The United States government entrusted the H4 Program to the aircraft manufacturer Hughes Aircraft during World War II. The objective of this initiative was to create an oversized seaplane to support the Liberty Ships, which were frequently the target of German U-boats during their transatlantic crossings. Nonetheless, this endeavor concluded in vain.
However, this concept continued to be significant, as the Defense Advanced Research Projects Agency (DARPA), the Pentagon’s innovation division, reportedly derived inspiration from it for its “Liberty Lifter” program eighty years later. This initiative aims to integrate the functionalities of a seaplane and an Ekranoplane to fabricate an aircraft capable of transporting 100 tons of cargo.
The objective is to design “a class of vehicles” capable of overcoming the “key operational limitations of traditional air and sea transport platforms,” DARPA explained in August 2021. Additionally, the missions of these aircraft were intended to go beyond simple freight or troop transport, with the Pentagon agency mentioning involvement in amphibious operations and, more broadly, roles in the Expeditionary Advanced Base Operations (EBAO) concept and Distributed Maritime Operations (DMO).
The Ekranoplan’s principle relies on an aerodynamic phenomenon called the “ground effect.” In simple terms, the lower an aircraft flies, the more lift it generates and the less drag it experiences. During the Cold War, the Soviets built the KM (Korabl Maket). Nicknamed the “Caspian Sea Monster,” this 550-ton, 100-meter-long aircraft could fly at an altitude not exceeding 14 meters and at a speed of 550 km/h. With ten engines, its range was 3000 km.
With such capabilities, this aircraft can fly below radar coverage and thus operate in contested environments. Furthermore, it eliminates the need for transport ships, which are potentially vulnerable to enemy submarines and naval mines. However, it has two drawbacks: it is not very maneuverable and can only be used in calm weather. Hence, DARPA needs to make the Liberty Lifter an aircraft with Ekranoplan and seaplane characteristics.
Frontier India speculates that the massive plane’s ekranoplan design would take advantage of the ground effect for takeoff, using the air cushion between its wings and the water surface at high speeds to lift its immense weight with a relatively short water runway. Once safely airborne, the wings and control surfaces adjust to provide lift for transitioning to conventional airplane mode at higher altitudes. As it approached landing, the Ekranoplan design would return to ground effect mode, allowing for a controlled descent back onto the water’s surface using its hull while minimizing the need for an extended water runway.
In February 2023, two companies, General Atomics and Aurora Flight Sciences (a Boeing subsidiary), were selected for this program.
Both presented radically different projects. General Atomics opted for a twin-fuselage design (similar to the experimental aircraft Messerschmitt Bf 109 Z Zwilling) with distributed propulsion based on twelve turbofans. On the other hand, Aurora Flight Sciences took a more “classic” approach, drawing inspiration from the Boeing Pelican ULTRA, a 152-meter wingspan aircraft intended to carry a payload of 1270 tons, whose development was halted due to budget cuts in 2003.
Ultimately, the “classic” design prevailed. On May 9, the Pentagon announced that Aurora Flight Sciences had been awarded a $8.3 million contract to “extend ongoing work under the Liberty Lifter program.”
Although complete design specifications are unavailable, various media resources provide little information on Aurora’s Liberty Lifter ekranoplan. The plane may not use conventional blended wing-body ekranoplans and instead use a high-wing monohull arrangement. This could help handle water surface and load modular freight. The Liberty Lifter’s design to operate in moderate to rough seas up to 13-foot wave heights suggests a more robust hull than earlier ekranoplans restricted to calmer waters. Additionally anticipated is conventional flight capability up to 10,000 feet, suggesting wings designed for both ground effect and higher altitude. The eight-turbine engines are geared toward producing a lot of power for large payloads in various flight regimes.
Among the cutting-edge strategies, Aurora might consider include hull designs with features for rough water stability, sophisticated control systems to handle changes between flying modes, and movable wing arrangements to maximize lift. Depending on ground effect vs. conventional flying requirements, a movable wing could alter its angle or profile. In particular, sophisticated controls would be needed to manage the intricate changeover stages in heavy seas. Design features could be included in the monohull to improve stability and reduce wave effects in rough water operations.
However, compared to its initial presentation, Aurora Flight Science’s concept has evolved, “transitioning from a T-tail to a cruciform tail,” which is “structurally more efficient for installing a ramp at the rear.” Additionally, the floats have been “moved from the side longerons to the wingtips” to balance performance “in terms of ground effect.”
The preliminary design review is expected to be completed by 2025, and the goal is to fly the prototype by the end of 2027.
