Could NASA’s SR-71 Blackbird Be Modified To Reach Mach 4?

During his time as the lead propulsion engineer for SR-71 Blackbirds at NASA’s Dryden Flight Research Center, Tim Conners looked into how to push the iconic recon plane to fly faster than it had ever flown before. The world record for any crewed jet-powered aircraft is officially Mach 3.3, or 2,193 mph. It was set by a Blackbird in 1976 “on a straight-line course over Edwards Air Force Base in California.” In a recent interview with TWZ, Conners said he investigated whether there were ways to get Blackbird to approach Mach 4, equivalent to roughly a whopping 2,660 mph at altitude. Fast forward to today, with the possibility that NASA could be returning the SR-71 in some configuration to its test fleet, as well as its potential missions, the question of just how fast could the airframe go has become far more relevant than just a hypothetical exercise.

Conners spoke to us about his knowledge of plans to restore SR-71 #844, which had mysteriously disappeared from NASA’s Armstrong Flight Research Center, as well as its potential uses, a deep-dive interview here. Prior to the revelations about the missing Blackbird, speculation about NASA’s renewed interest in the type was sparked last month, when NASA Administrator Jared Isaacman showed the silhouette of an aircraft that immediately drew comparisons to SR-71 during a speech at the annual All-In Summit in Los Angeles, California. Isaacman pushed back on a direct link to the SR-71 but said his agency is working hard to get “back in the business of flying high and fast again.”

NASA Administrator Jared Isaacman talks at the All-In Summit about new high- and fast-flying X-planes with the eye-catching aircraft silhouette in the background. All-In Summit/YouTube screencap

Given that, we wanted to know what limited the SR-71’s speed in its original configuration and how fast Conners thought it could go modifications.

“The study that I did at Dryden was purely conceptual,” Conners explained of his work on the topic there in the early 1990s. “I asked questions of the Lockheed folks and the Pratt folks regarding limitations on speed. So stagnation temperature. What can we get to at altitude?”

The Blackbird, he said, “flew a trajectory that was usually operationally limited to 450 knots equivalent airspeed. If you look at the flight envelope, that defines the right edge of the envelope.”

SR-71 Blackbird. (Courtesy photo via USAF)

“The engines, however, were optimized to fly at about 500 knots equivalent airspeed, 475 to 500, and for whatever reason, there was a mismatch between what the engines were designed to do and what the airframe was ultimately optimized for, which was flight at around 450 knots,” the engineer added. “And you end up with that limitation now, the equivalent airspeed curve hooks up and to the right as you go up in altitude, so it sweeps out Mach number, and as you sweep out Mach number, the stagnation temperature is going up on you.”

“So you do reach a point, even at 500 KEAS [knots equivalent airspeed], where you end up with a constant value up at altitude once you’re in the stratosphere,” Conners said. “The Blackbird was trimmed or limited at that speed on the right side of the envelope, and that equated to a certain stagnation temperature. That limitation was driven primarily by the material strength of the engine front frame, from what it was described to me, and that’s what drove the analysis we did at NASA.”

SR-71 tail number 844 during its service with NASA. (NASA)

To even consider pushing the aircraft past the world record speed, Conners said something had to be done to mitigate the tremendous effects of the extreme heat on the air entering the engines.

“We weren’t looking at active cooling of the frame by pumping coolant through it,” Conners recalled. “What we looked at was spray cooling of the incoming airflow to lower the bulk temperature, so it would only be used when you were accelerating beyond about Mach 3.4. You would only very briefly do Mach excursions to a higher Mach number, and then come back. That was the conops for that system.”

“This is a way to [get to a higher Mach range] briefly,” the engineer postulated. “You could probably get to a higher Mach number. The problem is that if you clog or freeze the system, if it fails while you’re at higher speed, you have to decelerate in a hurry to protect the assets.”

Among other impediments to breaking the speed record, the Blackbird’s existing power plant was not up to the job, according to Conners.

“So that weak link was the J58,” he explained. “It had the performance, but not the material strength. If we’re looking at alternate engines designed to handle the higher Mach capability, then that could put the limiter on the airframe at that point, which we were told at NASA was somewhere like up near Mach 4 for brief excursions.”

Aviation expert Paul F. Crickmore, who’s written several books about the Blackbird, told us that the aircraft was designed around a sustained cruise speed of Mach 3.2, with the primary critical limitation being aforementioned compressor inlet temperature (CIT).

Above 427 degrees Celsius (about 800 degrees Fahrenheit), the engines would be damaged, Crickmore noted. But if outside air temperature was colder, the aircraft could go faster before hitting that limit (in practice, Mach 3.3).

Temperature was just one factor in limiting how fast the SR-71 could travel, Crickmore explained. The shock waves from traveling so fast would also tamp down the Blackbird’s ultimate speed potential.

