Up next in commercialization: hypersonic testing
By Keith Button and Paul Brinkmann|November 2023
Stratolaunch, owner of the world’s largest aircraft, plans to start flying a reusable hypersonic testbed from this plane in a bid to remake itself as a provider of services to the U.S. Defense Department and others. After a brush with insolvency and four years of development, the first test flight is at hand. Keith Button and Paul Brinkmann tell us about the technology.
Under a blue sky and the treeless expanse of the Southern California high desert, a massive carrier plane sits on a tarmac. Even without the bright sun glinting off its white exterior and dual fuselages, the aircraft easily draws the eye due to its wingspan of 117 meters. On this September day, various panels in the plane’s airframe were open for ground checks, but most of the surrounding engineers and technicians were focused on something else: a smaller vehicle attached to the center pylon. It’s the first in a series of test vehicles that Stratolaunch LLC is counting on to prove the technical side of its new business model of selling hypersonic flight-testing services.
Four years ago, the approach of such a test flight appeared unlikely. Originally conceived as a launcher of small satellites, Stratolaunch was left rudderless by the death of founder Paul Allen of Microsoft fame in 2018, six months before the carrier plane flew for the first time. This aircraft was built at nearby Scaled Composites to carry up to three satellite launch vehicles between those fuselages — hence its name, Roc, a mythical bird in Arabian fairy tales that carried an elephant in its talons. Releasing the rockets and their payloads in the stratosphere meant that a smaller, less expensive rocket could do the job of getting them to orbit, and this was supposed to give Stratolaunch an edge in the competition for launch contracts.
Instead, by mid-2019 the company was on the brink of shutting down, which would have relegated its giant, twin-fuselage aircraft to a curiosity of history as the largest aircraft by wingspan ever to fly. At this time, Stratolaunch started pondering a shift toward hypersonic testing services, a decision that was finalized months later when the company was acquired by billionaire Steve Feinberg’s private equity firm, Cerberus Capital Management, well known for resurrecting distressed companies. Under Cerberus, one of the first steps Stratolaunch took was to hire Aaron Cassebeer, a longtime engineer at Scaled Composites. Cassebeer was tasked with building a team of engineers, now about 75, and a reusable, rocket-powered flying testbed that would make those testing services possible: Talon-A.
While Stratolaunch isn’t saying exactly when, plans call for releasing the first test vehicle, TA-1, from Roc over the Pacific Ocean before year-end to prove that the design can achieve hypersonic speeds. Another prototype, TA-2, is scheduled to be launched in early 2024 to prove that the design will be able to autonomously navigate and land at Vandenberg Air Force Base on three wheels.
It took an accelerated development program that drew lessons from hypersonic craft throughout history to get to this test period on schedule.
Cassebeer, who at Scaled Composites specialized in rapidly designing prototype planes, was given one year to design and start building the Talon-A prototypes. There were at least two interrelated reasons for the big rush. One was demand for hypersonic testing by the U.S. government and its contractors; the second was China’s and Russia’s purported hypersonic weapon capabilities, which was driving U.S. demand. In 2018, Michael Griffin, then under secretary of defense for research engineering, testified to the U.S. Senate Armed Services Committee that China was conducting 20 times as many hypersonic flight tests vehicle tests as the U.S. and had fielded or was close to fielding hypersonic missiles that could strike U.S. aircraft carriers thousands of kilometers away. The U.S. had no hypersonic weapons that could counteract them.
“It is among my very highest priorities to erase that disadvantage,” he told the committee.
Stratolaunch believes a small fleet of Talon-As can help the U.S. catch up.
“Our country is able to fly on the order of a handful of hypersonic flights a year successfully, and where the government would like to go is: They would like to be flying a flight every week; every seven days they’d like to have a hypersonic flight occurring in this country,” Cassebeer says. “The last vehicle that flew hypersonically often with a major flight test campaign — we’re talking hundreds of flights — was the X-15” in the 1950s and 1960s.
