The US Air Force is upgrading its fleet of Boeing F-15E Strike Eagles with the intention of keeping the venerable dual-role strike fighter in service beyond 2035. The service, however, has no idea what might replace those aircraft when the inevitable day comes when the 219 jets currently in the inventory wear out.

"There are no plans to replace the F-15E for the foreseeable future," the USAF said late last year.

Rebecca Grant, a noted airpower analyst and president of IRIS Independent Research in Washington DC, says that a replacement interdictor aircraft has been an unaddressed problem for a long time.

"The F-15E replacement has been a gap in air force strategic planning for a decade," Grant says. "Two decades really."

The problem has always been kicked down the road because compared with other USAF fighter platforms, the F-15E is relatively new, she says. The USAF's Strike Eagles were delivered between 1988 and 2004, Flightglobal's MiliCAS database shows.

There have been initiatives in the past for versions of the Lockheed Martin F-22 Raptor to replace the Strike Eagle, Grant notes. During the 1997 Quadrennial Defense Review (QDR), the USAF was initially set to buy 442 Raptors, but that number was reduced to three wings of F-22s, or roughly 339 aircraft, Grant says. However, along with the reduction in the numbers of original model air superiority-optimised F-22s came an option to buy two additional wings of strike-optimised Raptors. "At the time, the decision didn't seem urgent because in 1997 you're looking at F-15s that were young," Grant says.

The strike-optimised Raptors never materialised. "But it sure would have been nice to have those 1997 QDR-envisioned two more wings of F-22s," Grant says.

As for the air force's options, experts are divided over what course of action the USAF should take when the time comes to replace the Strike Eagle. Some, such as retired USAF Lt Gen David Deptula, who flew the F-15C Eagle during his time in service, say there is no need to directly replace the long-serving interdictor. Deptula says that between the Lockheed F-35 Joint Strike Fighter and the service's new Long Range Strike-Bomber (LRS-B) programme, the USAF will have the mission set covered. "People tend to think of individual aircraft being replaced by a follow-on aircraft, [but] that's sort of a last century way of thinking about building capability," Deptula says. "By reducing the different types of systems that we have, yet achieving greater capability, that also becomes much more efficient, as well as effective, and allows us to do more with limited resources."

Even though the F-35 does not have the range of the Strike Eagle, the aircraft will effectively conduct the overwhelming majority of the missions performed today by the F-15E. "In the 2030-plus timeframe we should have a robust fleet of F-35s, which will essentially be able to perform the functions that an F-15 assists in fulfilling today," Deptula says.

The stealthy F-35s would be able to provide intelligence, surveillance and reconnaissance (ISR) capabilities, in addition to the ability to perform precision strike. "They provide the capability of introducing an airborne ISR/strike complex, if you will, that becomes much, much more powerful than the way we have tended to use fighter aircraft in the past," Deptula says.


But the F-35 must be bought at a high enough rate to recapitalise the USAF's ageing fighter fleet, Deptula says. "It makes much more economic sense to increase and maximise the numbers that can be produced in any single year," he says. "But this is the challenge programmers face in an era of declining resources."

Grant agrees. "The F-35 is very capable in a lot of the roles: particularly the role we have seen the F-15E used in [over] the last 10 years," she says. "But we have hardly any of them."

It is difficult to say when the USAF will be able to reach a sizeable F-35 fleet given the production rate cuts of the past four budget cycles, she says.

But by the time 2035 rolls around, when the F-15E will start to be retired, the USAF should have the LRS-B in service to cover missions that require a longer reach than the F-35 can provide. "With respect to unrefuelled range, that is an asset that will provide that additional capability," Deptula says. "Furthermore, it will be part of this ISR/strike complex family of systems."

However, the USAF must build more than the 80 to 100 LRS-B aircraft that are currently planned, Deptula says. The air force must have enough bombers to deploy its aircraft on a rotational basis to meet US commitments around the globe, both during wartime and peacetime deployments, he says.

For the USAF, that means buying enough LRS-B aircraft to fill out the service's aerospace expeditionary forces, which would ­require 10 squadrons of 12 LRS-B aircraft each, Deptula says. In addition to the 120 combat-coded aircraft, Deptula says the USAF would need training and test aircraft, as well as an attrition reserve and backup inventory machines. This would bring the total up to between 150 and 160 LRS-B bombers.

Grant says, however, that the USAF should probably build more than even the 160 LRS-B aircraft Deptula envisions, because the service needs a fleet that can endure losses in combat. As such, she estimates the air force needs to buy at least 200 LRS-B aircraft.

