Why are there no Mach 4+ fighter aircraft? It seems that such aircraft would have massive advantages when it comes to being able to fire longer range missiles and evade return fire.
Today's fighter are designed with a max speed around mach 2 (max speed is generally calculated with no emport and 50% fuel, the operational top speed is way lower). They keep this ability to pass Mach 1 only to be able to respond fast enough to intercept / air policing missions.
Countries do not design very fast fighters (and planes in general) because :
- It's extremely hard to achieve.
- It does not give any advantage.
- It has no use anymore.
Back in the cold war period, radars and SAMs where not very sophisticated yet, they did not see very far and Ground to Air missiles were not able to get very high. So to protect their airspace from potential intruders / enemy bombers, countries were relying on interceptor aircraft, a fast jet able to catch up the target before it gets to its objective. At this time, going very fast, very high was an advantage : SAMs where not threatening. The US made a very successful reconnaissance plane exploiting that: the SR-71 Blackbird (max speed : mach 3.3). Many missiles were fired towards the SR-71, none reached its target.
Speed was used for intercept and reconnaissance / intel.
But radars and SAMs improved, getting more range, becoming able to threaten very fast planes. Also satellite images got better and better. The need for very fast interceptors decreased as radar range increased, and the need for very fast recon aircraft decreased as satellite images got better.
Now why does going very very fast not give any advantage to a fighter? Because you cannot turn. The thing to keep in mind is G force: force the same turn rate, the faster you go, the more g you have to take. And a human can sustain a very limited amount of g. This means a plane going at Mach 4 will barely be able to turn without blacking out its pilot. And if you can't turn, you won't evade enemy missiles, making useless the bit more range gained on the missiles.
And then there is the technical challenge, dealing with a little drag as possible (that means no missile / bomb / fuel under the wings), heat, airframe deformation and wear ... for an advantage which isn't there anymore.
Despite all that, the US are suspected to work on a Mach 6+ aircraft, probably unpiloted.
(Please keep in mind that I simplified a lot of things to not make this post too complex)
The planned successor to the SR-71 was supposed to reach Mach 4 to 5 but was never completed because of projected costs equivalent to almost 20 billion in today's dollars.
But that was a reconnaissance platform. A fighter has to do more than fly fast and shoot pretty pictures.
Agility would come first on this list: The realistically possible turn rate of a Mach 5 airplane would let it complete a full circle in about 800 seconds, that is almost a quarter of an hour, just for a single circle. If you want to turn any quicker, you better slow down, complete your turn, and speed up again.
(NASA) A Mach 4+ turn needs a lot ground, the SR-71 routinely crossed multiple smaller countries to execute a 180.
Visual contact is another requirement in todays muddled conflicts. In order to avoid collateral damage (and embarrassing political situations), it is often demanded that pilots visually identify a target before opening fire. Try that at Mach 4! Stand-off weapons will be of no use in this situation, and top speed will be irrelevant.
Small size would maybe come as a surprise, but the faster aircraft tend to be bigger and heavier, too. This will make them much more expensive and fewer will be procured. The specialty materials needed for the extreme flight envelope will make them maintenance-intensive, too. This will make them high-value assets that cannot be risked in combat. In the end, the fighting will be done by the smaller, more numerous and less expensive platforms. Why then fund such an expensive diva in the first place?
Operating altitude: If you look at the envelope of the SR-71, you will notice that flight speeds above Mach 3 required it to climb above 65,000 ft. Only then is air density low enough to lower drag sufficiently for prolonged Mach 3 flight. A Mach 4 design would need to climb to 90,000 ft to play out its design speed. What altitude will the adversary be at?
In addition to the other answers: at high speed, the aircraft is heated by air compression. At Mach 2.2, skin temperatures become so high you can't safely use aluminium anymore, you have to switch to steel or titanium, both of which are expensive to manufacture. The SR-71 was built in titanium. It also leaked like a sieve while on the ground, heat expansion at speed sealed those leaks. The SR-71 had to use a special fuel to make that an acceptable risk.
At Mach 4 this issue gets worse again, and you may have to use active cooling or fragile heat shielding to keep your airframe from melting.
Probably the best answer to this question is that, up to this point, there really has not been a need to build fighters capable of those speeds to counter the air forces of friendly nations.
Prior to the mid to late 1970s, the USAF was quietly making preparations at the behest of several aircraft OEMs to make the transition from manned aircraft into high speed missiles for both strategic and tactical uses, given the tremendous punishments both US and modern western air power endured against Soviet radar guided missiles in conflicts like Vietnam and Yom Kippur. The costs associated with developing a high speed manned aircraft which could evade these threats was prohibitive when compared with the costs of developing a disposable missile to counter it or any future threat, hence the reasons the XB-70, among other programs, were cancelled. However with the advent of stealth technology in the late 1970s and 1980s, the radar threat was effectively nullified and the exact details of how to build effective stealth aircraft remained out of adversarial hands for the past 20 years, giving us a tactical advantage over existing platforms and making the expenditure of national treasure on a high speed manned platform a moot point.
But that's not to say high speed aircraft are dead. The Lockheed Skunk Works is currently developing a new unmanned reconnaissance aircraft, dubbed SR-72, capable of Mach 6+, using a combined cycle engine. Hypersonic aircraft have been proposed over the years for troop or ordnance delivery and there is good evidence that the US and China are currently developing hypersonic air-to-air, surface to air, and ballistic missiles.
It may be that the quantum leap that stealth technology gave us is what delayed the development of high supersonic and hypersonic manned combat aircraft. And, as potential threat nations develop and perfect their own stealth technology, nullifying the advantages it gave us, that we begin to search of other options like additional speed to retain the tactical advantage.