Hyperloop and other vactrains

[Backshophoss]

Believe Mr Musk took a cue from the Beach air driven tube subway that was demonstrated back in the late 1800's in Manhattan.

[lpetrich]

Vactrain - Wikipedia -- trains running in evacuated tubes. Proposals of vactrain systems have been around for over a century.

Robert Salter proposed some vactrain systems in the 1970's:

The Very High Speed Transit System | RAND (1972)

Description of a very high speed transit (VHST) system operating in its own rarefied atmosphere in evacuated tubes in underground tunnels. Most cases considered took less time to go coast-to-coast (e.g., 21 min) than it takes an aircraft to climb to an efficient operating altitude. VHST's tubecraft ride on, and are driven by, electromagnetic (EM) waves. In accelerating, it employs the energy of the surrounding EM field; in decelerating, it returns most of this energy to the system. Tunnel systems would be shared by oil, water, and gas pipelines; channels for laser and microwave waveguides; electric power lines including superconducting ones; and freight systems. Environmental and economic benefits are substantial, and the technology for building and operating the system exists.

Trans-Planetary Subway Systems: A Burgeoning Capability | RAND (1978)

Describes a subway concept called "Planetran" comprising electromagnetically supported and propelled cars traveling in underground evacuated tubes, able to cross the United States in one hour. It is designed to interface with local transit systems, and the tunnel complex also contains utility transmission and auxiliary freight-carrying systems. Tunnels represent a major problem area and most of the cost. They will be placed several hundred feet underground in solid rock formations. It will require advanced tunnel-boring machines, such as hypersonic projectile spallation, laser beam devices, and the "Subterrene" heated tungsten probe that melts through igneous rocks. Planetran is rated as a system high in conservation of energy. For every car being accelerated, there is one decelerating in an adjoining tube. The decelerating cars return energy to the system. The tubes have a reduced atmosphere, making drag losses much smaller than for aircraft. Coast-to-coast energy costs are expected to be less than $1.00 per passenger.

Looks like he was proposing maglev trains.

[lpetrich]

Robert Salter proposed a NYC-LA routes that goes through Texas, Amarillo in the first one, Dallas in the second one, but the shortest-distance route is more northerly, going near Kansas City. The shortest route's length is the great-circle distance, about 2500 mi / 4000 km.

The U.S. Set a New Record for Airline Passengers in 2015 – Skift -- 900 million passengers that year.

Cheap Flights from Los Angeles LAX to New York John F Kennedy JFK| Skyscanner -- "More than 1.6 million passengers fly between Los Angeles International Airport (LAX) and John F. Kennedy International Airport (JFK) every year."


For comparison, I will use the Gotthard Base Tunnel - Wikipedia. It is a pair of single-track bores each about 57 km (35 mi) long that took 12 years to excavate, and it cost roughly $10 billion in 2015 dollars. It opened 4 years after both bores were done.

The average tunneling rate was 13 m/day in each bore, or 26 m/day combined (43 ft/day, 86 ft/day). The cost was $175 m/km or $282 m/mi, though that included adding tracks and overhead cables and signaling systems. For NYC - LA, that scales to $700 billion.


There are cheaper alternatives to tunneling, like elevated trackways. However, such alternatives are likely to provoke NIMBY opposition in populated areas.

[Triaxle]

Have any details been released about evacuation of passengers in the event of an emergency in the tunnel?

By employment of fast-response mortician teams.

[Triaxle]

Come to think of it, back then a lot of people thought steam-powered "horseless carriages" weren't feasible, and guess what--they were right.

The Stanley Steamer and competitors were serious competitors to gasoline engines, for a while.

[lpetrich]

I should note some tunnel-bore diameters.

• Gotthard Base Tunnel: 8.83–9.58 m (29.0–31.4 ft)
• BART Transbay Tube: 17 feet (5.2 m)
• Seattle University Link Tunnel: 21 ft (6.4 m)

So anything less than about 5 m will be a tight squeeze.

Vactrains have a certain fail-safety problem. If a tunnel leaks air, then trains using it will have to slow down from vactrain top speeds. There is also a problem with isolating the leak. One could do that with gates along the tunnel, but trains would have to stop for them.

-

I'll now calculate NYC - DC. Using NYC (Penn Station) and DC (Union Station), I get 205 mi / 330 km. Going to Philadelphia (30th St. Station) and Baltimore (Penn Station) gives 207 mi / 333 km. Adding more intermediate stops, like Newark, Trenton, and Wilmington, will make it even longer.

EM proposes to do NYC - DC in 29 minutes. That's about 680 km/h or 420 mph. Airliner cruising speeds are typically around 800 - 900 km/h or 500 - 560 mph, but airliners have airports as an important limiting factor. The vactrain stations will be likely be much more accessible, since they would likely be underneath Penn Station and Washington Union Station.

[electricron]

Vacuums will be hard to maintain in hundreds of miles in long tubes. It'll be much easier, and probably cheaper, to use hydrogen or helium gas at a low pressure, where if any leaks were to occur, the gas will leak out instead of the air leaking in. The electric power industry use hydrogen gas to cool generators (alternators) because it's easier and cheaper to do so than it is to maintain a vacuum.
Which is more deadlier doesn't' matter, because if exposed to either humans would die, but for some reason people think being enclosed in a vessel surrounded by a vacuum is safer than being enclosed in a vessel surrounded by hydrogen or helium - being an easier sale to potential customers.

