On April 14, 1986, the U.S. Air Force lost an F-111 supersonic jet in Libyan airspace. The aircraft had participated in a retaliatory airstrike against Libyan terrorists who had attacked American servicemembers, and while the Air Force called the airstrike a success, they hadn’t expected to lose the F-111 or its crew of two. Muammar al-Qaddafi, Libya’s despotic leader, had established a “line of death” across the Gulf of Sidra, a hornet’s nest of air defense systems that would aim to shoot down anything that dared to cross it. Now the threat had substance, and the Americans had to cross the line again. The following day, April 15, the Air Force assigned another jet to fly into Libyan airspace and assess the damage for U.S. intelligence officers. Because so little time had elapsed between this mission and the previous one, the line of death would be primed for another incursion. There was one aircraft capable of making such a daring flight: Pilots called it the Habu, the pit viper, but most of the world called it the SR-71 Blackbird. Lockheed Martin developed the SR-71 as a “black project” under supreme secrecy, and it was engineered around a simple concept: Nothing could shoot you down if it couldn’t catch you. The Blackbird’s top speed, Mach 3.2, set new records for jet engine aircraft, but on a pre-established flight path against Libyan surface-to-air missiles—thought to be Soviet-designed SA-2s and SA-4s capable of Mach 5 speeds—it would need to push past its limits. According to Major Brian Shul, the pilot of the SR-71, Libyan missiles shot into the sky as soon as the Blackbird crossed the line of death.

The jet was already screaming at its apparent maximum speed, Mach 3.2, but Shul pushed the SR-71 even harder, trying to beat the missiles to the turn in the flight path that would take the plane out of Libya and into safety. As the Blackbird accelerated, it tore past its safety limits, its two J-58 jet engines swallowing 100,000 cubic feet of air per second. Shul’s ground speed indicator ticked past Mach 3.31, faster than 2,500 miles per hour. More missile indicators lit up the cockpit. The SR-71 climbed above 70,000 feet, fleeing the rockets that might catch it from below. As it swung away from Libya, the plane’s speed indicator read Mach 3.5, faster than a bullet fired by an M-16. A sonic boom bellowed to the ground as the plane reached safety, an audible calling card of what remains the fastest jet-powered aircraft known to fly for any nation. The story of the SR-71’s improbable escape over Libya affirmed speed as a critical defensive tool. According to some particularly laudatory legends, the aircraft saw threats from more than 4,000 missiles during its three decades of service, and not a single one made contact. Today, stealth is the leading strategy for bypassing enemy air defenses, but as hypersonic missiles enter service in Russia and China, speed is seeing a resurgence in importance for the first time in almost four decades.

And while America’s hypersonic missile programs are lagging, the legacy of the SR-71 seems to have inspired a secret plan for the Air Force to win the hypersonic arms race in another fashion. The American savior might not be a missile, but an aircraft: the near-mythical Lockheed Martin SR-72 bomber. Little is known about the top-secret program developing the technology needed to bring the hypersonic plane to life, but we do know that it has a name: Project Mayhem. Through interviews with experts and analysts, poring over government documents and looking for clues in the scant public statements and images, we’ve pieced together the most complete picture of what America’s next hypersonic weapon may look like, and how well it stacks up against the best tech from Russia and China. The race for speed supremacy took on heightened urgency in March 2018, when Russian President Vladimir Putin announced that he possessed the world’s first operational hypersonic missile, the Kh-47M2 Kinzhal. Hypersonic weapons are certainly fast—the term “hypersonic” describes flight speeds in excess of Mach 5—but it’s these weapons’ unique coupling of speed with maneuverability that confounds modern air defenses. Even state-of-the-art defense systems can’t accurately intercept platforms moving at hypersonic speeds while changing course.

