Why is the maximum speed of current fighter aircraft lower than those from the 1960's and 1970's?

Question

While reading about the characteristics of the newest fighter aircraft, I observed that their maximum speed is lower than of the aircraft in the same roles half a century ago.

For example, the iconic F-14 and Mig-25 have a higher maximum speed than their modern equivalents.

After the Cold War ended, the major air forces are only engaged in ground support and bombing against low-tech opponents who use asymmetrical warfare, and no modern scenario would see the top fighter aircraft directly engaging each other any time soon. However, for any fighter aircraft in the interceptor role, I would expect speed should be an important characteristic. Also, military aircraft are designed and introduced over a long time period, and are expected to be used for many decades.

Why did maximum speed become less important? Wouldn't it be easier to design even faster fighters now than with the technology available 50 years ago?

Answer 1

Speed was life in air combat until the jet age. The pilots who became Air Force generals in the Fifties had learned their trade in the Thirties when speed was the most-desired quality in a fighter. Naturally, the ability to be faster than any adversary was very important to them. When the requirements for new fighter aircraft were written in the Fifties, those generals made sure that a higher top speed was part of the specification.

When those supersonic-capable aircraft were used in real conflicts, something very surprising and unforeseen happened: They did hardly ever fly supersonic. When the Air Force in the late Sixties accumulated the flight data from several years of Vietnam war air combat, they found that all aircraft had accumulated just minutes at Mach 1.4 and only seconds at Mach 1.6 out of more than 100,000 combat sorties¹. Never was even Mach 1.8 flown in aircraft which had been optimized for Mach 2.4 (F-104, F-105, F-106A, F-4D/E and F-111).

To cite from this study why speed stayed mostly subsonic:

The first of these reasons lies in the shape of the turn rate vs Mach number relationship for an aircraft. […] In combat, each pilot has the tendency to to fly his aircraft so as to maximize his turn rate. He thus gains angular position on the enemy which, in turn, may permit a missile launch or a gun firing. […] It can be seen that the pilot's urge to maximize his turn rate will unfailingly drive his Mach number to about 0.7. Thus, if the pilot is going to join in combat, […] his speed will inevitably drop to subsonic speeds. […] Note also, that even if the turn rate is held constant while increasing the speed, […] the turn radius and load factor increase, bringing with it increasing problems of keeping the enemy in sight.

The second reason given in the study is the dramatically smaller combat radius (the maximum distance the airplane can travel from its base, accomplish an objective and return) once the aircraft starts to fly at supersonic speeds. Even for flying into the combat arena supersonic speed was rarely advantageous. Northrop studied a multitude of intercept cases and found that speeds above Mach 1.1 were almost never helpful because they curtailed the combat radius severely.

Now you need to know that the top speed is an important driver in airframe design. Flying at Mach 2+ requires heavy and complex intakes, a heat-resistant structure, high wing sweep and heavy, low-bypass engines. This all degrades the combat qualities at high subsonic speed, which was where those aircraft were used the most. Building into them the capacity for Mach 2+ made them worse for what they were actually used for.

From the late Sixties on this lesson was incorporated into the newer designs like the F-16. Stealth again decreased the importance of supersonic capability, and the maximum sustained speed of the F-22 was actually reduced from Mach 1.8 to Mach 1.6 to reduce the heat load on the composite wing leading edge.

¹ Source: Northrop F-5 case study in aircraft design, AIAA Professional Study Series

Answer 2

Because while speed is one of the important characteristics, it is not the only important characteristic (or today, not the most important one) of a combat aircraft.

One important thing to note is that the conditions where top speed is reached is pretty restrictive conditions - high altitude, clean configuration, and afterburners - which are rarely if ever available together in combat.

The evolution of the speeds of combat aircraft in general and combat aircraft in particular were a direct result of the experiences of various air-forces in combat situations over the years.

• The main reason for the stress on speed of combat aircraft (at least in initial cases) was due to the lessons learned in the WWII, where extra speed can make the difference between life and death.

