A hyperloop is a theoretical mode of high-speed transportation sketched out by serial entrepreneur Elon Musk. Musk envisions the system as a 'fifth mode' of transportation: an alternative to boats, aircraft, automobiles, and trains. Musk, who has expressed his intent to develop a prototype hyperloop, stated that it "could revolutionize travel", but the technological and economic feasibility of the idea has not been independently studied.
Musk and a small group of Tesla and SpaceX engineers released an alpha-level design in August 2013. The alpha design calls for a capsule that would ride on a cushion of air forced through multiple openings on the capsule's bottom. The design proposes "a combination of active and passive means to reduce the negative effects of choked airflow."
According to the initial alpha design, released on August 12, 2013, a hyperloop would enable travel from the Los Angeles region to the San Francisco Bay Area in 35 minutes, meaning that passengers would traverse the proposed 354-mile (570 km) route at an average speed of just under 598 mph (962 km/h), and a top speed of 760 mph (1,220 km/h). The suggested route runs from the northern San Fernando Valley to the Hayward/Castro Valley area, paralleling the Interstate 5 corridor for most of its length. The "Alpha" proposal estimates a US$6 billion budget for a passenger-only version of the system, while a version allowing for transportation of both passengers and cars would cost US$7.5 billion. The alpha design's construction cost projections have been questioned by transportation engineers and others, who contend that the sum is unrealistically low given the scale of construction required and the level of unproven technology involved.
Musk first mentioned that he was thinking about a concept for a "fifth mode of transport", calling it the Hyperloop, in July 2012 at a PandoDaily event in Santa Monica, California. He described several characteristics of what he wanted in a hypothetical high-speed transportation system: immunity to weather, cars that never experience crashes, an average speed twice that of a typical jet aircraft, low power requirements, and the ability to store energy for 24-hour operations. Musk estimated at the time that the cost of the Castro Valley-Sylmar Hyperloop would be about US$6 billion.
Musk has likened the hyperloop to a "cross between a Concorde and a railgun and an air hockey table", while noting that it has no need for rails. He believes it could work either below or above ground.
From late-2012 until August 2013, an informal group of engineers at both Tesla and SpaceX worked on the conceptual foundation and modeling of Hyperloop, allocating some full-time effort to it toward the end.
A high-level alpha design for the system was published on August 12, 2013, in a whitepaper posted to the Tesla and SpaceX blogs. Musk has also said he invites feedback to "see if the people can find ways to improve it"; it will be an open source design, with anyone free to use and modify it. The following day he announced a plan to construct a demonstration of the concept.
Theory and operation
Developments in high-speed rail, and high-speed transport more generally, have historically been impeded by the difficulties in managing friction and air resistance, both of which become substantial when vehicles approach high speeds. The vactrain concept theoretically eliminates these obstacles by employing magnetically levitating trains in tubes kept at a complete vacuum, allowing for theoretical speeds of thousands of miles per hour. The high cost of maglev, however, and the difficulty of maintaining a vacuum over large distances, has prevented this type of system from ever being built. The Hyperloop resembles a vactrain system but operates at approximately one millibar of pressure.
The alpha-level design Hyperloop is proposed to operate by sending specially designed "capsules" or "pods" through a continuous steel tube maintained at a partial vacuum. Each capsule floats on a 0.5- to 1.3-millimetre (0.02 to 0.05 in) layer of air provided under pressure to air-bearing "skis", similar to how pucks are suspended in an air hockey table, thus avoiding the use of maglev while still allowing for speeds which wheels cannot sustain. Linear induction motors located along the tube would accelerate and decelerate the capsule to the appropriate speed for each section of the tube route. With rolling resistance eliminated and air resistance greatly reduced, the capsules are theorized to be able to glide for the bulk of the journey. In the Hyperloop concept, an electrically-driven inlet fan and air compressor would be placed at the nose of the capsule "actively transfer[ing] high pressure air from the front to the rear of the vessel," resolving the problem of high speed transport in a tube that is not a hard vacuum wherein pressure builds up in front of the vehicle, slowing it down. A fraction of the air is shunted to the skis for additional air pressure, augmenting that gained passively from lift due to their shape.
The capsules, which are proposed to be 2.23 metres (7 ft 4 in) in diameter for the passenger-only version, are projected to reach a top speed of 760 mph (1,220 km/h) so as to maintain aerodynamic efficiency, and the alpha design proposes that passengers will experience a maximum inertial acceleration of 0.5 g, well less than a commercial airliner on takeoff and landing. At those speeds, there would not be a sonic boom; with warm air inside the tubes and high tailwinds, Musk hypothesizes that the pods could travel at high speeds without crossing the sound barrier.
The alpha high-level design envisioned using turntables to turn capsules at each end of the tube. Industrial designer Serge Roux has proposed looping the tube around a circular station as in existing gondola lifts, increasing the time for passenger loading and unloading at each terminal.
In September 2013, Ansys Corporation ran computational fluid dynamics simulations to model the aerodynamics of the capsule and shear stress forces that the capsule would be subjected to. The simulation showed that the capsule design would need to be significantly reshaped to avoid creating supersonic airflow, and that the gap between the tube wall and capsule would need to be larger. Sandeep Sovani said the simulation showed that Hyperloop has challenges but that he is convinced it is feasible.
In October 2013, the development team of the OpenMDAO software framework released an open-source model of the Hyperloop's propulsion system. Their model allowed for a preliminary conclusion that the concept is feasible, although the tube would need to be significantly larger than originally projected—on the order of 13 feet (4 m).
