# Hyperloop : pie in the sky or is it time? **URL:** \ **Category:** Factual Questions\ **Created:** [August 13, 2013, 6:37am UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116 "2013-08-13T06:37:17Z")\ **Posts on this page:** 20\ **Page:** 6
**Author:** ![Terr](https://avatars.discourse-cdn.com/v4/letter/t/839c29/32.png) [@Terr](https://boards.straightdope.com/u/Terr)\ **Post date:** [August 14, 2013, 6:37pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/101 "2013-08-14T18:37:17Z")
> [@AdamF](#): > > Actually, the difference may be quite small. The pylons are supposed to be 30 meters apart. So each pylon needs to carry one capsule (15t) and 30 meters of the tube (38 t) for a total of 53 tonnes. With a high speed train, each pylon would carry the rails (3 t) and 30 meters long section of the train (about 68 t) for a total of 71 tonnes. I believe the question was about normal rail, not “high speed rail” - since high speed rail usually involves much higher costs for the rails themselves than the usual railroad’s. If we’re looking at maximum loads, train locomotives weigh 120 to 240 tons. A freight train car, loaded, can weigh in at 100-150 tons (if your line will carry freight). And the length of a train car and a locomotive is usually less than 30 meters, so you have to add some extra for that as well. That’s significantly more than the 53 tons maximum load for the proposed system. ---
**Author:** ![AdamF](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/adamf/32/6246_2.png) [@AdamF](https://boards.straightdope.com/u/AdamF)\ **Post date:** [August 14, 2013, 7:02pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/102 "2013-08-14T19:02:35Z")
> [@Terr](#): > > I believe the question was about normal rail, not “high speed rail” - since high speed rail usually involves much higher costs for the rails themselves than the usual railroad’s. The hyperloop was proposed specifically as a cheaper alternative to passenger high-speed rail. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [August 14, 2013, 8:03pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/103 "2013-08-14T20:03:48Z")
Good discussion. > [@Exapno\_Mapcase](#): > > And remember that his Space-X has been around for more than a decade, using proven technology, and has never put a person in orbit. How far out does that put a Hyperloop? The obstacles to Space-X putting humans in space are largely bureaucratic over technological. But that probably applies to this hyper capsule train thing as well, just a different set of bureaucracy. > [@Habeed](#): > > There’s a blade sticking out the bottom of the transit car, and a “slot” the blade must slide in to when it reaches the part of the track that has the linear motor. I didn’t see that in the discussion. > [@Blakeyrat](#): > > What he should have proposed is building this as an attraction for an amusement park-- imagine one these circling Disney World or Disney Land the way their monorail does now, for example. That gives them a chance to building a smaller scale prototype, resolve some of the issues, it’s not a big deal if it’s shut down for maintenance, etc. An amusement park sized trial system would perhaps shake out construction methods, but it wouldn’t be nearly large enough to provide the speeds and thus prove the concept. > [@The Great Sun Jester](#): > > Any problems with building a coast to coast prototype–NYC to Seattle–and just pushing freight? The challenge is that infrastructure costs go up with distance. The longer the run, the more is spent on the rail/tube/highway. I think your proposing the time benefit of freight run would be economical, but with longer distances, airplanes are going to have the same or cheaper prices, and already exist, even if we ignore his remark about developing supersonic aircraft. I think that’s why he has picked the LA/San Fran corridor as his proposed test case. It is long enough to allow the speeds desired to make the travel time improvement significant, but short enough to make it comparable with supersonic aircraft. That corridor has a high volume of regular traffic, meaning he can justify the payoff in 20 years versus 200 years. Whether his numbers really work is part of the point of the discussion, but trying to make an amusment park ride is not going to tell us anything and trying to make a cross-country freight transport is going to outcost conventional air traffic. Plus, he needs a corridor that is fairly straight yet already owned to reduce the land purchase prices required. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 14, 2013, 8:31pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/104 "2013-08-14T20:31:29Z")
> [@Chronos](#): > > The only reason I’m optimistic about the space elevator is that the carbon nanotubes needed would have a host of other applications, and thus are likely to be developed with or without the elevator. The first things built out of it will likely be sporting equipment and military hardware, then it’ll gradually start showing up in more accessible consumer projects, and in civil engineering like bridges. Eventually, once it’s a mainstream material, then we can start talking about the many additional challenges in using it to build a space elevator. I think they’re a fantastic idea… just that any cost estimate is currently outside the realm of reasonable speculation. When one can buy 100 tons of the stuff as easily as one can buy any other industrial material, we can talk more confidently about space elevators. But at the moment, it’s not far off from “unobtainium”. Nothing in the Hyperloop proposal is comparable. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 14, 2013, 8:36pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/105 "2013-08-14T20:36:47Z")