Reusing Talon-As could also reduce the costs of individual flights. A single hypersonic test flight by the U.S. government can cost between $60 million and $100 million, says Zachary Krevor, Stratolaunch CEO, while Stratolaunch expects to offer test flights for “single-digit millions” of dollars. The company also anticipates demand from companies developing civilian hypersonic aircraft for crew and cargo transportation.
“We can go Mach 3 to Mach 6, so we can fly a lot of different and operationally relevant trajectories and flight paths,” Krevor says. “Flying a variety of different Mach numbers and dynamic pressures is what the hypersonic community cares about.”
Among the first orders of business was to figure out what materials to build their aircraft from and how to protect it from heat. Cassebeer dreamed of becoming a Formula One race-car engineer before choosing a career in prototype aircraft design. He also honed an ability to assess risk in his aerospace decision-making through his pastime of technical rock climbing, which has included ascents of El Capitan and Half Dome in Yosemite National Park, California. As with other engineering decisions for Talon-A, the top priority in making the material decisions was the schedule — designing and building the aircraft quickly to get customers quickly.
“While they may prefer that we can fly faster and for a longer durations, they will take flying in the first place,” Cassebeer says. Cost and technical performance were priorities Nos. 2 and 3.
Thermal protection materials for Talon-A would need to hold up for as many as 20 flights without melting at hypersonic velocities, meaning greater than Mach 5 and faster than any bullet. Even for a short flight, the friction of an aircraft moving through the air at hypersonic speeds can generate temperatures of 500 to 1,100 degrees Celsius, hot enough to eliminate common metals in aviation from consideration.
“If you were to put a stainless steel material or a steel material into that environment, it’s going to be glowing orange and yellow. If you were to put an aluminum material in that flight environment, it’s going to melt; it won’t even exist,” Cassebeer says. “It really puts you into a situation as an engineer where you have to go to higher- and higher-performing materials.”
They considered thermal materials flown in hypersonic programs dating to the late 1950s. “Ideally, we wanted to choose materials that had already been developed in the past. We never wanted this vehicle to be the experiment itself,” Cassebeer says.
Three X-15s, the piloted rocket-powered aircraft that were flown 199 times by the U.S. Air Force and NASA from 1959 to 1968, were covered with a skin of Inconel, a nickel-chrome superalloy. NASA’s two unpiloted, expendable X-43As, the first hypersonic aircraft powered by airbreathing engines, each flew once in 2004 and were protected by a covering of composite carbon-carbon ceramic tiles; they dropped into the Pacific Ocean after their flights and were not recovered. The Air Force’s X-51As, also unpiloted with air-breathing engines, collected nine minutes of data during hypersonic flights from 2010 to 2013 while employing a silica-based material for thermal protection and ceramic tiles similar to those on NASA’s space shuttle orbiters. Flight controllers destroyed one X-51A after it stopped transmitting during a test flight; another was lost because of faulty control fin; the other two flew into the Pacific Ocean.
For Stratolaunch, the best option proved to be the thermal protection materials from the shuttle program, along with decades of engineering, manufacturing, and research and development data that NASA shared with the company via a Space Act Agreement. By early 2020, Cassebeer had chosen the “latest and greatest” of the thermal protection material installed on the shuttles over the years: the black ceramic tiles that were visible on the underbellies of orbiters coming in for landing; and a blanket material with a white surface coating, Felt Reusable Surface Insulation, which is the white covering on the shuttles.
For the Talon-A airframe — the internal structure — Cassebeer had to choose a material that could hold up for repeated flights under the aerodynamic loads of hypersonic speeds, provide a mounting structure for components carried inside the vehicle, and also adhere to the heat-protection materials on the surface. Metals employed in previous hypersonic and general aviation aircraft were considered, such as aluminum, Inconel alloys, stainless steel and steel options, and titanium. But Cassebeer decided early in 2020 to build the airframe from carbon fiber composite — not for its superior strength relative to the metal options, but for speed.
“It was the fastest possible way for our team to rapidly design and manufacture an airframe to go do these flights,” he says. Metal airframes, on the other hand, require longer lead times for tooling and setting up their assembly, plus they’re more difficult to modify once that assembly process has been set. On the plus side, it’s easier to mass produce airframes from metal. “If we had been designing a system that we expected to build 100 or 1,000 of the Talon-A, it’s very possible that we would have chosen a different path,” he says.
