Notes:

The F-15 is undoubtedly one of the most successful modern fighters. After several decades of service, it has scored dozens of victories, yet never been lost in combat. With an impressive combat history and plans to continue the program with improved versions, the design has clearly been a winner. In the 1970s, just as the original design rolled off the production line, there was already plans to improve upon what many believed to be the most promising airframe America had ever produced.

The United States Air Force wanted to build on the early F-15 airframe something of a demonstrator for what could be possible in future fighter designs. This coincided with other proposed research ventures in the mid 1970s, which were focused on understanding and utilising super-manoeuvrability. During the early 70s, many fighter aircraft designers were oriented around high-altitude, large-stores capacity for beyond visual range combat (known as BVR). Much of motivation for this research came from the surprise emergence of the Soviet MiG-25, an aircraft which shocked many in the West. While much of the focus was now on long range combat, some designers still believed that dogfighting and high agility would be necessary. This would result in the F-16 Fighting Falcon program in 1972. The success of the F-16 would help to redirect attention away from pure BVR combat, and towards a mixture of both long-range and short-range capabilities. The first of the F-15s to roll off the production line would indeed be manoeuvrable. Although it couldn’t keep up with the later F-16 at higher angles of attack, it was nevertheless a great platform, and researchers saw much potential in it.

By the mid 70s, it was believed that surpassing the constraints of conventional aerodynamic systems would be key in the future of aerial warfare. After all, supermanoeuvrability would allow for tighter turning circles, faster deceleration, and better control during unstable flight.

One major point of focus for manoeuvrability was “thrust vectoring” –adjusting the angle of the engine nozzle to aid in manoeuvring the aircraft, in ways that the conventional control surfaces could not. This idea came from rocket technology. Just as the introduction of variable jet nozzles allowed for better control of thrust, a vectoring element would allow for increased movement by redirecting thrust.

Throughout the late 70s, Langley Research Centre became the primary department researching thrust vectoring, and later implemented the technology into the F-15B airframe for testing. The approach taken with the modified F-15 would be square shaped nozzles, allowing for what would be known as “Two-Dimensional Thrust Vectoring”. Put simply, this meant that thrust could be directed on a single axis, helping aid the pitch of the aircraft. For context, this same type of two-dimensional thrust vectoring concept is the basis for slat engine nozzles on the F-22.

This early 1977 research program produced an F-15 with a powerful set of thrust vectoring engines, capable of 2D vectoring. These nozzles could redirect thrust up to 20 degrees up or down. The success of this early design gained the attention of others in the industry.

 

DEVELOPMENT

In 1984 McDonnell Douglas was awarded a contract to develop a variant of the F-15 which would use thrust vectoring technology. It would be called the F-15 STOL/MTD, referring to the short-take-off-and-landing technology that would be developed, and the improved manoeuvrability the program hoped to achieve. Taking the advances made at Langley, the same two-dimensional engine design was integrated into the aircraft, followed in 1988 by the addition of forward canards.

This technology demonstrator proved a success. The earlier F-15 would need over 7000 feet of runway to land, this prototype could land over just 1600 feet of runway. Take-offs were also shortened – capable of taking to the air at speeds as low as 42 miles per hour, the demonstrator was 25 percent faster than a standard F-15 at take-off.

In 1991, the aircraft underwent a series of tests to see just how capable these technologies actually were. Among the many impressive results were the ability to reverse thrust in mid-air, helping the aircraft to rapidly decelerate, and able to land on a 500m runway.

UPGRADES

With the program proving a success, and the results promising, in 1993 it was decided that the STOL MTD airframe would be rebuilt into what would become known as the F-15 ACTIVE system. The aircraft was handed over to NASA for further development. Initially, much of the aircraft’s agility capabilities were left unchanged, whilst designers used the aircraft to test cockpit tech for the new F-15E Strike Eagle. In fact, the entire cockpit was rebuilt to replicate that of the Strike Eagle.

After some time, however, NASA would begin implementing further technological improvements to the platform. The most important change to the aircraft were its engines. Having seen the impressive results 2-dimensional thrust vectoring, it was decided that these would be upgraded to 3-dimensions. A pair of Pratt and Whitney pitch-yaw balance beam nozzles, designated F100-PW-229, were selected. The nozzles were no longer limited to horizontal movement. They could move in a full 360-degree arc, redirecting outflow in any direction.

The NASA Active Flight Research Program Report stated that “The F-15 ACTIVE flying qualities are significantly improved over production F-15 aircraft. Aircraft response is crisp and heavily damped throughout the research flight envelope.“  In fact, in supersonic flight, they predicted that the addition of canards improved the stability of the aircraft by over 100 percent.

The ACTIVE program ended in 1999. The aircraft would be used in what was known as Quite Spike research, and later go on to become the F-15 IFCS, the testbed for further research. This time, the F-15 (still equipped with 3D thrust vectoring engines and canards) was used to develop a neural network system for identifying and correcting critical failures if they were to occur, as well as improving the safety of an aircraft during stable flight.

The aircraft – which was the 6th F-15 off the production line – retired in 2009. It was, at that time, the oldest F-15 still flying, having not only been used in various test programs, also helped produce the F-15E Strike Eagle. Although, it was never intended to be a test bed for F-15 upgrades, it was designed to give insight into future projects, and it is likely that it has influenced many modern designs.

Today thrust vectoring engines have made their way into a number of fighter aircraft designs in the United States and abroad. Multiple variants of the Sukhoi SU-30 developed for foreign nations, such as Malaysia’s Su-30MKM, and India’s SU-30MKI use a 2 dimensional thrust vectoring system. A 3 dimensional system is also used in the SU57.

Despite their proved success in both the ACTIVE F-15 and F-22 Raptor, western designers have not focused as much on thrust vectoring in recent years, however it will undoubtedly still have a role to play, as its benefits cannot be ignored.

It may be that not only American, but also Russian and Chinese designers have the F-15 agility program to thank – at least in some way – for the breakthrough research into two- and three-dimensional thrust vectoring and super-manoeuvrability.