> [@Sigene](#): > > I have a trip scheduled from my city to San Jose next month. I’ll get up at 5 am for the flight. Stop in Denver to change planes, then on to San Jose. I’ll land at 11am local time and spend an hour getting out of the airport and driving to my destination (noon local time; 2pm home time). Total flight time is about 4.5 hours. But my total trip time is 9 hours. I’m paying $450 for this trip. Hyperloop isn’t designed to replace longer plane flights. It is specifically for trips under ~1000 mi. Also, it should be the same price or lower than a plane flight of equivalent length. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 14, 2013, 8:41pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/106 "2013-08-14T20:41:36Z")
> [@](#): > > Nothing in the Hyperloop proposal is comparable. Agreed; the Hyperloop’s cost estimates are probably far on the low side, but there’s nothing in it that’s beyond current technology, or requires currently-unavailable materials. I didn’t mean to suggest otherwise, but was commenting purely on the side-topic of the space elevator. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 14, 2013, 8:44pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/107 "2013-08-14T20:44:45Z")
> [@AdamF](#): > > Actually, the difference may be quite small. The pylons are supposed to be 30 meters apart. So each pylon needs to carry one capsule (15t) and 30 meters of the tube (38 t) for a total of 53 tonnes. With a high speed train, each pylon would carry the rails (3 t) and 30 meters long section of the train (about 68 t) for a total of 71 tonnes. A train viaduct is a heck of a lot more than just the rails. I don’t know how much a 30 m section of viaduct weighs, but it’s obviously a heck of a lot more than 3 tons if it’s going to carry 68 tons of train. The tube, on the other hand, is strong enough on its own to be unsupported between pylons. Still, you have a point in that each pylon doesn’t need to carry the full train mass. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 14, 2013, 8:48pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/108 "2013-08-14T20:48:51Z")
> [@Irishman](#): > > The obstacles to Space-X putting humans in space are largely bureaucratic over technological. Yep. SpaceX has already had flights where a stowaway would have had a perfectly comfortable round trip flight. Obviously the safety issues shouldn’t be minimized–but the remaining long poles aren’t technical. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 14, 2013, 8:51pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/109 "2013-08-14T20:51:55Z")
> [@Chronos](#): > > Agreed; the Hyperloop’s cost estimates are probably far on the low side, but there’s nothing in it that’s beyond current technology, or requires currently-unavailable materials. I didn’t mean to suggest otherwise, but was commenting purely on the side-topic of the space elevator. Understood. I wonder if we shouldn’t build a space elevator on Mars first. It doesn’t require carbon nanotubes, and the relative lack of atmosphere resolves some problems. On the other hand, it’s not that useful unless we decide to have a strong Mars presence. Mars’s moons also pose some issues. ---
**Author:** ![Sigene](https://avatars.discourse-cdn.com/v4/letter/s/df788c/32.png) [@Sigene](https://boards.straightdope.com/u/Sigene)\ **Post date:** [August 15, 2013, 12:43am UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/110 "2013-08-15T00:43:29Z")
> [@Dr.Strangelove](#): > > Hyperloop isn’t designed to replace longer plane flights. It is specifically for trips under ~1000 mi. Also, it should be the same price or lower than a plane flight of equivalent length. Point taken, thank you. However it becomes less desirable for these shorter trips. I can fly to Denver in2.5 hours. But my entire trip will be about 5 hours by the time I get into and out of the station/airport. Reducing the flight time down to 1 hour means my trip is now 3.5 hours. How much is that worth? If its the same price and all else being equal, maybe I’d do it a couple of times. I like the view though from a plane , so maybe I’d sacrifice the extra time for a view and take the plane. But (and this may be our point of disagreement), I don’t believe it will be the same price or lower than a plane flight. I think it will need to be considerably more in order to recoup the cost of the infrastructure. It’s this extra cost that will make it uncompetitive. And this extra cost of a ticket is what will keep people flying instead of taking the tube ---
**Author:** ![The\_Niply\_Elder](https://avatars.discourse-cdn.com/v4/letter/t/8e8cbc/32.png) [@The\_Niply\_Elder](https://boards.straightdope.com/u/The_Niply_Elder)\ **Post date:** [August 15, 2013, 5:25am UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/111 "2013-08-15T05:25:45Z")
> [@AdamF](#): > > Actually, the difference may be quite small. The pylons are supposed to be 30 meters apart. So each pylon needs to carry one capsule (15t) and 30 meters of the tube (38 t) for a total of 53 tonnes. With a high speed train, each pylon would carry the rails (3 t) and 30 meters long section of the train (about 68 t) for a total of 71 tonnes. Sorry, but there is no physical way that 3 tons worth of steel rail can support a 68 ton weight, much less a moving train of that size at 200 mph. ---
