Imagine This: F-15 Fighters Armed with Hypersonic Missiles (It Almost Happened)

The F-15 fighter is clearly one of the best fighters on the planet today, with plans in the F-15EX to make it even better. But what if the F-15 was armed with some sort of hypersonic missile? Military expert Alex Hollings explains how NASA cooked up the idea back years ago and how it might have played out if taken to fruition: 

In the early 2000s, NASA came up with a plan to use the Air Force’s F-15 Eagle and the Navy’s famed AIM-54 Phoenix missile to slash the cost of experimenting with hypersonic flight. The effort wasn’t aimed at developing a new capability for America’s fleet of air superiority fighters, but rather at learning more about the unique challenges of flight at speeds in excess of Mach 5, or the hypersonic barrier.

Today, hypersonic weapons are all the rage, with multiple nations pursuing their development and at least three already in service around the world. But as we’ve discussed a number of times in the past, hypersonics are nothing new. The United States has been toying with hypersonic flight since before the Space Race began, and had a series of successful tests on different platforms in the 1990s and 2000s, before shifting focus—and funding—to the Global War on Terror.

Today, the U.S.’ hypersonic efforts continue to be hampered by the limits of America’s testing infrastructure and the immense costs associated with developing weapons and platforms capable of managing these extreme speeds. Back in 2006, those same concerns were the impetus behind NASA’s Phoenix Missile Hypersonic Testbed concept, which aimed to use the Navy’s legendary air-to-air missiles to gather data about hypersonic flight for a comparably tiny cost. The trick was, while the Phoenix missile was incredibly fast, it wasn’t hypersonic. But NASA felt confident they could pull a bit more speed out of the air-to-air legend.

By the time the idea was proposed, the U.S. Navy had already taken the AIM-54 Phoenix missile out of service, so if NASA could make their idea work, they’d have plenty of old missiles laying around to do it with.

F-15s and Hypersonic Missiles: The concept

NASA wanted to find a low-cost way to conduct hypersonic flight research, or more specifically, they were looking for an inexpensive step between ground testing and full-fledged flight demonstrators. The solution, they surmised, was to develop an air-launched system with programmable guidance that they could fly with different scientific payloads at a price point low enough to allow for multiple launches per year.

Because the Navy had a stockpile of decommissioned AIM-54 Phoenix missiles that already boasted a top speed of around Mach 4.3 (about 3,300 miles per hour), it seemed like the best possible candidate for the job. NASA assigned their Dryden Flight Research Center’s Thomas Jones to head the effort.

All Jones would have to do is find a way to pull another 500 or so miles per hour out of the weapon and they would have themselves a readily-available, inexpensive hypersonic flight test platform that fit smack between the supersonic tests they could conduct with fighters, and the Mach 7 and better tests they could conduct with platforms like their previous X-43 test platform and forthcoming X-51.

But it wasn’t just important questions about aerodynamics at hypersonic speeds NASA aimed to answer with what could have been the world’s fastest recycling program. They also believed they could learn important things about propulsion, from the fundamentals of combustion at 25 times the usual atmospheric pressure to validation of things like inlet design. Avionics and onboard computers could be tested, along with guidance systems and flight controls. Replacing materials would allow them to assess how material sciences can benefit hypersonic flight efforts, and as NASA’s own documents pointed out, they believed they’d find even more practical use for such a test bed once they had it in service.

Why the Phoenix missile?

The AIM-54 Phoenix missile was an air-to-air weapon designed specifically for engaging Soviet bombers at long ranges from the Navy’s in-development F-111B. The weapon was a continuation of the lineage of the AIM-47 Falcon missile that was to be carried by Lockheed’s YF-12 interceptor that never made it into service.

You can read our full profile on the YF-12, which was effectively an SR-71 Blackbird armed with air-to-air missiles, here.

Like the YF-12, the F-111B failed to find its way into Uncle Sam’s hangars, but the AIM-54 it was to carry was considered essential for both carrier defense (intercepting bombers before they could deploy anti-ship cruise missiles) and for preventing nuclear bombers from reaching American soil if such a conflict were ever to erupt.

