In the first part of this mini-series we looked at the background of the F-15’s gestation, the combat experience in Vietnam, the development of new Soviet fighters and how these all influenced the F-X programme which eventually produced the F-15. In this part we’ll take a look at the features and characteristics of the F-15, particularly versus its predecessor, the F-4 Phantom II. We’ll also take a look at the development of the air superiority F-15 through the latter stages of the Cold War and the decades that followed.
The F-15 first flew in 1972 and entered service in 1976. The original single seat model was designated the F-15 and the twin seat (and combat capable) training variant was known as the TF-15. At the time the USAF’s premier fighter was the F-4; specifically the F-4E which was developed during the Vietnam War and incorporated various improvements based on the combat experience from that conflict. The F-15 was meant to be a quantum leap over the F-4 and address all of the major shortcomings of the F-4 highlighted in combat over Vietnam.
Dimensionally the F-15 was slightly larger than the F-4E. The empty weight of the F-15 was around a metric ton lighter than the F-4E. At a typical loaded combat weight the F-15 was about a ton-and-a-half heavier. At maximum take-off weight the F-15 was about two-and-a-half tons heavier than its predecessor.
Now before we go any further I need to explain a couple of characteristics I’m going to use to compare these aircraft and their relevance to aerial combat:
- Wing Loading – This is the mass of the aircraft divided by the total wing area. An aircraft with a lower wing loading will be able to turn tighter than one with a higher wing loading.
- Thrust to Weight Ratio – This is the ratio of the total engine power to the weight of the aircraft. An aircraft with a high thrust to weight ratio affects acceleration, climb rate and the ability to maintain hard turns.
As I explained above, a loaded F-15 was slightly heavier than a similarly loaded F-4E, but the F-15 had 33% more engine power available. At a typical combat weight the F-4E had a thrust to weight ratio of 0.86 whereas an F-15 had 1.07. Likewise the F-15 had 15% more wing area than the F-4E and this yielded a wing loading at combat weight nearly 7% lower than the F-4E.
The F-15 could climb faster, accelerate faster, turn harder and faster and recover speed quicker than the F-4E. All crucial characteristics when in a dogfight.
The F-4 was already one of the fastest fighters of its era and the F-15 was marginally faster than an F-4, but remember the combat experience over Vietnam showed absolute speed wasn’t the be all and end all and the F-15 was deemed to be fast enough. In fact even today in terms of absolute speed it is still one of the very fastest fighters in service.
The F-15’s range was over twice that of the F-4E.
So already the F-15 is looking pretty impressive, but there’s still more areas where the F-15 improved significantly over the F-4E.
- The F-4 was never designed with cockpit visibility as a major concern. This makes life unnecessarily difficult for the crews in a dogfight. The F-15 was designed with a large bubble canopy with excellent all round visibility, minimal framing to obstruct visibility and a far better rearwards view than the F-4. The pilot also sat higher in the cockpit giving him a much better view over the side of the cockpit.
- The F-4 was not originally designed with a gun. The F-4E was the first model of the F-4 to incorporate an internal gun. The F-15 was designed from the outset with an internal gun. It was originally intended to fit the F-15 with the Ford-Philco GAU-7 25mm cannon firing caseless ammunition but development problems led to the M61 Vulcan 20mm cannon being fitted instead. This was the gun used on the F-4E (and in podded form on other F-4 models).
- The F-15 was fitted with the Hughes AN/APG-63 radar. This was a state of the art radar for its time and a real leap over the Westinghouse AN/APQ-120 radar in the F-4E. It levered advances in computer power to incorporate much more advanced signal processing to give significantly better performance, capability and usability. Critically the APG-63 was a new-generation “look down shoot down” radar which was capable of picking out low flying targets against the clutter of radar returns from the ground.
- Advances in avionics and ergonomics meant the F-15 had a drastically lower cockpit workload than the F-4. This allowed the crew to be reduced to just a single pilot.
It’s also worth mentioning the missiles. The service entry of the F-15 coincided with new and significantly improved variants of the Sparrow and Sidewinder becoming available:
- The AIM-7F Sparrow introduced a new rocket motor giving longer range and crucially the replacement of the old and extremely unreliable electronic components with new solid state electronics which vastly improved the reliability of the missile, as well as a larger warhead for more boom. For the RAF types reading this, the AIM-7F was the basis for the Skyflash.
