Tesla Motors CEO and the Undersea Supersonic Train

While most maritime news events involve ships, other maritime news events involve undersea tunnels such as the examples that between Sweden and Denmark, between the UK and France, between Japan’s island provinces. While some tunnels carry road and rail traffic, other tunnels carry only railway traffic. During the mid to late 19^th century, British engineer Isambard Brunel bored a tunnel partway under the Thames River in London. The second attempt resulted in a railway tunnel under the river. While most undersea tunnels were the result of drilling through the seafloor, it is also possible to prefabricate sections of tunnels.

A few weeks ago, the CEO of Tesla Motors announced the idea of a supersonic train being powered by air pressure while traveling inside a tubular tunnel. The history of atmospheric railway propulsion dates back over a century, with some installations using a large waterwheel-drive air pump that provided the propulsive energy for the train to travel over a short distance. All atmospherically powered trains of that era carried passengers over short distances within the confines of a city or town. The modern concept proposes to carry passengers between distant cities, including at supersonic speeds.

While the cost of drilling a tunnel of 300 to 500-miles in length would be prohibitive, there may be scope to build a large-diameter pipeline based on existing and proven technology. The oil and natural gas industry have built major pipelines that measure several hundreds of miles in length. When Hong Kong extended a railway line under a section of ocean, builders lowered preformed tubular sections from barges on to the seafloor. Builders then joined each new section of preformed tunnel section to the existing sections, to form a continuous tunnel with watertight seals.

Sealant applied on the outside surface of the joints prevents seawater from leaking into the undersea tunnels. Sealant applied to the inside surface across the interconnecting joints has the potential to prevent leakage of high-pressure air from inside the tunnel, into the surrounding seawater. A metallic surface applied to the tunnel interior along with a suitable lubricant, could allow for a piston with a flexible circumferential layer to slide along that surface. The combination of low air pressure ahead of a carriage and high air pressure behind the carriage would provide propulsion.

Grid-Scale Compressed Air Storage:

The natural gas industry pioneered compressed gas energy storage in giant underground caverns located several thousand feet below ground surface. They use seismic testing technology to pinpoint salt domes that can measure up to 1-mile in diameter by up to 6-miles in vertical height. The industry partially flushes rock salt from these caverns that can then hold pressure at up to 3,000-psia. More recently, compressed air has been pumped into such caverns, to store energy to generate electric power. The heat-of-compression is either used productively to boil water for interior heating, or pumped into thermal storage. 

Supersonic Air Flow:

It is possible to generate supersonic airflow across a converging-diverging cross sectional area, provided that the upstream pressure is at least double the downstream pressure. Sonic speed will occur in the region of minimum cross section, downstream of which it may accelerate to supersonic speed should the downstream pressure be sufficiently low. If a vacuum fan draws air out of the tunnel at the destination point, the pressure differential ahead and behind if a train inside the tunnel could enable it to travel at supersonic speed. Stored hydraulic or pneumatic energy may drive the vacuum fan.

There are several regions internationally with future plans to develop grid-scale compressed air energy storage (CAES), including Texas. A grid-scale CAES installation operates in Germany, with future plans to expand such technology. Other future plans revolve around possible development of CAES energy storage in the Middle Eastern, with potential to develop such energy storage technology at other locations across Asia. The combination of future CAES development plus prefabricated tunnel technology may see high-speed undersea train travel develop under the shallow seawaters around the Persian Gulf, Baltic Sea, Irish Sea.

Possible Undersea Routes:

Buenos Aires – Montevideo

Rio de Janeiro – Sao Paolo (Santos)

Houston - Tampa

Kuwait – Abu Dhabi

Kuwait – Bahrain

Mumbai – Karachi (during a time of long-term peace)

Singapore – Bangkok

Singapore – Djakarta

Singapore – Hong Kong (long-term option)

Hong Kong – Taiwan

Shanghai – Seoul

Shanghai – Taiwan

Conclusions:

There may be future scope for automated mass production technology to prefabricate sections of tunnels that ships may transport to offshore locations and lowered to prepared sections of seafloor. Transportation officials in China have already discussed the possibility of undersea railway tunnels connecting between major Chinese coastal cities, also across to Taiwan. The massive market for parcel freight and passenger transportation between several East Asian coastal cities, also select Middle Eastern coastal locations, could facilitate the development of long-distance undersea tunnels that carry fast trains driven by air pressure, including to supersonic speeds.

Harry Valentine can be reached at [email protected].