Boeing has revealed an updated concept for its Valkyrie reusable, unmanned hypersonic aircraft that indicates new design approaches for inlets and shaping.

Shown in an artist’s concept illustration provided to Air Force Magazine—and in the form of a model at this week’s American Institute of Aeronautics and Astronautics meeting in San Diego—the aircraft shows several design departures and revisions from Boeing’s previous concept, made public in 2018. The previous iteration showed an aircraft shaped much like the 1980s National Aero-Space Plane (NASP) concept, with a pointed nose, deep spine, and “2-D” rectangular intakes co-located under the fuselage. The new version shows more of a “waverider” shape with a blunt nose, flattened top, and rounded, separated intakes running the length of the aircraft to twin booms with nozzles apparently using aerospike technology.

Boeing provided a comment about the design to Air Force Magazine, saying that “over the past several years, we have advanced our vehicle design concepts, developed innovative integration solutions, and matured enabling technologies in the challenging areas such as propulsion, thermal, materials, guidance, navigation and control, and engine/airframe integration.” The company noted that it has “successfully designed, built, and flown hypersonic vehicles for the last 60 years” and that its research and technology units have been “developing hypersonic vehicle concepts and technologies in support of potential future defense applications.”

The previous iteration was revealed in a seeming effort to counter Lockheed Martin’s release of a design concept it dubbed the “SR-72,” intended to be a reusable intelligence, surveillance, and reconnaissance aircraft.

Though Boeing designed and flew the X-51, the first vehicle to sustain air-breathing hypersonic flight, it was not selected in a series of mid-2010s contracts for a variety of potential hypersonic vehicles and missiles. Last year, however, Boeing was funded to develop HyFly2, a Navy program for a hypersonic missile. The Valkyrie seems meant to remind the defense industry that Boeing is very much in the hypersonic game, even though the aircraft is not aimed at any known unclassified program.

Analyzing the Design

Air Force Magazine asked Mark J. Lewis, executive director of the National Defense Industrial Association’s Emerging Technologies Institute, to offer observations about Boeing’s concept. Lewis, a leading hypersonics authority, was director of defense research and engineering under the Trump Administration and had previously been briefed on the company’s hypersonics efforts.

“Boeing has been working on this for a while,” Lewis said. “I think that previous configurations that they were showing at conferences and meetings did not accurately represent the configurations that they’ve been developing.” However, the latest model is probably “a little bit closer to what they’ve been working on.”

Boeing’s aircraft is a combined-cycle aircraft, Lewis said, meaning the vehicle can take off from a runway using a gas turbine engine and, when sufficient supersonic speed has been achieved, transition to hypersonic air-breathing flight using a scramjet engine. Boeing officials have previously suggested that the gas turbine air flow path is closed off, allowing hypersonic airflow to move through the scramjet engine.

“Designing a propulsion flow path that goes from Mach 0 to Mach 5 or Mach 6” is a daunting task, Lewis said, and “the devil is really very much in the details. Boeing, I know, has put a tremendous amount of effort into studying that propulsion flow path. And … I think the most recent model reflects their understanding of the challenge.”

Lewis said Boeing’s design draws on lessons learned from NASA’s X-43 program.

“We did a large number of design iterations,” he said, “and we discovered that the most important driving factor in picking a new configuration was actually not the hypersonic performance, but actually the transonic performance.” Various hypersonic shapes, “once you got into the high mach range, they all did about as well. The real question was how much drag they had when you get to Mach 1, and could you even push through Mach 1 with a hypersonic shape? The Boeing folks really understand this well.”

While the overall waverider shape is consistent with other hypersonic concepts, Lewis said the unusual cranked-arrow wing planform and its odd step-join with the fuselage likely has to do with subsonic or transonic control and performance.

The departure from the “2-D” inlet to an ogival, or rounded “3-D” shape, tracks with what other hypersonics technology efforts are doing, Lewis said.

The 2-D inlets were used on the NASP—which never got to a flying prototype—in part because “they were easier to analyze,” but computational tools for more complex shapes make more efficient designs possible now, 40 years later, he said.

“NASP was a large enough vehicle that it was very well known that you’d have to do parallel engine modules,” he said, referring to the gas turbines to get up to speed and the scramjets for hypersonic travel.

“You couldn’t do one engine module to power the vehicle. And the best way to stack engine modules side by side is to have them be two-dimensional.” Also, NASP had to fly from Mach 0 up to Mach 25, so there had to be a variable-geometry inlet. Also, a rounded shape means less contact between the air and the inlet, and lower weight.

“For a given volume of air moving through the inlet, a round shape has less surface area than a squared-off shape. So that’s an advantage,” Lewis said.

