Based on publicly available information, it’s become increasingly clear that there are at least two (and likely three) hypersonic aircraft in active development within the secretive confines of American R&D facilities. If (or maybe more appropriately, when) such a platform enters service, it promises to upend the hypersonic arms race that America has been consistently characterized as losing to China and, to a lesser extent, Russia.
But what may be most surprising to some, is that America isn’t new to the idea of flying reusable hypersonic aircraft. In fact, the U.S. had a crewed hypersonic bomber in active development before the Soviets even put Sputnik into orbit.
America started developing hypersonic aircraft in the 1950s
Just prior to the launch of Sputnik in October of 1957, Boeing (aided by German engineers that arrived in the United States thanks to Operation Paperclip), began development on a hypersonic bomber that would come to be called the X-20 Dyna-Soar. The aircraft would launch atop a rocket before separating from the booster and using its lifting-body shape to bounce along the earth’s atmosphere, covering immense distances at speeds in excess of Mach 18 (though measuring exo-atmospheric speeds in Mach numbers isn’t necessarily appropriate).
At this same time, North American Aviation’s X-15, a manned, rocket-powered hypersonic aircraft, was also just beginning testing, with its first unpowered flight taking place in 1959.
In 1960, the Air Force set about choosing pilots for its new space bomber that was beginning to take shape, and among the first crop of selections was a 30-year-old Navy test pilot and aeronautical engineer named Neil Armstrong. Armstrong was already in high demand, going on to pilot the X-15 in April of 1962 before leaving the X-20 program altogether in favor of even higher speeds and altitudes with the still rather new NASA. He was assigned command of the Gemini 8 mission set to launch four years later, and three years after that he’d add another small feather to his cap by being the first human to set foot on the moon.
Once built, the X-20 Dyna-Soar’s first mock-up measured 35 and a half feet long with a 20.4-foot wingspan. As forward-leaning as the program may have seemed, it was considered to be entirely feasible with the technology of the day — though at a significant cost. And it was that cost, rather than technological limitations, that would ultimately lead to the Dyna-Soar’s extinction.
“If we had pursued it as a black-world program like the U-2, it might have gone ahead,” said Dr. Richard Hallion, former Air Force chief historian. “I never saw any technical issue that would have been a show stopper.”
The high-profile program was ultimately shelved on December 10, 1963, in favor of diverting more funding toward Armstrong’s new home in NASA’s Gemini program.
Modern hypersonic missiles are similarly hampered by cost
While the X-20 program may pre-date modern hypersonic weapons by about half a century, it shares a number of things with its modern counterparts. In fact, the X-20’s operation, i.e. being carried aloft by a rocket before descending unpowered back toward the earth at hypersonic speeds, could be seen as a combination of the boost-glide weapons Russia and China have in service as well as the fractional orbital bombardment system China tested in 2021.
And much like the X-20, the most significant limitation these modern hypersonic systems face isn’t any of the massive engineering hurdles inherent to high-Mach flight, but rather the huge costs associated with overcoming them. The United States recently assessed that its hypersonic missiles in development might cost as much as $106 million each. That means a single Mach 5+ missile — a weapon that can only be used one time — could ring in at millions of dollars more than a brand-new F-35A.
That kind of cost drastically limits the potential uses for these systems and explains why the hypersonics fielded by both Russia and China to date fall into the category of deterrent weapons. Deterrent weapons, which include systems like nuclear ICBMs, are primarily used as a looming threat. The real value of deterrent systems isn’t in their use, so much as in the threat of their use.
Russia’s nuclear Avangard missile, for example, is meant to assure that country can reach American soil with nuclear warheads regardless of America’s air defense capabilities. China’s DF-ZF is meant specifically to threaten America’s multi-billion-dollar aircraft carriers in the Pacific. Using either weapon would invariably mean large-scale war, and as such, they’re meant to serve as each respective country’s big stick during diplomatic negotiation.
In short, the huge costs associated with single-use hypersonics simply make them impractical for the vast majority of operations. But a reusable hypersonic platform that can deliver lower-cost munitions at a similar rate of speed, on the other hand, could dramatically shift that cost value proposition.
