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Comment by geocrasher | Hacker News Reader

An interesting look at an absolutely insane idea! The Foxbat was an interesting design because as the article mentioned, flying at Mach 3+ required essentially destroying the engines. The Foxbat was supposed to be able to catch the Blackbird, which is why it had giant engines and had skin with a high nickel content for enduring bursts to high Mach numbers.

Even so, the Blackbird could crank it up to about Mach 3.4- the Foxbat didn't have a chance to catch it. One could wonder- did the MIG design bureau know this, and suggest the biz jet version as a way to save the project? Either way, it was a success for the Soviet Union if nothing else as a propaganda machine.

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Foxbat project predated Soviet knowledge of the Blackbird. High speed, high altitude intercept was a role they anticipated needing based on American bomber evolution (B-58 and planned B-70) and extrapolating from U-2.

The Foxbat doesn't necessarily need to be as quick as its targets. In one way you could imagine the Foxbat as a gigantic first stage for missiles. Soviet missiles were technically capable of outspeeding even the Blackbird, but the challenge was the time and energy it took to get to altitude. High speed, high altitude interceptors could help mitigate those problems.

It's more complex than this though. At those speeds there is no margin for error for the intercepting aircraft and missile.

There are lots of accounts from SR-71 pilots about this. At Mach 3 slight changes in course result in large changes in position very quickly. Mach 3 at SR-71 altitudes is > 30 miles per minute. The pilots could make course changes that would put the plane off the original intercept by miles within seconds and it was very hard for a missile to adjust for that, missiles run out of fuel very quickly and have to burn their energy to change course. And they weren't very smart in the cold war.

The Soviets never managed to intercept an SR-71 even to scare it off by most accounts. The Swedes apparently claimed to have done it but only because they were friendlies and the SR-71 wasn't changing course to evade them.

The Swedes also "almost" developed a small fast business jet from a military airplane. The SAAB 105 "SK 60" military trainer had a development branch for a five seat civilian model [1]. I recall hearing it got cut sometime around the oil crisis back then.

It is a quite good plane, and surprisingly silent.

[1] https://en.wikipedia.org/wiki/Saab_105

There is ample margin of error. You forget that both airplanes are going at Mach 3+. The SR-71 might be going at Mach 3.4 and the MiG-25 at Mach 3.2, and this difference in speed is not anywhere near sufficient to prevent the interceptor from compensating.

>The pilots could make course changes that would put the plane off the original intercept by miles within seconds and it was very hard for a missile to adjust for that, missiles run out of fuel very quickly and have to burn their energy to change course. And they weren't very smart in the cold war.

No, that's not true. By the time the missile is fired, at a distance of, say, 30km, the SR-71 has anywhere between 10 seconds and 2 minutes before being hit.

From the SR-71 flight manual, we can find a turning radius of 105 nautical miles at Mach 3.2, and extrapolate to 140 nm at Mach 3.4. That means a 259km turn radius, and a 1700km turning circumference, meaning that every degree of rotation translates to, at 1180m/s, 4 entire seconds

So, the missile would have a very easy job. At a turn rate of 1 degree every 4 seconds, even in pure pursuit ("not very smart"), the missile would not have to expend much energy at all.

What I read is that the final Soviet tactic was to “surround” the SR-71 with MIG-25s so that it had nowhere to turn.

I suspect that getting to within 30km of the SR-71 would not have been easy for a single MIG. The MIGs would have had to accelerate to Mach 3 and also climb to altitude just the same as a surface to air missile aimed at the SR-71. In the time the MIG does that, the SR-71 could accelerate, climb, an|or change direction to evade the MIG.

Having intelligence as to from where and when the SR-71 was coming would probably be crucial. The MIGs could gain speed and altitude and time it to get to the SR-71 just as it arrived, giving it no time to evade. The MIG wasn’t going to loiter for long, and Mach 3 for the MIG was a one time use thing.

Lastly, the predecessor to the SR-71 did carry air to air missiles. That missile would have speed and altitude advantages over the MIG i.e. it would be launched from further away or reach its target first.

Of course, you cannot guarantee to intercept a target with a single plane, but that's hardly unique to the SR-71.

> The MIGs would have had to accelerate to Mach 3

Not really. In any realistic interception scenario, the 2.83 Mach safe maximum speed would be sufficient to intercept successfully. Going at Mach 3 or more would be most useful in case of emergency.

> could accelerate, climb, an|or change direction to evade the MIG.

We are assuming the SR-71 is already at maximum altitude and speed. Changing direction is a bad idea because the SR-71 turns very slowly at very high speeds, so it would have to slow down, which is counterproductive.

The MiGs were able to gain altitude around 4x as fast as the SR-71. They could get to the maximum altitude of the SR-71 and close to their maximum speed in around 10 minutes from the ground.

> the predecessor to the SR-71 did carry air to air missiles

The YF-12 never made it to initial service, it's production predecessor, the A-12, did not have armament. No version of the A-12 was ever made that could both do reconnaissance and carry weapons.

> Having intelligence as to from where and when the SR-71 was coming would probably be crucial.

Definitely. But again, you're just describing interceptions in general. Without having vectors and warning at least ten minutes (which means only 600km for the SR-71) in advance, you can't really intercept anything.

Of course, a single MiG-25 wouldn't have had the best of chances. But a pack of MiG-25s would definitely have very good chances. In actuality, SR-71s were indeed intercepted by MiG-25s in international airspace.