In fact, for a time, the U.S. Navy considered equipping their carrier strike groups with a new variant of the F-15 known as the Sea Eagle, thanks to its high top speed and incredible dogfighting capabilities, but it was the Phoenix missile itself that shot that idea down.

Even with all the power the F-15 had to offer, the massive Phoenix missile so thoroughly compromised the Eagle’s performance that converting it for duty aboard the Navy’s flattops just didn’t make sense. Instead, the Navy opted for the now legendary and incredibly powerful Grumman F-14 Tomcat.

Even when equipped with the Tomcat’s troublesome early TF-30 engines, this massive and powerful fighter could carry not just one, but six Phoenix missiles. The Tomcat’s onboard Hughes AN/AWG-9 radar could also identify and track up to six targets at a time, which when combined with the AIM-54’s range would allow a single Tomcat to engage six separate bombers in rapid succession at distances as great as a hundred miles.

The AIM-54 would close that hundred-mile gap at better than 3,300 miles per hour. In other words, from a hundred miles away, a Tomcat could potentially wipe a half dozen Soviet bombers out of the sky in just about the same amount of time it takes to sing America’s national anthem.

However, as fast as Mach 4.3 is, it isn’t hypersonic. So for NASA’s plan to work, it would take extensive modifications to the missile itself, some creative deployment strategies, and the fastest fighter America has ever put into active service.

Mounting a Phoenix Missile on an F-15

While the Navy was right that adding a 13-foot, 1,000-pound missile to the bottom of an Eagle compromised some of its maneuverability, the Eagle still had more than enough power to reach supersonic speeds with a Phoenix strapped to its belly. So, the first thing NASA had to do was create a means to mount the massive weapon to their F-15B research fighter.

Because the entire effort was meant to be a cost-saver, NASA attempted to re-use F-14 hardware wherever possible, making for a functional, if not aesthetically pleasing, mounting apparatus beneath the center-line of the F-15.

Converting the Phoenix into a hypersonic test platform for the F-15

The missile itself, however, needed a much more thorough redressing to push it past the Mach 5 barrier. First, all of the internal components related to the missile’s guidance system and explosive payload were completely removed, including its guidance computer and radar tracker, leaving just its propulsion and control sections at the rear of the missile intact.

With the armament and guidance sections gone, a new nose with slightly more sloping angles was added to what would now be primary and secondary payload sections with the same 15″ diameter as the original components. The primary payload section measured about 57 inches long and, based on the weight of the guidance section it replaced, could carry approximately 184 pounds worth of testing equipment.

The secondary payload bay contained both instrumentation, taking up approximately 12-18 inches of the missiles’ length, and the missile’s new guidance suite, occupying the remaining 14-20. While guidance systems had advanced since the AIM-54C went into service, the real space-saver was the simplicity of the missile’s new operational requirements. No longer would it need to chase bombers through the sky for dozens of miles. Now, the Phoenix missile needed only to capture as much speed as possible before expending its fuel and colliding with the ground (or ocean, as the case may be).

The result was a slightly leaner AIM-54 Phoenix missile that could be mounted and carried by an F-15B while offering around 5.5 cubic feet of payload space for testing instruments.

But all of these modifications still weren’t enough to make the missile hypersonic. To do that would still require a very specific approach to launching the massive weapon… at speeds many American fighters couldn’t even reach to begin with.

F-15, Fire! Launching a Phoenix missile at Mach 2 to help it reach Mach 5

By October of 2006, NASA has confirmed that they could stick a Phoenix missile to their F-15, but they still had a long way to go to prove they could make it achieve hypersonic velocities.

In order to be successful, their plan called for having the F-15 climb at speeds higher than Mach 2, or around 1,534 miles per hour with the Phoenix missile attached. Once it reached the appropriate speed and altitude, the missile would be released and have its solid-propellent rocket motor fire as its guidance system oriented the weapon’s nose down toward the ground at an angle designed for the highest attainable speed, rather than the fastest route.

