Quay Valley, California
In March 2015, Dirk Ahlborn, the CEO of Hyperloop Transportation Technologies (HTT) in El Segundo, California announced a plan to build a hyperloop test system at the Quay Valley site. Ahlborn indicated that construction could begin in 2016 to be completed in 2017 with the first rides to begin in 2018 if his company can raise $100 million in funding. The company has said that this track will be a fully working and operational creation of the hyperloop, but that running speed will be reduced on the shortened track from Hyperloop's full potential. It is not decided whether a 5-mile loop or straight section will be constructed, and it may cross the Interstate bisecting the site. Ahlborn said that speeds up to may be possible. Quay Hays expects to break ground on the new community in 2016, including a working version of the Hyperloop, estimated to cost $100–150 million.
Kantrowitz limit
The Kantrowitz limit is a fundamental concept in the Hyperloop, a high-speed transportation concept recently proposed by Elon Musk for rapid transit between populous city-pairs about apart. The Hyperloop moves passengers in sealed pods through a partial-vacuum tube at high-subsonic speeds. As the air in the tube moves into and around the smaller cross-sectional area between the pod and tube, the air flow must speed up due to the continuity principle. If the pod is travelling through the tube fast enough, the air flow around the pod will reach the speed of sound, and the flow will become choked, resulting in large air resistance on the pod. The condition that determines if the flow around the pod chokes is the Kantrowitz limit. The Kantrowitz limit therefore acts a "speed limit" - for a given ratio of tube area and pod area, there is a maximum speed that the pod can travel before flow around the pod chokes and air resistance sharply increases.