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Overview
The Hyperloop, an idea proposed by Tesla and SpaceX founder Elon Musk, is a high-speed transportation system in which pods filled with passengers or cargo travel through a near-vacuum tube at airline speeds. The pods would be propelled through the low pressure tube by linear induction motors placed along the tube. These motors would produce a magnetic field which would interact with the opposing magnetic fields developed by the pods, creating a linearly moving force which would allow for acceleration and deceleration. It has been proposed that these pods could potentially reach speeds up to 760 mph with passengers on board. The idea was unveiled by Musk in August of 2013 in a 57-page alpha white paper.

Kantrowitz Limit
The Kantrowitz Limit is a law which defines the minimum tube to pod area ratio necessary to avoid choking the air flow in a tube at a given speed. If moving at high-speeds, this limit requires a large diameter tube for the pod to travel through. If a small diameter tube is desired, then the pod must travel at much slower speeds than those expected from the Hyperloop.

In order to overcome this limit, Musk, in his original proposal, suggested mounting an electric compressor fan on the nose of the pod which would transfer high pressure air from the front of the vessel to the rear. The fan will act as a pressure reliever for the front of the vessel, allowing higher speeds while reducing tube diameter. This also produces a cushion of air surrounding the pod, allowing for a low-friction environment that favors high-speed travel.

Development
When Musk released his original paper on the Hyperloop he noted that, due to personal time constraints, he would make this an open-source project. By doing this, Musk allowed anyone to use his ideas and theories for the Hyperloop in order to develop their own prototypes. Multiple companies have been built around the Hyperloop and teams of students and engineers have begun constructing their own Hyperloop pods and tracks.

Hyperloop pod competition
As stated above, Musk, while proposing the idea of the Hyperloop, is not leading the effort in making this technology a reality. Instead, SpaceX is holding a competition in order to encourage students and professional engineers to develop prototype pods. In order to facilitate this competition, SpaceX built a one-mile test track in Hawthorne, California. In January 2017 and Summer 2017, teams will be able to test their human-scale pods.

Hyperloop One
Hyperloop One, a company cofounded by Shervin Pishevar and Josh Giegel, is leading the effort towards commercializing the Hyperloop. The company demonstrated aspects of the propulsion system in the Las Vegas desert in May 2016. The full system is set to be tested early 2017, also in Las Vegas. The company is considering constructing a Hyperloop track connecting Las Vegas to Los Angeles but nothing has been finalized as of now.

Hyperloop Transportation Technology (HTT)
Hyperloop Transportation Technology (HTT), an Hyperloop company cofounded by Dirk Ahlborn and Bipop G. Gresta in 2013, is also advancing quickly towards developing a fully functional Hyperloop system. HTT is made up of over 50 teams across 38 countries in the effort to construct the Hyperloop. HTT has reached an agreement with Slovakia for the country to become a regional hub in the development of an Hyperloop line connecting Vienna, Bratislava, and Budapest. There is no set date for when construction and testing of HTT’s Slovakia Hyperloop will take place.

Criticism
Many critics of the Hyperloop emphasize the experience of this type of transportation in which the passenger is riding in a compact, sealed, and windowless pod that is being launched at high speeds through a sealed steel tube. Also of note, with the pods reaching such high speeds the acceleration forces acting on the passengers would be significant and create potential health concerns. The air around the capsules are being compressed rapidly, which would also cause high noise levels. These factors support the criticism that the Hyperloop experience for the passenger could be unnerving and, potentially, traumatizing.

The design of the tube system has also been a point of stark criticism. James Powell PhD, co-inventor of the mag-lev train, notes that Musk’s original Hyperloop design allows for only three hundredths of an inch between the tube wall and the outer frame of the pod. This small gap allows for an air cushion that would stabilize the pod and allow it to hover in the tube free of noticeable friction forces. However,the necessity for this gap will also require a high-level of precision when the tube is being constructed. Powell believes that, due to this small gap, even a single bump displacing the pod just a millimeter high would trigger catastrophe.

Other criticisms include the vulnerability of the tubes providing a tempting target for terrorists, logistical concerns in the event of equipment malfunctions or emergency evacuations, and the implications of natural hazards such as seismic activity.