And it would seem that in 2006, Boeing took a look at their incredibly powerful F-15, and thought to themselves, “that thing might just work as a launch vehicle…“

THE F-15 GSE GLOBAL STRIKE EAGLE CONCEPT

On April 24, 2006, a Boeing team led by Timothy T. Chen, Preston W. Ferguson, David A. Deamer, and John Hensley attended the 4th Responsive Space Conference in Los Angeles with their unique proposal in hand. The proposal, which came in the form of a 17-slide PowerPoint and accompanying 10-page write-up, was built upon a previous study conducted by a team of seven Boeing staffers led by Payloads and Structures Engineer (at the time) Tom Mead.

Of course, there had been plenty of crazy proposals to come out of aviation firms over the years, from massive nuclear-powered flying aircraft carriers to genuine flying saucers — but despite targeting space itself, this proposal was decidedly quite grounded. The idea behind the Global Strike Eagle wasn’t to sell the U.S. government on developing a pricey new aircraft for space launch operations. Instead, Boeing aimed to take the equipment Uncle Sam already had laying around and assemble it in a way that would offer a groundbreaking new capability for an extremely low cost.

As the proposal puts it:

“The utilization of the F-15 as the initial stage of the launch system provides not only the expected performance benefits – reduced velocity requirements for the rocket stages, lower aerodynamic drag, and decreased atmospheric pressure but also the operational advantages inherent in using the existing support infrastructure.”

“Responsive Air Launch Using F-15 Global Strike Eagle,” Boeing, 2006

In keeping with that low-budget mindset, the proposal calls for using an existing F-15C or D with “high hours” as the initial technology demonstrator intended to serve as an airborne launch platform for a small rocket (likely from under-wing or center-line pylon), with plans to move on to using a more highly modified F-15E Strike Eagle as the basis for what would become the first true Global Strike Eagle.

HOW DO YOU ATTACH A 45-FOOT ROCKET TO AN F-15?

Of course, the first significant hurdle the concept would have to overcome was already apparent at this stage: despite the power and payload capabilities offered by the F-15 airframe, it would be utterly impossible to mount a rocket of this size or weight beneath the fighter. Previous studies of F-15E payload capabilities suggested that under-wing pylons couldn’t manage anything heavier than 220 pounds. NASA would successfully mount a 1,000-pound, 13-foot AIM-54 Phoenix missile on a custom center-line hard point devised for their F-15B in 2006, but their limited success in the effort only further confirmed that mounting a 45-foot, 30,000-pound rocket to the bottom of this aircraft would prevent it from rolling down the runway at all, let alone pitching up for take-off.

As a result, the decision was made to mount the rocket launch vehicle on top of the F-15, taking advantage of the wide tail span between upright stabilizers to allow for a larger diameter rocket. This new top center-line pylon concept was substantiated by testing originally intended for added ordnance, and according to Boeing, four internal bulkheads within the fuselage would provide adequate strength to support their massive rocket.

But even with the rocket now riding atop the fighter, its massive size still created problems for the Global Strike Eagle concept. The rocket’s nosecone would not only cause clearance issues with the cockpit canopy, it would interfere with the F-15’s ejection apparatus if used… and by interfere, Boeing meant that any pilot crazy enough to fly an F-15 with a rocket on its back would also be unable to eject without hitting the rocket itself. This seemed like reason enough to eliminate the pilot altogether and convert the Global Strike Eagle to use a communication link-based flight control system like those employed by Boeing’s X-45 or X-36 technology demonstrators. This change also offered the added benefit of not having to find a pilot with an affinity for rockets and a death wish.

Of course, operating an aircraft without a pilot is nothing out of the ordinary for the U.S. Air Force, which operates a wide variety of uncrewed platforms. The branch even has experience with converting crewed fighters into uncrewed jets, with none other than Boeing assisting in fielding QF-16 aerial target drones made from retiring F-16 airframes.

Because there would be no pilot onboard, the conversion from Strike Eagle to Global Strike Eagle would require very little in the way of avionics or system changes, and in fact, many systems, like onboard radar, could be removed entirely because of the aircraft’s non-combat role.

