There are rumours that a hybrid engine (turbojet/liquid-propellant rocket) has been developed by the RVSN academy in Serpukhov.
If you look at the chart in LMFSs chart you will see that for low speed flight... ie subsonic, the most efficient propulsion is turbofan, simply because this moves a lot of air and is very fuel efficient. If you have a big airliner or cargo plane a turbo fan can generate a lot of thrust... most of which is driven by the huge fans at the front, but if you look at the rear you can see the small turbojet engine that actually drives the big fans... the little turbojet propels the aircraft by turning the fans rather than generating the thrust itself.
If you want to go faster than the speed of sound however you can't use such a high bypass engine... it is called a high bypass turbofan because as I said most of the thrust is generated by the big fans at the front which don't actually go through the smaller turbojet engine inside it.
If you make the big fan at the front smaller and longer and add an after burner at the rear you have a medium or low bypass turbofan engine... turbofans can get a big boost in thrust with an afterburner because the bypass air is cold and has not gone through the hot section of the engine so it is still oxygen rich.
A turbojet engine is a tube with a shaft down the middle with fans on it.... the fans at the front are big and suck air into the engine... the tube gets narrower in the middle to compress the air and heat it up... fuel is added in the hot section and burned further heating the air and the air then blows out the back generating thrust.
To increase thrust fuel can be added in the rear exhaust area and burnt called an afterburner to increase thrust.
A turbofan uses a small turbojet to turn a much bigger fan at the front. The engine is fully enclosed so the air driven by the fan at the front goes into the turbojet but also around the outside of the turbojet. In a high bypass turbofan like on an airliner the turbojet might stick out the back of the short fan section... the big fan section generates the most thrust because it moves a lot of cold heavy air... the turbojet is just there to turn the fan and only adds a tiny amount of thrust.
A medium or low bypass turbofan has a smaller front fan and extends past the turbojet engine hot section... it generally has an afterburner and the cold bypass air has lots of oxygen so more fuel can be burned to create more thrust... generally a low and medium bypass turbofan can generate more thrust in afterburner than a turbojet because of this.
The engine of the SR-71 uses the bypass air as a separate type of engine.
The problem with a jet engine like the one in the MiG-25 or MiG-31 is that the rotational speed of the fan blades inside is limited by their strength... just like a rubber tire would shred itself if you put it on a supersonic car the engine blades in the MiG-25 and MiG-31 would shatter if you ever flew faster than about mach 3.2.
The SR-71 gets around that issue by pretending to be a ramjet.
A ramjet is a terribly simple concept... you keep the tube of the turbojet but you don't have a shaft or blades... it is just a hollow tube.
For a missile like a Kh-31 or the new Kinzhal that means that empty space where the fuel is burned normally you can put a big solid rocket motor to get the missile moving... once it has burned out there is empty space for the air to flow through.
So in a ramjet engine the air goes in the nose intake... the tube gets narrower and compresses, which heats up the air... in the hot section fuel is added and burned and so it leaves the rear at high speed generating thrust.
In the SR-71 when the plane takes off it is operating as a turbojet, but as it gets faster and faster it starts pushing air around the turbojet engine, but there are no fan blades pushing that air... just the force of the air flowing through the engine... when the engine goes beyond a speed... say mach 1.8, all the thrust is now being generated by the air flowing around the turbojet engine with fuel being added in the rear afterburner like a ramjet.
That makes the engines of the SR-71 a combined turbojet/ramjet.
With their experience with scramjet engines the Russians can go one better if they want... there are two main problems with ramjets... first, they don't really work from a standing start so you need either a jet engine or a rocket to get them moving... for a missile the empty space means a rocket makes sense, but for a reusable aircraft another type of jet engine makes the most sense. the second problem is that they can't burn fuel at supersonic airflows, so even if you are flying at mach 5 you need an air intake that constricts and reduces the airflow down to subsonic speeds before fuel can be added and burned to generate thrust.
