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

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

By Linda Sheffield Miller
Feb 26 2023
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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 Foxbat flew slower than the SR-71 Blackbird.

No reconnaissance aircraft in history has operated globally in more hostile airspace or with such complete impunity than the SR-71, the world’s fastest jet-propelled aircraft.

From 80,000 feet, the SR-71 could survey 100,000 square miles of Earth’s surface per hour cruising at Mach 3+ (three times the speed of sound) or over 2,000 mph.

The Blackbird’s performance and operational achievements placed it at the pinnacle of aviation technology developments during the Cold War.

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.

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

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

MiG-29K Fulcrum

‘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 story of the SR-71 Blackbird that slid on JP7 fuel residue while taxing into Kadena Hangar

‘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).

SR-71 print
This print is available in multiple sizes from AircraftProfilePrints.com – CLICK HERE TO GET YOURS. SR-71A Blackbird 61-7972 “Skunkworks”

‘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 Pages Habubrats and Born into the Wilde Blue Yonder 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

Lockheed SR-71 Blackbird model
This model is available in multiple sizes from AirModels – CLICK HERE TO GET YOURS.

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Linda Sheffield Miller

Linda Sheffield Miller

Grew up at Beale Air Force Base, California. I am a Habubrat. Graduated from North Dakota State University. Former Public School Substitute Teacher, (all subjects all grades). Member of the DAR (Daughters of the Revolutionary War). I am interested in History, especially the history of SR-71. Married, Mother of three wonderful daughters and four extremely handsome grandsons. I live near Washington, DC.

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