Key point: Both are capable jets, but are tailored to exceed at specific missions.

The U.S. military’s over $738 billion dollar budget is unprecedented in human history—but that only makes the struggles for a piece of that monetary pie all the fiercer.

One of the battles currently raging revolves over a seemingly simple question: should the Air Force devote a share of its funding to procuring a new version of its oldest fighter, the F-15 Eagle, or devote every available dollar to its newest, the F-35 Lightning.

Complicating matters is that the two aircraft are built to do different things. 

The single-engine F-35A is designed to launch penetrating strikes into defended enemy airspace relying on its stealth characteristics and powerful sensors to evade enemy forces. 

The Air Force’s soon-to-be-retired twin-engine F-15C fighters are air-to-air-only aircraft that patrol the airspace around the United States and foreign military bases, fending off intruders and potential attackers. The F-15 is non-stealthy as a fighter can get, but has a 33 percent higher maximum speed of Mach 2.5 and a longer range.

As foreign air forces have ordered more and more heavily modernized F-15s over the last few decades, Boeing is offering to an F-15EX variant that will incorporate all those new bits of technology (detailed here)—while also incorporating the multirole capability the F-15C lacks. 

Though the F-15EX’s proponents argue that the money spent wouldn’t detract from F-35 funding, Lockheed and most of the air force brass don’t see it that way and strenuously object. In the last few years, the Defense Department has repeatedly tilted towards Boeing in competition, and one of the acting defense secretaries was a former Boeing executive.

For now, eight F-15EXs are on an initial order for $1.1 billion—with that possibly serving as a precursor for 144 more.

So just how well can these aircraft perform various missions?

Penetrating Strike

Russian, China and their export clients are increasingly fielding powerful radars and surface-to-air missile systems that can detect and shoot missiles at conventional fighters like the F-15EX from over 100 miles away.

To survive those kinds of defenses, fourth-generation jets must engage in elaborate anti-SAM games of cat and mouse involving jamming aircraft, HARM anti-radar missiles and Wild Weasel strike planes, and use expensive long-range standoff missiles. Such intensive operations can open narrow windows of vulnerability, and over days or weeks, gradually dismantle an air defense network—but don’t initially permit round-the-clock strikes.

By comparison, because a stealth aircraft’s reduced radar cross-section means they will only become vulnerable to targeting radars at short range, they can more easily infiltrate between the “bubbles” of enemy air defense radars and employ cheaper, shorter-range guided weapons. That means the F-35s can more freely operate over enemy airspace on “Day One” of a conflict.

Furthermore, F-35 advocates argue that using stealth jets saves the need to deploy huge strike packages incorporating dozens of aircraft, as were fielded by the Air Force in the 1991 Gulf War.

Air Support and Interdiction in Permissive Environments

When fighting against foes like the Taliban or ISIS without high-altitude anti-aircraft weapons, stealth capabilities become superfluous. Instead, the ability to loiter at length over combat zones and precisely deliver large payloads of weapons is key.

Normally, an F-35 is limited to four or six munitions stored in its internal bay.  Theoretically, in a permissive combat scenario where stealth isn’t a factor, a “Beast Mode” F-35 can be loaded down with four to six additional air-to-surface weapons on external racks.  But this much-touted capability has not actually been developed yet.

While the old F-15Cs were not configured for ground attack, the F-15EX will be—and should be able to carry a heavier ground-attack payload underwing.

Still, the F-35 does bring a different advantage to the table: its networked sensor suit may be better suited for locating enemy positions and sharing that information with friendly forces.

Interceptor/Air Sovereignty

The F-15EX is primarily supposed to replace F-15Cs performing routine air patrols around U.S. airspace and near key overseas military bases. These frequently intercept Russian or Chinese aircraft that test the boundaries of the sectors they are assigned to defend. 

Therefore, the F-15C’s replacement must be judged by how efficiently it performs that mission in peacetime as well as how viable it is in a high-intensity conflict scenario.

The F-15 is well-suited for peacetime patrols because its greater combat range of 1,200 miles allows it to embark on long patrols, and requires less aerial refueling, compared to the 770-mile range of the F-35A. Furthermore, the fact the Eagle has two engines means the aircraft is less likely to be lost if one fails—a factor manifested in significantly higher accident-rates of single-engine F-16 fighters compared to twin-engine F-15s. 

In a shooting war, higher maximum speed and a larger maximum missile load—up twenty missiles using special quad-racks instead of the F-35’s six—allow an F-15 to intercept bombers faster and launch more missiles in an emergency scenario.

F-35 proponents argue, however, that stealth would give F-35s the advantage of surprise versus enemy bombers—increasing the likelihood of killing bombers before they flee—as well as protect the interceptor from falling victim to enemy escort fighters.

Air Superiority

Air superiority refers to seizing control of the skies by beating enemy fighters at their own game—not just playing defense against enemy intruders. Traditionally, an air superiority fighter must be maneuverable for within-visual range (WVR) combats, as enemy fighters will have decent odds of evading beyond-visual-range (BVR) missiles.

Undoubtedly, the F-15EX will be more vulnerable to enemy fighters in BVR, particularly advanced 4.5-generation jets like the Su-35, as well as stealth aircraft like China’s Chengdu J-20. Quite simply, while F-15s may be able to shoot at enemy 4.5-generation aircraft from afar—and launch many missiles due to their quad racks—they will, in turn, be equally vulnerable.

At short range—where most air-to-air kills have been scored historically—the F-15 would retain an energy advantage compared to the F-35, which would also be more susceptible to detection by infrared and radar. Compared to the highly agile F-22 Raptor stealth fighter, the F-35 is less optimized for WVR combat though it can reportedly maintain high angles of attack.

But the Air Force insists that its Red Flag military exercises have repeatedly shown that its F-35s consistently trounce non-stealth aircraft by lopsided 20:1 kill ratios.

The Battle of the Books: Taxpayer Burden and Bureaucratic Support

Contrary to one might expect, the F-35A is no more expensive than an F-15EX—at around $85 million each. That reflects success in cutting initially alarming F-35 costs thanks to massive economies of scale in its favor—the Pentagon expects to order over 2,000 F-35s.  

However, if the F-15EX has a cost advantage, it comes in terms of operating costs. The F-15EX is forecast to cost $27,000 per flight hour, while the Air Force is struggling to get F-35 operating costs to $34,000 an hour.  As the Air Force is also very familiar with F-15 after forty years of service, the F-15EX likely benefit from higher readiness rates than the notoriously low ones currently afflicting the F-35.

But the F-15EX’s fortunes may ultimately revolve more around political factors: it is perceived as being foisted upon the military by civilian appointees, rather than having a strong bases of support within the Air Force brass. Thus, the F-15EX program may not survive a change in administration.

If the Air Force does dispense with the F-15EX down the line, however, it should consider developing a longer-endurance model of the F-35 for the interceptor/air sovereignty mission—particularly as in 2019 the Air Force appears increasingly disinclined to develop an entirely new manned sixth-generation fighters for that job.

Sébastien Roblin holds a master’s degree in conflict resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring. This article first appeared earlier this year.

Image: Reuters.