This is another in a series of semi-regular pieces I do in which I try and set out what science and engineering can achieve and what it can’t do. In short, try and sort out science fact from science fantasy.Last month, Elon Musk made a somewhat surprising announcement of a plan to build a 570 km Vactrain system from Los Angeles to San Francisco, for the bargain price of $ 6 billion. The media quickly reported it with great enthusiasm, however this has turned to gradual and increasing scepticism as they’ve inevitably asked engineers their opinion and they’ve generally taken the view that it borders on science fiction, if not science fantasy.
Certainly it has to be said, Elon Musk has a habit of proposing wacky ideas and proving his critics wrong. When he first launched an electric car business he was told that he was crazy and consumers wanted petrol powered cars. Of course he has since proven that there is a market for electric cars, much to the annoyance of some republicans who’ve been trying to use legislation to block the business.Musk also took on the space launch establishment. When he founded SpaceX he was told that space was expensive because that’s just the way it is. Now while his current launch system is something of a cut down version of what he first proposed (the initially proposed five engine reusable first stage has ballooned into a nine engine expendable stage) and he’s now quoting costs considerably higher than originally forecasted (the original proposal worked out at around $2,800 per kg/LEO while SpaceX is now offering launches at around $4,100 per kg/LEO). But equally SpaceX are offering to launch payload for at least half (or less) the price of the established space industry and they have demonstrated that private industry can develop both a man-rated rocket and capsule for a fraction of the costs often quoted by the established space industry. So credit must be given where it is due. However the hyperloop proposal has the critics coming out of the woodwork for good reason. Firstly there is the sheer scale of the logistics. Hyperloop proposes a train line capable of over 1,000 kmph, 760 km’s long all for a cost of just $ 6 billion (about $7.9 million per km). By way of comparison Britain’s High Speed 2 (Phase 1) system will be 192 km’s long and is estimated at a cost of £20 billion (or about $35 billion dollars) or about $182 million per km (perhaps an extreme example given the high value of land over which the line will cross). The rival California High speed rail system proposes to build a conventional high speed rail for $ 68 billion (about $89 million per km, but it will involve upgrade of existing track where possible). Even the French quote a preposterously low figure of $15 million per km for their lines, and they often use various tricks to cut down on costs (such as long detours around difficult terrain, or simply upgrading existing lines to take HS trains, not an option for the hyperloop). So Hyperloop is proposing to be able to build a line capable of 4-5 times the speed for between 1/23rd and 1/11th the relative price. Indeed it is worth remembering that when it comes to the above quoted figures, those behind such projects are well aware that many rail, road and infrastructure projects have a nasty habit of coming in way over budget. Casing point, in the UK an upgrade programme of the railway lines into London saw them spend £850 million on upgrading Reading station, with a similar amount spent on Rugby Station (that’s 20% of Hyperloop’s budget blown just upgrading the track around one station!). Now before anyone starts going on and on about the excesses of public bodies squandering taxpayers money, the railway’s in Britain have been in private ownership since the 1990’s and both of these upgrades were performed by private contractors.
In part the reason for the $68 billion forecasted cost of the CHSR is the fact that local communities have demanded tunnels and viaducts at places where strictly speaking they aren’t needed. There’s also the issue of land purchases to consider as well as stations, car parks and public transport hubs at those stations (a large chunk of the cash for HS2 is to be spent on this) to service the line.
Furthermore, its worth considering that the only existing Maglev of comparable scale is the Shanghai system, which is only 30 km’s long , has a top speed of 430 kmph, yet it took two and a half years to build and cost $1.33 Billion or about $43 million per km (probably a vast underestimate, the Chinese government has a habit of hiding cost overuns seen as embarrassing to the regime). So Hyperloop proposes a system 25 times the distance at twice the speed, built 5 times faster and at least 1/6th the relative costs, even though the labour costs in China are vastly lower than they are in the US.
