Even with the F-22 on the ramp, if the F-35 program is delayed or killed, the U.S. and its allies will need more aircraft with a reduced radar signature or the ability to carry standoff weapons at a range to penetrate sophisticated air defenses.

The other requirements for these improved legacy aircraft are the ability to carry conventional and directed-energy weapons, advanced electronic warfare (EW) capabilities and a price tag that is a fraction of a specialized stealth design.

Proponents of the Joint Strike Fighter—including F-35 manufacturer Lockheed Martin—doubt whether legacy fighters, such as the F/A-18 and F-15 built by rival Boeing, or even Lockheed's own F-16 could be modified to match the capabilities that the JSF delivers.

Indeed, U.S. Air Force leadership remains adamant about maintaining the ability to take apart sophisticated air defenses, and few officials believe there is any substitute for specialized stealth designs such as the F-22 and F-35.

“To not incorporate the technology that is available to the U.S.—and growing in other nations around the world—does not keep pace with the requirements of today's fight,” Gen. Gary North, commander of U.S. Pacific Air Forces tells Aviation Week. “What most people don't understand is the growing increase in land- and maritime-based, surface-to-air missiles [which drive the requirement] to have stealth or reduced-radar-cross-section platforms in today's world.

“As air-to-air missiles develop longer ranges, the abilities to see an adversary [earlier in an engagement] and to work in an intensive electronic warfare attack environment are critical. Every nation has to decide what it needs for self-defense and how much they are willing to contribute to it,” North says.

But if the stealth fleet becomes too small, it has to be supplemented. That is the niche market being worked by Brad Jones, Boeing's director of F-15 mission systems. The program spans specialized, low-signature variants such as the Silent Eagle for international customers and upgrades to existing F-15C and F-15E aircraft for the USAF. A shrinking U.S. force structure also is part of the formulation, as is the need for international customers to fly interoperable aircraft in short-notice military emergencies such as the NATO-led Libyan campaign.

A first-order question is whether the F-15 is going to be around long enough to purchase new aircraft or upgrade the existing fleets. The evidence suggests that U.S. F-15s, at least, will still be flying combat missions at mid-century.

With new aircraft production slowing down and being cut, a fighter-shortage “bathtub” is looming, meaning there will not be enough to fill operational and training needs. Right now, the Air Force has about 350 F-15Cs and 222 E-model, two-seat strike aircraft. And the service is trying to move quickly to extend the airframe life of both.

A full-scale, F-15C fatigue test is underway at Boeing with the goal of extending the 9,000-effective-flight-hour life expectancy to 18,000 hr.

The Air Force is now also launching a fatigue test program for the stronger-wing, bomb-truck F-15E from its current 8,000-hr. rating to an effective service life of 32,000 hr.

Boeing has contracts to modernize the aircraft with active, electronically scanned array (AESA) radars that have ranges 2-3 times that of the original 56 nm produced by mechanically scanned radar, say radar specialists. It also has raised the mean time between failures to 2,100 hr. from less than 100 hr.

The APG-82(v)1 radar provides an ability to create high-detail maps for precision targeting of long-range, air-launched weapons. Designed to compound the advantages of the radar is the advanced display core processor (ADCP II) now in development. Moreover, there is a digital EW program in the 2013 budget plan.

“So we can see the Air Force's thought process,” Jones says. “The ADCP II is being loaded up with processors. That is the basis of what an aircraft needs for modernization. The Air Force is putting a foundation into these aircraft” for an extended operational future.

The F-15 can carry long-range, glide and powered weapons such as the cruise-missile-size Champ, which is critical for electronic attack. Moreover, the F-22s—operating at higher altitudes and deeper in the threat rings—can provide long-range targeting for the F-15s. The F-15s then supply a large off-board magazine of missiles for the F-22s, which can serve as command-and-control aircraft.

The ADCP II boxes, which are common to the U.S. Navy's F/A-18E/F Super Hornets, are bolted into the aircraft as structure. The core software also is the same so the services both benefit from upgrades.

Another F-15 upgrade option is the digital electronic warfare system (DEWS). It also has gigabits of data available and ports linking the radar, processors and EW systems. That provides the route for running information from DEWS to the radar and other emitters to tailor jamming and electronic attack.

“From the hardware standpoint, we're done,” Jones says. “We're now talking about software upgrades. The EW system could include electronic attack and other options. All the hardware is in the array. We put in everything we can. We've added more channels in the array to do the more exotic tasks.”

The possibilities are there to create a data beam, load it with algorithms and identify an enemy electronic target of interest. “All that has been thought of,” Jones says. “We've put in the processing power, the channels and the data buses. So what you need is the algorithms and the data base to go in the processor.”

The lower-signature F-15 Silent Eagle—with canted vertical stabilizers, specialized treatments and materials and other aids—is being proposed to South Korea for its FX-3 program.

“We've already installed the AESA, incorporated the DEWS and put in fly-by-wire,” Jones says. “All we're doing extra for [South] Korea is adding a large area display and a conformal weapons bay [for a decreased radar cross section]. But it gives your fighter Day 1, forward-sector stealth. In three hours, you can put the pylons, weapons and fuel tanks on, do your checks and be ready to go. After the special missions are done, you can return to carrying exterior payloads.

“We now have conformal weapons bays,” Jones notes. “The engine face has been considered in the changes. We did not change the engine intakes because that would require changing big structure. An option is grill work over the turbine face that is similar to what like was done with the F/A-18 Super Hornets. For a relatively low amount of dollars you can get a certain reduction in signature.”