Today, there are two publicly disclosed hypersonic aircraft programs in active development that have received DoD funding, as well as a third that hasn’t been confirmed but may actually be the most mature of the bunch.
Hypersonic Aircraft Program #1: The AFRL’s Mayhem program
The first we’ll discuss is the Air Force Research Laboratory’s Mayhem program. This effort managed to fly below the public’s radar for months, tucked away amid a laundry list of more than 70 hypersonic programs currently drawing funds from Pentagon coffers. But unlike the vast majority of these efforts, which are aimed at fielding new high-speed missiles of various sorts, the requirements for Mayhem suggest that it’s after something far more valuable than a single-use munition.
Mayhem’s focus is on developing the holy grail of hypersonic flight: a combined-cycle turbofan-scramjet propulsion system capable of propelling “larger payloads for longer distances than existing systems,” and all while taking off and landing like any other airplane.
The Air Force is calling for Mayhem to fly strike operations (delivering ordnance) as well as intelligence, surveillance, and reconnaissance missions with the aim of completing testing by October 15, 2028. In December of 2022, the Air Force awarded Virginia-based Leidos $334 million for Mayhem’s continued development.
“This program is focused on delivering a larger class air-breathing hypersonic system capable of executing multiple missions with a standardized payload interface, providing a significant technological advancement and future capability,” reads the Air Force’s contracting announcement.
Scramjets, or supersonic ramjets, aren’t new technology — they’ve been under testing for decades, but to date, no country has ever managed to field an operational scramjet in a missile or aircraft. Air moving through a scramjet flows at supersonic speeds, which makes ignition very difficult — it’s been likened by some to keeping a match lit in a hurricane.
“Scramjets in particular are still an immature technology. It’s really hard to get them to work right, and there are some fundamental problems like unstart, fuel mixing, and flame holding that we’re still figuring out,” Dee Howard endowed professor of hypersonic and aerospace engineering at the University of Texas at San Antonio Dr. Chris Combs, told Sandboxx News.
But that’s not to say that nobody’s managed to make scramjets work. Over the years, the U.S. has had repeated successes with scramjet technology demonstrators, including NASA’s X-43A, which reached Mach 9.64 in 2004, and Boeing’s X-51 Waverider in 2013. More recently, both Raytheon and Lockheed Martin have had successful scramjet tests under DARPA’s Hypersonic Air-breathing Weapon Concept, or HAWC, missile program, including one record-setting run for Lockheed that dethroned the X-51 for longest sustained scramjet-powered flight.
But fielding an operational scramjet alone isn’t enough to power a reusable hypersonic aircraft. Scramjets don’t function efficiently at lower speeds and can’t function at a stop — so these propulsion systems have to rely on another system to take off and accelerate.
“The problem with ram/scramjets is they need supersonic flow to function,” Combs explained. “Rockets can get you from zero to orbital velocity but relatively speaking they are less efficient because you have to carry oxidizer. So it’s a tough problem.”
Hypersonic cruise missiles leveraging scramjets in development rely on conventional rocket boosters to bring them to sufficient altitude and speed for the scramjet to kick in, but in order to build an aircraft capable of taking off and landing under its own power, Mayhem is marrying a scramjet to turbofan engine like the one you might find in a modern fighter jet instead.
Mayhem would work by taking off and accelerating under turbofan power, likely to speeds in excess of Mach 2. Once flying at sufficient speed for the scramjet to function, airflow would bypass the turbofan and feed directly into the scramjet, accelerating the aircraft beyond Mach 5 and potentially well beyond Mach 10.
Hypersonic Aircraft Program #2: Hermeus’ Darkhorse
Another publicly disclosed program aimed at fielding a reusable hypersonic aircraft for the U.S. military comes from Atlanta-based aviation firm, Hermeus.
Here at Sandboxx News, we’re no strangers to the mind-boggling progress Hermeus has made toward fielding not only the world’s first reusable air-breathing hypersonic aircraft at a fraction of the cost of other high-profile defense efforts. In 2021, Hermeus received a $60 million contract from the U.S. Air Force to continue development on hypersonic propulsion systems, with Defense giant Raytheon following suit in 2022 with an undisclosed sum of money invested into the firm.
