Vacuum Tube Transportation
Low-pressure air train systems show promise as futuristic transportation. Initial calculations predict increased speeds and efficiency. Although the system is potentially limited by maximum distance, with some development it is a viable candidate for intercity and interstate commuting.
The Commute of the Future
A businesswoman steps out of her San Francisco apartment and hurries to her new commuter vehicle. She sits down, plugs her earbuds in, and closes her eyes. Less than two hours later she arrives at her final destination - Los Angeles [1]. This is the commuter of the future, and she doesn’t make her trip by plane or on a high-speed rail. For her commute, she sits floating on a cushion of air traveling at twice the speed of sound. Imagine train-sized mail-tubes sending a mass of commuters smoothly and speedily across whole states. Vacuum train transport is advantageous because the passenger capsules are significantly lighter and more efficient than current transit systems, with reduced maintenance and a centrally powered system, which reduces energy consumption.
Vacuum trains have seen resurgence in popularity because of innovator Elon Musk, who is already breaking new ground in transportation with Tesla Motors, and his recently released design proposal, ‘Hyperloop Alpha’, which endorses vacuum train technology. The concepts behind these vacuum-powered trains have been floating around since the mid-1800s, however, and were first put into practical use by the Samuda brothers and Samuel Clegg in the 1840s [2]. This early design has served as the basis for multiple air trains over the last two centuries, including the most successful current system, Brazil’s Aeromovel [3]. These first designs, however, do not place the passenger compartment inside a vacuum tube. Instead they had a smaller channel running beneath the passenger compartment where the air pushed against a flat plate. The bottom-channel design used by Aeromovel is illustrated in a graphic from Aeromovel’s patent documentation in Fig. 1 below. The flat panels the pressure forces act on (17) can be seen in the pressure channel, and the compressors stations are shown beneath the track (23 - 28). A notable difference between Aeromovel and most other designs is continued use of wheels to reduce friction (19). These designs, while efficient on a small scale, are limited in range because of their low top speeds [3].
Figure 1: Illustration of Aeromovel Propulsion System [4]
Why Vacuum is Better
More recently, vacuum trains have seen resurgence in popularity because of support from inventors like Elon Musk with the Hyperloop, or Daryl Oster from ET3. These new designs predict speeds and efficiencies that would dwarf even large jetliners like the 747 [1]. Musk is the groundbreaking innovator behind Paypal, SpaceX, and Tesla Motors. He endorses air cushion trains because of their numerous advantages over current mass transit systems (i.e. electric rail, maglev, diesel, airliner) and recently improved manufacturing techniques, which make large-scale production feasible.
The efficiency of an air cushion train lies mainly in the low energy cost required to accelerate capsules to speeds of 760 mph (200 mph faster than a Boeing 747 [5]). This advantage can be attributed to two main design features. The first is that enclosing the train in a partial vacuum reduces the drag on the capsule. Imagine a standard high speed train traveling at standard atmospheric pressure: no amount of aerodynamic design can eliminate the drag effect of the 2 square meters of air the train is pushing against every second. However, in a low pressure environment the train still has to push through the same volume of air, but at a significantly lower density. The difference between atmospheric pressure and partial vacuum to a high-speed vehicle is easier to visualize if you consider the difference you would feel between swimming in molasses and swimming in air. This phenomenon is also why jetliners typically cruise in the low-pressure air of the upper atmosphere.
The second advantage vacuum trains have is the cushion of air they float on. It may not seem intuitive, but floating on a cushion of air offers significantly less resistance to movement, even when compared the most efficient wheel. Think of an air hockey table, and it becomes obvious – the pucks bounce around the table much longer than a marble hit with the same force would. In a similar way air-cushion trains experience much less resistance to movement than their wheeled counterparts. Air drag and wheel friction are the two major forces inhibiting movement in high-speed trains. Vacuum trains conveniently avoid both of those. The result is an incredibly efficient transportation system, with minimal drag, which can be accelerated to supersonic speeds and only requires minimal additional energy to maintain those speeds.