The airframe was built in Mojave from layers of resin-impregnated carbon fiber textiles that were hand-laid into forms, bagged and vacuum pumped, and hardened in an oven. Acquiring the composite materials and tools for a single prototype and designing it took weeks, compared to months or years that would have been required to set up production for a metal airframe and then fabricate it.
Talon-A’s carbon fiber composite airframe can be easily modified or repaired — to add reinforcement to a specific part of the structure, for example. Stratolaunch took a “concurrent design-build” approach, Cassebeer says, meaning that engineers started building parts of the aircraft while still making adjustments to its design. By continuously evaluating the airframe design, they made changes as it was being built.
Even with NASA’s voluminous data about the shuttle’s thermal protection materials, Stratolaunch was missing a key piece of information: How would those materials be attached to Talon-A’s carbon composite airframe? On the shuttle orbiters, the thermal protection materials were glued to their aluminum airframes, but Talon A’s airframe would be carbon composite. The bond would need to hold up over dozens of flights, Cassebeer says.
The engineers tested potential adhesives. They glued pieces of ceramic tile to fist-sized samples of the carbon composite airframe material, then set each glued sample in a load frame to pull from both sides of the bond and measure how much force it took to pull it apart. They also repeated the test across a range of temperatures, up to 175 Celsius — the maximum temperature that the carbon composite airframe was expected to be exposed to. If the ceramic tile broke before the adhesive bond did, the bond was a success.
The adhesive that won out was unexpected (a common material that Cassebeer declined to identify for proprietary reasons). Meanwhile, an adhesive that the engineers thought would be their choice “ended up falling on its face” for a different reason: It was strong enough but dried too fast.
“Imagine building a model aircraft or car, and the glue dries two seconds after application, and you have almost no time to put two pieces together,” he says.
Flash forward to present day: The challenging hypersonic test flights are still to come, but Cassebeer and company are getting closer. In late 2022, they flew their first hardware, TA-0, a propulsionless version of Talon-A that was clasped in Roc’s center pylon during a series of captive carry flights. And in May, Roc carried and released TA-0 to fall into the Pacific Ocean. This “separation release test,” as Stratolaunch calls it, was the first test toward the business plan that calls for flying versions of Talon-A repeatedly on a variety of missions for customers. Currently, the company has four Talon-As built or partially built, including one ready for flight. Stratolaunch also plans to modify the aircraft if needed between flights or build a new version with a design change if that’s necessary to accommodate a customer’s in-flight experiment.
Some customers will test electronics carried inside Talon-A — either as payloads or as avionics controlling its flight; others will want their test items on the aircraft’s surface to experience the hypersonic flight conditions there. “Think about materials, instrumentation, windows, optical seekers, things of that nature,” Krevor says. That could require engineers to modify the Talon-A fuselage or nose cone, for example, or make other changes that weren’t considered in the original design.
“We can make modifications to our thermal protection system or even the underlying airframe structure,” Cassebeer says, “and we can integrate new technologies of payload into the vehicle in very interesting ways, which sometimes require some surgery.”
Potential customers have already expressed interest in testing modified versions of Talon-A, says Krevor. Some may want to try alternate nose shapes, or an airflow experiment on the bottom surface of the aircraft, or alternate rudder shapes. Other potential customers, such as the U.S. Missile Defense Agency and Space Development Agency, may want to test their hypersonic missile or space vehicle tracking capabilities with Talon-A.
As for speed and endurance, Stratolaunch expects Talon-A to eventually fly at Mach 6, remaining in hypersonic flight for at least two minutes. A launch could be done every week.
In the nearer term, Stratolaunch has booked customers to fly their tests aboard TA-1 — the first hypersonic flight targeted for later this year. That Talon is to crash into the Pacific Ocean after being released from Roc, and none of the hardware onboard will be recovered.
“That highlights the real demand and need in this country for hypersonic flight testing,” Krevor says, and also, perhaps, hypersonic bargain-hunting: He notes that the first-flight customers will get a “screaming deal.”