In the aviation business, cabin pressurization, heating, and air conditioning are quaintly called “environmentals.” It’s not the only euphemism in commercial aviation. The passenger briefing describes a “water landing.” The reality is only six times has an airliner alighted on water without the loss of life. In every case the event happened either in a river in a metropolitan area or near a rescue ship, where rescue personnel were minutes away.
Sugar-coating doesn’t change the facts
Hyperloop will likely follow a similar path, sugar-coating the risks. If engineers stay close to Elon Musk’s 2013 white paper, it might be more appropriate to call these capsule systems life support. The environment in the tube will be evacuated to 1/1000 the pressure at sea level. That’s equivalent to an altitude of 160,000 ft, four times higher than an airliner flies. To survive in that environment, a human needs a pressure suit and to breathe oxygen under pressure. Therefore, a Hyperloop capsule needs to provide a pressurized, comfortable environment for passengers.
Cabin pressurization is well-developed technology. Since the introduction of the B-29 Superfortress and its derivative the B-377 Stratocruiser, pressurization has been a feature of every airliner. A key design objective is graceful failure. In the case of depressurization there are three modes: slow, rapid, and explosive. An explosive decompression happens faster than a human can expel air from her lungs. An airliner by virtue of its large cabin is unlikely to suffer explosive or rapid decompression. The smaller size of the Hyperloop cabin and higher differential pressure makes the design challenge more like a spacecraft.
Time of useful consciousness
Forty thousand feet is a practical limit for humans to survive a decompression. There are several factors to consider. The first is time of useful consciousness (TUC) or effective performance time. TUC is the period a young, healthy person can continue routine tasks without supplementary oxygen. At 40,000 ft, TUC is typically 15-20 seconds. For most people, that would be time enough to don an oxygen mask. The second factor is the partial pressure of oxygen. Above 40,000 ft, 100% oxygen at ambient pressure as you would get from an overhead mask is insufficient to sustain consciousness for indefinite periods. If the aircraft descends or the tube is repressurized, oxygen will extend consciousness for some period. At higher attitudes still, the body bloats. You will blow up like a body builder. It won’t kill you, but for that you need a pressure suit, something Hyperloop customers are unlikely to want.
The Hyperloop capsule needs to maintain something like sea-level pressure. Airliners typically operate with a cabin pressure equivalent to 6,900 ft. The FAA limit is 8,000 ft. The FAA has become more stringent about cabin altitude over the last two decades. Before 1996, it certificated aircraft to fly at 45,000 ft, which would subject passengers to that atmosphere for a short time while the aircraft descended to 40,000 ft. Since then, only the A380 has been approved for flight at 43,000 ft, with the proviso that it can descend to 40,000 ft within a minute. Hyperloop is likely to be subjected to the same kind of regulation. By what agency, we don’t know yet.
My tires hold 30 psi
The design challenge for Hyperloop engineers is to build a capsule that can maintain a 6,900 ft pressure altitude (78,000 Pa) in a tube that is evacuated to 40,000 ft (18,000 Pa) or 160,000 ft (100 Pa). The optimist will say, “That’s only 12 psi. My car tires hold 30 psi.” The regulator will say, “Decompresssion is a potentially fatal event. We have duty to protect the public.”
It’s not difficult to demonstrate a 12 psi pressure vessel. The challenge is to build doors that can contain it. It’s a challenge to build a capsule that will withstand thousands of pressurization cycles. The Boeing 787 is designed to withstand 44,000 cycles. In real life, those airframes will likely be retired after about half that. A Hyperloop capsule is likely to see 16 trips per day. That’s 6000 cycles per year, far more than an airliner sees. A Hyperloop pressure vessel needs to be stout or designed for relatively frequent replacement.
Once you have a vessel, you need to pressurize it with breathable air. From the beginning of the jet age, airliners used bleed air from the engines to pressurize the cabin. The air at 40,000 ft has plenty of oxygen; it’s just at a low pressure. The Boeing 787 instead uses electric motors to pressurize cabin air, a strategy that Hyperloop could use. It’s not clear however that the air inside the tube will be oxygen-rich. Unless the tube has its air refreshed, it could become stale. In that case, the capsule would need to carry oxygen and CO2 scrubbers. There’s nothing wrong with that, but as spacecraft designers are painfully aware, it is more weight in the vehicle.
Doors are not so simple
Doors are simple to understand but difficult to build. Airliners use a design called a plug door. Even though the door appears to open outward on its hinges, it is a stopper that is forced into the jambs by the air pressure in the cabin.
In the event that a capsule does suffer a decompression, there are two immediate tasks. First, the tube needs to be repressurized so that passengers will have air to breathe before the capsule oxygen runs out. Second, the tube needs to be opened to get the passengers out. Assuming the capsule is otherwise undamaged, it might be commanded to proceed at low speed to an emergency tube exit. If the capsule is disabled, rescue personnel would need to open the tube, likely using plasma cutting technology. Rescue techniques are the topic of another monograph.
Liability and Regulation
The first generation of space tourists knows that they are engaging in an inherently dangerous activity like skydiving. They will likely sign away their rights to sue in the case of accident. Even if they don’t, the judge will invoke the doctrine of unavoidably unsafe.
Hyperloop falls into a different category. The operators of Hyperloop are providing common carriage – offering transport to all comers. They will be expected to provide a reasonable level of safety. Certainly regulators in Washington will see it that way. It will much easier for them to no than to say yes.
Hyperloopers needs to anticipate the rush to regulation and meet it face on, before Washington builds a bureaucracy that stifles the industry. The Acela is an example. Amtrak originally intended to buy tested, lightweight train technology from France. After the Federal Railroad Administration mandated impact standards that anticipated a head-on collision between Acela and a freight train, the French engineers renamed the project cochon (pig) for the weight it added. Insofar as Hyperloop and high speed rail both depend on low weight for speed and economy, misguided and unnecessary safety regulations could doom both in the United States.
Hyperloop is lucky. It doesn’t share its guideway with other vehicles. It doesn’t have a legacy of regulation. Hyperloopers need to keep it that way. It will face many technical challenges, many related to safety. It doesn’t need artificial challenges before it launches.