Hyperloop: Traveling in Sonic Speed About the author
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Richard Fu October 3, 2013
Richard was a senior majoring in electrical engineering with an interest in transportation systems. He enjoys skiing, swimming, and reading. Abstract Hyperloop is a hypothetical high-speed transportation system recently proposed by American entrepreneur and inventor Elon Musk. Hyperloop consists of a low pressure tube with capsules that are transported throughout the tube. The capsules carry passengers and even vehicles, and they are able to travel at 760 miles per hour, which is near the speed of sound. With Hyperloop, a trip from Los Angeles to San Francisco would only take 35 minutes [1]. Hyperloop has the potential to revolutionize our transportation system. The Ideal Mode of Transportation What would be the ideal mode of transportation? According to Elon Musk, an ideal transportation system should be fast, low-cost, safe, immune to weather changes, and sustainably self-powering. Hyperloop, as designed and proposed by Elon Musk, is able to meet all these standards. Hyperloop travels in sonic speed and makes it possible to travel from Los Angeles to San Francisco in only 35 minutes. A one-way ticket for this trip only costs 20 dollars, and . Hyperloop includes a pair of tubes placed on pylons, making it safe during weather changes and even earthquakes. Hyperloop is powered by solar panels on the top of the tube and does not require other power inputs [1]. Elon Musk, the founder of rocket company SpaceX and co-founder of electric car maker Tesla Motors and online payment business PayPal, is sometimes referred as “the Iron Man in real life”. Musk explains in detail his vision of Hyperloop in the document Hyperloop Alpha, released on August 12, 2013. The technology of Hyperloop still remains hypothetical, but it looks promising, partially because of who Elon Musk is.
One motive of Musk to design Hyperloop is his dissatisfaction with the California High-speed Rail project. This current project proposes a $68 billion USD cost, a travel time of 2 hours and 38 minutes between San Francisco and Los Angeles, and a one-way ticket price of $105. Musk is disappointed in the project, describing it as “one of the most expensive per mile and one of the slowest [1]”. He claims that the Hyperloop saves much land because it is built above ground on pylons. A ground based rail system, on the other hand, requires a 100 ft wide and very long piece of land, impedes other ground transportations, and is noisy. Hyperloop is hypothetically much faster and cheaper than the California High Speed Rail system. Overall Structure Hyperloop consists of two long tubes connecting Los Angeles and San Francisco, and the capsules are transported throughout the tube in both directions. The tubes are supported on pylons placed every 100 ft. The tubes are covered by solar arrays to provide power to the system, making it friendly to the environment. Figure 1 shows the overall structure of Hyperloop [1]. The design of the tube and the capsule will be discussed separately. ![c:\users\richard\appdata\roaming\tencent\users\625381410\qq\wintemp\richole\)nr({3cqbb%5j{5][[]7]k1.jpg](6652_html_m74ad6603.jpg)
Figure 1: Overall structure of Hyperloop The Tube The design of the tube is essential to the Hyperloop system because it determines how Hyperloop would work. Musk has considered several options for the tube environment. One approach is to have a normal air pressure inside the tube, and another is to create a vacuum. In the first case, fans would push the air at high speed and propel the capsules. However, if a sonic speed is to be reached, this approach is practically impossible due to the extremely high friction of the air against the tube. In the second case, the vacuum inside the tube would be able to get rid of the problem of friction. However, this is not a possibility either. It is extremely difficult to maintain the near vacuum environment, because a small leak in the 700-mile round trip tube can destroy the functionality of the system [1]. However, Musk proposes a third approach which turns out to be a viable solution. It is to have a tube with a low air pressure system. This system is adaptive to variations in air pressure, and it would be possible to overcome both air leaks and air friction. The system still faces a problem, which Musk calls the Kantrowitz Limit. This concept shows a minimum tube to pod area ratio below which the air flow will be choked. It means that when the capsule travels at a high speed, air is not able to pass around it and the capsule would push the entire air column forward, unless an unrealistically huge-diameter tube is built [1]. Musk was able to find a solution, which is the air compressor in front of each capsule, which will be discussed later. The resulting low pressure Hyperloop tube will be operating at the pressure of 100 Pascals, which is 1,000 times less than the atmospheric pressure at sea level. Two low pressure tubes, made of steel, will be welded together to allow capsules to travel both directions. The tube will be supported by pylons placed every 100 ft, and it will be covered by solar arrays on the top [1]. The Capsule The passenger-only Hyperloop capsule holds 28 passengers in side-by-side pairs and departs every two minutes. The capsules are 4.4 ft wide and 3.6 ft tall and travel at 760 mph. The doors on each side will open in a gullwing or sliding manner, and the luggage compartment will be at the front or rear of the capsule. The interior of the capsule, as Musk proposes in Hyperloop Alpha, is designed for passenger comfort and safety. The seats will make passengers comfortable during accelerations. The cabin would “display beautiful landscape” and provide entertainment systems to each passenger. Musk also adds the possibility of a passenger plus vehicle version of the capsule, which is able to accommodate 3 vehicles in addition to the passengers [1]. Figure 2 shows the structure of the passenger version capsule [1]. ![c:\users\richard\appdata\roaming\tencent\users\625381410\qq\wintemp\richole\]1b{f2euci5hv}gsg_vl9yk.jpg](6652_html_71c2b644.jpg)
Figure 2: Hyperloop Capsule subsystem notional locations One important part of the capsule is the air compressor, which allows the capsule to travel without too much air drag. The device cools and compresses air that is bypassed through the capsule, with a compression ratio of 20:1. At the same time, it supplies air to air bearings at the bottom of the capsule (Suspension in Figure 2), which makes the capsule float. The compressing system includes a water tank used for cooling of the air. The cooling process produces steam, which are stored until the capsule reaches the station [1].
