|Fig. 1: France's TGV. (Source: Wikimedia Commons|
Elon Musk, co-founder and CEO of SpaceX and Tesla, first introduced the idea of the Hyperloop in May 2013 at the All Things Digital conference as a faster alternative to high-speed railways, describing it as a cross between the Concorde, a rail gun, and an Air Hockey table.  Reportedly, Musk became interested in high-speed, efficient travel after researching California's high-speed rail project and marveling at how slow and expensive it would be (record-setting in both regards). In the railway's defense, affordable yet high-speed travel poses many challenges because it requires substantial power to overcome air resistance and friction. In addition, any transit system must compete on the grounds of not only speed and cost but also convenience, comfort, weather immunity, environmental impact, and safety.
Currently, the hyperloop is imaginary. This report will provide an overview of equally or less imaginary forms of transportation for mid-range travel (e.g., between cities) and then examine how the hyperloop would perform in comparison, following the proposed solution of Elon Musk, for the Los Angeles to San Francisco route.
Several of the fastest trains in the world are wheeled; they run on conventional tracks, including the first Japanese Shinkansen train, China Railways CRH380A and the TGV Reseau in France.  The Shinkansen was first to enter the mainstream and did much to popularize the concept. All the high-speed railways run on traditional steel rails, are powered by electric current, and have proven over decades to be remarkably safe. These trains are popular; their convenience, price, and comfort are hard to beat, even for highways.  However, these trains' top speeds are practically limited by their wheels; minor imperfections in the wheels and tracks become magnified into intense vibrations and accelerated wear and tear. Even without operating at exorbitant speeds, maintenance costs are substantial. Maglev was created to succeed conventional wheeled trains and overcome these shortcomings.
California's planned high-speed conventional railway would connect Los Angeles and San Francisco via the Central Valley with possible extensions to link San Diego or Sacramento.  Running at 220 mph, it is a far cry from being the fastest train, but the journey would only last 3 hours. Several sources have indicated that the manner of its construction is unnecessarily pricey, especially in contrast to similar European or Japanese projects. 
Magnetically levitated (maglev) trains were first developed in the 1960s.  Maglev trains float a short distance above their guide tracks, suspended and propelled by a magnetic field.  Eliminating mechanical contact means the only friction these trains encounter is wind resistance. This enables achieving higher speeds and acceleration or deceleration more smoothly and quietly with reduced maintenance and ongoing costs (but greatly elevated system cost).
|Fig. 2: SCMaglev at a test track in Yamanashi Prefecture, Japan. (Source: Wikimedia Commons)|
Maglev railway systems and trains achieve levitation in two ways: electromagnetic suspension (EMS) and electrodynamic suspension (EDS). The Chinese train uses EMS; these have C-shaped arms that wrap around a guideway. Electronically controlled electromagnets in the train are attracted to the magnetically conducting (steel) track, which pulls the bottom of the arms up toward the track while pushing the rest of the train above it ; this method has the advantage of stationary levitation but the need for a complicated feedback system to maintain appropriate levitation height.  The Japanese trains use EDS: above 32 km/h, the train's superconducting magnets induce a magnetic field in the U-shaped guideway's sides, which lifts the train from the track.  This system is self-correcting for levitation height but requires wheels for low speeds. Propulsion is provided by the tracks rather than the train, making these trains light, fast, and efficient but the tracks extremely expensive.  For EDS, the propulsion coils sit behind the levitation ones; these track- mounted coils or sheets of aluminum periodically flip polarity which propels the train. 
Currently, despite decades of research, only two maglev systems are in operation. One is the world's fastest train, China's Shanghai Maglev Train, which can achieve 504 km/h but operates at 430 km/h; the other is the Linimo in Japan which was largely built for publicity but continues to serve a small community.  However, despite what these trains can achieve, the systems are often accused of being expensive, technological publicity stunts.  The power required for levitation is minimal compared with that needed to overcome air resistance. Energy losses in maglev systems are due to aerodynamics; aerodynamic drag increases with speed, and power increases as speed cubed. The transit system imagined to combat this problem is the vactrain, discussed in the next section.