• After the jets went supersonic, the main reason to have high speeds is for the purpose of interception of enemy aircraft. Speed was the most important requirement in case of combat aircraft (interceptors) half a century ago as the interceptor had to bring the intruder under its range (whether guns or guided missiles). However, the (interceptor) aircraft rarely flew supersonic and as the radars and missiles became advanced, the importance of speed diminished.

• Another important reason was to escape from the missiles, especially the surface to air missiles. The idea was that the aircraft can outrun the missile, at least at an altitude. However this was a lost cause from the beginning as the missiles flew far faster than the aircraft. For example, the S300 missiles have a top speed of > Mach 7, more than 2 times that of the aircraft it was designed to shoot down. While the combat aircraft are required to be in service for decades, the missile systems can be developed and fielded much more quickly, erasing any speed gains made. As a result, most of the aircraft initially configured for high speed-high altitude attacks switched over to low altitude penetration tactics.

• As the USAF absorbed the lessons learned in the sixties, it became painfully obvious that the combat aircraft were spending less and less time at very high speeds (> Mach 1.5), while most of the combat devolved into close quarter dogfights (in part due to rules of engagement), where maneuverability, rather than speed is more important.

• Also, the experiences of the Vietnam and Yom-Kippur Wars, where the western aircraft went against sophisticated Soviet Air defense systems reinforced the fact that the suppression of enemy air defenses is more important than speed.

• Designing the aircraft for speed adds significant penalties in the aircraft design- the need for complex air intakes (which adds weight), high temperature materials (which increases cost and maintenance requirements), high powered low-bypass engines (which are usually not fuel efficient). This degrades the other performance parameters of the aircraft in low speed (high subsonic) regime, where most of the operations take place.

• As it became clear that the combat aircraft are not going to defeat an air defense system by virtue of speed (or maneuverability for that matter), the strategy again changed- not getting detected by the system in the first place. This emphasis on stealth further reduced the importance of speed (you can only heat the composite airframe so much and afterburners for speed are like lighting a torch in the dark for IR sensors) and enhanced that of the avionics and sensors. Today, the stress is on finding the enemy before getting detected and firing the first shot.

• Another reason for high speed is that combat aircraft were also used for reconnaissance, where speed is important (again, for escaping from missiles). However, satellites have taken over most of these duties, further reducing the need for speed.

The epitome of fast combat aircraft is the Soviet Mig-25, which could reach speeds in excess of Mach 3. However, this speed came at a cost- the high temperatures experienced (~300°C) meant that steel had to be used for airframe, requiring complex insulation and cooling systems for avionics and the aircraft rarely flew at this speed due to engine overheating and control issues.

The results (of training and simulation) so far seem to indicate that the modern (fifth generation) aircraft (like F-22) perform better than their fourth generation counterparts (like F-15) in most situations, 'shooting' them down before even getting detected.

Answer 3

One important factor is that the threat model changed as ICBM's replaced bombers as the primary delivery mechanism for nuclear weapons over the course of the 1960s.

Being a few seconds late in intercepting a bomber carrying several tons of conventional bombs isn't good and could easily result in a few hundred deaths. There's a huge difference between that and being a few seconds late in stopping a bomber carrying nuclear weapons where a single bomb can kill tens or hundreds of thousands of people.

Of particular relevance to the your question the MiG-25's mach 3.2 speed was driven by the threat of the mach 3.0 B-70 nuclear penetration bomber program. The US cancelled plans to mass produce and deploy B-70 bombers but the Soviets kept the MiG25 alive. While the MiG25 was first deployed a few years after the XB70 program was cancelled; the USA continued to build two XB-70's and used them for research purposes throughout the 60s. The threat of a future supersonic bomber based on the research could not be counted out; and an interceptor significantly faster than the Mach 2 B58 would have remained a valuable part of their nuclear defense plans until the B58 was retired in 1970.

Answer 4

Maximum speed is not the best speed to fight. Two major factors in a fighter must be taken into account:

1. Wing loading.
2. Thrust-to-weight ratio (T/W ratio).

Wing loading is the weight of the airplane distributed across the total area of the wing. This tells the aircraft's ability to turn sustained.

