The Hyperloop: flawed fantasy or achievable challenge The tubes themselves would be elevated above street level, in which the capsules would float on a cushion of pressurised air expelled from tiny holes in skis attached to the bottom of the capsule. A magnetic linear accelerator at various points along the tubs would accelerate the capsules, each containing 28 people. For a Hyperloop link between Los Angels and San Francisco, Musk estimates the journey would take just half an hour and could transport 7.4 million people each way a year. All this he promises for just $6 billion (£3.88 billion). While it all sounds feasible, there are some potential pitfalls that stand in the way: Heat damage Compressing air and expelling it can produce heat, which could potentially damage the capsule and the surrounding tube. Musk predicts cooling the air from temperatures of 283 degrees C after compression and that temperatures of around 125 degrees C will be created immediately beneath the skis. Sam Jaffe, a senior research analyst at Navigant Research, said that pods would need to carry quite large coolant systems to counteract this. This would increase their weight and reduce their efficiency. He said: "The biggest concern with this plan has to do with temperature. The pod will be compressing air and expelling it downwards and backwards. All that air compression creates an enormous amount of heat, which can damage the pod and its machinery. Musk's solution is to add to each pod a water tank that will capture that heat and turn it into steam to be collected at the next station. Although the thermodynamic calculations are correct, a small pod with only a few cubic feet of room for a heat exchanger leaves little space for an efficient exchange of heat. That means that the flow of water must be increased, requiring a lot more water on board. There may be an elegant solution for this challenge, but it's not in Musk's current paper. Hyperloop capsule in tube cutaway with attached solar arrays. Wind stress Tall structures are prone to wind shear - just ask any sky-scrapper architect. The differences in wind strength at the top of a building compared to the bottom can place large forces on them and cause them to sway, if not appropriately designed. Musk proposes building the Hyperloop tube and its solar panels on top of pillars ranging from 20 feet to 100 feet. In analysis for Navigant Research, Mr Jaffe said: "Wind stress is another challenge. Any structure elevated 100 feet off the ground is going to be under a lot of wind pressure, which will act on it in weird and sometimes multiple directions. If that structure is a heavy tube stretching hundreds of miles in either direction, you effectively have a big sail. Will the concrete pylons be powerful enough to resist that pressure? Land cost The pylons themselves will not take up much land, but anyone attempting to turn Musk's vision into a reality will need to seek permission to build along a long thin 400 mile long strip of land. As some have pointed out, the $6 billion cost of building the Hyperloop does not include the cost of buying land, which would surely see the project costs rocket. Even if this was possible, getting planning permission to build such a construction is yet another hurdle. This is not as simple as it seems. Look at the problems with building wind turbines and facing the route of the High Speed 2 Rail link in the UK. Now imagine trying to do that with something that will be elevated above the landscape and visible for miles around. Even if they were to build it along the route of an existing highway, then the traffic delays caused by the construction would be difficult to sell to commuters and politicians. Hyperloop passenger capsule version with doors open at the station. Energy supply Musk proposes using solar panels mounted on top of the tubes to provide the energy required to power the Hyperloop. John Hansman, a professor of aeronautics and astronautics at the Massachusetts Institute of Technology, told the MIT Technology Review, that he had reservations about the energy it would require. He said: "It would be enormously expensive." My questions aren't could you do it, but could you do it in a way that makes sense from an energy efficiency standpoint and makes sense from an economic standpoint. He added that Musk's proposal to use Tesla electric vehicle batteries to suck air in through the front of the pod, compress it and force it out through skis on the bottom required a lot of energy. He said it was unclear whether the batteries would hold enough energy to do this for the entire trip. Practicality To transport 7.4 million people a year, Musk predicts the Hyperloop would need to have one capsule departing once every minute for 12 hours a day every day. However, he also raises the prospect of airport style security, which combined with the inevitable technical hitches that will create delays, could dramatically reduce the numbers that could travel. Equally, there will be peak times, during the hours when commuters want to reach their place of work and return home, so there will be times when it will be more congested and others when it will be quiet. So, the chances of carrying the maximum number of passengers Musk is aiming for diminishes. With the High Speed Rail link between San Francisco and Los Angeles that so riled Musk into dreaming up the Hyperloop in the first place predicted to carry at least 16.5 million, then the Hyperloop is unlikely to fulfil the area's transport needs. However, if Musk's estimate for a cost of $6 billion is correct, compared to the $68 billion for the high speed rail project, then he could build several Hyperloops to meet the demand. Hyperloop passenger transport capsule conceptual design rendering. G-forces Provided the capsules are accelerated slowly, then the forces exerted on the human bodies inside the capsules should actually be fairly low. No more than that of a sports car. However, the tubes would need to be as flat as possible and without any steep corners to ensure those forces stay low. If the capsule banks with the corners then those forces can be vertical, making the ride comfortable. Musk's other commitments Elon Musk himself has admitted that he is already over stretched with running his two companies - SpaceX and Tesla. He is also on the board for a solar installation company called Solar City. With five children at home, he has said he is not able to dedicate the time that would be needed to see a Hyperloop realised. Instead he has said he may attempt to build a small-scale prototype but will rely on others to take the idea through to fruition. Sadly, as Jay Yarow of Business Insider points out, a project of this scale needs someone of Musk's vision, influence and wealth to make a reality. Mr Yarow said: "Sadly, his 57-page plan for the Hyperloop is missing the key element for its success: Elon Musk."