Indeed, if launched at a high enough altitude and velocity, something like the R-40 would have been significantly faster than the SR-71 at over Mach 4. The other part of the equation is that at high speeds such an aircraft would be an easy target - any serious evasive action would require it to slow down unrecoverably, or simply destroy it. Meanwhile, the massive heat signature would mean any use of ECM or stealth characteristics would be unlikely to succeed, as each plane carried both radar and infrared versions of these missiles.

Most likely, unless the Soviet air defences didn't manage to detect the SR-71, it would have been shot down, which is presumably why it never overflew the USSR.

The reason it didn't overfly was political. The Gary Powers incident being the main initiator of that policy. It overflew Vietnam and had loads of missiles shot at it by Soviet SAMs with Soviet SAM operators. Mainly it flew too fast and too high for those to hit it.

As far as I'm aware, the MiG-25 is not a surface-to-air missile. There were no MiG-25s in Vietnam either.

The SR-71 was fast enough to avoid surface-to-air missiles, but it was much too slow to avoid the air-to-air missiles carried by a MiG-25.

On paper, yes, it would seem a MiG-25 would work against an SR-71, but those are max characteristics and not sustained performance. Mach 3.2 was best case while testing. Real world speed is more like Mach 2.5, especially when carrying missiles. The AA-6 was huge and massive drag on performance. Further, the AA-6 had very limited targeting and engagement electronics, which meant that the MiG-25 was not very effective against the SR-71.

This was my understanding too as they would only fly along the international border but could use side look down radar to peer deep inside the country. But once satellite tech was good enough then the RS-71 became superfluous and too expensive ($300m per year per RS-71) to keep flying.

Actually, the MiG-25 was designed to catch the XB-70 Valkyrie, which is why top speed and cost were the priority requirements. Speed because it needed to catch it, and cost because there had to be a lot of them built if they wanted to protect the entire territory from supersonic nuclear bombers. And it should've been able to catch the supposed mach 3.1 speed of the XB-70. The Valkyrie was cancelled of course, but the Soviet Union kept using (and selling) the jet.

The remaining XB-70 would be reason enough to visit the US AF Museum (Dayton, Ohio) - it's a stunning plane. That said, the museum is easily worth two days.

Oh, my memory did not serve me well in this case. Thanks for the correction! The XB-70 is an amazing aircraft.

Didn't the XB-70 become the B-1?

Kinda not really? The XB-70 cancellation left a hole (next generation strategic bomber) that was eventually filled by the B-1. But the initial B-1(A) was envisioned to be somewhat lower speed, but still fast (Mach 2), and specifically a low altitude penetrator (XB-70 was a high speed, high altitude plane). It then got reworked into the B-1B which is uh... weird role. It's even slower than the B-1A, somewhat more stealthy, and really supposed to be used as a cruise missile truck. Arguable how much value the B-1B really provided.

The B-1B seems to be a really good smart bomb truck.

I seem to remember having read in a German print magazine (https://www.fliegerrevue.aero/) that Mig-25's stationed in East Germany (or was it Poland?) were supposed (and trained) to intercept incoming Blackbirds over the Baltic Sea, which could have theoretically worked even if the Mig would be slightly slower by using the right "approach path" and coming in behind the Blackbird in weapon range for a short time window.

The MiG-25 doesn't need to be faster than the SR-71. It just needs to have a certain fraction of the speed to be able to intercept it at an angle.

At the high altitudes and speeds the Blackbird would have to go to evade a MiG-25, it would have no hope at all of evading a missile.

> At the high altitudes and speeds the Blackbird would have to go to evade a MiG-25, it would have no hope at all of evading a missile.

I thought the SR-71 was faster than missiles?

https://nationalinterest.org/blog/buzz/forget-stealth%E2%80%...

From your own article:

> In effect, by the time it took a SAM system could lock onto an SR-71 traveling at Mach 3 and launch a missile, the Blackbird was already moving beyond the effective range of the missile.

It wasn't purely faster than the missiles, it was fast enough to get out of range of radar and tracking for ground based missiles and beyond the effective range of the missile before it could reach the jet. Fired from an already moving jet at altitude missiles would have a longer effective range as it would spend less fuel getting to speed and gaining altitude.

It was too fast for surface-to-air missiles, because by the time they got to it's altitude and caught up, they would have lost most of their kinetic energy. Of course, modern SAMs would be able to hit it.

The Soviet Union at the time had the R-40 series of air-to-air missiles, that could got Mach 4.5 at very high altitudes if launched by a fast enough, high altitude aircraft, which is much higher than the Mach 3.4 the SR-71 could hope to achieve. The reason it could go that fast is because it didn't have to expend its kinetic energy to climb much, and because it already started up at Mach 3.

Missiles are faster but start in a far disadvantaged position. The missile has to start from 0 speed and 0 altitude. By the time the missile is at speed and altitude, it is too late given the speed of the SR-71.

SAMs have to start at 0 speed and 0 altitude. An R-40 launched from a MiG-25 would start at Mach 3+ and 30km altitude.

As many other people pointed out a slight change in direction would be able to put the SR-71 way off course of a MiG-25 intercept path.

No, it would not. The SR-71 is only very slightly (~200km/h) faster than the MiG-25, and if it were to make any sizeable turn, it would have to slow down significantly, perhaps even to a lower speed than it's pursuer.

It is highly doubtful a MiG-25 is just waiting up in the air for a plane to intercept and just happened to be going at it’s top speed. It is much more likely they will have to scramble a few jets for the intercept. I believe that SR-71 didn’t fly over Russian airspace due to the U2 incident. It’s also likely an AWACs or two are around and could detect the jets scrambling.

Bit it would also put the SR-71 of course by quite a bit. If that happens early enough any hypothetical MiG-25 mission would have been a success either way.

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