"We are looking at scenarios in the Pacific where there will be attrition," she says. "It may be on the ground, it may be in the air, but it will happen, and we need to start buying aircraft with that in mind." The days of fighting in "incredibly permissive" airspace are over, Grant adds.

But if the LRS-B is going to be used for F-15E-like missions, Grant says that the bomber must also be armed with air-to-air missiles. "It needs to be wired for that [Raytheon AIM-120] AMRAAM family of missiles," she says.

The LRS-B should also be wired to carry future hypersonic strike weapons and to mount airborne laser weapons as they become available, she says.

While Grant agrees that the F-35 and LRS-B can cover most of the Strike Eagle mission set between them, she says the service needs to start looking at a sixth-generation replacement for both the F-22 Raptor and the F-15C and E versions of the Eagle. "It is time now to start thinking about the F-15E and F-22 replacement," Grant says.


That follow-on platform should be initially developed for the air superiority role before being developed into a strike aircraft. "The air force has had really good luck with designing for air superiority and then quickly modifying to the fighter-bomber role," Grant says.

Indeed, the classic example of this concept is the Strike Eagle itself, which was developed from the air superiority-optimised F-15C/D model fighter. Another example is the now-retired Grumman F-14 Tomcat, which towards the end of its service in the US Navy was adapted to the strike role. "That has tended to be a sure bet," Grant says. "So I think they're right to start with that."

But from a budgetary standpoint, the USAF is not remotely close to being ready to undertake such a sixth-generation development, she says. Nonetheless, both the USAF and USN, along with the US Defense Advanced Research Projects Agency, have begun preliminary studies on developing such an aircraft.

Industry officials say that if the money is not there to develop a new clean-sheet strike aircraft design, it would be possible to modify the F-35 into an intermediate-range strike platform. Lockheed has in the past looked at two-seat variants of the F-35, going back to the time of the X-35 concept demonstrators in the early 2000s. The company had even studied two-seat variants of the US Marine Corps' short take-off vertical landing variant of the jet. Later studies focused on a two-seat or extended-range variant of the navy's F-35C; the company found that the aircraft could accommodate either an extra crew station or more fuel as a result of the addition of a 40-50in fuselage plug.

In previous years, Lockheed had pitched a larger-winged, extended-range variant of the Raptor to meet the USAF's bomber requirement, called the FB-22. However, with production of the F-22 at an end, there is almost no chance of resurrecting a Raptor variant to replace the Strike Eagle.

Lt Col Joseph Siberski, Air Combat Command's (ACC's) F-15E programme element monitor, says he does not know what will follow the Strike Eagle, but that the service has a robust plan to keep the jet relevant for the remainder of its time in service. The USAF is currently working with Boeing to conduct a full-scale durability test on both the F-15C and F-15E airframes to assess the aircraft's maximum potential life spans.


On the F-15E, the testing will assess the airframe out to 65,000h, Siberski says. Testing to this mark will allow the USAF to keep the jet in service until it hits 32,500h worth of flying hours. "That gets our fleet to be viable from a structural standpoint through 2035, given assumptions for utilisation rates," Siberski says.

F-15s are unique, Siberski says, because the jets undergo extensive depot maintenance every five years, which helps keep the airframes in good shape. Making sure the F-15E airframe is structurally viable lays the groundwork for upgrading the jet's avionics.

The most pressing upgrade for the Strike Eagle is to replace the jet's increasingly obsolescent mechanically scanned Raytheon APG-70 radar with a new active electronically scanned array (AESA) Raytheon APG-82(V)1 set, Siberski says.

To fit the new radar into the Strike Eagle, the jet's environmental control system had to be modified, as did several interfaces with the F-15E radar warning receiver and some other components. Developmental testing for the radar modernisation was completed in 2012 at Eglin AFB, Florida, while operational testing is ongoing at Nellis AFB, Nevada. A total of five aircraft have been modified so far.

"It's a big mod and we're doing the first LRIP [low-rate initial production] installs. [The aircraft] are going to roll into the hangar here in the next couple of months now up at Mountain Home AFB, [Idaho]," Siberski says. "So that mod is starting in this fiscal year."

The other major sensor upgrade on the F-15E is the Eagle Passive/Active Warning and Survivability System (EPAWSS), which will be shared with the F-15C. The new systems, which first started receiving funding in the 2013 budget, will replace the Strike Eagle's obsolete Tactical Electronic Warning System (TEWS). However, Siberski says, the specifics of the EPAWSS capabilities have yet to be determined. The ACC is currently undertaking an analysis of alternatives on what capabilities the system should have, based on the available funding. The study should be complete by the autumn of 2013, Siberski says. In the meantime, Boeing is working on a radio frequency characterisation study on the F-15E to pave the way for whatever system is picked for the EPAWSS, says Brad Jones, the company's F-15 mission systems director.

