As of late, there has been talk about a "sixth-generation jet fighter," the conversation apparently getting a shot in the arm from the halting of the U.S.'s order of the F-22 fighter at 187 planes (making industry watchers look ahead to the next round of big programs, and feeding speculation in some quarters that a more advanced plane is due to come out of a black program somewhere).
Of course, this way of dividing the development of the jet fighter into phases is not unproblematic. This kind of classification - which assumes the existence of generations one through five, with their distinguishing features - is a fairly recent development, and the categories are only approximate (as is the case with almost any system when looked at closely enough). There are differences in ideas about what were the most essential traits, and sometimes which generation a particular aircraft belongs to.
Nonetheless, there is a core of rough consensus about these phases, and what they entail, which I'm offering below (with the key traits of each generation highlighted in bold lettering).
First-generation jet fighters appeared in the '40s (with the British Meteor and German Me-262 at the end of World War II) and continued to serve in front-line roles through the '50s (like the MiG-15 and F-86, famous from the Korean War). Powered by turbojets, they are capable of subsonic maximum speed, and generally armed with guns (cannon or machine guns), optically targeted without the aid of electronic sensors. Where broader innovation is concerned, this was the first generation of aircraft to be equipped with ejection seats, while swept wings also became commonplace at this time, and hydraulic flight control systems were in the process of supplanting earlier, mechanical control systems.
Second-generation fighters, which started appearing in the 1950s, were built with an emphasis on nuclear war-fighting, for which reason their designers deemphasized dog-fighting (and the agility that went with it) and ground attack capability (left to dedicated "fighter-bombers") in favor of fast, quick-climbing, point-defense interceptors, the design of which was often dedicated specifically to countering the threat from nuclear bombers. They are powered by afterburning turbojet engines that give them maximum speeds in the supersonic (Mach 2) range while in level flight at high altitude. Radar (range only, meaning it just gave a target's location), infra-red sensors, and air-to-air missile armament (generally short-range, usually infra-red-seeking, and primarily effective in the tail chase mode) entered into the package. (Other features, like delta wings, also began to appear at this time, while hydraulic control systems were standard at this point.) Planes like the French Mirage III, the British Lightning interceptor, the Soviet MiG-21 and the American F-104 Starfighter are examples of this stage of development. (Their fighter-bomber counterparts included planes like the American F-105, and the Soviet Sukhoi-7.)
Third-generation fighters, most of which emerged out of the 1960s, tend to be multi-role in contrast with the specialized interceptors of the previous era, in part due to higher performance-not improving their maximum speed much (still generally in the Mach 2 range), but enabling them to carry larger payloads and cover greater ranges. (Compare, for instance, the third-generation F-4 Phantom to the F-104 Starfighter as they stack up against one another in these respects.) They were equipped with more sophisticated, pulse-Doppler radar, enabling them to determine not only a target's location, but its radial velocity (range-rate); and also to fire medium-range, semi-active-radar guided missiles at beyond-visual range targets. It is also in this generation that innovations like Heads-Up Displays (HUDs) and variable-geometry wings begin appearing, while electronic countermeasures started to play something like their present role. Besides the aforementioned F-4, the Soviet MiG-23, and the French Mirage F-1, are good examples of such fighters.
Fourth-generation fighters, coming out of the 1970s, represented the next stage of advance, embodied in the "air superiority" fighters that have dominated the skies ever since. Building on the multi-role capabilities of third-generation fighter jets, there was a renewed emphasis on dogfighting capability in response to the actual conduct of battle over Southeast Asia and the Middle East (one aspect of which was the development of more agile aircraft, able to handle 8, 9 and 10-G maneuvers). It was also with this generation that fighters started to become truly effective in beyond-visual range engagements. (In most of the air wars of the '60s and early '70s, most of the kills achieved by the second- and third-generation fighters involved were actually achieved at short-range, with guns and infra-red-seeking missiles. By contrast, the U.S. Air Force in the 1991 Gulf War achieved most of its aerial kills using radar-guided Sparrow missiles.)
