THE LEVITATION CHALLENGE The original concept outlined in Hyperloop Alpha utilized large low-pressure air bearings to provide a high-speed, low-friction system for supporting the pod. These bearings used both the relative motion between the pod and the tube, and pressurized air supplied by a multistage compressor to provide the airflow necessary to lift the pod. One of the major challenges of using such a fluid film bearing is the extremely small clearances that are required to keep flow requirements reasonable, maintain stability, and allow for reasonable hydrodynamic effects. Commercial air bearings typically operate at clearance of approximately 10 microns, whereas the bearings outlined in Hyperloop Alpha were designed for a much larger clearance of 1 mm. Initially, the team pursued a similar levitation strategy for the MIT pod. Using a combination of simple axisymmetric viscous flow models, 2-D CFD, and a multidisciplinary sizing tool, we determined a feasible maximum gap height for our pod. We considered using both compressed air cylinders and centrifugal compressors to supply the pressurized air to the bearings. The cubic dependence of mass flow rate on clearance caused tank volume or compressor power requirements to quickly reach impractical levels for clearances larger than 1-1.5 mm. Surface imperfections in an air bearing surface should not exceed approximately Âź of the total clearance, and this led to a maximum tolerable surface finish for the test track of approximately 250-400 microns. SpaceX released final tube specifications in November, which allowed for up to 1 mm step discontinuities between sections of the track on which the air bearings would ride. This forced us to move towards the passive eddy-current maglev system, which is currently used by the pod. This system allows for clearances an order of magnitude larger those of an air bearing while still maintaining low drag and feasibility at high speed.
Published on Nov 18, 2016
Annual magazine review of MIT Aeronautics and Astronautics Department research and educational initiatives.