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Overall Document & Proposal.
This is a very comprehensive proposal of nearly sixty pages, which includes not only the overall design but most of the individual required elements and construction methods, which have all been detailed and costed. I've ignored the specifics of the route, which has it’s own particular difficulties to surmount, and comment only on the unveiled concept of Hyperloop travel. The design uses knowledge from two Musk companies, namely lithium ion batteries from Tesla to power the pods, and extreme altitude technology from SpaceX which is behind the near vacuum within the tubes concept.
Mr Musk’s original intent was to create a better and cheaper method of transportation between the two cities of Los Angeles and California and so slash the expected cost of the Government’s High Speed train link. In Hyperloop Alpha he has delivered a concept to meet his intention and whilst this would be a great way of travelling between those cities, it is not the complete transport system such as I was hoping to find. It is not a fifth mode of global transportation. Instead it falls somewhat short - not being an integrated system, more a collection of individual links between conurbations. For it to become a world girdling system it will need a design feature for junctions between tubes - the equivalent of a set of points or switches on a railway. A part of the Musk proposal includes a brief comment referring to stations between the two terminals (page 10) but the rest of the proposal and concept as a whole, is concerned with a loop between just the two city terminals.
This is a very ambitious and high profile design. It is one of a number of designs currently being proposed based around the generic concept of tube transport, which I believe in time will become the fifth mode of travel. If Hyperloop tries and fails, it will set the concept back and I feel it has to be improved and modified before it progresses much further. I sincerely hope the scientific community will work together to build on this great leap forward in Tube Transport Travel which Mr Musk has initiated.
At first sight, in addition to the omission of junction details, there appear to me to be another two specific technical problems concerning the pods which are raised below.
Air pressure in the length of the tube is proposed to be 100 pascals (Pa) whereas ambient pressure is around 100,000 Pa. Tube air pressure will therefore be one thousandth that of ambient pressure and equivalent to being at an altitude of 150,000 feet, or 30 miles high.
1. The pods will therefore have to be pressurised for the comfort of the passengers. However part of the propulsion system will be sited within the pod namely, the forward main axial flow compressor which will have a quoted compression ratio of 20:1 thus raising the incoming air pressure from 100 Pa to 2,000 Pa, and a secondary compressor increasing the pressure to 10,400 Pa using a quoted 5.2:1 compression ratio. This will be used to propel the pod forward but is still only a tenth of ambient air pressure, not enough to sustain life, and so another air supply method must be used which caters for the passengers, although I can see no mention of how this is achieved in the proposal.
2. Understandably wheels won’t be used, and instead skids riding on a cushion of air are proposed. This will push air bearing design much further than hitherto, not that this in itself is a bad thing. The skids, with 28 fixed to each pod and 1.5m long by 0.9m wide, are lifted above the tube surface by combining compressor derived lift, and aerodynamic 'ram air' lift. However, the air gap or cushion is very small. Over the whole of the speed range from 0 to 760mph, the gap will be maintained between 0.5 and 1.3 mm (0.020 / 0.050 ins). A paper clip by comparison has a diameter of approximately 0.6mm. All 700 miles of the tube will need to be absolutely and completely clean of every single piece of grit to ensure the skids don't catch. Grit in an oyster is generally considered a good thing, but not here inside the Hyperloop.
General observations - design
1. The proposed passenger pods, each carrying 28 people will have a mass of 15 tonnes. Of this the payload will be around 2.8 tonnes, allowing 100kg per head (75kg / person + 25kg luggage each). This gives a payload / total mass ratio 2.8 / 15 = 0.18. To be as energy efficient as possible, the perfect, albeit unachievable, ratio is unity at 1.0.
2. It is also noted that the compressor and the compressed air will be cooled using the latent heat of change of state of water to steam. So water, superheated steam, batteries, passengers and luggage will need to be changed at the station at the end of each journey. In addition, all this is packed into the same pod as the travellers, who must be kept safe from the internal equipment under all circumstances.
It all boils down (excuse the pun) to an enclosure containing passengers, their luggage, superheated steam and other heavy equipment, travelling at speeds over 700mph in an evacuated tube which can’t sustain life where a single piece of grit inside the tube could cause a failure of the air bearings.
3. Currently the Hyperloop is only designed to travel over a distance of 350 miles or so with a journey time of 35 minutes. To achieve this, the onboard batteries hold a charge to cover a journey time plus a safety margin of 10 minutes. This has created a self limiting system in that extensions of the Hyperloop to other cities, countries and continents are limited to base stations set at around half an hour travel distance from each other, or less where there are minor stations linking into the central system. To increase the travel distance would require an increase in the onboard equipment and major scaling up of the components.
4. The joining points or switches will themselves be a major problem to the integrity of the near vacuum sealed tubular system not only if Hyperloop is extended to other cities but also for the minor stations joining the route. It is a problem which Mr Musk has not addressed but one of sufficient importance to cause the concept to succeed or fail. I believe I have a design which could possibly resolve this dilemma.
As I said earlier considerable expertise, cutting edge technology, time, money and resources have gone into the completed and costed Hyperloop Alpha proposal. I want to believe in it, I want the impetus and interest it has generated to gain momentum and bring about a finished and successful Tube Transport Travel system in the near future. But I just don’t believe this Hyperloop design is the way forward.
I would be very interested in meeting with the engineers and designers involved in the Hyperloop and discuss these points. I’d also like to know their opinion of my design, which I feel has distinct advantages partly from using an external propulsion system and also the avoidance of a near vacuum within the tube itself.
Text © Andy Marks 'A Man in A Shed'
Andy is also a partner in Skyscan a British aerial photography company.