As soon as word hit the media about the Pentagon applying pressure to the U.S. Air Force to look into procuring new F-15X aircraft, defense officials and even defense industry journalists have lamented the idea of purchasing new fourth-generation airframes in the era of stealth fighters like the F-35 Joint Strike Fighter. Critics argued there’s no place for dated platforms like the F-15 in modern warfare, arguing what we really need is more F-35s.

At first glance, that line of thinking seems logical enough. The F-35 and F-15 are technically both fighter jets and the F-35 objectively offers a wider variety of combat capabilities than the older design offered by even a brand new F-15X. The problem is, passable as that line of thinking seems at the surface, it starts to fall as soon as you dive into the nuts and bolts of the idea.

The F-35 is a fifth-generation multi-role fighter purpose-built with an emphasis on air-to-surface engagements in highly-contested airspace. The F-15, on the other hand, is a purpose-built air superiority fighter that boasts a much higher top speed and a combat-proven track record that shows unequivocally it can stand and swing with most of the (also dated) fighters being fielded by America’s competitors. When it comes right down to it, these aircraft may both be fighters, but the roles they fill are quite different.

You wouldn’t take a car out of the NASCAR oval and drop it into the Indy 500, nor would you build an open wheel Formula One car for the Daytona 500. Sure, they’re both race cars, but they’re built to go about the act of “racing” in very different ways.

Where these two worlds could intersect, however, has been in the funding. The F-15X program would involve purchasing brand new F-15s that have already been updated with more weapons mounting points and modern radar and flight systems for export to nations like Qatar and Saudi Arabia. They would be used to replace some of America’s existing and long-dated F-15C and F-15D platforms. According to Boeing (which built the F-15), the operational cost savings along would pay for the procurement in just a few years, thanks to the newer fighters being so much cheaper to fly. The problem is, however, America doesn’t want new old fighters, it wants to build a fleet of advanced, next generation fighters, and some were concerned that diverting funding toward the F-15X could mean slowing procurement plans for the F-35.

Now, however, that argument has been shot out of the sky.

“In terms of the F-35, it is well supported across the board, regardless of whatever platform the Department of Defense is looking at,” Lockheed Martin CEO Marillyn Hewson said this week. “If they chose to have an order of the F-15, it won’t be at the expense of the F-35 quantities. I’m hearing that directly from leadership in the Pentagon, and I think that’s an important point for me to make. It’s not just our suspicion, but I’ve been told that directly.”

The need for the new F-15, was echoed by Air Force Chief of Staff Gen. Dave Goldfein, who made it clear that he’d love to field more F-35s, but the force has to meet its operational needs in a budget-friendly way.

“I’m not backing an inch off of the F-35” Goldfein said. “The F-35 buy that we’re on continues to remain on track. And I’m not interested in taking a nickel out of it when it comes to buying anything else in the fighter portfolio.”

“If we had the money, those would be 72 F-35s. But we’ve gotta look at this from a cost/business case.” Goldfein added. “An F-15 will never be an F-35. Never. But I need capacity.”