The Hyperloop includes a variety of functions for it to operate at the required performance. Two important functions are the suspension system and the propulsion system, which will be discussed. The Suspension System One technical challenge is the capsule’s suspension. The traditional wheel and axle system is impossible to use at the proposed high speed. Magnetic levitation may be a viable solution, but Musk could not tolerate the cost to build it. The alternative Musk has developed is the air bearing suspension system, which is stable and at reasonable cost. The system includes air bearing skis at the bottom of the capsule, which produces a high pressure to push the capsule away from the wall [1]. The suspension system includes two mechanisms: aerodynamics and external pressurization. In the aerodynamic method, the front tips of the skis are slightly elevated so that a thin film of air is trapped between the ski and the tube. The resulting increased pressure below the ski gives a force that raises a portion of the capsule’s weight. In the external pressurization, highly pressurized air is injected in supplement of the aerodynamic pressure so that sufficient lift is generated to support the capsule [1]. Both mechanisms contribute to the suspension of the capsule.
Hyperloop uses linear accelerators similar to those used in maglev trains [2]. The physics in the linear accelerator is not difficult to understand. As Figure 3 shows, a body containing a current loop is placed between two lines of electromagnets with alternating polarity [3]. The dots indicate current flowing out and the crosses indicate current flowing in. Looking at the body at the bottom, the current surrounding the body creates an N pole to the right and S pole to the left, repelling the magnets near the top and attracting those near the bottom, so that the body is propelled downward. Whenever it passes a pair of electromagnets, the current is reversed so that the same process repeats [3]. 
Figure 3: The basic layout of a linear motor Hyperloop uses these accelerators to accelerate the capsules to the speed of 760 mph. In the Hyperloop system, the current carrying body is called the rotor. It is a large aluminum blade 49 ft long, 1.5 ft tall, and 2 in. thick, placed underneath each capsule. The track of electromagnets is called the stator, which is mounted on the bottom of the tube. The two rows of electromagnets are laid out symmetrically on each side of the rotor. The components are shown in Figure 4 [1].
Figure 4: Rotor and stator 3D diagram Cost Analysis Musk estimates the cost of building 40 capsules to be $54 million and the tube to be $5.4 billion, making a total of $6 billion as the cost of the Hyperloop passenger transportation system, as shown in Table 1 [1]. Musk believes that one key advantage of the Hyperloop is that it can be built above ground on pylons. Moreover, the land costs can be mostly eliminated by building the Hyperloop alongside the Interstate 5 highway. By his plan, the system would only occupy spaces comparable to telephone poles. [1] Figure 5 shows the route of the Hyperloop project [1]. 
Table 1: Total cost of the Hyperloop passenger transportation system ![c:\users\richard\appdata\roaming\tencent\users\625381410\qq\wintemp\richole\]eina[brhox$6}rn00%~~ul.jpg](6652_html_76564626.jpg)
Figure 5: Overview of Hyperloop route from Los Angeles to San Francisco. Some critics argue that the $6 Billion price tag is largely underestimated. Alexis Madrigal, a senior editor at The Atlantic, points out the “42,424 acres of land the Hyperloop would need to acquire” is one obstacle to Musk’s budget [4]. The process to acquire land for the project is indeed an issue for the project to be realized. In the development of California’s high speed rail system, many local communities have either demanded for extra viaducts and tunnels or insisted they not be bypassed, which all contributed to the expensive budget of the project. Hyperloop is probably not immune to such issues, but Musk has not put them into consideration [4]. Another critic, Michael L. Anderson, who is an associate professor of agricultural and resource economics at the University of California, Berkeley, gives a more pessimistic prediction. He estimates the cost of the entire project to be closer to $100 billion, which is even higher than the California high-speed railway system [4]. It seems uncertain how much it would actually cost to build Hyperloop, since it is an entirely new transportation system. Hyperloop faces both technical and political uncertainties. They will be solved only when Hyperloop is actually implemented. We must wait until then to know the precise cost of Hyperloop. Conclusion The proposed Hyperloop has many advantages as a transportation system, yet it still remains hypothetical. If Hyperloop could succeed, there is no doubt that it would revolutionize transportation. Hyperloop is able to create megaregions of cities that are initially far apart, improving regional economy. The speed and convenience of the system can significantly benefit people living in California. Hyperloop is also a leap in technology. Even if it fails, it would be a worthwhile exploration because it can give insights to future inventions. However, Elon Musk, who is working on his Tesla and SpaceX, is too busy to work on this project that he proposed. Therefore, the Hyperloop project still remains pending. Musk has currently left this concept open to anyone who wishes to develop deeper into this project, but it is more likely to be himself who will turn his vision into reality [5]. Bibliography[1] E. Musk. (2013, Aug.). Hyperloop Alpha. SpaceX Corp., CA. [Online]. Available: http://www.spacex.com/sites/spacex/files/hyperloop_alpha-20130812.pdf |