The T/W ratio is the amount of thrust developed by the engine divided by the weight of the aircraft, and this will tell the acceleration the aircraft is able to achieve in level flight. When both things are taken into account, you get a second parameter that is the aircraft ability to climb (climb rate).

When air war developed in the WW1/2, one rule of thumb for fighter pilots was that speed/altitude was life. If you examine the aircraft as a physical system of energy, you have two kinds of energy at your disposal: Kinetic energy (the actual rate of aircraft movement) and potential energy, the altitude of the aircraft. When you dive, you convert potential energy into kinetic energy. When you climb you turn kinetic energy into potential energy, until the point where your engine is the only thing that is inputting energy into the system (so you reach the stage where instantaneous climb rate ends and you start to see the sustained aircraft climb rate).

When your aircraft turns, it will be slowed a little bit, and the increase in induced drag generated by the turn will decrease your speed slowed. In the fighter pilot jargon, you are burning energy. There are two (again) kinds of turn, instantaneous and sustained.

The fighting styles where divided into two kinds: Turn and Burn and Boom and Zoom.

Turn and burn means that both fighters try to outturn each order in order to get into their six o'clock position in order to fire, it's a horizontal fight in circles. Boom and Zoom is a climb fight where the higher positioned aircraft dives into the other in order to make a pass, fire and then climb again in order to not lose airspeed (and thus energy).

Quickly the boom and zoom style was adopted. Experienced pilots only entered combat when sure to have higher altitude than their enemies, because altitude was a reservoir of energy to be exploited in combat.

A turn and burn styled fighter/pilot would quickly slow down and be unable to either attack or flee from a higher positioned aircraft.

So far so good.

After the war, all efforts went on to produce ever faster aircraft and aircraft able to reach high altitudes quickly. This has dual advantage of giving a better high altitude position and allowing such fighters to engage incoming bombers.

All changed when better missiles and onboard radars became available. Not that current fighters don't need to be able to dogfight, but because the missile became the major combat element. Two kinds of missiles come to my head at this stage. AAMs and SAMs.

The increasingly powerful radars (even onboard) meant that aircraft might very well attack things that are above it, meaning that the boom and zoom tactics might go wrong. Secondly, a radar allows one to see the enemy way before visual contact is made, giving more time to climb to a similar altitude, etc.

If you are flying very high, you will be seen on enemy radar far away, and missiles directed by those radars will come from various directions. To enter enemy airspace, you need to stay low, where air resistance is bigger and most aircraft can't reach the stated figures of speed. (See F-111 or Su-24 for example).

Bombers won't be flying at stratospheric levels like B-29 anymore, due to the same reasons. They need to enter enemy airspace at low altitude in order to have a chance of surviving. If they use standoff weapons, those will fly low too. So, if you are escorting a bunch of bombers in a mission, or you are trying to intercept those, they will not be very high.

When a onboard radar tries to lock against a low flying aircraft, there is a problem. Earlier radars where not able to do so at all. More modern versions (called look-down/shot-down) are able to lock a target against terrain, but not with the same performance as when they are looking up. Clutter (false echoes) is removed via software. Infrared detectors work better at high altitude (where air is colder and a hot fighter is more visible), than against the hot ground below, and so on.

So, all in all, fighter combat in intensive theaters will be going to be done at middle or low level, where speed is slower due to higher drag. Even then, all fighter aircraft have something called corner velocity, where the aircraft is able to sustain their most tight turns, and those speeds are usually far from being top speed.

Another factor is engine consumption. Only recently did supercruising aircraft came to be operational. Forth-generation aircraft where not able to break the sound barrier without using afterburns, which both increase aircraft detectability (due to increased heat signature) and fuel consumption.

The same aircraft that might travel 1000 km without using afterburners, might very well be unable to do more than 300 km while afterburning all along.

All things taken into account, an aircraft with more missiles, better radar and longer loiter time is preferred above fighters like the British Lightning or the F-104 which had a very high speed and climb rate, but were short-legged and lugged only a pair of missiles.