In 1863, Jules Verne wrote about the idea of suspended transatlantic tunnels through which trains could be sent using air pressure. Pneumatic tube delivery was a fashionable idea then, and small scale pneumatic delivery systems were commonplace until the late 20th century – I remember a few shops using them to transport change around. In 1935, the film ‘The tunnel’ featured another high speed transatlantic tunnel, as did another film in 1972, ‘Tunnel through the deeps’. Futurists have often discussed high speed mass transit systems, often featuring maglev and vacuums (no, Elon Musk didn’t invent the idea, his Hyperloop is justifiably famous for resurfacing and developing this very old idea and is likely to see its final implementation).
Anyway, I have read quite a bit about supercavitation over the last years. First developed in 1960 as a military idea to send torpedoes at high speed, it was successfully implemented in 1972 and has since developed somewhat. Cavitation happens when a surface, such as a propeller blade, moves through water so fast that a cavity is left until the water has a chance to close back in. As it does, the resultant shock wave can damage the propeller surface and cause wear. In supercavitation, the cavity is deliberate, and the system designed so that the cavity encloses the entire projectile. In 2005, the first proposal for people transport emerged, DARPA’s Underwater Express Program, designed to transport small groups of Navy personnel at speeds of up to 100 knots. Around that time, a German supercavitating torpedo was reaching 250mph speeds.
More promising articles suggest that supersonic speeds are achievable under water, with less friction than going via air. Achieving the initial high speed and maintaining currently requires sophisticated propulsion mechanisms, but not for much longer. I believe the propulsion problem can be engineered away by pulling capsules with a strong tether. That would be utterly useless for a torpedo of course, but for a transport system would be absolutely fine.
Transatlantic traffic is quite high, and if a cheaper and more environmentally friendly system than air travel were available, it would undoubtedly increase. My idea is to use a long string of capsules attached to a long graphene cable, pulled in a continuous loop at very high speed. Capsules would be filled at stations, accelerated to speed and attached to the cable for their transaltlantic journey, then detached, decelerated and their passengers or freight unloaded. Graphene cable would be 200 times stronger than steel so making such a cable is feasible.
The big benefit of such a system is that no evacuated tube is needed. The cable and capsules would travel through the water directly. Avoiding the need for an expensive and complex tube containing a vacuum, electromagnetic propulsion system and power supply would greatly reduce cost. All of the pulling force for a cable based system would be applied at the ends.
Graphene cable doesn’t yet exist, but it will one day. I doubt if current supercavitation research is up to the job either, but that’s quite normal for any novel engineering project. Engineers face new problems and solve them every day. By the time the cable is feasible, we will doubtless be more knowledgeable about supercavitation too. So while it’s a bit early to say it will definitely become reality, it is certainly not too early to start thinking about it. Some future Musk might well be able to pull it off.