WASHINGTON — In 1959, the U.S. Air Force and Navy partnered with NASA to fly a piloted hypersonic test aircraft, the X-15, for the first time.
During that flight, the high-speed vehicle designed to travel at speeds of at least Mach 5 was dropped from under the wing of a B-52 bomber flying over the Mojave Desert in Southern California. Pilot Scott Crossfield carried the aircraft to an altitude of 52,341 feet and reached a peak speed of Mach 2.11.
The flight kicked off a robust testing effort, and over the next nine years, three X-15 aircraft flew 199 times. The program eventually surpassed performance targets and achieved what is still the nation’s the fastest piloted hypersonic flight at Mach 6.72, or 4,520 miles per hour. Though it ended in 1968, discoveries from the program continue to inform the government’s hypersonic vehicle research.
In a 1964 report detailing the program’s accomplishments, NASA researcher Wendell Stillwell wrote: “As long as Earth’s atmosphere exists, wherever men fly that fast, they will be traveling in a region whose secrets the X-15 was first to probe.”
More than a half century later, the X-15 remains the Defense Department’s most rigorous hypersonic testing endeavor — a testament to the program’s success, but also a result of periods of restrained investment in high-speed vehicle research. Whereas in the 1960s the X-15 flew an average of one airborne test every 18 days, today the department only supports about a dozen hypersonic flight tests in a good year.
Despite early technological breakthroughs, the U.S. has viewed operational hypersonic technology as a future capability and debated its role in the military arsenal. In the meantime, China and Russia have made progress in recent years developing and fielding hypersonic systems.
Those advances are driving a sense of urgency within DoD to field hypersonic weapons of its own and boost funding for enabling technology, including advanced materials and propulsion systems. The Pentagon expects to spend $15 billion on hypersonic programs between 2015 and 2024 across 70 different efforts, according to estimates from the Government Accountability Office. Its fiscal 2023 budget called for $4.7 billion for hypersonic weapon development and $225.5 million for hypersonic defense.
As part of the strategy, Principal Director for Hypersonics Mike White wants to bring back that former testing rigor. The more programs test, he told C4ISRNET, the more they learn and the faster they’ll deliver.
“We really want to open the aperture to allow the national team to learn at the pace of discovery and not at a pace limited by the availability of flight-test range windows,” he said in an interview.
Last year, White gave the defense testing community a challenge: Increase the hypersonic test cadence to one flight per week. The target sounds ambitious, but that’s the point, he said. While the shift won’t happen overnight, the hope is that setting a stretch goal will push the enterprise to change the way it operates.
“The goal of one test per week was established to drive us toward a paradigm shift with respect to how and where we test, versus trying to just do more of what we’re doing,” White said. “To get there will take some time, but we are already well on our way.”
Focused on flight test
Over the last year, DoD has made some pointed moves toward improving its hypersonic testing capabilities and moving toward a more regular flight-test cadence.
Those initiatives include a Defense Innovation Unit program called Hypersonic and High-Cadence Airborne Testing Capabilities, which is leveraging commercial technology to develop a test aircraft that could fly in the next two years.
The Test Resource Management Center — a Pentagon-level office that supports test events and infrastructure across the department — also partnered with the Navy to develop the Multi-Service Hypersonics Test Bed, which will make it easier to demonstrate and validate hypersonic technology. And TRMC made progress expanding its SkyRange program, which is converting 24 decommissioned Global Hawk drones to support hypersonic testing.
Along with these new programs, the department is working to expand its access to hypersonic test ranges. According to the Congressional Research Service, DoD has 11 open-air ranges that can support test flights, and White said DoD is partnering with industry and international allies to increase that number.
The department has also upped its investment in its notoriously under-resourced test infrastructure. In a report to Congress this year, the Pentagon identified a $5.7 billion funding gap for lab and test infrastructure, including some $817 million in unfunded hypersonic military construction projects.
While TRMC’s annual funding request doesn’t clearly identify hypersonic testing budgets for security reasons, director George Rumford told C4ISRNET the organization is prioritizing funding for that work.