As you can imagine, not having to slow down the air flow and being able to burn fuel in the air as it moves supersonically through the engine means a much more power engine that is much simpler in terms of intakes and very simple in design... few moving parts except fuel injectors really...
That was a very expensive and specialized plane and was limited to 3.2 M,
It was completely made of Titanium... it was expensive and difficult to work on. Its fuel had a very high flashpoint and everywhere it went on the ground it leaked that fuel everywhere... it didn't have fuel tanks as such it used the inside structure of the aircraft as the fuel tank, but its speed performance meant when it was cold there were enormous gaps between each plate meaning the fuel poured out when it was cold on the ground.
The first thing it had to do when it took off was refuel...
I wonder how the Russians plan to go beyond that and also why this is so important. In the end the future threats are going to be hypersonic missiles way faster than 4 M, reaction times for interception are going to be just very few minutes to begin with..
Contrary to popular belief you don't need to move as fast as a bullet to intercept a bullet... Russia is not interested in protecting Europe or Africa or Asia for that matter... just Russia and it can position its defensive pieces so the threats have to come past them...
That being said having a faster aircraft means interception further from the target and also faster reactions to attacks... not to mention an R-37M missile might have a normal interception range of 300km from a MiG-31, but launched much higher and much faster from a MiG-41 that reach might double without any modification at all.
They don't seem to want longer range so I suspect northern bases will be used to reach out to the arctic... but I would also expect that a MiG-41 possibly with internal weapons carriage capabilities could carry Kinzhal internally to deliver a blow to any US ships operating in the arctic trying to swat down ICBMs... if that is the case then time is critical... assuming a US first launch the Russians wont have much time to start an attack on those ships before Russian ICBMs start passing over the north pole...
Of course amusingly many of the new ones will be heading over the south pole... it will likely take about an hour longer to reach their targets in the US but the US will have no idea they are coming.
Note most satellites take about 90 minutes to orbit the earth... if you watch the ISS go over it comes over again 90 minutes later... so 30 minutes from Russia to the US directly would mean another 60 minutes going the other way... roughly.
It seems the PDE engine would be more capable for different speeds. This seems the most promising option, but I guess it is at least a decade away from any practical use, only tests in laboratory done until now.
Well the question is whether PDE can be applied to all types of jet engine... in might not work with SCRAMJETs, but for mach 4.2 as I said you could do that with ramjet technology...
Scramjet technology would still be preferable...
This would be relevant if they indeed plan to make a near-space weapon. What amount of fuel and oxidizer would the aircraft need to carry in that case to accelerate beyond the limits of the turbojet and operate outside of the atmosphere? That would be a huge load...
The ideal design would be a powerful medium bypass turbofan like an NK-32, but with the bypass component modified to operate as a ramjet or a scramjet.
On paper there is no top speed limit for a scramjet, but as you exit the atmosphere of course the issue of no air to burn... so you block off the air intake and pump hydrogen and oxygen through the scramjet like a rocket engine.
Or even better... keep the NK-32 but with the scramjet have a nuclear reactor to heat the air when the scramjet is operating on its own... that means takeoff with fuel but once operating at mach 1+ you could activate your nuclear scramjet and accelerate to orbital speed... now when you leave the atmosphere your scramjet will stop propelling you because there is no air flowing through... then you pump nitrogen through the hot parts of the nuclear reactor to generate high energy gas flow = thrust. Nitrogen is abundant and not a fire risk as it is totally inert... 70% of the earths atmosphere is nitrogen, and by cooling it to liquid state carry it in high density... in the worst situation where you need to get back into the atmosphere you could blast some nitrogen forward to slow down to below orbital speed to reenter the atmosphere safely.
Yeah, I simply don't know what scenario and threats they are considering, the plane is not going to be ready before say 20 years if everything works as usually.
Speed will always be useful for an interceptor... the problem has been the enormous cost simply because the turbojet and turbofan engine simply were not up to the challenge.
Ramjets, or more specifically scramjets make it viable... once all the issues of heat are dealt with, but then they have said they have already developed heat resistant aluminium alloys...
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