The Air Force already operates more than 400 F-15 Eagles and F-15E Strike Eagles the world over, so the vast majority of the infrastructure required to operate the Global Strike Eagle already exists, and many of the aircraft’s systems could be maintained by existing Air Force personnel. This offered both a means of reducing costs and of ensuring the Air Force could have orbital launch capabilities from practically any airfield found under a friendly flag, and just as importantly, it could be done quickly.

Theoretically speaking, a Global Strike Eagle and its support team could be flown to any airstrip with a few thousand feet of runway, prep their aircraft for launch, and deploy a payload into orbit in fairly short order. When conducting these operations from military airstrips with existing F-15 infrastructure, these launch operations could be easily hidden, as most of the launch preparations that would take place outside the hangar would look like any other F-15 sortie.

HOW TO LAUNCH A ROCKET FROM AN F-15

Like the fighter launch platform, the rocket carried by Boeing’s Global Strike Eagle would also be a bargain. Boeing intended to use off-the-shelf solid rocket motors in conjunction with solid rocket motors already produced for America’s arsenal of ICBMs. Using rockets America already had a supply of wasn’t just a cost-saving measure, however, it would substantially reduce development time.

The first stage of the Global Strike Eagle’s “Launch Vehicle” rocket would carry an SR-19 solid rocket engine sourced from the second stage of a Minuteman II ICBM, producing 60,300 pounds of thrust for 287.5 seconds. The second stage would carry an Orion 50XL sourced from the third stage of the same Minuteman II, which would provide another 34,500 pounds of thrust for the next 289 seconds. Finally, the third stage would pack an Orion 38 rocket motor used in a variety of small rockets, which would provide 10,600 pounds of thrust for 289.6 more seconds of powered flight, finally putting the payload into Low Earth Orbit.

The rear portion of the rocket would carry a cone-body during flight for improved aerodynamics that would be ejected almost immediately after the rocket separated from the F-15. According to Boeing’s study, the F-15’s aerodynamic design would not only sustain adding a rocket to its back, but some elements even seemed well suited for it.

“Preliminary computational fluid dynamics (CFD) modeling was conducted on the F-15GSE/ LV configuration to ensure no aerodynamic show- stoppers,” the Boeing proposal reads.

“The analysis indicates only minor reduction in F-15 lift due to the payload/ LV and increased vertical tail loading, which is compensated by a change of aircraft’s angle of attack by 1 degree.”

In order to eventually ferry rockets as big as 30,000 pounds into the sky (with payloads as large as 1,200 pounds), Boeing suggested incorporating “JATO” (Jet-Assisted Take Off) rocket boosters on the F-15 itself. They also suggested leveraging a technique known as mass injection pre-compression cooling (MIPCC) for a bit more power throughout the flight envelope. MIPCC is effectively a water or coolant injection ahead of the engine’s compressor that evaporates and cools as it passes through. This cooling effect allows the engine to operate at higher velocities and altitudes than the heat produced by the engine would normally allow.

It’s not quite like Dom hitting the NOS in his fart-canned Supra, but it is a cheap and effective way of pulling a bit more power out of an existing jet engine.

Boeing’s approach called for the Global Strike Eagle to begin the launch process by pitching upward at a 40.4-degree angle and increasing speed to Mach 1.7 at an altitude of 27,700 feet. At 47,800 feet and a speed of Mach 1.35, the rocket launch vehicle would detach from the F-15 and give the fighter enough time (about four seconds) to pitch down and away before the first stage rocket motor ignited.

From there, the rocket would do what rockets have long done best: burn through each successive stage until payload separation occurred in Low Earth Orbit approximately 400 seconds (an ominous 6.66 minutes) after the F-15 began the launch maneuver.

IF THIS CONCEPT COULD WORK, WHY HAVEN’T WE SEEN IT HAPPEN?

To be clear, despite the proposal offering a number of promising claims about the feasibility of the Global Strike Eagle concept, that seems to be as far as this concept has gone, at least publicly.