Histories and documentaries of aircraft tend to just plainly list aircraft performance. The next generation of aircraft is just better than the previous one, but it’s rarely explained how this is possible.
This is especially curious regarding aircraft like the F-4 and F-15, both manufactured by the same company, McDonnell Douglas.
Eagle
Well, to not bury the lede, the biggest difference is simply the engine. F-4 used two GE J79 engines, while F-15 uses two Pratt & Whitney F100. The J79 engine had a thrust to weight of 3.1 while the F100 has 4.5. This is a mind boggling difference. With afterburner the numbers are 4.6 and 7.3. This means the F100 without afterburner is as good as the J79 with afterburner. This affects the whole airplane, resulting in an empty dry thrust to weight 0.8 for the Phantom and 1.0 for the Eagle, or 1.2 and 1.7 with afterburner.
There is a lot of discussion around the big wing of the F-15 and how it enables maneuverability, which certainly is great. But that maneuverability is much more useful with the high thrust to weight ratio of the plane, which is possible only thanks to the much higher high thrust to weight ratio engine. The F-4 with a bigger wing would not have been nearly as useful. The F-15 also has great aerodynamic design (NASA did a lot of base research), good flying qualities, radar, ergonomics and electronics etc. But the engine really makes the plane.
So the most interesting question should be, what made the engine so much better? Certainly it’s a turbofan compared to the previous generation’s turbojet, and there’s materials advantages, but let’s discuss that further down.
Raptor
What about the jump from the F-15 to the F-22? The Pratt & Whitney F119 engine in the Raptor has an astounding dry thrust to weight ratio of 6.6, and 8.8 with afterburner. Again, large strides from the F100:s 4.5 and 7.3. The F-22 needs internal weapons bays to maintain stealth, that should hurt weight, yet the whole airplane has empty T/W 1.2 dry and 1.6 with afterburner. The plane can accelerate vertically without afterburner, much better than the F-15 that can only maintain speed. This is one of the reasons why the Raptor can supercruise, ie fly supersonically without afterburner. Afterburner doesn’t bring as big a gain as in the F-15 but it’s still impressive.
The F-22 of course has stealth, thrust vectoring, all computerized flight controls, synthetic array radar in the wings etc. Even the F-15 has not been won in air combat and has hundreds of kills. The F-22 then is so good it’s nott really even deployed anywhere so that adversaries can’t test their sensors against it, and it has never been exported.
Engines
So, the most interesting question is, how did the engines get so much better? The United States Government has run multiple long very high budget programs to research and mature new engine technology: IEDP, IHPTET, VAATE, ITEP, ADVENT etc. The results have been shared to engine manufacturers, and the manufacturers have been contracted to create demonstrator engines.
So, the reason, at some level, is government committing large sums of money over a long term into engine research and development. The technical aspect to fighter engine development would warrant its own research topic.
The work has continued, and the next fighter engine after F119, the Pratt & Whitney F135 in the Lockheed Martin F-35 Lightning II has a 7.5 dry and 11.5 afterburner thrust to weight ratio. Later programs have been AETD and AETP.
As comparison, the Eurojet EJ200 in the Eurofighter has T/W 6.1 dry and 9.2 with afterburner, relatively close to F-119.
All data is sourced from Wikipedia.