Hyperloop UPV
The Hyperloop UPV concept is a levitating rail-free pod design. Using magnetic repulsion and air cushioning for levitation is expensive and can produce unpredictable forces. The pod is equipped with a hybrid system of permanent magnets and electromagnet units, being able to control lift, so it can hover from the top of the tube. That way, the pod doesn't need so much electricity to be lifted by itself. Propulsion is obtained through expansion of the air with a nozzle, located at the rear of the pod, achieving speeds of up to 1000 km/h. The turbine recovers energy from the flow allowing a more efficient journey. This clean tube layout is said to cut costs in up to 30% of the whole Hyperloop project (about 180m USD) and the possibility to comfortably fit more passengers.
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.