There has been some criticism of the hyperloop concept, mainly revolving around the requirement of maintaining vacuum (or near-vacuum, specifically 1 mbar) over long distances. The criticism is on-point. It is in fact very hard to maintain low pressures over long distances. That is the reason that vacuum trains have not yet been implemented in any kind of large scale. The problems include:
- Thermal expansion of the tube. This means that expansion seals will need to be included at regular intervals along the tube, and the expansion seals will need to be airtight. An alternative is to bury the entire tube underground, however this is probably too expensive to be worth doing.
- Vibration. The pods traveling in the tube will cause vibration and the structure needs to be able to absorb this without risk of failing.
- Earthquakes. Self-explanatory.
- Thermal buckling of the tube. Temperature differences across the top and bottom of the tube will result in differential length change. The structure of the pods, as well as the expansion seals, will need to be able to deal with this.
- Malicious acts. The tube would be a fairly easy target for terrorists or troublemakers as even a rifle bullet would be more than enough to pierce the tube and compromise the vacuum.
- Any loss of vacuum has the potential to be disastrous, as a shock front of air would travel down the tube and impact any pods in its way in a fairly violent fashion.
It seems that any practical design would have to assume that vacuum will be lost occasionally, perhaps even frequently, and would have to deal with this in a graceful way and without loss of life.
One option that has been pointed out by others is to use the pods themselves to provide the vacuum. Is this feasible or not? If so, it would solve several problems at once: The problem of creating vacuum, the problem of maintaining vacuum, and the problem of vacuum loss.
The proposed idea is to put compressed air tanks inside each pod. As each pod moved through the tube, more and more of the air in the tube would be collected. At the same time, the pods are designed to operate at any pressure between 1 and 1000 mbar. At 1000 mbar they travel slowly (well under the speed of sound), to avoid the large aerodynamic forces of moving at high speed. As air pressure in front of the pods decrease, they move faster, and at 1 mbar they reach their maximum speed.
This covers the process of starting with an air-filled tube and proceeding to an evacuated one, but what about loss of vacuum? In this case, the main priority is to make the loss as gradual as possible, to protect against rapid deceleration of the pod which could cause structural failure or injury to occupants. We can assume that at the point of vacuum compromise, we have no control over rate of pressure loss, and we also have to assume the rate of pressure loss would be large (a 1 atm shock front moving down the tube at sonic velocity). But we can control the rate of vacuum loss on the non-compromised sections. Under vacuum, the pods are moving rapidly, so by creating a series of very small pressure increases along the length of the tube, the pod can be made to experience gradual pressure increase. Each section of the tube would have a pressure sensor, and if a section of the tube detected loss of vacuum, neighboring sections would open valves to the outside and let air in as well, in a controlled manner.
The number of pods needed to evacuate the tube depends on a lot of factors. I get a rough ballpark estimate of ~2500 trips, or 20-30 days, to evacuate the tube using this method.
In short, more research is needed to see if this idea has merit and could work. If anyone has any links on studies relating to this concept, I’d love to hear about it in the comments.