Judging from his tweets, President-elect Donald Trump thinks the Pentagon would be well advised to defer a few F-35 fighter jet purchases from Lockheed Martin (NYSE:LMT) and invest in some cheaper fighters from Boeing (NYSE:BA) instead -- and the Air Force may agree with him.

Earlier this week, I reviewed for you the soon-to-be-president's musings about a potential "comparable F-18 Super Hornet" that Boeing might build. Granted, over at the Air Force, they don't have a lot of use for Boeing's F/A-18 Super Hornet (which is, after all, a Navy and Marine Corps plane). But they are awfully fond of Boeing's F-15 Eagle fighter ... which is kind of a shame.

Because about half of those planes may be forced to retire.

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Boeing's F-15 Eagle -- will it sink or soar? Image source: Getty Images.

Retirement age for warplanes

According to data from FlightGlobal's World Air Forces 2016 report, the U.S. Air Force currently has 416 F-15 fighters (models C through E) in its inventory. More than 180 of these are recent-model F-15Es. But Defense One reports that 235 of the planes are older F-15C and D models.

Now here's the problem: The basic F-15 design has been flying since the 1970s, and some of these C- and D-variant warbirds are getting a bit long in the beak, and need to be either upgraded or retired. To keep them flying for their full hoped-for life spans (which won't run out for another 25 years), the Air Force says it must spend at least $12 billion (Defense One estimates the cost at "tens of billions of dollars") to refurbish the F-15 fleet with upgraded electronics, new wings, bigger fuel tanks, and other structural replacements.

Even these upgrades won't succeed in transforming a fourth-generation F-15 into a stealthy fifth-generation combat jet, of course. But they'd increase the plane's range, endurance, lethality -- and life span. And over at Boeing, they're all for the idea. Steve Parker, Boeing's vice president of F-15 programs, contends that teaming up upgraded F-15s with stealthy fifth-generation F-22 fighters from Lockheed Martin would enable the Air Force to put a lot of metal in the air, and ensure air superiority "into the 2040s."

But at what cost?

The high cost of late-life medical care -- for warplanes

At the low end of Defense One's estimated "tens of billions of dollars," $20 billion spent on upgrading 235 F-15C and D models implies an upgrades cost of more than $85 million per plane. That's nearly enough money to buy a Lockheed Martin F-35 -- or three brand-spanking-new Scorpions from Textron (NYSE:TXT), with money left over for missiles and jet fuel.

Looked at that way, the prospects of getting the Air Force to pay for upgrading the F-15 fleet -- or getting Congress to approve the funds -- seem pretty dim. The same money that would be needed to upgrade 235 older-model F-15s could instead be spent buying 235 brand-new F-35s from Lockheed, or if Textron's plane is ever deemed worthy of induction into the air fleet, as many as 700 new Scorpions.

A second alternative suggested by Defense One -- and this is an idea the Navy has been mooting as well -- might be to take the money the Pentagon would otherwise spend on F-15 upgrades and invest it into developing a sixth-generation jet. Tentatively titled the "Penetrating Counterair" fighter by some, and "F-X" by others, such a 6G jet couldn't possibly be ready to enter service before 2030. But once it does, it would arrive at hypersonic speed, and packing laser guns!

The upshot for investors

These features would admittedly be pretty cool -- 14 years from now. But honestly, given that the U.S. military needs to put more iron in the sky today, I think the choice here is more likely to come down to this: Spend the money refurbishing old F-15 Eagles, or spend the same money buying brand-new F-35s. (And we've already seen how the Air Force has handled similar debates in the past.)

Ultimately, this probably puts the ball in Lockheed Martin's court. Lockheed has committed to getting the price of its F-35 Lightning II stealth fighter down below $85 million a copy. If it succeeds in doing that, then advocates of spending a similar sum to upgrade a fourth-generation F-15 to still-not-fifth-generation status won't have much of a leg to stand on. Buying F-35s instead of upgrading F-15s will become a no-brainer.

On the other hand, if Lockheed Martin can't (or won't) drop the price of its F-35, then this could prove a closer call. Crazy as it might seem (and as frustrating as it must feel for Boeing), the fate of this "tens of billions of dollars" upgrades program probably rests entirely in the hands of Lockheed Martin.

Rich Smith does not own shares of, nor is he short, any company named above. You can find him on Motley Fool CAPS, publicly pontificating under the handle TMFDitty, where he's currently ranked No. 346 out of more than 75,000 rated members.

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