The U.S. Air Force says it is buying 80 updated F-15C fighters to complement the new F-35 and (unofficially) make up for the fact that the air force is not getting as many F-35s as it believes it needs. There will be two variants, the single F-15CX and the two-seat F-15DX. Normally the two-seat version is for training but in this case the DX version provides the option to provide a GIB (Guy in the Back) to help with the use of the F-15X as a bomb truck for F-35s and F-22s. These two stealthy aircraft, especially the F-35, have demonstrated the ability to remain silent (few radio and radar transmissions) to detect targets and have less stealthy aircraft (or missile units on the ground) launch weapons to destroy the target. Israeli F-35s, the only ones with combat experience, may have already demonstrated this technique over Syria.
The X version would combine years of experience and upgrades for American and foreign F-15s to produce a more capable (better electronics and larger bomb load) and durable (built to operate for 12,000 flight hours) aircraft. The X model would carry up to 14 tons of bombs and missiles and be built to avoid a lot of the structural fatigue problems F-15Cs encountered. While the two-seat F-15E fighter-bomber is still available for sale new, all existing F-15Cs are at least 30 years old.
On paper, the F-35 will eventually (5-10 years) become cheaper to maintain and operate than the F-15C. The F-15X will have a cost advantage immediately. The manufacturer is willing to produce them at a fixed cost (about $72 million each) and absorb any unforeseen costs. The manufacturer has additional incentives to make the F-15X work; export sales.
Meanwhile, the F-15E continues in service and demonstrates how well an updated F-15 design can work. But the production of the F-15E is scheduled to end in 2022 and the X model would keep F-15 production going as the two-seater F-15X can be equipped to operate as a more advanced F-15E. This willingness to produce the X model should not be a surprise as there have been signs for a long time that an updated F-15 would be useful.
For example in 2008 the air force announced plans to operate its 36 ton U.S. F-15E for at least another ten years, and probably longer. In service for twenty years now, the F-15E can carry up to 11 tons of bombs and missiles, along with a targeting pod and an internal 20mm cannon. It's an all-weather aircraft that can fly one-way up to 3,900 kilometers. It uses in-flight refueling to hit targets anywhere on the planet. Smart bombs made the F-15E particularly efficient. The backseater handles the electronics and bombing, and the F-15E remains a potent air-superiority fighter, making it an exceptional combat aircraft. This success prompted Israel, Saudi Arabia, South Korea, Singapore and currently Qatar to buy it, paying about $100 million per aircraft. In the U.S. Air Force, the F-15E is one of the most popular aircraft for combat pilots to fly, even more so than the new F-22.
The F-15E entered service in 1989 and within a decade Russia decided this novel fighter-bomber design was worth emulating. Russia developed its own F-15E, the 45 ton Su-34, which was yet another variant of the 33 ton Su-27. Su-34s to cost $36 million each (less than half the cost of an F-15E) and included a full set of defensive and offensive sensors (radars, targeting cameras, laser designators) and electronic warfare gear and could carry eight tons of missiles and smart bombs. Although development work began in the late 1990s, the Su-34 did not enter service until 2014 and proved capable in Syria.
Meanwhile, the older F-15s, which entered service in 1976, are falling apart. In 2009 the air force retired the last of its 384 F-15A fighters. Long flown only by reserve units, these were old aircraft and all built in the 1970s. Air force reserve units got the F-15As in the 1980s and 1990s, as active duty units got the new F-15C. But by 2009 the F-22 is was entering service, and more F-15Cs were going to the reserves. Many of those F-15As flew for over 30 years.
Unfortunately, the later model F-15s are not aging well. In 2007 the air force grounded all of its 442 remaining F-15As and Cs (and the smaller number of two-seat B and D trainer models) for 18 days, then grounded them again, all because of suspicions that portions of the aircraft structure have been weakened by stress (lots of maneuvering during combat training).
Before that, the air force halted non-critical flights of its F-15C (the interceptor version) fighters after a National Guard F-15C crashed. It appeared that the crash was the result of structural failure. In 2002 an F-15C traveling at high (over 2,000 kilometers an hour) speed crashed when its left tail fin broke off. At the time the F-15Es operating in Afghanistan were not grounded initially, but soon were when it was realized that the problem might be a design flaw, not age, that caused the 27 year old F-15C to go down. The F-15Es were restored to flight status after about a week, once each aircraft had undergone an extensive structural examination (taking about 13 man hours each). Most F-15Es were less than ten years old. But some F-15Cs were over twenty years old.
This time around, the F-15Es were not grounded, because metal stress in the older F-15s would not occur in the F-15E, which is somewhat different in its internal structure and was designed to avoid the fatigue problems the F-15A and C models encountered. Structural failure is more common in older fighters that have lots of flight hours (over five thousand) on them. When originally designed, the F-15 was believed to have a service life of only 4,000 hours. But new materials and design techniques increased that to 8,000. In peacetime, F-15s are in the air 250-300 hours a year. But because of the 1991 Gulf War, the 1990s "no-fly-zone" patrols over Iraq, and the operations after 2001, the F-15 fleet piled up the hours more quickly, and many are approaching the 8,000 hour mark much more rapidly than planned.
If weak components are detected, they can be replaced with stronger ones, made of materials not available when the F-15 was originally built. But you want to find the weak components before they fail. While scanning technology has improved, it's still not good enough to detect all the F-15 components possibly weakened by years of use. As a result, flying an F-15C became a bit more stressful from then on. To some in the air force, this situation has a bright side. One can now make a more compelling case to build more F-22s, to replace F-15 that are wearing out faster than expected. That did not happen and the delays in getting the F-35 into mass production kept the idea of designing an “improved F-15” to fill the gap. Because of that, the F-15X was not a recent idea but the accumulation of proposals that have been showing up for over a decade.
Another area that is getting a lot more attention from engineers than journalists is the impact of heavy and sustained stress on combat aircraft. This component failure problem is not unique to the F-15 and has been occurring with increasing frequency among aging fighter aircraft all over the world. The end of the Cold War in 1991 led to the cancellation of many warplane replacement programs. Air forces were compelled to make do with thousands of increasingly older aircraft. Whenever an aircraft goes down because of a structural failure, you have to ground all planes of that type until you know exactly what caused the loss, and made any needed repairs to other aircraft of that type. Pilots are a pretty sharp lot, so governments don't dare try to play games with this. If the pilots suspect they are being set up to fly dodgy aircraft, they will not fly them, or not fly them in a useful (stressful) way.