Take A Look At The Hyperloop Competition Entries
- by Hackaday
- Jan 29, 2017
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January 29, 2017 at 1:17 am
Would work well in subsurface tunnels for a moon colony, because you have a full vacuum already, but the idea seems rather daft for use on Earth. Actual engineer who works in aerospace here, I’ll offer my opinion…
Hyperloop is beyond stupid!
1) Plain unreinforced steel tube is vulnerable to crumpling if it receives a small dent. I actually went as far as running some FEA sims of the tube being destroyed – although its got a safety factor of around 10 (probably not a coincidence) at rest with zero distortion, as soon as the tube is knocked out of circular it can crumple in a cascading failure that takes out an entire tube section up to the adjacent expansion joints (something like 5 to 10 meters length in this design). Meanwhile a 7 to 10cm thick reinforced concrete tube would cost slightly less, is an off-the -shelf item designed for storm water drains and available with certified crush strength that gives a safety factor of approaching 100, with cascading failure being almost impossible and about an order of magnitude greater impact energy required to cause a perforation.
2) The air bearings idea in the original hyperloop white paper is the biggest con since eestor and emdrive. Because a) turbomachinery is not cheap, and a compressor of the size described even less so. b) Mr Musk describes using water -> steam to cool/intercool the compressed air, this has the _slight_ problem that the resulting steam volume is huge, as in many time larger than the pod volume. c) the batteries need to be recharged, doubling the number of pods that have to be manufactured, unless swappable batteries are used, in which case it “just” doubles the number of very expensive batteries. d) Lithium batteries last at most a few thousand cycles, ok for a Tesla car, but Hyperloop the batteries will last less than 18months, a huge operating cost penalty.
Thats not to say that semi evacuated tube transport is a bad idea, I would say it can easily be made to work for a tiny faction of the cost of hyperloop. Here’s how:
1) Use reinforced concrete tube, probably not off-the-shelf sewer or storm water pipe, as its design requirements differ slightly from pure compressive forces, increasing its cost. For a two tube system redesigned tube should cost less than $1k/meter in v large quantities.
2) Use conventional steel rail with two continuously welded rails mounted inside the tube using conventional rail clips.
3) Run of order +-5kV down the rails, this is a high enough voltage that it can supply power to multiple pods over several km between each AC->DC step down unit (perhaps powered by 15kV 3 phase running through conventional aluminium conductors in a external cable tray between the tubes, although this would require a higher voltage grid interconnection every 100km or so). However, the electric field between the two rails would not be strong enough to cause breakdown.
4) Reduce the operating speed to 500km/h or so. Although this makes the journey a bit longer, its around 250% of the speed of the best high speed rail, and only 50% longer journey time than Musks “design”. This reduction in speed massively simplifies the aerodynamics, its now possible to use interconnections between the two tubes to reduce “pistoning” effects. This is how the problem is solved in the channel tunnel. Interconnections every 50 to 100m could be made using a precast reinforced concrete module. These modules would also be a perfect place to install emergency escape ports.
5) The DC down the rails means that conventional solid steel axle boogies cannot be used, some sort of electrical isolation would need to be incorporated, but some conventional high speed rail (e.g. ICE) already has electrically non conductive axle systems. A slightly more tricky issue would be powering the propulsion motor, climate control system, lighting and infotainment. One solution would be to have a spinning propulsion motor with spinning drive electronics on one axle (similar to designs used for high power generators), and aux power take off generator/emergency propulsion motor on the other. Alternatively brushes mounted inside pressurized axle covers could be used for power takeoff, and all the electronics mounted on the chassis (some soviet era satellites used similar arrangements for tracking antenna connections).
6) The main problems limiting the speed of conventional high speed rail at present are a) pantograph stability at high speed – solved here using DC down the rails, and b) boogie stability. Maintaining boogie stability over a wide speed range with only passive techniques requires progressively more excessive track straightness requirements as the speed increases. However, this problem can easily be bypassed using active steering, or to a lesser extent just with active damping. Bombardier have done a lot of research in this area and aiui have tested a 550km boogie with “steering assist”.
In a concrete tube the potential for a catastrophic derailment is massively reduced, so the consequences of a system failure are less severe than if such a system was used in conventional rail at 500km/h.
Some issues that were almost completely side stepped in the white paper are climate/oxygen/CO2 control and safety/evacuation, but it should be possible to make a hyperloop style system that is much safer than an aircraft, in fact the tube should reduce the risk of derailment, offer a carefully controlled environment and potentially allow it to be safer than conventional high speed rail. Climate control is outside my area of expertises, possibly some sort of rebreather style system might work? Also maybe using water/slush ice as a heat sink might be most cost effective, there will be tens of kW of water heat to get rid of.
tl;dr Hyperloop is a non starter but simpler, similar designs may work with a lot of effort.
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