I understand the SR-71 had to deal with a lot of issues in order to keep its speed (special fuel which leaked on the runway, titanium fuselage and probably other stuff). But wouldn’t the same type of engine be able to power a relatively slower fighter jet capable of easily cruise at match 2-2.5, so it doesn’t have to deal with so much friction as the SR-71 at match 3.
But while the engines exist since the 1960s, relatively few fighters go faster than match 2 and it took all the way to the 21st century to have fighters capable of supercruise (and still below match 2). So I guess there has to be a reason for that.
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Turboramjets are substantially larger and heavier than a standalone turbofan engine. You basically have to build two separate engines, then stick the turbojet inside of the ramjet. That also leads to the engine being much more expensive and difficult to maintain than a standalone turbofan.
Also, the ability to go that fast is of limited usefulness nowadays. When the SR-71 came out, networked radar didn’t exist – every AA battery was a standalone station that could only coordinate with other radar sites by having the operators talk to each other over the phone. That meant that it was really only practical for a battery to fire on a target once it came within range of that battery’s own radar, which is limited to 35ish miles, depending on how high the plane is.
The missiles themselves were also pretty dumb and would basically just try to stay centered on their current target. For a high flying, fast target, that meant that the missile was constantly turning and bleeding off speed.
The theory behind the SR-71 was that the SR-71 was flying so high and so fast that a missile fired from 35 miles away didn’t have the time or speed to reach the SR-71’s altitude and then catch up to it.
Modern radar is networked, which means that a missile battery can fire on a target that it can’t see, but which a radar hundreds of miles away can. Missiles are also a lot smarter and can target an empty area of space where they calculate they will intercept the target at some point in the future. The result of this is that speed offers no protection from SAMs anymore and flying high just exposes you to more radar.
High speed does still offer a benefit, since it allows you to get to an enemy quicker or run away from a slower enemy. But its not worth the substantial cost of the engines. To give you some context – the SR-71 cost $34 million to build, much of the cost of which was the engines, at a time when top of the line fighter jets cost ~$5 million.
That latter point is especially true if you’re only looking to go Mach 2, since you can get to that speed with a much cheaper turbofan engine. The reason that most jets don’t go that fast is, again, there’s just very limited utility to doing so outside of the interceptor role and most fighters are now built as multirole fighters, rather than interceptors.
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