Like the hyperloop, vacuum tube trains, or vactrains, are also imaginary and are introduced here because the hyperloop works on related concepts and is often compared to them. The 'vactrain' concept was first described by rocket pioneer Robert Goddard in the early 1900's, and physicist R. M. Salter published "The Very High Speed Transit System" in 1972. [9-11] The trains Salter describes would be magnetically levitated in evacuated tubes that would allow theoretical speeds of thousands of km/h. A vacuum-sealed high-speed transit system theoretically eliminates or reduces the air resistance and friction obstacles of high-speed travel.  A major difference between vactrains and hyperloops is the latter has an air cushion suspension and does not use maglev. Unfortunately, constantly pulling a hard vacuum is power-hungry, hazardous, and difficult and expensive to maintain; besides, maglev is also quite expensive.  Currently, a vactrain system has not been successfully implemented.
Musk unveiled a report on the hyperloop concept in August 2013 in a blog post titled "Hyperloop" with a detailed feasibility study linked in the "Hyperloop Alpha" report. What follows is an overview of the system Musk envisions; full details may be found in the feasibility study. Musk's proposed system would be inexpensive (transport 840 people an hour, each paying $20 a ticket), incredibly fast, environmentally friendly, ready when the customer wished to travel, and a tenth of the cost for California's high-speed railway. 
The hyperloop would theoretically work equally well above, on, or below ground, and has several distinct components: a sealed pod (vehicle), parallel steel tubes, propulsion system, and a route.  The basic concept resembles both a vactrain system and a maglev one: it entails a pair of steel tubes elevated on pylons through which pods carrying 28 passengers each coast at nearly the speed of sound on thin air cushions in air pressure one-thousandth that at sea level to reduce wind resistance and friction.  The pods would be accelerated and decelerated via magnetic linear accelerators and given period boosts using linear induction motors- the same as in maglev trains. Because rolling resistance is eliminated and air resistance is reduced, the pods theoretically should require little energy to sustain high velocity.  A large acceleration upon entry would be required plus periodic reboosts every 110km or so followed by long periods of coasting.  Thus, despite the high speeds, the low acceleration would make it feel like riding a plane or train.
Musk hypothesizes that the passengers would experience a maximum inertial acceleration of only 0.5g, and the pods are restricted to subsonic speeds.  The pods would also have water- cooling systems to help combat the heat generated during air compression. The main anticipated sources of energy loss are aerodynamic and bearing drag plus choked flow. Choked flow occurs where there is a build-up of air mass leading the pod which results in increased drag. Because power increases as speed cubed; its losses are substantial, so it will play a dominant role in design of the route, pod design, tube, propulsion system, and the speed of the transit system. The route would be optimized to avoid sharp bends in order to minimize losses and preserve the comfort of the passengers; the hyperloop would also likely have less visual and noise pollution than the planned high-speed railway. 
This all sounds great, but consideration of the actual implementation and construction raises some flags. Although experts say the idea is technically feasible, proving the technology and implementing it will be both difficult and expensive.  Additionally, economists say that even Musk's $7.5 billion price tag (or $6 billion, for the pedestrian version) is impossible. 
|Fig. 3: Presumed hyperloop capsule: Air compressor in the front, passenger compartment in the middle, battery compartment at the back, air bearing skis on the bottom. (Source: Wikimedia Commons)|
An economist at UC Berkeley predicted the hyperloop project would cost closer to $100 billion.  Historically, large infrastructure projects almost always exceed their budget substantially. There is the cost of construction; materials aside, land acquisition and appeasing the communities the transit system passes through are quite expensive. One of the major factors in the high-speed railway's enormous budget is the necessity to acquire 1,100 different pieces of land, which costs an estimated $7 billion alone for the land between Fresno and Bakersfield, plus the demands of local communities along the route which require the railway to add additional stops, viaducts, or tunnels that are not strictly necessary.  Although Musk believes the hyperloop's minimal profile and elevated tubes along the median of the Interstate 5 may eliminate much of the cost for his 570 km hyperloop (he estimated only $1 billion for acquiring rights of way), there are non-negligible costs associated with implementing any form of land-based transportation nearly as long as the state of California. Although the hyperloop will definitely save on tunnel boring, his estimates have been described as wildly optimistic by many.  There are legal hurdles associated with constructing a hyperloop over someone else's land, and foreseeably, the hyperloop would be no less immune to political pressures than the railway despite its smaller footprint.  None of this even accounts for the cost of developing and implementing a demonstrative prototype, a nontrivial task in itself.