To illustrate this, he pointed to a section of TRMC’s budget that identifies testing resources for strategic technologies like artificial intelligence, cyber and hypersonics. In fiscal 2018, the department’s five-year forecast predicted TRMC would need just $470 million to support strategic technology testing in fiscal 2022. TRMC’s actual fiscal 2022 funding came in around $1.5 billion, more than three times what it forecast. That includes funding for ground testing infrastructure such as wind tunnels and labs to support hypersonic development.
That growth wasn’t accidental, Rumford said, but was part of a concerted effort to boost testing support for key technology development areas.
“Within the last five years, the department has made a strategic plan to triple the amount of investment in test infrastructure to accelerate the national defense strategy,” he said. “Hypersonics is the big player in that.”
According to White, these focused initiatives and investments are starting to pay off. In July, three U.S. hypersonic programs conducted successful tests: the Defense Advanced Research Projects Agency’s Operational Fires missile and its Hypersonic Air-breathing Weapons Concept and the Air Force’s Air-Launched Rapid Response Weapon, or ARRW. Over a period of about three months, between November and the end of January, the department expects to conduct seven flight tests, he said.
As part of that recent string of tests, ARRW completed its first full prototype, or all-up round, flight test Dec. 9 in Southern California. The test extended a streak of successful flights for the program, which had three failures in a row in 2021.
White said the department has seen performance improvements across several of its hypersonic programs in the last year, which he attributes to greater systems engineering rigor from the companies developing those weapons.
“We have experienced a dramatic turnaround in our flight-test results,” he said.
Making the right investments
Former DoD officials and industry analysts say that while the department’s testing ambitions may be within reach, it needs to invest in the right areas.
Mark Lewis, director of the National Defense Industrial Association’s Emerging Technology Institute and an expert in hypersonic capability development, told C4ISRNET he’s optimistic DoD can reach its testing targets — and its history on the X-15 is evidence of that.
“We can, because we have in the past,” he said. “But if you’re going to test at that pace, you need to do it differently than the way we’ve been doing it so far in many of our hypersonics programs.”
Lewis has served in a number of roles within the Pentagon’s research and development and science and technology community, including director of defense research and engineering for modernization and Air Force chief scientist – positions that included oversight of hypersonic development efforts. He said he’s encouraged by TRMC’s investments, but thinks the department needs to put a greater emphasis on recoverability and creating more realistic testing opportunities.
On recoverability, Lewis said that while retrieving a full system after test would be ideal, even collecting parts of the vehicle has advantages for data analysis.
“I might want to test new high-temperature materials. If I can put them on the outside of the vehicle [and] if I can get that vehicle back, I can take them off. I can look at them. I can see how they survive,” he said. “Otherwise, I’m just relying on sensors and instrumentation.”
Realistic flight testing — or as Lewis calls it, “wind tunnels in the sky” — should also be a priority for the department. While ground-based wind tunnels that replicate the in-flight environment are an important part of a program’s test plan, they have limitations.
“Wind tunnels are always making some compromise,” he said. “The very fact that you’re in a tunnel and your model is sitting in a chamber and that chamber has walls — that’s different than a vehicle that’s flying out in the atmosphere.”
Lewis and White both noted that developing a more robust modeling and simulation infrastructure will lessen risk, but that it’s not a replacement for flight testing.
“The more you can test on the ground, the more you reduce risk for flight testing, but there is no facility that can fully simulate hypersonic flight,” White said. “So, we must fly.”
Roman Schweizer, an analyst at defense research firm the Cowen Group, said that while a “build and test and fail quickly” model is important if the department wants to move fast, frequency also comes with a cost — and there needs to be a push to reduce that even as the testing cadence picks up.
“You need to keep cost as a function of that,” he told C4ISRNET. “Whenever anyone has introduced new technology, but particularly the military, there’s just a lot of trial and error that goes into it. And if the trial and error are super expensive, you’re not going to be able to do a lot of it.”