[andrewjw]

If the gas is at lower pressure than the atmosphere, a leak would still be inwards...

[lpetrich]

That cannot possibly work.

If the tube's internal pressure is less than that of the surrounding air, then it still risks air leaking in, no matter what gases are in the tube.

Turning to having a pressure balance, one can easily calculate the necessary density for hydrogen or helium. To a first approximation, one can use the ideal gas law:

(pressure) = (density) * (Boltzmann's constant) * (absolute temperature) / (average molecular weight)

Absolute temperature = temperature relative to absolute zero (Kelvin, Rankine).

A.M.W. = about 29 for air, 4 for helium, and 2 for hydrogen. So for the same temperature as the surrounding air, one gets a density improvement of 14.5 for hydrogen and 7.25 for helium. One can get a factor of 2 improvement by bumping up the internal gas's temperature by a factor of 2 -- in absolute temperature, of course. So for 20 C (68 F), one gets 313 C (596 F).

Large-size drag is approximately proportional to (density) * (velocity)^2, so for air at 300 km/h, I find for the same drag 808 km/h for helium and 1142 km/h for hydrogen.

This will require a *lot* of helium or hydrogen, and while hydrogen gives the best performance, it burns in air, and many people will bring up the destruction of the Hindenburg.

[CLamb]

Depending on how high a vacuum is needed outgassing could be a problem. Not only could the outgassing from the tunnel and the vehicles compromise the vacuum but it can also degrade the materials giving off the gas.

[Motorman]

If you're interested in weird railroad technologies (and if you can read German), I recommend the book
"Rollen, Schweben, Gleiten"- Unkonventionelle Verkehrsmittel gestern-heute-morgen im spurgebundenen Verkehr
("Rolling, Hovering, Sliding"-unconventional means of transportation yesterday-today-tomorrow by guided traffic.)
alba buchverlag, Düsseldorf, 1975 ISBN 3-87094-033-6

Sit down, have a drink, and relax. This book covers most of the (then known in 1975) "projects".
Flash Gordon and his colleagues would be pleased, and sometimes you think "what were these smoking/drinking lately", when conceiving such oddities like rocket-powered supersonic tube-trains and many others...

[lpetrich]

Depending on how high a vacuum is needed outgassing could be a problem. Not only could the outgassing from the tunnel and the vehicles compromise the vacuum but it can also degrade the materials giving off the gas.

One may not need a very hard vacuum. Elon Musk himself once proposed 1% of external pressure (0.01 atm) for his Hyperloop. That would make a 300-km/s-equivalent of drag at 3000 km/s.

Outgassing is only a problem for much lower pressures, about 10^(-12) of external pressure (10^(-12) atm) (Ultra-high vacuum - Wikipedia), and such low pressures will likely not be needed for vactrains.

Vacuum - Wikipedia has some definitions.

[bostontrainguy]

Hyperloop is already outdated . . .

http://www.dailymail.co.uk/sciencetech/ ... 00mph.html" onclick="window.open(this.href);return false;

[lpetrich]

Titled link: China's supersonic 'flying train' could travel at 2,500mph | Daily Mail Online -- it will be a vactrain.

Here are some reference speeds:

• Highway driving: 100 km/h
• High-speed train: 300 km/h
• Airliner (long-distance): 900 km/h
• Hyperloop (vactrain): 1200 km/h
• Chinese "flying train" (vactrain): 4000 km/h
• Low Earth orbit: 28,000 km/h

Numbers in mph: 62, 186, 559, 746, 2490, 17400.

I included low Earth orbit because that is the case where a vehicle's speed will provide enough lift to balance out the Earth's gravity.

A problem is how far it takes to accelerate to those speeds. If it takes more than the travel distance, the vehicle will not reach its intended speed, and if it reaches that speed, it must decelerate to stop. I'll use 1 g (9.81 m/s^2) as my reference value, since it produces a vector sum of 1.4 g 45d downward. Much more acceleration than that will get very uncomfortable. 0.3 g is more pleasant, and it adds up to about 1.04 g at an angle of 17d from vertical.

The acceleration distance is v^2/(2*g), where v is the target speed.

The resulting distances, in km: 0.0393, 0.354, 3.18, 5.66, 62.9, 3080
For acceleration and deceleration together, in km: 0.0787, 0.708, 6.37, 11.3, 126, 6170
The above, for 0.3 g, in km: 0.262, 2.36, 21.2, 37.8, 419, 20600

For China, some city distances are Beijing - Shanghai: 1069 km, Beijing - Hong Kong: 1961 km, Beijing - Chengdu: 1526 km (Distance Calculator Find Distance Between Cities). So with the 4000 km/h version, one could do Beijing - Hong Kong in 1/2 hour with 10% or less of the distance used for accelerating and decelerating.

For the US, NYC - DC is 328 km, too short for the 4000 km/h version, though acceptable for a 1000 km/h version. That would give a time of 1/2 hour. However, NYC - Chicago is 1146 km, NYC - Dallas is 2208 km, and NYC - LA is 3940 km, far enough for a 4000 km/h version.

[BandA]

Scot and Brendan: also remember that people thought electric cars were practical, briefly, at the turn of the 20th century before abandoning the technology for something better. I'm not convinced that Hyperloop won't suffer the same fate - being technically possible, but economically unnecessary and less useful than other tech.

Electric cars were better behaved than internal combustion vehicles, and one was still used in the late 1920's or 1930's by a little old lady to visit her friend. Electric cars with their original Edison batteries still working still exist.