**Author:** ![AdamF](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/adamf/32/6246_2.png) [@AdamF](https://boards.straightdope.com/u/AdamF)\ **Post date:** [August 15, 2013, 1:56pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/112 "2013-08-15T13:56:15Z")
I grant that my earlier post on the weight difference was quite off. But I have another bone to pick. The proposal seems to skip over the comfort and safety of the passengers. The capsule seems very short on amenities. I’ll grant you that the trip is only about 45 minutes, so a cab is a better comparison than a train or a plane, but even so. Sitting in a metal window-less tube only 4 feet wide, with another guy squeezed next to you seems claustrophobic at best. There is also no mention of any air-conditioning. And with 28 people squeezed in a tiny space, it is going to get toasty very quickly. Even elevators have fans these days. There is also no mention of supplying fresh air. Is the air only being recirculated? As to safety, most issues seem to be waved by saying that you’ll be in the destination soon anyway. Decompression is a real problem. In a plane, you always have the option of breathing pure oxygen. With the tube at near vacuum, oxygen wouldn’t help. Your only hope is that the leak is slow enough that you reach the destination before the internal pressure drops too low. And what about a fire on board? We’ve seen with the Boeing 787 that even high-tech batteries can catch on fire. ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [August 15, 2013, 2:10pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/113 "2013-08-15T14:10:02Z")
He pends some time talking about air supply in the PDF, but no mention of the other comfort issues. Decompression is covered too, including popout oxygen masks like planes have. I can imagine people sitting still for 30-45 minutes if there’s good entertainment or high speed internet or similar. But no moving, no getting up, no restrooms, doesn’t seem likely beyond that. ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [August 15, 2013, 5:34pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/114 "2013-08-15T17:34:03Z")
Just for fun I did some research into how thermal expansion is handled in large pipelines. None of it really applies to Hyperloop. The primary method seems to be to build the pipeline in a zig-zag fashion, with the supports at the bends having enough play to allow for expansion to move the pipe back and forth a few inches. That way, the expansion doesn’t accumulate. That’s obviously a non-starter for Hyperloop, given the radius required for the bends, and the fact that if you put in a bend every mile or so you’d drive the people in the pods insane. The other method that’s used is a variety of expansion joints, but they are complex and require regular maintenance so you want them near terminal ends where there is good access. And even then, they are limited in size and they do not have smooth interior surfaces. The most common type is a form of bellows. There are ‘slip’ type expansion joints as well, but they require more sophisticated seals since one end is moving against the seal whereas in a bellows type the seals can be fixed in place. I couldn’t find any expansion joints in the sizes needed for this scale, and none of them have the ability to take the forces of a heavy car passing through them at 600 mph. I don’t know that any of the seals available would keep out air with a 14.7 psi differential. In short, it’s a really difficult engineering problem, because these expansion joints would be up on high pylons and hard to get to for maintenance and inspection, and there would have to be hundreds or thousands of them. They would be expensive and the maintenance and inspection costs would drive the operating cost of hyperloop through the roof. Until I see a solution to the thermal expansion problem, all the other problems with hyperloop are irrelevant. This is a major show-stopper. Maybe someone will come up with a creative solution, but until they do the whole thing is pie in the sky. ---
**Author:** ![Habeed](https://avatars.discourse-cdn.com/v4/letter/h/a587f6/32.png) [@Habeed](https://boards.straightdope.com/u/Habeed)\ **Post date:** [August 15, 2013, 6:17pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/115 "2013-08-15T18:17:28Z")
> [@Sam\_Stone](#): > > Until I see a solution to the thermal expansion problem, all the other problems with hyperloop are irrelevant. This is a major show-stopper. Maybe someone will come up with a creative solution, but until they do the whole thing is pie in the sky. Agreed. Thanks for answering my ultimate question. It’s pie in the sky until someone demonstrates a working expansion joint that is inherently reliable and can hold the pressure on an actual section of pipe that is large enough to be on the same scale. Or until Musk demos a prototype. ---
**Author:** ![AdamF](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/adamf/32/6246_2.png) [@AdamF](https://boards.straightdope.com/u/AdamF)\ **Post date:** [August 15, 2013, 6:24pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/116 "2013-08-15T18:24:31Z")
> [@aNewLeaf](#): > > Decompression is covered too, including popout oxygen masks like planes have. Oxygen masks do not solve the problem. It takes time to put on an oxygen mask. At 35,000 feet, you got 30-60 seconds. This is why most planes are not allowed to fly above 40,000 feet. Because after a rapid decompression, pilots would loose consciousness in less than 15 seconds, which may not be enough to put on an oxygen mask. And most decompression accidents happen in seconds, there being about [50](http://en.wikipedia.org/wiki/Uncontrolled_decompression#Notable_decompression_accidents_and_incidents) of them every year. In hyperloop, such accident would kill everybody aboard. Because even if you have time to put on an oxygen mask, the outside pressure is low that you will be unconscious in 6 seconds, and brain damaged 5 minutes later. 100 kPa = normal pressure 80 kPa = pressurized airplane 19 kPa = pressure at 40,000 feet (15 seconds of consciousness) \<3 kPa = 6s of consciousness even if breathing pure oxygen 0.1 kPa= pressure in the hyperloop tube ---