- The AIM-9L Sidewinder was the first version of the missile to feature an all-aspect capability. Earlier Sidewinders needed to be fired from astern the target where the infra-red seeker could see the hot engine exhaust gas. The Lima had a vastly more capable seeker which could see the target from any aspect and could be launched at a target from any angle, including head-on.
Both of these missiles were significant improvements over the Vietnam era versions which performed so poorly and these new missile variants became the standard armament of the F-15.
That’s not to say the F-15 was without its problems. Most notable were the engines. The F-15 was powered by a pair of Pratt & Whitney F100-PW-100 afterburning turbofans. These were very advanced engines for their time and had some serious teething problems. The designers had seriously underestimated how frequently an F-15 pilot would move the throttles during manoeuvring and the frequent and rapid changes in engine power had the effect of increasing wear and tear on the engines beyond the expected levels. Failures were frequent and overhauls were needed at far shorter intervals than were anticipated. More importantly the F100 was susceptible to stagnation stalls. These occur when the airflow into the engine is significantly disrupted, such as during high angle of attack manoeuvres and the airflow through the compressor stages into the engine core is disrupted. The core turbine slows and causes the engine to both lose power and overheat which can not only cause damage to the engine but also cause it to fail in flight. Stagnation stalls in the F100 could also be caused by a “hard” afterburner start (sorry to the RAF types here, I know “afterburner” is an American term and the correct term in in fact “reheat”, but seeing as we’re talking about an American bit of kit I suppose I’d better use the American terminology). In a hard afterburner start the fuel sprayed into the jet exhaust duct doesn’t immediately ignite and instead a build-up of unburnt fuel occurs before igniting in an explosion which sends a pressure wave back through the engine causing it to stall.
The standard procedure for recovering from a stagnation stall is to shut the engine down and allow it to cool before restarting the engine. Clearly this isn’t something you want to be doing in the middle of a dogfight.
What’s more problems with the supply of new engines were experienced as Pratt & Whitney attempted to solve the above issues. To make things worse strikes at two of the major sub-contractors supplying engine components slowed down deliveries even further. By the end of 1979 the USAF was accepting new F-15s without engines fitted.
Eventually the frequency of stagnation stalls and hard afterburner starts were greatly reduced, but not eliminated entirely. Upon service entry in 1976 the F-15 was experiencing around a dozen stagnation stalls per 1,000 flying hours but by 1981 that had fallen to around 1.5 per 1,000 flying hours.
There was also one more very serious issue with the F-15.
The F-15 was to date the most expensive fighter ever built. Taking into account inflation, an F-15 cost very nearly three times more than an F-4. The USAF realised very early on that it wouldn’t be able to replace the F-4 with the F-15 on anything near a one-for-one basis. This led to the USAF starting the Light Weight Fighter (LWF) programme which eventually resulted in the F-16 as a cheaper supplement to the F-15. It also resulted in the F/A-18 Hornet, which is another story entirely…
In 1978 the new F-15C entered service and was joined by its twin seat companion the F-15D. By now the original single seat F-15 was re-designated the F-15A and the TF-15 had become the F-15B. The F-15C featured a slew of improvements including the Production Eagle Package 2000 which increased the maximum take-off weight by 2,000lb, increased the internal fuel capacity and added the necessary provision for a pair of conformal fuel tanks on the sides of the fuselage. The increased maximum take-off weight allowed the F-15C to take off with full internal fuel, full weapons load, full conformal fuel tanks and three full external fuel tanks. Another important upgrade for the F-15C was the AN/APG-63 radar was fitted with a new programmable signal processor (PSP), the first of its kind to enter service. This allowed the radar to be upgraded with better performance, new modes and new weapons integration simply through software changes rather than having to physically replace parts of the radar. The F-15C also got strengthened landing gear, a new digital central computer and an overload warning system if the pilot was exceeding the maximum G load for the aircraft.
In 1985 the first F-15C with the Multi Stage Improvement Program (MSIP) was delivered. This featured a further upgraded central computer, a Programmable Armament Control Set (PACS) which allowed for more advanced versions of the Sidewinder and Sparrow to be used as well as making provision for the AIM-120 AMRAAM which was then in development. The Tactical Electronic Warfare System (TEWS) was also upgraded to provide better threat detection and self-protection capabilities.