As a result, Russia, China, and the United States are competing to field different types of hypersonic missiles. Russia and China claim to have some in service; the U.S. is apparently falling behind. One possible reason for that lag, analysts say, is that America’s plans for hypersonic technology are more ambitious than simply propelling a warhead beyond Mach 5. One goal is to develop a reusable hypersonic aircraft that could thwart enemy air defenses while also averting the prohibitive cost of single-use hypersonic weapons. The U.S. has pursued this capability for decades; by this measure, it once led the world in hypersonic technology. In 1957, Boeing proposed the X-20 Dyna-Soar spaceplane for bombing and reconnaissance (Neil Armstrong was among the pilots chosen for the program), and in 1967, Air Force test pilot William “Pete” Knight flew his rocket-powered North American X-15A-2 at a speed of Mach 6.7. By 2004, NASA’s scramjet-powered X-43A reached Mach 9.6, and in 2007, reports surfaced about Skunk Works and the Air Force working on a pilotless dual-mode scramjet-powered follow-up to the SR-71, dubbed the SR-72. By 2015, the SR-72 was no longer hidden behind a “black project” veil—its page on Lockheed Martin’s website advertised that it could be in service by 2030.

Up until March of 2018, Lockheed was public about its efforts to field the SR-72, but immediately after Putin’s speech announcing the Kinzhal, the company redacted any outward-facing mention of the plane, either because it gave up on the effort or because it was pushed back into the shadowy world of projects using classified funding. Meanwhile, the hypersonic weapons fielded by America’s competitors thus far are similar to ballistic missiles, albeit much faster. Known as boost-glide vehicles, they’re propelled to hypersonic speeds by a rocket before detaching and gliding back toward their target at speeds as high as Mach 20. How boost-glide weapons shift course remains a secret, but experts like Chris Combs of the University of Texas at San Antonio, who specializes in hypersonics and aerospace engineering, suggest that it’s likely done using a combination of gas-powered thrusters and hydraulic control surfaces. The other type of hypersonic weapon, which has yet to reach service for any nation, is the hypersonic cruise missile, powered by a supersonic-combustion ramjet engine, or scramjet—the same experimental propulsion technology believed to be tapped for the SR-72.

A hypersonic cruise missile flies similarly to an aircraft or suicide drone, but it doesn’t work well at low speeds because its engine’s compression system relies on fast-moving air entering the engine. “Air-breathing, scramjet systems typically require a rocket booster to get started,” Combs says. And currently, all hypersonic weapons share a common downside: cost. A recent Pentagon assessment revealed that America’s current crop of hypersonic missiles in development might cost between $89.6 million and $106 million each. That’s more than an F-35A fighter jet—expensive to the point of being prohibitive. However, in August 2020, the Air Force created a new program to its growing field of hypersonic efforts (70 projects in total, as of 2021). This one, known as Mayhem, aims to field a “multi-mission cruiser” powered by an “air-breathing hypersonic” propulsion system—a technical way of describing a ramjet or scramjet. In terms of cost alone, a hypersonic multi-mission cruiser could be a significant game changer. In terms of capability, it could shift the world’s balance of hypersonic power back to America.

According to an Air Force contract notice released in December 2021, Mayhem aims to field a “hypersonic multi-mission ISR [intelligence, surveillance, and reconnaissance] and strike” platform that can carry at least three kinds of payloads. Two of them are types of weapons, listed in Air Force documents as an “area effect” and a “large unitary payload,” while the third would be dedicated to the SR-71’s old specialty, reconnaissance. Based on those documents, it’s likely that the weapon being developed under Project Mayhem is not a hypersonic missile, but a hypersonic drone with the same operational parameters as the rumored SR-72. Further, it appears that the military is developing a new type of engine to propel the SR-72. As NASA has learned through years of experimental testing, a traditional scramjet engine wouldn’t meet the needs of Project Mayhem’s multi-mission purpose, as scramjets function only at high speeds. Instead, Lockheed Martin appears to be developing a combined-cycle scramjet that incorporates a traditional jet engine into its design. An engine of that type would be a massive leap forward from what’s available now, and require sophisticated engineering to solve the many design challenges.