**Author:** ![Habeed](https://avatars.discourse-cdn.com/v4/letter/h/a587f6/32.png) [@Habeed](https://boards.straightdope.com/u/Habeed)\ **Post date:** [August 15, 2013, 6:40pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/117 "2013-08-15T18:40:17Z")
I guess there’s no point in an oxygen system then. You’d have to deal with decompression with a system aboard each hyperloop car to signal for a re-pressurization of the entire tunnel. Sam Stone mentioned earlier that this would have to be done with explosive blow-out panels that would blow at points along the tunnel so that air can rush in everywhere evenly and not accelerate down the vacuum into shockwaves. The other side of the equation, then, since a decompression would be so expensive (and fatal if the communication system between the individual cars fails to signal properly, or if there are pressure sensor failures, and so forth), is to minimize the probability of it happening. As I understand it, airliners have a lot more structures on them subject to stress, such as between the wing box and the wings, the windows and the body, and so forth. You could design the pressurized section of the tube car to be significantly stronger than the body on an airliner. In fact, if I understand the math right, making the body of the tube-car heavier won’t increase the energy consumption of the hyperloop significantly. This is because the decelerators at the end reclaim nearly all the kinetic energy in the tube-car, and making the pressure vessel heavier won’t increase the friction between the car and the air in the tunnel significantly. So, changes because of this : 1. No Gull wing doors. You need hatches like on an airliner that cannot open if there’s a positive pressure differential. 2. No emergency oxygen masks, they are a waste of time. At least this means no stupid briefing at the start of every journey on how to use them. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 15, 2013, 6:49pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/118 "2013-08-15T18:49:29Z")
Frankly, I’m not sure why the proposal ever had gull-wing doors to begin with. “Because they look cool” isn’t good enough, and I’ve never heard any other justification for them, on any vehicle. ---
**Author:** ![Terr](https://avatars.discourse-cdn.com/v4/letter/t/839c29/32.png) [@Terr](https://boards.straightdope.com/u/Terr)\ **Post date:** [August 15, 2013, 6:55pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/119 "2013-08-15T18:55:10Z")
> [@Chronos](#): > > Frankly, I’m not sure why the proposal ever had gull-wing doors to begin with. “Because they look cool” isn’t good enough, and I’ve never heard any other justification for them, on any vehicle. The pop-inwards-then-slide-to-the-side type of door seems to be much more suitable and it is self-sealing with the air pressure on the inside. I guess they are called “plug doors” - but the designs I see online open inwards instead of sliding to the side. ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [August 15, 2013, 7:01pm UTC](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116/120 "2013-08-15T19:01:36Z")
> [@Habeed](#): > > Agreed. Thanks for answering my ultimate question. > > It’s pie in the sky until someone demonstrates a working expansion joint that is inherently reliable and can hold the pressure on an actual section of pipe that is large enough to be on the same scale. > > Or until Musk demos a prototype. I’m sure it would be relatively easy to design such an expansion joint, at least as a prototype. A bellows-type unit with a sliding track inside would do it. The track moves back and forth so the pod has a smooth surface to ride over while the bellows move back and forth. The problem isn’t building such a device - that’s fairly straightforward engineering. The problem is building a _system_ of thousands of such devices, up on elevated towers, that are cheap enough to build that you don’t totally blow out the budget of the design, which are robust enough to handle years of constant expansion and contraction without failure, which can be inspected and maintained without destroying the business model of hyperloop, and which can be removed and replaced very quickly and painlessly when they have to be. Think about this: every time you have to take an expansion joint out of service for inspection or maintenance, _you’d have to lose the vacuum in the tunnel_. That means everything comes to a stop until you’ve replaced/repaired the expansion joint, plus the time it takes for the pumps to evacuate 350 miles of air. Even if these things had an average service life of 10 years before you had to break their seals, with 5,000 of them you’re taliking about 500 shutdown events per year - more than one per day. This is a massive logistical problem. [Previous page](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116.md?page=5) [Next page](https://boards.straightdope.com/t/hyperloop-pie-in-the-sky-or-is-it-time/666116.md?page=7)