Liberty Lifter concept

Concept art of DARPA’s future Liberty Lifter vehicle. (Courtesy DARPA)

WASHINGTON: The Pentagon’s premiere research agency is moving forward with an effort to build an unusual kind of plane that, if successful, will rival the lift of the Air Force’s storied Globemaster, cost half the price and not be constrained by a traditional runway. That’s if the Defense Advanced Projects Research Agency can get right what the Soviet Union got wrong.

Dubbed the “Liberty Lifter,” DARPA says the seaplane will use a trick of physics to serve as “a long-range, low-cost X-plane capable of seaborne strategic and tactical lift.”

Alexander Walan, a DARPA official overseeing the program, told Breaking Defense the agency decided to move forward with the Liberty Lifter program after receiving enthusiastic feedback from industry in response to a request for information published last summer.

While DARPA has an early, dual-hulled concept in mind (as seen in the image above), Walan said the agency expected to receive proposals from industry this week in response to a solicitation for conceptual and preliminary design work, with two contracts up for grabs and scheduled to be awarded later this summer.

DARPA hopes to build and fly a prototype vehicle within five years.

“The biggest thing I took away [from industry’s responses] is that US industry was ready to do this,” he said in a May 24 interview. “They had some innovative ideas. They were excited about it. And there was an industrial base out there that we could tap into.”

Rivaling The C-17

The plane itself will likely be a similar in size to a C-17 Globemaster, weighing somewhere between 500,000 to 600,000 pounds. Meanwhile, DARPA wants the price tag to be half that of the Air Force plane, which costs roughly $340 million each. The Liberty Lifter should be capable of carrying two Marine Corps Amphibious Combat Vehicles, which weigh roughly 35 tons each, or six 20-foot conex storage containers.

The lower price tag will come from a focus on affordability. Walan described it as more comparable to building a B-17 Flying Fortress than a C-17 Globemaster. “Less touch labor, less exquisite materials, lower manufacturing costs,” he said.

C-17

The price tag and capabilities of the Air Force’s C-17 Globemaster are heavily influencing the goals for DARPA’s Liberty Lifter program. (U.S. Air National Guard photo by Master Sgt. Matt Hecht)

Critical to designing this plane is a concept known as the “wing-in-ground” (WIG) effect. The premise of the effect is that when an aircraft is flying at low altitudes, the air being forced underneath the wings presses upward from against the ground giving the aircraft extra lift.

“If you’ve ever been on an airliner or watched a 737 land, sometimes they come in close, and then they almost kind of like go to a hover,” Walan said. Large birds passing over bodies of water while searching for food also serve as an example of the wing-in-ground effect.

During those moments of low-altitude updraft, the aircraft experiences less drag and can more easily maintain what little altitude it has left. “For very long-range operations, a few percentage points [of increased lift and reduced drag] actually start really adding up” in terms of fuel efficiency, he said.

But while it might be hard to maintain low altitude over varying terrain, there is one area where the “ground” is more or less consistent: at sea. A key element of the Liberty Lifter will be “runway independence,” the capability to take off, land and maneuver in shallow waters. Walan said DARPA’s initial vehicle designs had drafts between six and 12 feet. The draft is important because DARPA wants the plane to be capable of entering ports and being as close as 100 meters from the beach.

The result is a cargo plane that matches the C-17’s lift capabilities for a fraction of the price to build and fly, an appealing offer for the US military which for several years now has struggled with maintaining enough airlift and sealift capacity — that is if DARPA can make it work.

But if the wing-in-ground effect is so potent, why isn’t the US military already producing planes that utilize it? There are some serious challenges standing in the way.

Studying The Soviets And How They Got This Wrong

WIG Lun-class ekranoplan

An artist’s concept of a Soviet wing-in-ground effect vehicle. (Archive.gov)

To get any benefit from the WIG effect, Walan said a plane’s altitude needs to be less than that of its wingspan. In other words, if the plane’s wingspan is 100 feet, then the WIG effect demands the aircraft be less than 100 feet off the water.

“The challenge is you’re low, and you’re going fast,” he said. Rough waves, small islands and other ocean-going vessels are all potential hazards to a future Liberty Lifter. Walan said DARPA feels confident they can overcome the obstacle avoidance problems through advanced sensor technology.

Another issue is how rough waters could result in a choppier ride for the plane. Larger waves passing by the aircraft will interrupt the natural air flow underneath the plane’s wings, resulting in turbulence. On a traditional aircraft, turbulence is an inconvenience, but heavy turbulence for a WIG vehicle means losing fuel efficiency — defeating the whole purpose of trying to utilize the ground effect.

But Walan argued modern flight controls could help to keep the plane steady even while near rough water. The Liberty Lifter will be designed to fly in sea state 5, a universal measurement of the ocean’s conditions that work on a scale from 1 to 9. Sea state 5 indicates the presence of waves as high as 13 feet.

The physics, of course, isn’t new, and there have been scattered attempts by both American companies, such as Boeing’s Pelican concept, and other nations to design vehicles that effectively utilize the WIG effect. But Walan said DARPA is mostly studying the Soviet Union’s doomed efforts.

“The Soviets really showed how not to do this, like a lot of things, right?” he said.

The Soviet Union made two key errors when trying to build WIG vehicles, Walan argued. The first problem was their aircraft, one of which was called the Caspian Sea Monster, had short wings.

Because the WIG effect is partly dependent on wingspan, the Caspian Sea Monster had to fly as close as five to 10 feet off the water to see any meaningful lift or drag benefits. The result was that even mildly rough seas threaten to damage or disrupt it while in flight.

The second problem is the Soviets focused explicitly on designing aircraft that utilized the ground effect, resulting in a vehicle that could not ascend to the higher altitudes where traditional planes cruise. That greatly limited where the Caspian Sea Monster could go.

Walan emphasized the Liberty Lifter will utilize the WIG effect, but the ground effect is just a means to an end. If the water becomes too crowded or too rough, DARPA’s aircraft will be able to climb to safer, higher altitudes.

“Fundamentally, we think you have to be able to operate out of ground effect,” he said. “We’re not trying to shoehorn in to this one narrow concept.”

As for which branch might transition the Liberty Lifter into a program of record, it’s up in the air. The program is still relatively nascent, and Walan said a formal transition agreement won’t be discussed until later in the program. But for now, he added, the Liberty Lifter has attracted the interest of all the military services.



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