By traveling at such high speeds to begin with and then firing its motor until it depleted all of its fuel, NASA believed the Phoenix missile would achieve hypersonic velocities before impact, giving its suite of onboard sensors an opportunity to gather much-needed data about the nature of flight at such high velocities, all for the cost of nothing more than a decommissioned missile the Navy had a stockpile of laying around.

Using the most aggressive flight conditions, NASA believed they could consistently and reliably achieve approximately eight seconds of hypersonic flight per launch, with two launched per year proposed at approximately $500,000 per test, which while seemingly expensive, would have offered a great deal of information about not only the high temperatures associated with hypersonic flight, but importantly, the high pressures missiles experience while pressing through the air at these speeds.

A missed opportunity

There isn’t a ton to be found about NASA’s Phoenix Missile Hypersonic Testbed program, and that’s probably because it never made it past a few captive-carry test flights. And as little as there is to be found about the effort, there’s even less about the justification for its termination… but the timelines here may be telling.

By 2006, America’s priorities had shifted toward combat operations across multiple theaters in support of the Global War on Terror. No longer was the United States investing heavily in advanced technologies that might deter near-peer opponents, as the high cost of conflict took priority over the lofty technologies of tomorrow. 2006 saw the end of the Air Force’s plans to field an F-22-based fighter-bomber, dubbed the FB-22, for fiscal reasons, as one example, with the F-22 itself finding its way to the chopping block just three years later.

Reviewing an audit of America’s Prompt Global Strike program, which aims to field conventional weapons that can strike any target on the globe in short order, reveals that in 2006, a shift in focus began to emphasize the use of submarines-launched conventional cruise or even ballistic missiles to meet America’s Global Strike needs, seemingly downplaying the value of expensive hypersonics in favor of existing technologies. While research into hypersonics continued, it seems clear that America wasn’t interested in heavily investing into this technology field at the time.

So, it seems likely that, even at $500,000 per test, NASA’s crazy idea to strap Phoenix missiles to F-15s and launch them at the ground may have still seemed too expensive at the time for just eight or so seconds worth of valuable hypersonic flight data. Of course, today, as the United States continues to try to close the capability gap presented by Russian and Chinese hypersonic weapons, the decision seems awfully short-sighted. After all, America’s military has only conducted 16 hypersonic test flights since 2010, with only six deemed completely successful. It seems likely that an extra two flight tests per year could have gone a long way toward advancing America’s efforts. After all, $1 million may sound like a lot, but it’s still less than the price of a single Tomahawk cruise missile.

Interestingly, that’s not quite the end of this story, because Russia’s own hypersonic Kh-47M2 Kinzhal missile isn’t actually all that far off from NASA’s concept for the Phoenix. The Kinzhal is an air-launched ballistic missile that leverages a solid-fuel rocket motor for propulsion, just like the Phoenix did.

The Kinzhal reaches hypersonic speeds via a ballistic flight path as well, though the Kinzhal certainly offers greater range and likely, higher speeds thanks to its larger motor and fuel stores. In fact, just the Kinzhal’s nuclear warhead weighs more than the entire Pheonix missile did, but the concept behind it is largely the same. While the hypersonic Phoenix missile would have been a repurposed AIM-54, the Kinzhal is also a largely repurposed weapon, though in its case, it’s the 9K720 Iskander short-range ballistic missile it shares components with.

America currently has two new air-to-air missiles in development that may share some similarities with the Phoenix missile as well. Lockheed Martin’s AIM-260 Joint Advanced Tactical Missile (JATM) is a beyond-visual-range air-to-air missile intended to replace the AIM-120 AMRAAM. Like the Phoenix, the JATM is designed to engage aerial targets at triple-digit ranges, and likely will fly at high supersonic speeds, not hypersonic ones. Raytheon’s Long-Range Engagement Weapon is likewise aimed at engaging airborne targets at such long ranges, and although it’s been announced as a two-stage rocket-powered missile, it too will likely be a high super-sonic weapon.

Alex Hollings is a writer, dad, and Marine veteran who specializes in foreign policy and defense technology analysis. He holds a master’s degree in Communications from Southern New Hampshire University, as well as a bachelor’s degree in Corporate and Organizational Communications from Framingham State University. This first appeared in Sandboxx News.