One good SR-72 Darkstar AKA "SOB" (as in "Son of Blackbird," i.e. the SR-71 Blackbird, not the profane kind of "Son of...") deserves a timely follow-up. As I noted in my previous Simple Flying article on this still-hypothetical (at least officially) futuristic warbird, it not only captured the imagination of the Western moviegoing public but concurrently freaked out the top brass of Red China's People's Liberation Army Air Force (PLAAF) via its fictitiously embellished appearance in the 2022 box office blockbuster "Top Gun: Maverick."

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That article also reported that the prospective manufacturer, Lockheed Martin, has plans to put the SOB through its first test flight sometime in 2025. To cynics familiar with the history of other U.S. Department of Defense (DOD) experimental technologies, this projected timeline for the maiden flight of a plane whose physical existence hasn't even been officially confirmed yet may sound too good to be true.

At the risk of sounding like naysayers and cynics ourselves, Simple Flying looks at the potential hurdles to the SR-72 making the quantum leap from cinematic fantasy to operational reality.

Technical Challenges Part I: The Airframe

As in, can the airframe take the heat, or will it need to stay out of the (metaphorical) kitchen?

The SR-72 will supposedly be able to attain a max airspeed of anywhere from Mach 5.21 (3,997.472 mph; 6,433.308; 3,473.708 kn) to Mach 6.0 (4,603.61 mph; 7408.8 km/h; 4000.43 kn), with the latter estimate coming courtesy of Maya Carlin in The National Interest). That would generate a tremendous amount of heat, as Ms. Carlin (an analyst with the Center for Security Policy and my former colleague at 19FortyFive) notes:

"At speeds greater than Mach 5.0, the resulting high temperatures can melt conventional metallic airframes. To rectify this, engineers are researching alternate composites including high-performance ceramic, carbon, and metal mixes."

To put that in both numerical and historical context, the SOB's "father" (so to speak), the SR-71 -- which still remains the fastest air-breathing aircraft ever designed, even after all these years of retirement -- generated a "mere" (sarcasm intended) Mach 3.56 (2,731.478 mph; 4,395.887 km/h; 2373.59 kn).

In the process, the Blackbird's speed would heat the aircraft's external surface well beyond 500 °F (260 °C) and the inside of the windshield to 250 °F (120 °C), and an emergency ejection at Mach 3.2 would subject crews to temperatures of about 450 °F (230 °C)!

In addition, such high operating surface temperatures would plague the aircraft with ease of detectability by enemy radar systems, as noted by Dave Majumdar of USNI News:

"Stealth would be of limited value in an aircraft like the SR-72, given that radar absorbent materials would not survive the extreme temperatures. Moreover, the jet would have a huge infrared signature, which would negate most signature reduction measures. The aircraft might also leave a wake that might be tracked on radar—much like the SR-71."

Therefore, the plane would *need* to rely on speed for survival, as invisibility would not be an option. To quote a popular saying among pilots. "Speed is life. Altitude is insurance"

While the SR-71 is the world's fastest **air-breathing** aircraft, the fastest airplane of any kind was the rocket-powered X-15 , at 6.7 Mach (5,140 mph; 8,273 km/h; 4,467 kn)

Technical Challenges Part Deux: The Engines

Okay, so once the issue of the heat resistance of the airframe's skin is resolved, that still leaves the question of what sort of engine design can generate that "Ludicrous Speed" (obligatory homage to Mel Brooks's "Spaceballs" right there; my apologies for misquoting it as "Ridiculous Speed" in my previous Dark Helmet, er, Darkstar article).

Drawing a comparison with the SR-71, that iconic spy plane was powered by the unique Pratt & Whitney J58 turbojet engine, which aviation experts often refer to as a “turboramjet.”

However, as impressive as the Blakbird's turboramjet was, even it wouldn't be adequate to the needs of the SOB's speeds. Christopher McFadden of Interesting Engineering explains why:

"Conventional turbojet and turbofan engines can provide enough power to an aircraft during takeoff and landing at subsonic speeds. However, they cannot sustain hypersonic speeds. While there are jet engines that can power an aircraft at hypersonic speeds, they cannot be used during takeoff and landing. To this end, the SR-72 requires an engine that can do both."

So then, how do the engineering geniuses of Lockheed Martin's legendary "Skunk Works" program propose to work around this problem? We turn to Mr. McFadden again:

"The SR-72 program is focused on developing a fully reusable turbine-based combined cycle (TBCC) hypersonic propulsion system. This propulsion system is a kind of air-breathing jet engine that combines the turbofan engines used in many modern tactical aircraft with a supersonic combustion ramjet (also known as a scramjet) that is capable of achieving and sustaining speeds above Mach 5 and even potentially exceeding Mach 10."

The financial (budgetary) challenge

So, even if the SR-72 makes it past the technical hurdles, there's still the even greater hurdle of money, i.e. dollars and sense. For historical precedent that those aforementioned cynics are sure to call to mind, one need look no further than another Lockheed Martin Skunk Works brainchild, the F-35 Lightning II.

Yes, that controversial plane has been operational for several years now, but those oh-so-dreaded "cost overruns" (one of the most common cliches in defense budgeting jargon) put the plane a decade behind schedule and $183 million USD over original cost estimates, according to the US Government's General Accounting Office (GAO).

Members of the US Congress who control the proverbial "power of the purse" and who have long memories of the F-35 and other long-delayed, hyper-expensive defense technologies may very well balk at the price tag that the SR-72 could conceivably ring up. To quote Mr. Majumdar again:

"But only time will tell if the Pentagon has the financial wherewithal to fund Lockheed’s dreams of the SR-72, even if the Defense Department wants such an aircraft."