Here’s why the MiG-25 Foxbat was never as fast as the SR-71 (and why Soviets never developed their own Blackbird)

The MiG-25 (NATO reporting name “Foxbat”) could outrun any fighter in the air, and indeed any military aircraft other than the SR-71 Blackbird.

In the late 1960s, the USSR revealed the existence of the aircraft that appeared to be the world’s deadliest fighter, the MiG-25 (NATO reporting name “Foxbat”). This aircraft could outrun any fighter in the air, and indeed any military aircraft other than the SR-71 Blackbird.

The MiG-25’s development has been attributed at various times to the threat posed by either the B-70 Valkyrie, or to the SR-71.

However even after the B-70 was cancelled, the Foxbat’s development and production went forward. In fact as explained by Lou Drendel in his book SR-71 Blackbird In Action, the Habu posed a much more formidable threat, with its demonstrated ability to sustain cruise speeds above Mach 3 at over 80,000 feet.

The MiG-25 has presented as much an enigmatic image as the Blackbird, at times being reported as invincible, and at other times being dismissed as practically prehistoric in its approach to high speed and altitude flight.

But why didn’t the Soviets come up with their own “SR-71” and build a counterpart warplane to the Blackbird?

‘The Soviets built the MiG-25,’ says Iain McClatchie, an aviation expert, on Quora.

‘Notice how huge those air intakes are? Compare that against a Mach 2 airplane:

‘Dynamic pressure is what holds the aircraft up and causes drag. It scales with air density linearly, and with velocity SQUARED.

‘Air mass flow rate through the engines is what is needed for thrust. It scales with air density linearly, and with velocity LINEARLY.

‘Since the wing works with dynamic pressure, dynamic pressure can’t vary more than a factor of 5 or so during the flight. So, if you want to go fast, you have to go high, where the air is less dense. For mass flow to keep up with drag, you’ll need big fat intakes. These aren’t going to work well at lower speeds.

‘The moment we saw the MiG-25, we knew it was designed to go fast like an SR-71/A-12, whose intakes are each nearly the size of the fuselage.

‘The other problem that a high and fast jet faces is hidden. The engine intake slows the air to subsonic flow before it goes into the compressor. The air’s kinetic energy turns into heat… the temperature rise goes as the SQUARE of velocity. The engine then has to compress that air (heating it more), and then add lots of heat before expanding it through the turbine. The turbine has to survive air heated in all three ways.’

McClatchie continues;

‘The J58 engine in the SR-71 was the first engine to use directionally solidified turbine blades, which were able to resist creep at higher temperatures than any blades before them. This is a critical technology. Without high temperature turbine blades, you cannot make a jet engine fast.

‘In fact, the SR-71 and MiG-25 are both thermally rather than power limited. Both have reserve power to climb (rapidly) at full speed. Their speed is limited by the temperature of their turbines, and not by power.

‘(These days, most turbine engines have hollow turbine blades, with “cooler” air from the compressor blown through them and out tiny holes in their leading edges. The air forms a film over the blade, insulating it from the heat of the surrounding gas. Neither the SR-71 nor the MiG-25 had these blades. Neither had the ceramic coatings we use today. These changes would have enabled an operating speed increase, but probably not more than Mach 3.6 or so).

‘As it turns out, the Soviets did not have the technology to make blades that could tolerate as high temperatures as the J58 turbine. As a result, the MiG-25 flew slower than the SR-71 (Mach 2.83 rather than Mach 3.4), and its engine did not last the 400 hours between overhauls that the J58 managed.’

McClatchie concludes;

‘That speed limit, btw, is a very abrupt speed limit. NASA squeezed a tiny performance increase out of the SR-71 by reducing engine life from 400 to 50 hours. A MiG-25 was tracked at Mach 3.2 over the Sinai in March 1971. That speed excursion would have only lasted minutes and the engines were scrapped after the flight.’

Be sure to check out Linda Sheffield Miller (Col Richard (Butch) Sheffield’s daughter, Col. Sheffield was an SR-71 Reconnaissance Systems Officer) Facebook Page Habubrats for awesome Blackbird’s photos and stories.

Top SR-71 Blackbird photo: Stuart Freer

Photo credit: NASA, U.S. Navy, Alex Beltyukov and Dmitriy Pichugin via Wikipedia