A rounder inlet is also a “more efficient pressure vessel. So if you’re worried about loads inside the inlet leading into the combustor, round shapes have advantages.”

While it was initially thought that rounded inlets had to be optimized for a particular speed, that has since turned out not to be true, Lewis noted. It was also thought that rounded inlets would be prone to engine unstarts, also since proven not to be true, he said.

“The bottom line is, if you look at almost every … hypersonic configuration that we’re looking at today … they’re all rounded inlets. So it’s not surprising” that Boeing’s fresh concept has them. “And in fact, I think it’s safe to say that the squared-off inlets were always kind of a red herring.”

Pluses and Minuses

He called the separation of the inlets an “interesting design choice,” with a “lot of pluses and minuses.” While separating the engines offers “packaging advantages” providing “a lot more internal volume,” separated engines pose a danger of sudden violent yaw torque if one engine unstarts. This was a periodic problem with the triple-sonic SR-71. “The pilots describe that as a fairly harrowing experience,” he observed.

The biggest challenge with a combined-cycle aircraft such as Boeing is working on “is having those engine cycles overlap with each other. The big technical issue you face is … when the gas turbine engine stops working at a mach number below the mach number at which a scramjet can take over, you have to bridge this gap in some way.”

One option is to push the gas turbine to as high a mach number as possible; the other is to reduce the startup speed of the scramjet as low as possible. Or, “You might have what’s called a dual-mode operations, so you’ve got one engine which can be both a ramjet and a scramjet.”

Declining to offer further comment on whether Boeing has solved this conundrum, Lewis said the company “has paid a lot of attention to this.”

A lot of the solution is “not just the propulsion, but it’s the entire flow path. So it’s optimizing design of the inlet, optimizing the design of the nozzle, being able to adapt the inlets and the nozzles to whatever mach number you’re operating in.”

The SR-71, which flew at a high mach number but not at hypersonic speed, accomplished this with an inlet cone that shifted position depending on its speed.

To deal with the high temperatures encountered at hypersonic speed, Lewis speculated that Boeing has resorted to a “hot structure.” He explained that a “cold structure” uses materials that don’t handle high temperatures well but have some kind of thermal protection. The space shuttles, for example, had an aluminum structure protected by silica tiles that absorbed and dispersed heat.

The SR-71, the F-15, and the never-built X-20 all used a “hot structure,” in which the materials themselves are high-temperature tolerant. They are also better for maintenance, an important consideration—“remember what a nightmare those space shuttle tiles were. But I honestly don’t know which approach the Boeing folks have converged on.”

The SR-71’s materials expanded during the high temperatures in flight, meaning that it had to have gaps on the ground and leaked a lot. “But you know, our structural design tools are so much more sophisticated than they were back in the 1950s when the SR-71 was designed.”

The half-conical boom-like exhausts are “very straightforward,” Lewis said. “That’s a classic design for any of our individual hypersonic configurations.” The reason has to do with equalizing the pressure of the exhaust with the ambient air.

Unlike a rocket bell, “you essentially do what’s called an aerospike design,” he explained. “An aerospike design is, you take the nozzle and you turn it inside out, so instead of having a bell-shape, and the flow is expanding inside the bell, you’ve got a spike, and the flow is expanding along the spike.” While at slower speeds, the thrust would come out somewhat sideways. “The nature of a supersonic flow is such that you’ll maintain basically a contiguous stream of air … coming out of the engine, and it will gradually expand.” The stream is contained “because it’s moving so fast … And the beauty of that is if you do it right, it expands to atmospheric pressure, or pretty close to it.”

So, “for any hypersonic configuration, you’re almost certainly going to use an external expansion nozzle of the type the Boeing folks show.”

Lewis declined to offer much comment on the canted verticals, saying they could have something to do with yaw stability or “artistic license.”

The flattish shaping of the vehicle is also good for stealth, Lewis said, and its thermal signature will not be as much of a giveaway as one might think.

It’s “kind of a misnomer that you often hear, that a hypersonic vehicle is going to be glowing super hot. … But if you have designed it correctly, really, only the leading edges are going to be super hot. In fact, the sorts of shapes that lend themselves to efficient hypersonic flight tend to actually have relatively small cross section. So they tend to be these really slender shapes.”

Lewis agreed that the Valkyrie release is likely a reminder that Boeing is still a player in hypersonics.

“Remember, Boeing built the X-51. They know how to build a hypersonic airframe. They were eliminated from the DARPA competition. They are now back in the game,” he said. “The Joint Hypersonics Transition Office is funding Boeing to do a hypersonic air-breathing missile concept called HyFly2. And they are, to put it bluntly, kicking butt in their design. They’re doing a really nice job.”