In more strictly technological terms, while the performance of fourth-generation fighters as measured by speed, range and payload did not change much, turbojets were largely supplanted by more efficient turbofans, and earlier hydraulic control systems by lightweight, redundant fly-by-wire systems. Additionally, look-down/shoot-down capability became standard in their radars. Aircraft controls also changed, with hands-on-throttle-and-stick (HOTAS) configurations (putting buttons and switches on the throttle and control stick so the pilot can access them without removing their hands from them) and Multi-Function Displays (in place of earlier strips and dials) increasingly the norm, bringing about the advent of the "glass cockpit." Datalinks also became a regular feature, the better to facilitate the coordination of large-scale battles by airborne command and control systems (a function performed in the American case by the Joint Tactical Information Distribution System).
Most of the fighter jets in front-line service today-such as the American F-15, F-16 and F/A-18, the French Mirage 2000, the British-German-Italian Tornado, and the Soviet MiG-29 and Sukhoi-27-fall into this category.
Fifth-generation fighters are only beginning to appear now, with the only one actually in service the American F-22. (No other proposed fifth-generation plane has even flown as a prototype, though the Russian-Indian Sukhoi-PAK program has reportedly produced three prototypes, with the plane expected to fly within the year.)
Fifth-generation fighters are distinguished from fourth-generation fighters (on the multi-role character of which they build) primarily by their incorporation of all-aspect stealth and supercruise capability (essentially, they can fly at supersonic speed without using afterburners, not changing their maximum speed much but making for a much higher cruising speed, 1,140 miles an hour for the F-22, compared with just half as much for the F-15). The thrust-vectoring capabilities that were once unique to specialized aircraft (like the Harrier), helmet-mounted displays, Active Electronically Scanned Array (or phased array) radars (the use of multiple radiating elements, which not only permits more contol over scans, but makes their signal difficult to detect passively, and more difficult to jam), Infra-Red Search and Track (IRST) systems in place of older Forward Looking Infra-Red (FLIR) sensors and "sensor fusion" (integrating data from multiple sensors), also seem to be standard on these.
That said, it seems easy enough to tell the difference between a fifth-generation F-22 and a first-generation F-86, for instance, or a fourth-generation MiG-29 and a second-generation MiG-21, but there are plenty of points in between where the line is harder to draw. Aircraft upgrades confuse the issue, in instances, by giving the aircraft of one generation some of the capabilities of more modern aircraft; for example, later versions of the first-generation F-86 were equipped with the air-to-air missiles associated with second-generation aircraft. Likewise, there are cases where technologies that only later became standard appeared in an earlier generation of aircraft-like the HOTAS control configuration or the supercruise capabilities of the second-generation Lightning, or the helmet-mounted sights and IRSTs featured in fourth-generation Soviet jets like the MiG-29 and Sukhoi-27 (while in other respects lagging a bit, the MiG-29 relying on older-style hydraulics rather than fly-by-wire).
Some aircraft also represent intermediate stages from their beginnings on the drawing board, as is now the case with many of the planes competing for contracts with premier air forces-like the American F-35, the French Rafale, the Eurofighter Typhoon, and the Swedish Saab-39 (all of them "generation 4.5" or "4+ and 4++" jets), just as the "Super Hornet" derivative of the F-18, and the later variants of the MiG-29 and Sukhoi-27 do. Simply put, they are designed with a mix of fourth- and fifth-generation capabilities-for instance, supercruise capability and phased array radar, but only limited stealth (as with the Typhoon).
Nonetheless, despite the ambiguities it seems that the divisions still have some value, and make the point that a sixth-generation fighter would have to incorporate not merely quantitative improvements over a fifth-generation plane like the F-22 (for instance, its having more bandwidth), but qualitative changes in its propulsion, sensors, controls and other fundamental areas of its design and performance comparable to those described above.
What might those be? For the time being the writing on the subject is rather vague and speculative, but John A. Tirpark writing in Air Force Magazine in October 2009 mentions key traits as including the capacity to change its shape in flight, hypersonic speed and directed-energy weaponry. (Incidentally, those who check out his article can examine a slightly different version of the schema given above, which gives special attention to the middle ground between generations four and five.)