You can watch our full interview with Hermeus COO Skyler Shuford here:
Hermeus has kept most of the details about Darkhorse close to the chest, though we can make some assertions with a great deal of certainty. They are taking a slightly different approach to hypersonic flight than Mayhem, leveraging a ramjet rather than a scramjet in its combined-cycle engine. This not only reduces the engineering headaches associated with hypersonic flight, but it also dramatically reduces cost.
“One of the pillars on which we started the company was that the technology, at least at the component/subsystem level, is mature enough to build a hypersonic aircraft today,” Hermeus Chief Product Officer and Founder Mike Smayda told Sandboxx News via e-mail. “The biggest technical challenge is bringing it all together into a system that is efficient enough to accomplish useful missions.”
Ramjets function very similarly to scramjets but use an inner body (sometimes called a defuser) within the jet inlet that forces the inflowing air to slow down to subsonic speeds to make ignition more manageable.
In December, Hermeus announced that it’ll use the Pratt & Whitney F100 turbofan as the base for its combined cycle Chimera engine — the same family of engines leveraged by air-to-air legend, F-15 Eagle.
In 2022, Hermeus successfully demonstrated its Chimera engine’s transition from turbofan to ramjet power in a wind tunnel. That powerplant is destined for service in the firm’s Quarterhorse technology demonstrator set to fly later this year. While Chimera will power Quarterhorse, Hermeus plans to field an even larger, more powerful engine for the military’s Darkhorse.
Using a ramjet means Darkhorse will likely be limited to speeds below Mach 6 or about 4,600 miles per hour. Practically speaking, such speeds would be sufficient to defeat any modern air defense system Darkhorse may come across, and because deploying munitions at hypersonic speeds is a massive technical undertaking in itself, higher hypersonic speeds may limit a platform’s payload options.
To be clear, in our discussions with Hermeus, the company did not overtly indicate plans to carry weapons in its Darkhorse aircraft, and it was careful not to presume the Defense Department’s needs. But Shuford did indicate the possibility of deploying sensor nodes, so carrying payloads of some sort is certainly already in the cards.
Hermeus intends to fully unveil their hypersonic Darkhorse in 2025.
Hypersonic Aircraft Program #3: Lockheed Martin’s SR-72
While both the Air Force’s Mayhem program and Hermeus’ Darkhorse may be secretive, they’re both publicly disclosed efforts. The third aircraft on this list, Lockheed Martin’s SR-72, however, is not. And while discussing such an aircraft may creep toward “what if” territory, there’s good reason to suspect that there may be an operational platform tucked behind the Black Budget curtain.
As we’ve discussed at greater length here, Lockheed Martin was very open about its effort to field a hypersonic successor to their legendary SR-71 Blackbird beginning in 2013, when they launched a (now defunct) website for the platform. As the years pressed on, Lockheed continued to update the SR-72’s website, prompting so much interest in the program that Popular Science even made it a cover story in 2015.
Two years later, in 2017, Aviation Week reported on eyewitness claims of an uncrewed SR-72 technology demonstrator spotted flying near the U.S. Air Force’s Plant 42 in Palmdale, California — notably the same location as the headquarters of Lockheed Martin’s legendary Skunk Works.
Aviation Week reached out to Lockheed Martin’s Orlando Carvalho, executive vice president of aeronautics, at the time.
“Although I can’t go into specifics, let us just say the Skunk Works team in Palmdale, California, is doubling down on our commitment to speed,” he told Aviation Week in 2017.
“Hypersonics is like stealth. It is a disruptive technology and will enable various platforms to operate at two to three times the speed of the Blackbird… Security classification guidance will only allow us to say the speed is greater than Mach 5.”
By early 2018, Lockheed Martin officials were openly talking about the SR-72 at events, discussing it as though it was not only an existing platform… but one they had already flown in testing. Perhaps the most notable of these statements came from Lockheed Martin’s Vice President of Strategy and Customer Requirements in Advanced Development Programs Jack O’Banion, during an American Institute of Aeronautics and Astronautics event in Florida.
