Scramjet-Powered Cruise Missile Emerges As New U.S. Priority

Fielding an operational scramjet-powered cruise missile has emerged as a new priority for the U.S. Defense Department’s proliferating portfolio of maneuvering hypersonic weapons.     

Senior defense officials are putting together a program to develop an operational follow-on to DARPA’s Hypersonic Air-breathing Weapon Concept (HAWC), which currently supports competing scramjet-powered missile demonstrators designed by Lockheed Martin/Aerojet Rocketdyne and Raytheon/Northrop Grumman Innovation Systems teams. 

• Pentagon officials seek hypersonic air-breathing weapon follow-on
• Awareness of boost-glide challenges sinks in

“We are in the process of trying to figure out what [an operational program] would look like,” says Mike White, assistant director for hypersonics in the office of the under secretary of defense for research and engineering. 

As the U.S. military rushed after 2017 to respond to Russian and Chinese hypersonic advances, air-breathing hypersonic cruise missiles fell to the bottom of the priority list. Funding for operational programs favored boost-glide technology over the seemingly less mature field of weapons powered by scramjets (supersonic combustion ramjets). 

But that assumption is being challenged. Along with the flight-test experience accumulated a decade ago by the Air Force Research Laboratory’s (AFRL) X-51 scramjet vehicle, recent ground tests and simulations indicate scramjet technology is more advanced than previously understood. In September, the AFRL announced it had achieved thrust levels over 13,000 lb. with a Northrop-designed engine at speeds “above Mach 4” in a hypersonic wind tunnel. In June, Raytheon reported the maturity of its scramjet-powered HAWC demonstrator had exceeded that of its boost-glide design.

In December 2018, Michael Griffin, under secretary of defense for research and engineering, described hypersonic cruise missiles as “further out” than boost-glide weapons. But the technology advanced so quickly that another official, Air Force acquisition chief Will Roper, concluded seven months later the HAWC program would be “a nearer-term not a far-term capability.”

“We’d like to see HAWC transition to a fully operational system,” says Mark Lewis, the Defense Department’s director of research and engineering for modernization. “It’s probably the issue that our hypersonic team is spending most time on right now.”

Awareness is also growing for the technical challenges still facing medium-range boost-glide missiles in the class of DARPA’s Tactical Boost Glide (TBG) missile demonstrators. The Air Force’s 2017 decision to launch the AGM-183A Air-launched Rapid Response Weapon (ARRW), an operational follow-on to the TBG, helped legitimize the Defense Department’s revived interest in hypersonic weapons, White says. 

“I think people underestimate the importance of this decision of the Air Force [to launch ARRW] in the hypersonic community,” he says. “We’ve always been kind of stuck in the [research and development] realm. The Air Force in 2017, they were the first service that said: ‘Hey, we want hypersonic weapons.’”

But the TBG-derived ARRW represents a particularly difficult technical challenge. The design uses a higher lift-over-drag ratio wing shape, which has never been successfully tested by the U.S. government. By contrast, the axisymmetric shape of the lower lift-over-drag glider developed for the Common Hypersonic Glide Body (C-HGB)—the front-end designed for the Air Force Hypersonic Conventional Strike Weapon, the Army’s Long-Range Hypersonic Weapon (LRHW) and the Navy’s Intermediate-Range Conventional Prompt Strike (IRCPS)—has logged several successful flight tests since the late 1970s. The winged TBG’s greater maneuverability, albeit with shorter range, makes it far more challenging to design. 

“It’s DARPA-hard, and TBG is hard,” Lewis says.

Ongoing studies by the Air Force’s Warfighting Integration Capability are also starting to highlight the operational benefits of cruise missiles compared to medium-range boost-glide systems. A cruise missile still requires a booster rocket to accelerate to hypersonic speed, but it does not need to carry as much oxidizer and fuel as a boost-glide rocket because it remains within the atmosphere. Air-breathing cruise missiles’ smaller size means a single aircraft, such as a Boeing B-52, can carry them in much greater numbers.  

“For a hypersonic boost-glide vehicle you can get two, maybe four, on a B-52,” White says. “But you can get 15 or maybe 20 hypersonic cruise missiles [on a B-52] because the size is much smaller.So you can carry them internally in the rotary rack. There are significant advantages for the air breathers, but they offer different technical challenges.”

The smaller size and increased packaging advantages of air breathers would give the Air Force significant tactical advantage, Lewis adds. “The No. 1 question we should be asking is: ‘How do we deliver lots of these things?’ In my mind, one way to do that is to fit a lot of them in a weapons bay. Getting 15-20 per bomb bay is a lot, but if I’m [launching them from] a single mobile launcher, I’m not sure I can deliver the numbers I need. We are not interested in capability when we build two and declare it a success—that doesn’t do anything.”

The Pentagon’s hypersonic weapons portfolio emerged in a blur of bureaucratic activity between 2017 and 2018. The first step was the Air Force’s decision to launch the medium-range ARRW program in 2017 as the follow-on to TBG. Shortly afterward, the Air Force also decided to launch the longer-range HCSW. In November 2017, the Navy conducted a successful test of the proposed C-HGB, which prompted the Navy and the Army to support funding toward the operational prototypes of the IRCPS and LRHW—for submarine and ground launch, respectively.

As it stands now, the portfolio includes air-launched medium-range and long-range boost-glide systems, an intermediate-range submarine-launched missile and a long-range weapon launched from a tractor trailer. If an operational follow-on of the HAWC is approved, with Air Force and Navy concepts under consideration, new air- and surface-launched options for medium-range targets could become available. 

In addition to the offensive programs, the Defense Department’s road map also includes development of a counter-hypersonic system—starting with the Missile Defense Agency’s Regional Glide-Phase Weapon System as well as multiple programs for booster development and continued funding of basic science and technology. Additional DARPA programs include the ground-launched Operational Fires, which seeks to integrate a TBG front-end on a two-stage booster stack that includes a throttled upper stage, and the Advanced Full-Range Engine, a dual-mode ramjet that could power a future hypersonic aircraft. 

Such a diverse yet overlapping  road map has prompted criticism. In July, the chairman of the House appropriations subcommittee on defense, Rep. Peter Visclosky (D-Ind.), warned defense officials that they “need to better define the strategy for the investment in these systems.” Visclosky’s committee proposed cutting some funding for the Army’s hypersonic program, but a joint conference committee of Congressional appropriators ultimately restored the funding and added more for other hypersonic programs. 

Lewis believes the development of a multitude of hypersonic missile programs is justified.

“Too many people think hypersonics is just one thing,” Lewis says. “They think, for example, [it’s just for the long-range, conventional prompt strike mission]. But no, it’s a range of capabilities.

“Even at the tactical level it’s, for lack of a better phrase, a high-low mix,” Lewis adds. “We should probably have a mix of air breathers and boost-glide systems. They probably have different capabilities, different ranges and so on. We have F-16s and F-15s, and they have different roles, and that should be the same with tactical hypersonic systems as well.”