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The Ekranoplan (Wiki) is a type of Ground-Effect Vehicle. It is essentially an aircraft, which flies just meters off of the surface of the ground, using a cushion of air to provide additional lift. These vehicles are amphibious, but can only travel over a flat surface. The are predominately used over water and ice, although they could travel over flat land as well. Ekranoplans are capable of aircraft speeds, but, due to the “free” lift provided by the ground effect, they are capable of carrying about 50% more weight while burning 40 – 50% less fuel. I believe these vehicles are sorely under utilised in a world which is hampered by rapidly dwindling natural resources.

Ekranoplans are in a position to compete directly with not only commercial aviation, but also passenger ferries and commercial shipping as well, due to their larger cargo capacity. Ekranoplans can also be much bigger than regular aircraft (The first Ekranoplan, the Soviet “Caspian sea monster” was 92 meters long, with a wingspan of 37.6 meters, and a max takeoff weight of 544,000 kgs!) and could therefore be used to carry vehicles or heavy cargo, or additional passengers.

To my knowledge, no ekranoplan has ever been built to break the sound barrier, but this should be absolutely possible. I think there could be a real niche here for a high-speed relatively low-cost passenger transport, similiar to Concorde. The main reasons Concorde was eventually retired were it’s high cost, and low passenger carrying capacity. A supersonic ekranoplan could significantly alleviate both of these issues.
Research and Development of a supersonic ekranoplan would likely be much cheaper than for a supersonic transport (SST). There are several mechanical and engineering challenges which would be either easier to solve, or absent entirely, with a supersonic ekranoplan.

For example, from the wikipedia article on Concorde:

“At the time it was known that the drag at supersonic speeds was strongly related to the span of the wing.^{[N 1]} This led to the use of very short-span, very thin rectangular wings like those seen on the control surfaces of many missiles, or in aircraft like the Lockheed F-104 Starfighter or the Avro 730 that the team studied. The team outlined a baseline configuration that looked like an enlarged Avro 730, or more interestingly, almost exactly like the Lockheed CL-400 “Suntan” proposal.

This same short span produced very little lift at low speed, which resulted in extremely long takeoff runs and frighteningly high landing speeds.^[8] In an SST design, this would have required enormous engine power to lift off from existing runways, and to provide the fuel needed, “some horribly large aeroplanes” resulted.^[9] Based on this, the group considered the concept of an SST unfeasible, and instead suggested continued low-level studies into supersonic aerodynamics.^[9]”

The project that later became Concorde addressed these issues by using a new type of wing planform, (The “slender delta”) however, an ekranoplan would likley have fewer difficulties in this area. Due to the ground effect, ekranoplans require a much lower wing span to generate usable lift, therefore the drag at any speed would also be far less than for a conventional aircraft. Additionally, ekranoplans don’t Takeoff or land like conventional aircraft, and their “runway” is open ocean.

Another issue with Concorde is it’s higher cruising altitude. This can potentially increase radiation exposure to passengers during periods of higher than normal solar activity, or result in serious injuries or fatalities in the event of a loss of cabin pressure. Obviously, neither of these issues would exist for a craft travelling just above sea level.