In 1986 the F100-PW-220 engine became available which solved all of the reliability issues of the original F100-PW-100 engines outlined earlier. In 1997 the remaining F100-PW-100 engines in service were modified to the F100-PW-220E (Equivalent) standard.
The final 43 F-15C to roll off the St Louis production line were fitted with the new AN/APG-70 radar which had been developed for the F-15E (covered later in the series). The existing F-15C MSIP aircraft with the AN/APG-63 were upgraded to the AN/APG-63(V)1 which provided similar capabilities and improvements as the AN/APG-70.
The F-15C has been superseded by the F-22 Raptor as the USAF’s premier air superiority fighter. However, the Raptor is a whopping four times more expensive than an F-15C (or 12 times more expensive than the F-4E…. that’s the military industrial complex Eisenhower warned us about). Even the mighty USAF with its seemingly bottomless pit of money couldn’t afford to operate a large number of Raptors and production was halted after only 187 had been built. Of these 125 were assigned to front line units and the Raptor is quite the hangar queen; the latest figures reveal a little less than 50% of these are available to fly at any given time; the worst availability and readiness rate of any aircraft in the USAF inventory. The USAF was greatly concerned there were simply not enough Raptors and so it was decided in 2007 to upgrade a number of F-15C. These were to be hand-picked airframes selected on the basis of their maintenance history and reliability. They would be upgraded with the AN/APG-63(V)3 Active Electronically Scanned Array (AESA) radar and fitted with the Joint Helmet Mounted Cueing System (JHMCS). An explanation of an AESA radar is beyond the scope of this article, but suffice to say AESA radars have become the standard with the latest jet fighters and are a lot more capable and flexible than non-AESA radar sets. The JHMCS is a system that allows the pilot to cue missiles onto a target simply by looking at them, again another capability which is rapidly becoming standard across all modern jet fighters. These upgraded F-15C were colloquially known as “Golden Eagles” and would supplement the F-22 force. The USAF intended to upgrade 178 aircraft along these lines.
In 2015 Boeing unveiled the “Eagle 2040” upgrade designed to keep the F-15C competitive for another two decades. The upgrade includes the AN/APG-63(V)3 as well as the JHMCS already mentioned, but additionally includes an as yet undeveloped Infra-Red Search and Track (IRST) system and the replacement of the 1980’s era TEWS system with a new Eagle Passive/Active Warning Survivability System (EPAWSS) developed by BAE Systems.
However, all of the above was thrown into serious doubt in 2017 when it was revealed the USAF was considering retiring the air superiority F-15 fleet completely and replacing them with upgraded F-16C models. Budget realities began to seriously encroach on the US military under the Obama Administration and the USAF in particular was looking at being forced to retire entire fleets of aircraft it could simply no longer afford to operate. The recent budget passed by President Trump significantly increased defence spending and it’s not yet known what effect this may have on the mooted retirement of the F-15C.
Twenty years ago just about everyone would have agreed the F-15C was the best air superiority fighter in the world. Nowadays it’s not so straightforward. The F-15C has been superseded by the F-22 as the USAF’s premier air superiority fighter and there are other fighters available which are at least as good as an upgraded F-15C; notably the Eurofighter Typhoon and some of the more advanced derivatives of the Sukhoi Su-27. That’s not to say we can write off the F-15. Remember it still has some very potent capabilities, it’s flown by pilots who know their trade very well and will have received some of the best training anywhere in the world, it will be accompanied into combat by the full panoply of force multipliers the USAF can bring to bear, and it will of course be joined by the F-22 when it goes to war. Despite its age the F-15C is still very much a premier league air superiority fighter.
Ah, did you think I’d get this far and not mention the Streak Eagle?
Between 16th January and 1st February 1975 a specially modified F-15A dubbed the “Streak Eagle” set eight time to height world records. The Streak Eagle was a specially lightened F-15A with all unecessary equipment removed and the paint stripped from the airframe. The Streak Eagle today resides in the National Museum of the United States Air Force (albeit now painted a more familiar grey to protect against corrosion.
Here’s some visualisations of the Streak Eagle’s flight profile on its record breaking flights:
In the next part of this series we’ll take a look at the combat record of the air superiority F-15 and see if the designers got it right.
© Æthelberht 2018