“Without the digital transformation, the aircraft you see there could not have been made,” O’Banion told the audience in 2018 while standing in front of an SR-72 render.
“We couldn’t have made the engine itself — it would have melted down into slag if we had tried to produce it five years ago. But now, we can digitally print that engine with an incredibly sophisticated cooling system integral into the material of the engine itself, and have that engine survive for multiple firings for routine operation.”
When later pressed about these statements by Bloomberg, O’Bannion doubled down.
“The aircraft is also agile at hypersonic speeds, with reliable engine starts,” he told Bloomberg.
In 2022, Top Gun: Maverick hit screens with a fictional hypersonic aircraft, called the Darkstar, which was actually built by Lockheed’s Skunk Works. In our interview with the film’s executive producer Jerry Bruckheimer, Sandboxx News broke the story that China had allegedly re-oriented spy satellites to get a better look at the platform, under the apparent assumption that it was a real aircraft. If true, the existence of a real hypersonic drone with similar characteristics could certainly have played a role in China’s interest.
Operationally speaking, the SR-72 is alleged to work much like the Air Force’s Mayhem aircraft — using a combined cycle turbofan scramjet engine. In 2018, Lockheed Martin said on the SR-72 webpage that they were working with Aerojet Rocketdyne on just such a system. (Interestingly enough, Lockheed Martin would go on to attempt to purchase the engine maker for $4.4 billion, though regulatory roadblocks ultimately shot the attempt down.)
But then things changed.
On March 1 of 2018, Russian President Vladimir Putin delivered a speech that has since been considered the onset of the modern hypersonic arms race. In the speech, Putin claimed that Russia already had one hypersonic missile in service with a second following closely behind, and while the veracity of those claims is worthy of debate, it’s clear that the U.S. took his threats seriously.
Immediately after, Lockheed Martin purged their website of any and all mention of the SR-72 program, plunging it back into the darker recesses of the rumor mill just as the U.S. began redoubling its efforts to match and exceed the hypersonic capabilities of its foreign competitors.
So… while the SR-72 may now exist as little more than a rumor, this is one rumor that might actually be able to break Mach 5.
When will the US field an operational hypersonic aircraft?
Hermeus intends to officially unveil Darkhorse in 2025 and the Air Force Research Lab intends to have testing on its Mayhem program completed in 2029. As for Lockheed Martin’s SR-72… it may already be flying, or it may never fly at all.
But regardless of the exact date, the introduction of hypersonic aircraft will be a pivotal moment in the hypersonic arms race, pulling hypersonics out of the realm of exquisite deterrents that are too expensive to use for anything short of World War III and placing Mach 5+ capabilities squarely within America’s conventional operating forces.
Using a hypersonic aircraft to deliver munitions would allow the U.S. to limit pricey development on hypersonic weapons in favor of fielding a high-speed aircraft that can deliver lower-cost, already existing ordnance to targets anywhere in the world with the same urgency. And thanks to the aircraft’s hypersonic speed and ability to maneuver, it could defeat even the latest integrated air defenses in the same way the SR-71 once did: by simply outrunning them.
But delivering ordnance isn’t the only value these platforms could deliver to the U.S. military. They could also serve as a rapid means of intelligence gathering over areas with limited satellite coverage, among other essential high-speed operations.
“When we’re getting into strategic competition, near peer adversaries where we really need to cover long distances quickly, where we need to get eyes on target quickly or reconstitute networks when something like a comm node went down,” Hermeus’ Skyler Shuford told Sandboxx News. “To be able to get there quickly, drop off a new comm node — those are starting to be the more interesting conversations that we’re starting to have.”
Throughout much of the 20th century, the air was ruled by hot rods that could scream across the sky at two or even three times the speed of sound, but the stealth revolution changed all that — prioritizing low observability over brute force and bringing operating speeds down. Now, however, it’s beginning to look like stealth may have to share the spotlight in the 21st century, as the sheer power of speed makes its comeback.
Feature image courtesy of Hermeus
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