’t the Pentagon asked for enough tactical aircraft to replace retiring airframes? Does the United States field sufficient airlift and cargo aircraft? What can be done to ensure a sufficient pipeline of pilots and maintainers in the future?

American sovereignty over the skies (and space) has ended in some areas, and it is coming to an end in others. The Pentagon and its overseers in Congress would profit from spending much more time on the hardest questions facing the Air Force. Picking either aircraft will not win or lose the next war; the comparative savings or delays are measured in low the billions of dollars over decades or mere months of readiness lost. But squandering an entire budget cycle debating F-15X or F-35 delays a necessary reckoning about the future of American airpower.

Rick Berger is a research fellow at the American Enterprise Institute (AEI), where he works on the defense budget, the National Defense Authorization Act, military appropriations and acquisition reform, as well as on other national security budget-related issues. Follow him: @bergerrichard.

This first appeared in RealClearDefense here.