Why aren’t turboramjets (like those in the SR-71) used for other aircraft?


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.

In: 522

Basically there is no demand for such aircraft. It’s still too expensive to use as fighter or bomber and for reconnaissance drones and satellites are good enough. Also all modern anti aircraft solutions are designed to kill fast moving jet planes while primitive slow drones (like those Ukrainians used to blow up a refinery in Russia) are extremely hard to hit due to tiny radar and heat signatures.
Only reasonable way to practically utilize hypersonic engines would be to “skim” the boundary of our atmosphere to reduce drag (plane would fly almost in vacuum) and increase range this way. But you’d need a scramjet for that. Such engines are in development and testing but so far (at least publicly) those are only considered for missiles.

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.

Fun fact, the SR-71 was actually named RS-71 but the name was botched during it’s reveal to the public . The name stuck.

Edit: still holds speed records. I love that thing. It’s a technological wonder. And it was designed all the way back in the 60s. Just crazy far ahead of it’s time. I wonder what’s top secret nowadays, if that tech is over 50 years old. So many good docs about it and stories.

LA speed story is so funny.

There have been many experimental aircraft whose sole purpose was going fast and some of them flew faster than the SR 71. They were going fast more or less for the sake of it, since all they were made for was to study aircraft behavior in such high speeds.

But when an aircraft has to be produced in large numbers for civilian or military use, it has to serve a purpose other than just going fast. The SR 71 was not a fighter plane, it was a reconnaissance aircraft. It was an incredibly expensive and complex aircraft for what was essentially a flying camera. At the time it was developed that was the best and safest option for reliable spying, but now it’s obsolete since satellites and UAVs are much cheaper and more effective at the same task. When it was new it could fly so high and so fast that no interceptor aircraft or even ground to air missile could catch it. However it didn’t take long for such aircraft and missile systems to be developed for that exact purpose.

The military shifted its focus from having a wide variety of highly specialised single role aircraft to having more versatile multi purpose fighter jets. Such an aircraft can’t minmax and sacrifice everything for speed, especially since missile technology has advanced to a point where there’s no way to make a plane fly faster and higher than a missile. So the simple answer is that there’s simply no reason to make other aircraft that fly so fast. No role a modern fighter jet undertakes requires it to fly so fast, and it’s much better to have an aircraft that does a lot of things well than one that does one thing extremely well and is bad at everything else.

Fuel consumption in the SR-71 was gigantic. Sure, it could go really fast, but the cost to fuel it is also really large. There isn’t much demand for speed at that cost.