Other attributes that contribute to the efficiency of vacuum train systems are derived from the advantages of having an externally powered capsule. With the exception of minor onboard air and electrical circuits, or possibly the compressor generating the air cushion, all of the major elements driving vacuum trains can be stationary. Both the linear induction motor (an electromagnetic acceleration device) and the pumps used to generate a partial vacuum environment in the track area are stationary [5]. This allows both of these elements to be powered from the local energy grid, rather than by less efficient and heavy on-board batteries or fossil fuels. The Aeromovel system has already demonstrated the energy saving effects of using stationary pump stations [3], albeit for a different purpose, and the principles of the linear induction motor have already been implemented effectively in more common magnetic levitation (maglev) trains, like the Transrapid systems in both China and Germany [6].
Vacuum trains also save costs because of the relatively minor mechanical wear they suffer per unit distance. In a standard vehicle there are factors like rail wear, wheel wear, and mechanical stresses that slowly break apart key components, all of which reduce efficiency and drive up maintenance costs [2]. Vacuum train capsules are much lighter because of their centrally powered systems [3], and are isolated from stress by the air cushion they rest on [1]. The reduced weight and dampened vibrations protect the capsule from the stresses and wear associated with most current transit systems. The result is cost saving on repairs as well as train service time. In Joseph Samuda’s early paper on the efficiency of vacuum trains, he predicts that the costs of maintaining a vacuum system are roughly 40% of the costs of maintaining an equivalent rail system [2]. In both Musk and Oster’s more recent calculations, the predicted cost is even smaller, at only 20% the cost per passenger per mile [5]. Even if these predictions are unrealistically optimistic, there is still a huge gap between the running costs associated with vacuum trains and the costs of rail and air transport.
The Catch
Of course, there are hurdles to overcome with high-speed vacuum trains. One issue that has limited Aeromovel and similar systems is the difficulty in maintaining a pressure difference over long distances. Any tunnel or tube designed to span long distances at a realistic cost is victim to leaks and gaps, which would cripple the system. While this design flaw hasn’t been addressed by ET3, the Hyperloop Alpha proposal accounts for a loss of vacuum and suggests that the system is still feasible in a sustainable low-pressure environment. Sandeep Sovani, director of Ansys software’s land transportation strategies, has independently verified the feasibility of the Hyperloop Alpha design [7]. In spite of this assertion it is hard to quantify the rate at which the pressure difference will degrade without experimental data, and so the ability to sustain low pressure along inter-city lengths is still the main criticism of vacuum trains.
There is also a stigma associated with using high power electromagnets in transportation. This is mainly due to legitimate concerns with electronic interference (magnets can wipe a hard drive or permanently distort display screens), but also because of the novelty of maglev systems in an industry where reliability and comfort are demanded. Although relatively new, many maglev designs are already implemented and used reliably without any harmful impact on passengers or their electronics. Maglev trains are also consistently considered one of the most comfortable transit systems available [6].
Conclusions
Although there is still unresolved debate about the efficacy of maintaining a vacuum tunnel over the scales proposed by Musk and Oster, there is no doubt that the efficiency of the system demands further research into vacuum train construction. The reduced drag and wear allow for speeds not yet seen in mass transit schemes, and greatly reduced travel times to go with it. The energy efficiency of using stationary power stations precludes a greener transportation network and lower overall cost. Whoever can prove their system and gain the support necessary to bring their design off the drawing board and into reality has a good chance of changing the future of transportation and would surely earn the hearts of millions of daily commuters.
References
[1] D Oster. (2013). ET3 [web publication]. Available:
[2] J D Samuda, A Treatise on the Adaptation of Atmospheric Pressure to the Purposes of Locomotion on Railways: With Two Plates, 1st ed. London, UK, 1841.
[3]Aeromovel Inc. (2002). Aeromovel [web page]. Available:
[4]Slot sealing system for a pneumatic transportation system guideway, C A Campani, O Hans, W Coester. (1997, Nov 13). US 5845582 A [web publication]. Available:
[5]E Musk. (2013, August). Hyperloop Alpha. Space X, California. [Print article]. Available:
[6]J Kluehspies. (1998). The International Maglevboard e.V. [Web Page]. Available at:
[7]A Vance (2013, Sept 19). Simulation suggests Musk’s Hyperloop ‘quite viable’. Sf Gate [Web Publication] Available: