Not sure this new aircraft is going to be very stealthy... its design requirements of high speed and heavy long range missiles means it needs a huge powerful radar... which is going to make any stealth measures pretty pointless.
The focus will be speed and range and payload capacity first... high speed means it can fire its missiles at targets further from their targets which makes it more likely it will be shooting down platforms before they release their standoff weapons.
Range means it can also engage targets further away from their targets and works together with speed to mean you are more likely dealing with aircraft rather than missiles that have already been launched.
At such flight speeds internal weapons makes sense and that will reduce drag and improve stealth but again for speed and range reducing drag is rather more important than stealth.
Turbofans can't operate fast enough to form the primary propulsion at mach 4 or even mach 3.
The SR-71s engines run at idle speeds when flying fast, it uses bypass air in the turbofan as a ramjet... those big pointy nose cones sticking out the front of the engines moves at higher speeds forward so the airflow does not go through the turbofan, it goes around the turbofan where the bypass air goes... no spinning blades... just airflow and fuel injection and burning for thrust.
A good new engine for the PAK DA will be a different type of engine optimised for very high flight speeds that will possibly be a low bypass turbojet for raw thrust, but with a design where air coming in to the engine bypasses the turbo jet at higher speeds and it acts like a ramjet instead.
Engine power is not critical... a turbojet and a turbofan burn fuel in the airflow at subsonic speeds so the current engine... the air flows in at mach 2.83 when flying at top speed... well that is not true... according to reports the top speed has increased it used to be 3,000km/h but currently it seems to be 3,500km/h due to various improvements so it is probably a mach 3 plane currently, but that has been its limit due to the limits of the rotational speeds of the blades in its jet engines and its design.
To stop the engines choking on supersonic air the intakes contract at high speed to reduce the airflow and slow it down to subsonic speeds when it goes through the hot section where fuel is added and burned and is directed out the rear with bypass air coming through and burning more fuel in the after burner.
For a ramjet the airflow needs to be slowed down so the fuel can be burned but with no moving parts there are no problems with blades spinning too fast and breaking.
With a scramjet engine... even a very small one the airflow is not slowed down... the air goes in and is compressed and heated... fuel is added and burned and it leaves generating thrust.
The PAK DA might have a small scramjet engine in addition to cheap simple current technology engines, where the standard turbofans or turbojets are for takeoff and landing and climbing and descending and then the ramjet or scramjet is lit accelerating to super high speeds... might even be a single turbofan or turbojet and two ramjets or two scramjets, but obviously it would be more elegant if you can have multimode engines that operate as a turbojet and ramjet...
It is more about efficiency... a good ramjet is not supercruising in dry thrust but will give much better high speed flight performance than the most amazing turbojet or turbofan engine... because they simply can't operate at mach 4... rotational speeds would rip the blades apart.
With Scramjet engines or engines that can operate in scramjet mode there would be no supercruising, but the fuel performance would be as good or better than turbojets at lower speeds.... which would mean a huge increase in flight range.
Currently the MiG-31 has a flight radius of about 720km at mach 2.6, and about 1,500km at subsonic speeds... the core difference is that when flying fast it goes a shorter distance but it reaches 720km in about 13 minutes... at subsonic speed the same distance would likely take 3 times longer, or its new radius would take 6 times longer to reach. In terms of an interceptor flying at full speed means being 6 times faster.
Last edited by GarryB on Wed Apr 12, 2023 5:22 am; edited 1 time in total (Reason for editing : Fixed mistake (in bold))