Legacy F-15 Eagles Boost NASA Test Fleet
The ‘legacy’ F-15C/D may now be a dwindling presence in the U.S. Air Force, but the jets still support vital test work with NASA’s Armstrong Flight Research Center. NASA has long flown different F-15 variants for numerous kinds of missions and recently added to its fleet with another pair of jets cascaded down from the Air Force. Meanwhile, older F-15s are also continuing to take on new test assignments with NASA, having already contributed enormously to its military and civilian research programs, including flying alongside legacy F/A-18 Hornets.
Earlier this month, NASA confirmed that it had received two twin-seat F-15Ds, serial numbers 81-0063 and 84-0045, previously operated by the Oregon Air National Guard’s 173rd Fighter Wing at Kingsley Field. This is the Air Force’s F-15C/D ‘schoolhouse,’ which, as we have reported in the past, will replace its Eagles with F-35As, overturning a previous plan that would have seen the 173rd Fighter Wing assume responsibility for training pilots for the new F-15EX Eagle II.
NASA’s windfall provides new equipment for its flight research fleet at Edwards Air Force Base, California. However, only one of the F-15Ds will go into active NASA service, with the other serving as a source of spare parts for the maintenance-heavy Eagles.
One of the missions that the F-15D will be involved in is tests of NASA’s remarkable-looking X-59 Quiet Supersonic Technology experimental test aircraft, or QueSST, which made its first flight in October last year and will be flown out of Edwards. Much is resting on the test program that has now been kicked off, with the future of supersonic passenger flight arguably dependent on its successful outcome.
The QueSST project is one that TWZ has covered in detail over the years and which is planned to demonstrate how careful design considerations can reduce the noise of a traditional sonic boom to a “quieter sonic thump.” If that can then be ported over to future commercial designs, it could solve the longstanding problem of regulations that prohibit supersonic flight over land.
“These two [F-15Ds] will enable successful data collection and chase plane capabilities for the X-59 through the life of the Low Boom Flight Demonstrator project,” explained Troy Asher, director for flight operations at NASA Armstrong. “They will also enable us to resume operations with various external partners, including the Department of War and commercial aviation companies.”
X-59 Team Reflects on Completing First Flight
“NASA has been flying F-15s since some of the earliest models came out in the early 1970s,” Asher added. “Dozens of scientific experiments have been flown over the decades on NASA’s F-15s and have made a significant contribution to aeronautics and high-speed flight research.”
As part of its diverse test fleet, NASA’s F-15s provide an ideal platform for test and chase duties that demand high-speed, high-altitude capabilities. At the same time, the Eagle’s impressive load-carrying ability means that various experimental payloads can be mounted on it externally, either under the wings or on the fuselage centerline, benefiting from the jet’s generous ground clearance.
The legacy F-15’s 1970s-era technology is also fairly straightforward to modify, meaning that new or adapted software, systems, and flight controls can be integrated to meet particular test requirements.
NASA has also ‘tweaked’ its F-15s to better optimize them for high-performance test work.
Back in 2022, NASA announced that it had made modifications to two of its earlier F-15s to support X-59 chase flights.
The two-seaters received new emergency oxygen bottles and regulators, for the pilot and back-seat technician, to reduce the risk of hypoxia — a lack of oxygen reaching the brain and other tissues of the body, which can happen as the aircraft climbs.
The new positive-pressure breathing system was developed for the F-22 and provides additional pressure compared with the F-15’s original life support system. It means the F-15 can operate safely at up to 60,000 feet.
The X-59 is designed to hit this altitude and cruise at 55,000 feet.
In fact, the revised life support system also shares many components with the X-59. Flight crews wear the same gear, the same panel-mounted regulator, and the same device that reduces the pressure flow from the liquid oxygen tanks to the regulator. The same modification is being made to NASA’s newly acquired F-15D.
This will all help NASA’s QueSST test program, which aims to push the X-59 to a speed of Mach 1.4, equivalent to around 925 miles per hour, over land. Ahead of this, multiple sorties will be flown over the supersonic test range at Edwards, accompanied by F-15s.
In the meantime, NASA researchers continue to utilize earlier Eagles — including NASA tail number 836, a 1974-vintage F-15B, a variant of the jet long since discarded by the Air Force. This particular jet was obtained by NASA in 1993 from the Hawaii Air National Guard.
Earlier this week, NASA announced it had completed a high-speed taxi test of its F-15B after modification for the Crossflow Attenuated Natural Laminar Flow (CATNLF) test.
The CATNLF concept is intended to boost laminar flow over a wing surface, therefore reducing drag and improving efficiency.
For the tests, the F-15B has been fitted with a three-foot scale model of a CATNLF wing design, mounted under the belly, in a vertical position. Earlier this month, the F-15B was taxied at a speed of 144 miles per hour with the wing model fitted. A first flight in this configuration is planned in the coming weeks.
The CATNLF wing is tailored to address a key problem of laminar flow technology, namely the effect of crossflow, an aerodynamic phenomenon that occurs on angled surfaces. Even large, swept wings of the kinds found on most commercial airliners have crossflow tendencies.
According to earlier NASA studies, the CATNLF wing design, if incorporated in a large, long-range aircraft like the Boeing 777, could result in annual fuel savings of up to 10 percent.
While the legacy F-15 continues to provide valuable service to NASA, the Air Force has recently moved to adapt its plans for the phase-out of the jet.
Already, the Air Force has given up its last active-duty F-15C/Ds. The final active-duty squadrons to be deactivated were at Kadena Air Base, Japan, which you can read about here. A handful of test jets remain in use, with all other F-15C/Ds now assigned to the Air National Guard.
Previously, the Air Force’s Fiscal Year 2024 budget request detailed plans to divest the entirety of the F-15C/D fleet by 2026.
As of last October, however, the service said it planned to retain some of its F-15C/Ds until 2030. The Air Force determined these aging jets are still needed for the homeland defense mission, something it laid out in its Long-Term Fighter Force Structure report.
The report was mandated by Congress in the 2025 National Defense Authorization Act, which called for the Air Force to clarify its long-term fighter plans.
Under these plans, the Air Force wants to keep 42 F-15C/Ds as part of its combat-coded total aircraft inventory through 2028. Thereafter, a reduced fleet of 21 of the youngest jets will continue to serve with the California Air National Guard’s 144th Fighter Wing until 2030.
At this point, the Air Force’s legacy Eagles should be fully replaced. The last F-15C/Ds are slated to be superseded by the F-15EX, while some others will have been replaced by F-35s; one A-10 unit is also receiving them. However, it should be noted that the Air Force itself has described its Long-Term Fighter Force Structure document as highly aspirational, and such plans are, by their nature, liable to change.
Whatever the future brings for the legacy F-15 with the U.S. Air Force, the recent arrivals at the Armstrong Flight Research Center confirm the continued value of the Eagle for NASA’s exacting test missions.
Contact the author: thomas@thewarzone.com