Inside the cockpit, the USAF hopes to add the new Advanced Display Core Processor II (ADCP II) computer, which is set to be installed on the jets from 2016. Siberski says the new computer hardware is needed to process the data being generated by the aircraft's new sensors. The same computer will also be used on the F-15C, with means the USAF will be able to share software between the Eagle variants, Siberski says. The release of the new processor will coincide with the release of the F-15E Suite 8 software load.

But what the F-15E currently lacks are modern displays to allow the pilot and weapons systems officer to effectively utilise the new sensor systems. "Our actual glass displays in the F-15E are the same ones we had when the airplane fought in Desert Storm" more than 20 years ago, Siberski says. "They are, quite frankly, Reagan-era technology."


Aircrew cannot take full advantage of the resolution provided by, for example, the Lockheed Sniper targeting pod that the F-15E routinely carries into battle. "Replacing and modernising displays is right now our number one unfunded modernisation effort," Siberski says. But before anything can be done about the displays, the ADCP II must be installed onboard the jet.

Jones says that the USAF has a number of options available to it. The service could replace the existing displays with colour displays, or with a single large area display in each cockpit. The optimal solution in Boeing's view in terms of cost versus effectiveness is to replace the rear cockpit displays with a large area display and switch the three front cockpit displays to a new colour format, Jones says. "That was the most cost-effective quick study that we did," he says.

Along with the modified hardware, the F-15E will need updated software. Along with the ADCP II, the aircraft will receive a Suite 8E software load, Siberski says. The updated software will include support for both the Raytheon AIM-9X Block I and II air-to-air missiles; Advanced Targeting Pod-Sensor Enhancement units built by both Northrop Grumman and Lockheed; a rear cockpit Joint Helmet-Mounted Cueing System; Link 16 datalink upgrades; small diameter bomb II capability; and software enhancements to the APG-82.

Beyond that, the USAF is defining the F-15E's Suite 9 software package for release in fiscal year 2018, Siberski says. Potential additions in Suite 9 include enhanced combat identification capabilities and electronic protection measures, he says. More significantly, Suite 9 is likely to be the software load that will integrate the EPAWSS, he says. "Right now we're going through the requirements development for Suite 9, but those are the big potentials for that software suite."

The F-15E community is also looking at potentially adopting a new infrared search and track system pod that is currently under development for the F-15C, Siberski adds.

Other currently unfunded but mandated capabilities include adding an automatic dependent surveillance-broadcast (ADS-B) system onto the jet, as otherwise it will not be able to operate inside national airspace. Additionally, the satellite communication systems will have to be upgraded to use the Mobile User Objective System constellation and all of the jet's radio frequencies will have to be remapped.

Other than the modifications that the USAF has already mapped out for the F-15E, there are a host of advanced technologies that have been developed by Boeing for international customers that could potentially be added to domestic Strike Eagles. "We always have the US Air Force in mind," Jones says. "So any modification that we design, we always try to make it backward compatible."

While it would be a huge modification, the fly-by-wire system being developed for Saudi Arabia's new and upgarded F-15SAs could be retrofitted to existing aircraft, Jones says. While this would be difficult to retrofit, activating the existing outboard wing weapons stations on the USAF's F-15Cs and F-15E would be a relatively simple matter, he says. The extra weapons stations are rated for 454kg (1,000lb) each.


If the USAF chooses to do so, there are also technologies from Boeing's F-15SE Silent Eagle project that the service could adopt. For example, the Silent Eagle's radar cross-section reducing conformal weapons bays (CWBs) fit onto the same interfaces as the F-15E's conformal fuel tanks (CFTs). The CWB, which carries a total of four weapons, holds 867kg of fuel, compared to the 2,250kg of fuel capacity of the CFTs. The CFT and CWB can be switched as mission requirements dictate, says Howard Berry, Boeing's capture team lead for South Korea's F-X campaign, where the company is pitching the Silent Eagle.

Additionally, the stealth coatings on the F-15SE could potentially be retrofitted to older jets. Jones says that the USAF has not expressed interest in any of the Silent Eagle technologies, but is keeping an eye on the performance of the new variant.

There are a number of countries that have expressed an interest in the Silent Eagle. "We continue to have dialogue with customers on a variety of products in the tactical aircraft field, both in the Middle East and other parts of the world," Boeing says. "We are working with our allies to determine what growth strategies and technologies make sense for weapon systems that will be in service for the next 30 years."

Source: Flight International