And the chances are that if anything the costs of the Hyperloop proposal will be a lot higher than those for a conventional railway, or indeed even a conventional maglev system. A normal railway line after all doesn’t need an enclosed, pressurized cylinder around the train. Much of the line will have to be cut and cover tunnels or viaducts, with fewer options to “go around” obstacles. This is in part enforced by the fact that maintaining an air tight seal is going to be difficult (particularly so if the tube has to follow lots of contours as that means more joints and opportunities for leaks).
While opting away from a complete vacuum does make things a little easier, it still means you’re going to have problems with air leakage into the tunnels. Consider that Brunel trialled a similar idea, his atmospheric railway, back in the 1840’s. He only enclosed a small “pusher” element rather than the entire train and even this proved too much for technology at the time. In all probability you’d need pumping stations every couple of km’s along the route to regulate the air pressure and pump out excess air.Then there are other issues, such as the problem of choked flow and transonic turbulence around the vehicle. This has always been a problem for high speed flows (aircraft, gas turbine engines, engine manifolds, gun barrels, etc.). While the Hyperloop team claimed to have resolved it, I’ve not seen anything that indicates rigours scientific study of this problem. This is significant because the sort of buffering forces such a train would be exposed to are enormous, and the slightest instability could lead to enormous friction forces on the vehicle (which would require a lot of energy to overcome) not to mention discomfort for passengers (NVH related issues) or even catastrophic failure.
Also there is some debate as to the energy efficiency of maglev’s. Although they are potentially a lot faster than conventional railways. Mc Kay (2005) suggests Maglev’s are slightly more efficient than conventional high speed rail, but of course the figures for high-speed rail is backed up by decades of experience & thousands of miles of track, while the data set for maglev’s is considerably smaller (and under fairly tame conditions).
Maglev’s are also a lot less fault tolerant, vastly more expensive to install and harder to maintain (this is what brought down the Birmingham Airport Maglev). There are also issues such as noise to consider. And putting a maglev in a tube will if anything make these problems worse not better.
The Hyperloop also seems to consist of lots of small pods rather than a multi-carriage train. One has to question the wisdom of this. The whole reason why trains, planes or buses tend to be as large as economically possible on any particular route is to maximise economies of scale (i.e. it takes 2 pilots to fly a ATR-42 with 50 passengers, same number as it takes to fly an A380 with 500 on board), as well as to reduce the number of vehicles in service. The logistics of having a hundred small pods in transit, against a half a dozen multi-carriage units is obviously going to be a lot more complicated.
Remember in the real world things go wrong. Trains break down, faults develop with the track (there’s a huge problem in the UK with thieves nicking signal cable), “passenger incidents” (suicides, heart attacks, bomb alerts, people being slow to board/get off), all of which serve to slow down trains. On the highly congested UK West coast line, a single incident can have a ripple effect such that it adds multiples of delays to other trains (5 minute delay to one train can lead to a 30 minute delay to a following train) and can lead to whole services having to be cancelled. And accidents involving maglev’s have proven to be a lot more serious, largely due to the combination of high speed and lightweight construction. Also, returning to the transonic flow issue, any fast moving object through a narrow tube is going to produce considerable wake turbulence, even in a near vacuum. You would have to leave a considerable distance between pods to even out these effects.However, perhaps the big problem I can see is the background engineering and research. I’m reluctant to argue either way as to whether or not the Hyperloop is technically feasible, as there simply isn’t enough research data in the field to give a reliable answer. Completing all of that research to fill in these gaps would take both time and money. The German and Japanese Maglev systems (both considerably less ambitious) consumed many tens of billions to develop. And no bank or investor is going to commit money at this sort of scale until such research has confirmed the feasibility.
Don’t get me wrong, I’d love to say that a solar powered mass transit system was a great idea. However there are some serious holes in this proposal. Not insurmountable problems, but certainly some major question marks that need answers. I certainly would be in favour of research on this area, but as things stand, if you want to move people rapidly between the Californian coastal cities, I’d stick to conventional high-speed rail for the time being.