Transport at low altitude

The Liberty Lifter concept aims to combine the advantages of ground effect vehicles and flying boats into a high-performance system. Ground effect vehicles are aircraft that fly exclusively just above the sea surface, utilising the increased lift and reduced drag created by the interaction of wings and water surface. Flying boats, on the other hand, have the same flight behaviour as conventional aircraft, but use a hydrodynamically shaped hull or floating pontoons for water landings. The Liberty Lifter could switch between these two flight modes according to mission requirements.

The project, which was initiated in mid-2022, is being developed by the US Defense Advanced Research Projects Agency (Darpa) as a cost-effective heavy-duty transport platform with a long range. The performance requirements specified by Darpa include a payload capacity of at least 90 tonnes, an operational range of 4,000 nautical miles with a payload of 45 tonnes and a range of 6,500 nautical miles. By way of comparison, the payload capacity of a Boeing C-17 Globemaster III is 77.5 tonnes with a range of 2,400 nautical miles without refuelling in the air. The transport capacity is to comprise at least two armoured combat vehicles or six standard containers. The payload is to be unloaded using the roll-on/roll-off method via rear or nose ramps.

In ground effect mode, the Liberty Lifter will fly at a height of up to 30 metres above the sea surface and reach a flight speed of 200 knots. In normal flight, altitudes of around 3000 metres above sea level are to be achieved. The aircraft should still be able to take off and land in sea state 4. Ground effect flight and operation at sea should be possible up to sea state 5. Operation on the water should be possible for at least four weeks before the aircraft has to return to the harbour for maintenance.

The manufacturing techniques are to be based on the more cost-effective processes used in the shipbuilding industry compared to aircraft construction. Lightweight but durable materials will be used to reduce the weight of the aircraft and increase performance and efficiency. Darpa has not yet provided any information on potential procurement quantities, but care is to be taken during the design planning stage to ensure that materials are available on a larger scale.

The basic concept is aimed at a new combination of speed, range, payload and flexibility that does not currently exist in this form. Payload or passengers could be picked up both in harbour and at sea. This would allow transport and logistics ships to remain at a safe distance from enemy defences on the high seas, while the payload reaches the task forces much faster. Ground effect flight would allow the transporter to fly under the enemy's radar belt so that soldiers and payloads could be dropped undetected close to the front line. This would reduce the risks associated with flight operations compared to the use of conventional transport aircraft. Not only supply flights are to be carried out. The Liberty Lifter is also intended to bring combat troops to a beach to support amphibious assault landings.

The aircraft could be deployed in the Western Pacific in the event of a war against China. The aircraft could transport heavy equipment, fuel, equipment and ammunition, including air defence and anti-ship missiles, as well as combat troops between the individual islands of the East and South China Seas in a short space of time. The new deployment concept of the US Marine Corps, which envisages a rapid and constantly changing occupation of the islands in this region, would take on a completely new level of mobility in line with the Pentagon's expectations. For this reason alone, the versatile ground effect aircraft is considered a strategically significant concept.

In principle, however, the aircraft would be suitable for supporting mobile warfare in various geographical zones. Interestingly, an animated film produced by Darpa shows a ground effect aircraft flying over a sea with ice floes. In addition to military use, applications for civilian and humanitarian purposes are also conceivable, for example in the field of disaster relief or to supply remote research stations.

On 1 February, Darpa awarded contracts to two competing consortia of companies to design independent concepts for the new aircraft. The teams are made up of companies with expertise in the fields of marine technology, naval architecture and hydrodynamics.

A consortium is being led by General Atomics Aeronautical Systems (GA-ASI). The most important partner is the maritime engineering firm Maritime Applied Physics Corporation. The proposed basic concept envisages two parallel, equally sized aircraft fuselages connected by a central wing. This design emphasises stability on and just above the water. Twelve turboshaft engines are planned for propulsion.

The second team is led by Aurora Flight Sciences, a subsidiary of the Boeing Group. Support comes from the ship design companies Gibbs & Cox and ReconCraft. The rather conventional-looking concept envisages a single hydrodynamically shaped fuselage and is similar to the C-17 Globemaster built by Boeing, although Aurora describes the concept as "half boat, half aeroplane". Eight turbine engines are mounted on the wings of the high-wing aircraft. The outwardly angled "gull-shaped" wings serve to stabilise the aircraft during descent and increase lift during ground effect flight.

The first project phase initiated with the award of the contract has an estimated budget of 55 million dollars. This 18-month programme phase will be used to develop the aircraft design and define the feasible capability profile. The first six months are for concept development. Nine months of detailed design work will follow, followed by a three-month transition phase during which remaining issues will be resolved. The final drafts are expected by mid-2024. In the subsequent second phase between mid-2024 and the end of 2025, the detailed design of one of the two concepts will be continued and preparations will be made for the construction of a prototype. Individual critical on-board systems will be designed and tested in advance. During the third phase, which will begin in early 2026, the prototype will finally be finalised and tested on the ground and in the air. Flight testing is scheduled to begin at the end of 2027. The transition to a development and procurement programme for a production aircraft is then planned for mid-2028. Darpa assumes that international partners will also be involved in the development and test programme as well as the expected series production of the aircraft.

Sidney E. Dean