In November 2013, MathWorks published a blog post with analysis of the route proposed in the initial proposal. The analysis focused on acceleration forces that would be experienced by passengers along the journey and the necessary deviations from public roadways in order to keep the accelerations reasonable. The conclusion was that the route was mainly feasible. The analysis did highlight that maintaining a trajectory along I-580 in eastern San Francisco at the planned speeds was not possible without significant deviation into heavily populated areas. 
The suggested main route for the Greater Los Angeles Area to the San Francisco Bay Area system used in the alpha design document would begin around Sylmar just south of the Grapevine, approximately follow the I-5 highway to the north, and arrive at a station near Hayward on the east side of San Francisco Bay. Several proposed branches were also shown in the alpha design document, including Sacramento, Anaheim, San Diego, and Las Vegas.
While terminating the hyperloop route on the fringes of the two major metropolitan areas would result in significant cost savings in construction, it would require that passengers traveling to and from Downtown Los Angeles and San Francisco – and any other community beyond Sylmar and Hayward – transfer to another transportation mode in order to reach their final destination. This would significantly lengthen the total travel time to those destinations.
Political and economic considerations
The proposal assumes that cost savings versus conventional rail will come from a combination of several factors. Its small profile and elevated nature mean that it can travel for most of its journey down the median of the I-5 highway, with small detours when the highway bends too much. The low profile reduces tunnel boring requirements and the light weight of the capsules versus a train is claimed to reduce construction costs. It is claimed in the design paper that there would be less right-of-way opposition and environmental impact as well due to its small, sealed, elevated profile versus that of a rail easement; however, other commentators have noted that a smaller footprint does not guarantee less opposition. In criticizing this assumption, mass transportation writer Alon Levy said that "In reality, an all-elevated system (which is what Musk proposes with the Hyperloop) is a bug rather than a feature. Central Valley land is cheap; pylons are expensive, as can be readily seen by the costs of elevated highways and trains all over the world." Michael Anderson, a professor of agricultural and resource economics at UC Berkeley, predicted that "You're talking $100 billion to build what they're proposing." The 19-mile (30 km) Shanghai Maglev line is entirely elevated on pylons and cost US$1.2 billion to build, a cost of US$63 million per mile. At that price, building a 354-mile (570 km) maglev route in China would cost US$22.3 billion. This does not factor in the costs of developing the Hyperloop's new technology or the significantly higher cost of construction in the United States. EC Harris has estimated that construction in the U.S. is between two and three times more expensive than in China.
The Hyperloop alpha-design white paper suggests that US$20 of each one-way passenger ticket between Los Angeles and San Francisco would be sufficient to cover initial capital costs, based on amortizing the cost of Hyperloop over 20 years with ridership projections of 7.4 million per year in each direction and does not include operating costs (although the proposal asserts that electric costs would be covered by solar panels). No total ticket price was suggested in the alpha design. The projected ticket price has been questioned by Dan Sperling, director of the Institute of Transportation Studies at UC Davis, who told Al Jazeera America that "there's no way the economics on that would ever work out."
The early cost estimates of the Hyperloop in the alpha design are a subject of debate. A number of economists and transportation experts have expressed the belief that the US$6 billion price tag dramatically understates the cost of designing, developing, constructing and testing an all-new form of transportation. The Economist said that, at the very least, the estimates are unlikely to "be immune to the hypertrophication of cost that every other grand infrastructure project seems doomed to suffer."
Political impediments to the construction of such a project in California will be very large. There is a great deal of "political and reputational capital" invested in the existing mega-project of California High-Speed Rail. Replacing that with a different design would not be straightforward given the California political economy.
Building a successful Hyperloop sub-scale demonstration project could reduce the political impediments and improve the accuracy of cost estimates. Musk has suggested that he may be personally involved in building a demonstration prototype of the Hyperloop concept, including funding the development effort.
Efforts at development
A startup company, JumpStartFund, has announced its intent to pursue the development of the hyperloop. The effort reportedly has the approval of SpaceX President Gwynne Shotwell although neither the company nor Elon Musk is involved. A company called Hyperloop Transport Technologies will be formed, led by Dr. Marco Villa and Dr. Patricia Galloway. A number of companies and institutions have also indicated they will participate including ANSYS, GloCal Network Corporation and the UCLA Architecture and Urban Design school. Initial plans include the release of an updated white paper by March 2014 and, if the conclusions are favorable, thereafter to construct a prototype.
The Crystal Palace pneumatic railway operated around 1864 and used large fans, some 22 feet (6.7 m) in diameter, that were powered by a steam engine. The tunnels are now lost but the line operated successfully for over a year.
In 1869 the Beach Pneumatic Transit was an early one-block-long prototype of an underground tube transport public transit system in New York City. The system worked at near-atmospheric pressure, and the passenger car moved by means of higher-pressure air applied to the back of the car while somewhat lower pressure was maintained on the front of the car.
In the 1910s, vacuum trains were first described by rocket pioneer Robert Goddard. While Hyperloop has significant innovations over early proposals for reduced pressure or vacuum-tube transportation apparatus, the work of Goddard "appears to have the greatest overlap with the Hyperloop."
Swissmetro was a proposal to run a maglev train in a low pressure environment. Concessions were granted to Swissmetro in the early 2000s to connect the Swiss cities of St. Gallen, Zurich, Basel and Geneva. Studies of commercial feasibility reached differing conclusions and the vactrain was never built.
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