To transfer hyperloop pods from the low-pressure tube environment to a station to allow passengers to exit, some sort of airlock is needed. There are several ideas about taking the pod out of the tube, as well as leaving the pod inside the tube. The main objective is to have a smooth passenger transition from pod to station. In this article, the two most promising airlock concepts will be explained and a comparison between the two is made.

The low-pressure tube environment is crucial for the hyperloop and is what makes the hyperloop unique and stand out. A pod arriving at a station cannot simply open the doors and let passengers out, because of the harmful low-pressure environment. In order to let passengers safely exit the pod, the inside of the pod should be in direct contact with atmospheric pressure. Multiple options and innovative ideas have been developed by people around the world. All these ideas can be divided into two main categories. 

The first option is a chamber in between the two states of pressure, called an airlock chamber. In this chamber, the pressure will vary, depending on which direction the pod goes. If the pod goes from atmospheric pressure (station) to the near-vacuum tube environment, the chamber will start with atmospheric pressure and will depressurize once it is sealed. When the pressure in the chamber is the same pressure as in the tube, the chamber will open on the side of the tube, allowing the pod to continue.  The other way around works as well with this method.  This is the same principle that was used in the Space Shuttle. 

The second option involves bridge doors at the platform that will lock onto the pod doors, similar to a jet bridge. In this case, the vacuum tubes will continue into the station and the pod never leaves the low-pressure environment. The bridge doors will connect the inside of the pod to the station atmosphere.    

The air in the bridge or in the airlock when the pod departs will dissipate into the tube. This is achieved by opening a vent in the door. From within the tube, the air will be pumped out by the vacuum pumps.  Both options have advantages and disadvantages, these are listed in the table below. 

Pod freedom at the station is understood as the ability to easily move the pod around and to allow for possible maintenance. The perceived safety is lower for the latter option due to passengers directly stepping into a sealed off low-pressure tube. Next to these (dis)advantages, the mechanisms can be compared in operation time, safety and structure. 

Looking at the operation time, both concepts are expected to be in the same range. For the airlock option, two big doors need to be moved, one to close the chamber and one to open the chamber. It is estimated that one door movement will take around 30 seconds. The removal/addition of the air is expected to take around one minute. The option with bridge doors also has two door/bridge movements which are also expected to take around 30 seconds per movement. The bridge doors are smaller, however, they require a better sealing for the safety of the passengers. The removal/addition of the air will take less time than the airlock since the volume is smaller (only around the door versus around the whole pod). 

In terms of safety, the airlock chamber is preferred. Humans are not able to survive a (near) vacuum environment. In an airlock chamber, the passengers are protected from the low-pressure environment by the pod at all times. However, when walking through the bridge used in the other option, passengers will pass close by the connection from the bridge to the pod. On the other side of this connection is the low pressure.  A perfect seal should be guaranteed to avoid fatal accidents.  

Structure-wise, the airlock is preferred as well. A hyperloop station requires up to 12 platforms to accommodate all incoming pods and to provide enough time to embark and disembark. Thus, pods should be able to move from the tube to (at least) half of the platforms, and desirably more. A large transfer area between the tubes and the platforms is needed. With the bridge doors option, this means that a transfer area from one tube to at least six platforms will have to be placed inside the low-pressure environment. This results in large spans of the low-pressure tube resulting in extreme forces.  

The airlock also has its structural problems, due to the repetitive cycle from atmospheric pressure to near vacuum and back, fatigue issues will occur. However, the airlock can be designed to keep the stresses in the tube below the endurance limit, nullifying fatigue effects. 

To conclude, the airlock chamber concept and the bridge doors concept both have their advantages and disadvantages looking at the operation time, safety and structures. The bridge doors concept wins it in terms of the time it takes to transfer people from the tube to the station using the bridge doors. However, the bridge doors concept performs worse than the chamber concept in terms of safety.  The bridge doors concept has more possible fatal failures, whereas, with the airlock, the passengers are protected by the pod. In terms of the structures, the large low pressure transfer area for the bridge doors option seems to be unrealistic. All in all, for now it is opted for the airlock chamber concept since it seems the most realistic and reliable solution.

By Delft Hyperloop, March 2019