Next, route is inconvenient: it would have two stations, one in Sylmar, Los Angeles and one in Hayward, San Francisco. The anticipated capacity would be 840 passengers/hour, for which there is significant demand. However, termination of the hyperloop route on the fringes of the cities is inflexible and inconvenient; it requires passengers to use a different transportation mode to get to and from the hyperloop stations, significantly lengthening the total travel time and possibly the cost as well. Although trains require this as well, adding stops is more easily implemented.
Third, maintaining even a partial vacuum is nontrivial and expensive. If a leak were to occur, the entire tube would shut down. In addition, the extremely thin air cushion is worrisome. The tolerances provide little factor of safety: best case, the passengers experience uncomfortable bumps; worst case, the outcome is devastating. 
Fourth, the comfort of the passengers matters: people love riding the bullet trains for the scenery, the relaxing smoothness, the efficiency, and the atmosphere. The hyperloop would have a different hype- the concept is outrageous. However, the comfort one could expect is questionable. The compressor fans and air bearings are likely to be extremely noisy, and there will be no scenery at all apart from what's shown on computer screens. In addition, should a passenger have to evacuate their bowels, receive medical attention, or get up from their seat, etc., there is little recourse for action. Although emergencies are unlikely to occur and require resolution within a half-hour ride, they are not impossible. Emergency evacuations would be nearly impossible. Musk mentioned escape hatches, but these would likely create undue leakage. Perhaps the larger version of the hyperloop (intended to carry cars as well as pedestrians) may accommodate these needs, but the proposed hyperloop did not. A possible remedy for the noisiness of the fans is to implement multiple-pod "trains" so they could draft off one another, but the other issues require more consideration.
In summary, Musk raises some valid points concerning the proposed high- speed railway project, but the hyperloop may not be the answer now or ever. This author would prefer to see Musk turn his attention to maglev and make it a system as cost-effective as high-speed railways can be but as environmentally friendly, comfortable, and awesome as his electric cars.
© Natalie Burkhard. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 E. Baig, "Elon Musk Talks About High Speed 'Hyperloop'," USA Today, 31 May 13.
 D. Gross, "Hyperloop vs. World's Fastest Trains," CNN Tech, 7 Apr 14.
 "U.S. High-Speed Rail 'Myths' Debunked," CNN, 13 Apr 11.
 A. Nagourney, "High-Speed Train in California Is Caught in a Political Storm," New York Times, 6 Jan 14.
 E. Pfanner, "Japan Pitches Its High-Speed Train With an Offer to Finance," New York Times, 18 Nov 13.
 J. Powell and G. Danby, "MAGLEV: The New Mode of Transport for the 21st Century," 21st Century Science and Technology Magazine, Summer 2013.
 "Ideas Coming Down the Track," The Economist, 1 Jun 13.
 B. Einhorn, "Don't Hold Your Breath Waiting for Japanese Maglev Trains to Arrive in the U.S.," Bloomberg Businessweek, 19 Nov 13.
 "The Future of Transport: No Loopy Idea," The Economist, 17 Aug 13.
 D. Lavrinc, "Elon Musk Thinks He Can Get You From NY to LA in 45 Minutes," Wired Magazine, 15 Jul 13.
 R. M. Salter, "The Very High Speed Transit System," Rand Corporation, P-4874, August 1972.
 A. Vance, "Hyperloop Physics 101 with Elon Musk," Bloomberg Businessweek, 12 Aug 13.
 N. Statt, "One Year Later, Hyperloop Remains a Fantasy," CNET News, 13 Aug 14.
 S. Hargreaves, "Hyperloop: San Francisco to L.A. in 30 Minutes," CNN Money, 13 Aug 13.
 N. Bilton, "Questioning the Feasibility of Hyperloop," New York Times, 19 Aug 13.