It’s likely what’s holding the project back. Conventional jet engines use a centrifugal compressor, or spinning fan blades, to compress the air flowing in before mixing it with fuel and detonating it for propulsion. Scramjets forgo a compressor in favor of using the immense pressure of high-speed airflow into the engine. As a result, scramjets sustain higher velocities than jet engines, but at lower speeds the inflowing pressure of air isn’t enough for the scramjet to function well. That means a scramjet-powered platform can’t fly slow enough to land, and as such, can be used only once. But a combined-cycle scramjet that incorporates a traditional jet engine could fly just like an aircraft, and feasibly see repeated use. “This type of engine could revolutionize air travel and defense,” Combs says. Rather than destroying an expensive scramjet by using it to power a single-use missile, Mayhem may use a combined-cycle scramjet to power a drone. This system would use the traditional jet engine to take off and accelerate to around Mach 3 before transitioning to a scramjet that would launch it beyond Mach 5.

Once in enemy airspace, Mayhem could deploy inexpensive conventional bombs and missiles at targets, or conduct reconnaissance, before flying home to rearm and do it all again, just like Brian Shul’s SR-71 over Libya. It’s enticing in theory, but confounding and even paradoxical from an engineering standpoint. The compression system of a turbine impedes airflow by design, but a scramjet needs unobstructed airflow to operate. A successful engine would have to merge two seemingly contradictory designs. “It’s hard to build hypersonic cruise aircraft that can take off from a runway,” says Skyler Shuford, chief operating officer of Hermeus, a start-up developing a reusable ramjet-powered aircraft known as Quarterhorse. (In ramjet engines, combustion takes place at subsonic speeds.) “What’s optimal for takeoff is very different from what’s optimal to break the sound barrier, and both are very different from what’s optimal at hypersonic speeds. Many of these [factors] are extremely sensitive and hard to predict on the ground.” The challenges inherent to developing even normal scramjets are immense, particularly when it comes to igniting an air-fuel mixture as it passes through an engine at faster than the speed of sound. (Combs calls this challenge “flame holding.”) To date, no nation has managed to field a scramjet propulsion system in a missile, let alone an aircraft, though the U.S. has run some encouraging tests.

During the fall of 2021, Northrop Grumman conducted a successful flight of a missile-sized scramjet as a part of DARPA’s Hypersonic Air-breathing Weapon Concept (HAWC) program, and in March 2022, Lockheed Martin followed suit as a part of the same effort. The technology powering Mayhem is an extension of scramjet systems currently found in HAWC, but the scramjets used for HAWC offer only half of the combined-cycle recipe needed for a platform like Mayhem, or the SR-72. In order for Mayhem to fly like an aircraft, the Air Force still needs to develop a way to incorporate a traditional jet engine into the mix without blocking airflow into the scramjet or making the aircraft too heavy to fly. The hypersonic arms race runs parallel to stealth aircraft technology, but it nonetheless affects how militaries develop and utilize that tech. America is the obvious global leader in this area. It boasts two stealth fighters in service, the F-22 and F-35, and the world’s only operational stealth bomber, the B-2 Spirit. The U.S. has two next-generation stealth fighters in development, plus a successor to the B-2.

But other nations are beginning to field effective counter-systems. Stealth technology is limited by physics when utilized on a fighter jet. In order to offer the aerobatics necessary for the fighter class, these jets need vertical tails, moving control surfaces like flaps and ailerons, and open mouths to feed the jet turbines inside the fuselage. These elements are prone to producing easily readable radar returns. Stealth jets aren’t actually invisible to radar; they just reflect minimal energy. In fact, many can be easily spotted using lower-frequency radar bands. Savvy detection systems partner these low-frequency arrays with other instruments and surface-to-air missiles (in 1999, this combination shot down a stealth F-117 Nighthawk with a Soviet-era SA-2) or multistatic radar arrays that can read returns from multiple areas, not just from where the signal was broadcast. The added sophistication has troubled the Air Force; its experts have assessed that beyond 2030, even the F-22 will no longer be survivable in contested airspace.

That leaves an opening, and an urgent need, for Project Mayhem’s speed-first approach. Multi-cycle scramjet propulsion would make an aircraft as impossible to stop as a hypersonic missile. The drone aircraft produced by Mayhem could fly into enemy territory, engage a target or gather intelligence, and fly out again without being shot down. And as Air Force Secretary Frank Kendall has explained about other drone programs in development, not having a pilot onboard can allow the aircraft to take greater risks. That would represent a significant cost saving over $100 million hypersonic missiles. Most of America’s arsenal of air-launched missiles and bombs range in cost from tens of thousands up to a few million dollars each. Rather than these weapons being seen as outdated, they could remain just as effective if delivered to targets at hypersonic speeds from inside the bomb bay of an aircraft. A combined-cycle scramjet aircraft such as Lockheed Martin’s SR-72 could allow the U.S. to leapfrog the high-Mach capabilities being fielded by Russia and China, and it could carry many of America’s existing munitions with it into the hypersonic age. Alas, putting different payloads on a hypersonic plane requires more finesse than simply lashing them to a wing the way one might tie down camping gear atop a minivan. “Going Mach 5 and faster generates extreme levels of heat, driving the need for innovative materials, sensors, and electronics to withstand such speeds throughout its journey,” says Dave Berganini, vice president of hypersonic and strike systems at Lockheed Martin Missiles and Fire Control. “These systems must be able to maintain consistent communication connections to perform precise maneuverability techniques and overcome a range of defense systems and extreme environments.” In other words, you need to keep the bags on top of the minivan while driving at 3,800 miles per hour, steering around potholes, and dodging oncoming traffic at the same time.

While official information is scant, some clues are emerging that the U.S. Air Force and its Mayhem partners may be further along than many initially thought—or even believed possible. In January 2018, about a month before Putin kicked off the hypersonic missile race, Jack O’Banion, Lockheed Martin’s vice president of strategy and customer requirements for advanced development programs, discussed the SR-72 on the record at a conference as if it already existed. He seemed to suggest the aircraft’s scramjet propulsion systems had already been tested, and when an artist’s rendering of the SR-72 appeared on the screen behind him, O’Banion told the crowd that the aircraft could only have been built thanks to the “digital transformation.” “[The engine] would have melted down into slag if we had tried to produce it five years ago,” O’Banion said. “But now we can digitally print that engine with a sophisticated cooling system integral into the material of the engine itself, and have that engine survive for multiple firings for routine operation.”

Later, O’Banion told the press that the “aircraft is also agile at hypersonic speeds, with reliable engine starts.” Although Lockheed Martin removed any mention of the SR-72 from its website just a few months later, the U.S. has since tipped its hand to imply that Project Mayhem might have some intersection with the long-rumored plane, or at least a hypersonic jet in general. In December 2021, the Air Force Research Laboratory in charge of developing new warfighting technologies released an updated contracting document for Mayhem that removed key terms like “expendable” and “demonstrator” from its description. It also stated that the Mayhem vehicle will need to execute “multiple missions” with an air-breathing hypersonic system. The relationship between the SR-72’s development and the Project Mayhem aircraft is unclear: The projects could be borrowing from one another, or not related at all. It’s possible that when Lockheed Martin wiped the SR-72 from its public-facing list of projects, it was giving up on the effort.

It’s also possible that the high-stakes context of a hypersonic-capable Russia and China have pushed the SR-72 to the secret depths of the Air Force’s vault of classified programs. Lockheed Martin won’t say, but on June 2, the contractor released a statement about the fictional, combined-cycle, scramjet-powered aircraft named Darkstar that is featured in this summer’s aviation blockbuster, Top Gun: Maverick. Not only did Lockheed Martin’s Skunk Works help design and build a full-scale mock-up of the hypersonic aircraft for filming, but it also hinted in a statement that the plane’s performance might go beyond movie magic: “Darkstar’s capabilities could be more than mere fiction. They could be reality.” Whether Mayhem ultimately produces something like the SR-72 or another hypersonic platform, it seems clear that the program’s multi-cycle scramjet technology is a matter of “when,” not “if.” But as America’s stealth advantage narrows and its potential near-peer opponents gain technological strength, the question of “when” is becoming more important than ever.

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