DARPA To Demonstrate Flying, C-17-Size Liberty Lifter Amphibian
New DARPA projects come about for various reasons, but sometimes they start with an impossible question. Five years ago, Alexander Walan, a program manager for the agency’s Tactical Technology Office, started asking if it was possible to design an ocean-crossing cargo vessel with a 100-kt. cruise speed.
“The short answer was: not really,” Walan tells Aviation Week.
- Liberty Lifter concept unveiled by DARPA
- Daylong Pacific crossings sought with 80-ton loads
By pursuing that line of inquiry for several years, however, Walan settled on what became the newly unveiled Liberty Lifter concept. Starting with a $31 million budget request for fiscal 2023, DARPA hopes to demonstrate a C-17-size, seagoing strategic airlifter by 2028.
In fact, DARPA’s proposed design calls for a hybrid configuration. The Liberty Lifter would be a wing-in-ground-effect aircraft that skims on a ground-effect air cushion over the tops of waves at up to 200 kt., but would also be a flying boat capable of ascending up to 10,000 ft.
If the Liberty Lifter concept works in five years, the U.S. military would have the option to field a new type of cargo transport and amphibious assault ship. The seaborne airlifter would cross the Pacific Ocean in about a day—not 2-3 weeks, as required for a seagoing freighter. The DARPA concept also could land and take off from bodies of water rather than the easily targeted runways and dirt strips used by Boeing C-17s.
That unique operational profile is the entire point for a military reorienting itself to strategic competition in the Indo-Pacific area, but it also creates an organizational dilemma. As the project gets started, DARPA officials are not yet clear on whether the operator for the Liberty Lifter should be the Air Force’s aircraft-focused Air Mobility Command, the Navy’s ship-oriented Military Sealift Command, or both.
“We’re probably doing something right if the services like it but can’t figure out where it fits,” Walan says. “That means it’s new and different. So that’s an opportunity and a risk.”
The concept evokes the Cold War-era “ekranoplan” fleet, which included the Soviet Union’s A-90 Orlyonok transports and Lun-class anti-ship missile launchers, but the Liberty Lifter comes with a major difference. Due to a design limited to ground-effect flight only, the Soviet vessels were limited to flying over calm seas.
DARPA’s concept calls for operating in ground effect over waves up to 18 ft. high, covering about 85-90% of maritime conditions, Walan says. Moreover, if the military decided to move a Liberty Lifter from the Atlantic Ocean to the Gulf of Mexico, it would be better to fly overland rather than around the tip of Florida, he adds. So DARPA added a requirement for up-and-away flight to 10,000 ft.
Bidders for a Phase 1 study could propose alterations, but DARPA came up with a design for a Stratolaunch Roc-style twin-fuselage arrangement, with canards in front of a long wing powered by 10 turboshaft engines positioned on the trailing edge. Most ekranoplan designs position engines in front of the wing to stimulate the ground-effect airflow, but DARPA rejected this approach. Mounting the engines on the wing’s trailing edge protects the blades and turbines from corrosive saltwater spray, and the Liberty Lifter’s wings can generate enough of an air cushion for ground-effect flying anyway, Walan says.
As the concept blurs the line between aircraft and boat, so does DARPA’s concept for the structure. The Liberty Lifter should be built from medium-grade aluminum, splitting the difference between lighter aerospace alloys and cheaper, heavier metal forms used in ships. A risk reduction project by DARPA manufactured a center section wing using 3D-printed aluminum panels, which were joined using friction stir welding, Walan says.
The proposed amphibious aircraft will need an advanced control system, including fly-by-wire and wave-monitoring sensors. Any given sea state involves more than simply the height of the waves, including the distance between the waves and the interaction between them. “If I know what the waves look like—not only the crests, but also wavelength, etc.—I can optimize how high I want to fly,” Walan says.
The DARPA project comes amid a revival of interest from startup companies in wing-in-ground-effect vehicles. Although Flying Ships and Regent have held discussions with DARPA, both companies say they are not participating. For his part, Walan expects industry teams composed of aviation and maritime companies.
“We kind of gave a heads-up last year that this might be coming,” Walan says, “to let people start working those relationships.”