One might also guess that its stealthiness will extend to new spectra, perhaps to include invisibility to the naked eye. (Going by an article I read in the May 1997 Popular Science which described, among other things, an experiment with a modified F-15-"Hiding in Plane Sight"-I figured we'd be way past that point by now.) Given the strain the performance of these aircraft may place on a human pilot, and what is suggested about the progress of artificial intelligence, they may be unmanned and even autonomous.
Perhaps. However, I find myself wondering if there really will be call for a sixth-generation fighter circa 2030, for three reasons.
#1. The Actual Rate of Technological Change (Slower or Faster).
On the one hand, it may be that the requisite technologies will never quite be realized-the incorporation of hypersonic flight, for instance, proving to be further off than expected. Indeed, the argument has already been made for the "senility" of major conventional weapons systems like combat aircraft.
On the other hand, it may be that so much else will take place between now and then with regard to high-end war-fighting systems that even a substantially more evolved fighter aircraft will have little place in the battlespace-for instance, because of a rapid evolution of space war-fighting systems that renders old-fashioned air superiority moot.
#2. Economic Stagnation.
Regardless of the uncertain limitations to technological advance over the next generation or two, it is worth noting that each succeeding generation of jet aircraft has been more expensive, resulting in fewer purchases and shorter production runs, even as the number of types built as part of any one generation has shrunk-which cannot but have consequences. It can be pointed out, for instance, that while an F-22 may cost five times as much as an F-15C, it can do things five F-15s cannot-but at the same time, there will be situations where the larger number of aircraft has its uses, and these may be more plausible, so that the combination of budgetary constraints, cost-and mission (see below)-will make a larger number of less-sophisticated aircraft more attractive.
It is conceivable that the rising cost of military systems (especially if the world economy stagnates in the 21st century) will result in a next-generation fighter being prohibitively expensive for even the largest powers. As things stand, even the most affluent governments increasingly rely on international collaboration and international sales to cover the costs of their high-tech weapons programs. Britain, which once produced its own jet fighters domestically, had to partner with Germany and Italy to produce the Tornado, and an even larger-scale collaboration to produce the Typhoon. Russia, even with its economy bouyed by higher energy prices, partnered with India to produce its fifth-generation fighter. The U.S. is relying on a broad international partnership (including the U.K., Italy, the Netherlands, Turkey, Canada, Australia, Denmark and Norway) in the development and production of the F-35 (as well as the foreign sales they will bring). This seems especially likely to be the case in the event of the above two factors being operative, especially should they also combine with the stagnation of the world economy in the 21st century (or depending on one's outlook, its continuing to stagnate).
#3. Changing Military Missions.
Large-scale conventional warfare between states-and especially great powers-may exercise a smaller influence over the priorities of the major militaries. Of course, it is commonly considered ahistorical complacency to deemphasize interstate warfare, but the fact remains that two decades have passed since the Cold War's end without the U.S. trading fire with a "large peer competitor." A host of factors, like the spread of nuclear weapons (and in particular, robust strategic nuclear arsenals complete with second-strike capabilities) and international economic integration (most evident in Europe, but significant elsewhere), work against such a prospect ever getting closer. Meanwhile, counterinsurgency, peacekeeping and other, "alternative" missions-to which late-generation fighters contribute little-continue to dominate the actual, practical life of the major militaries. This trend may not be irreversible (indeed, it has been speculated that intensified resource competition, or the hijack of a major state by a radical clique may "turn the '20s into the '30s"), but the shift would nonetheless make a difference to which projects got R & D money, which systems got ordered. Not only is this worth noting given the current high profile of generation 4.5 aircraft, but signs of renewed interest in even much less-sophisticated systems, with the U.S. Air Force (admittedly, at civilian direction) taking a new interest in small war gear like "light fighters.") In short, fifth-generation systems would be just as irrelevant as if they had been superseded by a fundamentally new technology.