Paul Crickmore’s latest book on the SR-71, Lockheed Blackbird: Beyond the Secret Missions. The Missing Chapters. (Osprey Publishing)

“On the SR, you have to think three-dimensionally,” Crickmore explained. “But looking in just two dimensions, you would have an oblique shock wave on the nose, the first shock wave, and it would be at 90 degrees on either side. As the airplane got faster, that cone became more sharp until you got down to about 32 degrees. Now, at that speed, you’re at [Mach] 3.2. That oblique shock wave was still outside the envelope of the airplane – you don’t want to get that shockwave going anywhere near the flight controls. The faster you go, the narrower that cone, and the closer it gets to the wingtips, which is not good. This is the thing: People say, ‘Oh yeah, we can go maybe Mach 6.’ Well, right. But it’s not just about that. It’s about a lot of other things.”

The limiting factors imposed by the CIT value and the shockwave effect are also recalled by the late SR-71 pilot David Peters, recounted on the Habubrats SR-71 account on X.

“A little clarification on the speed. The issue is that I have been limited to less than Mach 3 on a few occasions because the outside air temperature was entirely above standard, and 427C came up at about [Mach] 2.95. On other occasions, like the Murmansk deal I got above 3.4 (3.49 on one occasion) and wasn’t close to 427C. The actual limiting airspeed is around 3.55; that is where the spike, being at full retraction loses the intercept on the shock wave and can no longer position it correctly in the inlet. Also, the overflow of the shock starts to go over the wing and interfere with the flight controls. So the limiting speed as configured would be about 3.55 so long as you don’t exceed 427C.”

For his part, Conners said that “shock effects at the inlet can be accommodated through rescheduling the inlet spike movement and perhaps making changes to the bypass schedules. None of that would necessarily be simple, and any changes would require careful envelope expansion. But it would not be impossible.”

Regardless of any other factors, Conners, the Blackbird engineer, remembered that his effort in boosting the jet’s speed never took off.

“It never got beyond me just sizing tank volume, considering different fluids that might work as a coolant, and then just gauging the relative appetite of NASA hierarchy for something like that,” Conners stated. “They weren’t interested in the risk. Nobody was asking for that Mach range.”

But that may be changing. It seems clear that one of the biggest challenges of getting the Blackbird back in the air will be its engines. They have sat for a long time, and getting them into a flyable condition could be the limiting issue. Beyond that, equipping the SR-71 with new engines also may represent the biggest opportunity for NASA getting a return on investment from its revived Blackbird.

(NASA)

In our interview, Conners told us that the feedback he heard about the remaining Blackbird J58 engines was “discouraging” because “the engines are indeed unserviceable.”

“I don’t believe that is based on an actual attempt to run them,” he explained. “It is based on inspection. Probably no surprise. They’ve sat idle, you know, for 27 years. That is what led to questions going back and forth regarding how the airframe would be powered. And that’s what led into, if you connect the dots, that’s where the airframe would be used as an engine test bed.”

Different engines “would have to be high-Mach bypass systems,” he noted. “We can speculate on which engine company might be developing those systems, but yeah, I don’t want to give it too much away because I don’t want us to lose the inside information that we got.”

Pratt & Whitney J58 engine. (National Air and Space Museum)

Also, the flight regime approaching Mach 4 would likely be one of the areas NASA is most interested in. It’s here where a combined cycle engine capable of hypersonic flight, one that contains both turbojet and ramjet or scramjet functions, would ‘hand-off’ propulsion from the turbojet to the other that is optimized for extreme speed. This is the same concept behind Lockheed’s SR-72 concept. Having an aircraft, and especially one that is not mired in deep classification and hidden from view, that can actually test this transitional flight regime could be of extreme value in unlocking a future where hypersonic flight is more accessible.

“Yeah, that could very well be what we’re looking at here,” Conners told us when we asked whether Blackbird’s open classification level is a driving factor in using it as a test bed for this research. “I’ve had friends ask that question this week. We were talking about the SR-72. Where is that at? I don’t know. I know people who went to work on it years ago. I’ve never heard anything since. So it could be that that’s so deep black that there’s no way that platform could be used as a test asset. So I think it’s precisely what you said. The Blackbird’s no longer classified, and that it’s possible that the engines that might be used in Blackbird wouldn’t be classified either.”

Lockheed’s ‘SR-72’ hypersonic plane concept. (Lockheed Martin Skunk Works)

So, reengining the SR-71 may solve two birds with one stone — providing a modern powerplant that won’t require the J58s to be revived and taking care of the inlet temperature issues discussed above that would allow the SR-71 airframe’s full potential to be exploited in testing. The fact that the jet can carry a massive payload on its back could allow it to provide critical testing of the handoff for dual cycle engine concepts.

This is just one use, of course. With the explosion in high-speed flight for defense purposes over the last decade, there are many other reasons to push a Blackbird to higher Mach numbers.

So with all this in mind, if we get the Blackbird back at all, maybe it will roll out with some major modifications that could indeed push it to new velocities.

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