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does a train use more engery than a plane as it goes faster ?


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#401 Catalina Park

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Posted 28 July 2013 - 08:00

Train drivers are taught to apply the brakes with one small application to compress the train then to make the proper amount of brake.

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#402 gruntguru

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Posted 29 July 2013 - 01:16

Interesting that Mat said that the front of the train is braked first, with varying delays to the rear. Railway engineers must know what they are doing, but perhaps there are cases when simultaneous braking, or rear-end-first braking would help in a situation like the Spanish crash.

I assume the front to rear order of braking is a natural consequence of the braking signal having to travel backwards down a pneumatic hose.

#403 mariner

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Posted 29 July 2013 - 13:19

All about rail bogie forces at high speed

http://www.diva-port...1657/FULLTEXT01

and some maths in it to keep Greg etc. happy!

Section 3.6 discusses " conicity" and why it leads to "hunting" - potentially unstable rotation around the bogie centre pivot point. This explains the massive horiziontal dampers you will see on high speed trains.

http://www.traintest.....se-up NRM.jpg

#404 Tony Matthews

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Posted 29 July 2013 - 13:28

I assume the front to rear order of braking is a natural consequence of the braking signal having to travel backwards down a pneumatic hose.

We demand the return of the brakeman!

#405 MatsNorway

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Posted 29 July 2013 - 20:16

There are systems taking care of it now. Im not sure about how common it is.

#406 gruntguru

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Posted 21 August 2013 - 04:30

Surprised no one has mentioned the Hyperloop concept here yet. All the arguments about "why" disappear if you could actually build something that is faster and uses 1/10 the energy of air travel.

 

http://www.teslamoto...rloop-alpha.pdf


Edited by gruntguru, 21 August 2013 - 04:31.


#407 Wuzak

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Posted 21 August 2013 - 09:27

Surprised no one has mentioned the Hyperloop concept here yet. All the arguments about "why" disappear if you could actually build something that is faster and uses 1/10 the energy of air travel.

 

http://www.teslamoto...rloop-alpha.pdf

 

Has a trial system been built?

 

One advantage of a conventional HST (like the TGV) is that they can use existing terminals.



#408 WPT

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Posted 21 August 2013 - 16:24

Trouble with the Hyperloop is it runs along the surface of the Earth. Dig a staight tunnel from where you are to where you want to go, 42 minutes oneway. And, gravity does all the work.    WPT



#409 RogerGraham

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Posted 21 August 2013 - 16:24

Capacity of 840 people/hour seems very low, if there is expected to be such a thing as "peak hour" on that line?



#410 bigleagueslider

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Posted 22 August 2013 - 04:51

Surprised no one has mentioned the Hyperloop concept here yet. All the arguments about "why" disappear if you could actually build something that is faster and uses 1/10 the energy of air travel.

 

Uh...OK.  A method of travel that is both faster than commercial aircraft while using just 1/10th of the energy over the same distance?  I'd bet that you're also a big believer in Keynesian economic theory.



#411 gruntguru

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Posted 22 August 2013 - 23:56

Either you don't bother reading a post fully before responding with sarcasm or you have a very limited grasp of the English language.



#412 bigleagueslider

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Posted 24 August 2013 - 03:07

Either you don't bother reading a post fully before responding with sarcasm or you have a very limited grasp of the English language.

 

I read both sentences in your post in their entirety.  And even though I only have the benefit of a California public school education, I still think I have a fairly good grasp of written English.  And in the interest of civility and global harmony, I'll drop the sarcasm.  But my question to you still remains- Please provide a coherent technical explanation of how this HyperLoop concept would be faster than commercial air travel while using just 1/10th of the energy.

 

Even Mr. Musk himself admits that the economics behind Hyperloop do not make sense for distances beyond about 300 miles or for less heavily traveled routes.



#413 gruntguru

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Posted 24 August 2013 - 08:14

The bit you didn't read or understand was the phrase "if you could" .

 

There is a technical dissertation in the link. This comes courtesy of Tesla Motors and Spacex. Apart from guessing the likely energy cost of maintaining a 99.9% vacuum (0.001 bar abs) in 700 miles of tube, I can't find a problem with their energy sums.

 

Interestingly the claim is not just a faster CBD to CBD time (where rail has obvious advantages). The claimed top speed is 700 mph - significantly faster than any commercial aircraft.

Please provide a coherent technical explanation of how this HyperLoop concept would be faster than commercial air travel while using just 1/10th of the energy.



#414 gruntguru

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Posted 24 August 2013 - 08:16

Capacity of 840 people/hour seems very low, if there is expected to be such a thing as "peak hour" on that line?

Peak capacity is stated as one 28 passenger vehicle departing every 30 seconds ie 3360/hour.



#415 bigleagueslider

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Posted 26 August 2013 - 05:22

The bit you didn't read or understand was the phrase "if you could" .

 

There is a technical dissertation in the link. This comes courtesy of Tesla Motors and Spacex. Apart from guessing the likely energy cost of maintaining a 99.9% vacuum (0.001 bar abs) in 700 miles of tube, I can't find a problem with their energy sums.

 

Ah, so all of the claims are qualified by the phrase "if you could".

 

I took a quick look at the "technical dissertation" linked.  And frankly many of the claims seem questionable.  One obvious issue is how will there be adequate heat transfer in a near vacuum to cool a several thousand horsepower electric propulsion system? Another issue is how will the vehicle deal with the torque reactions resulting from a seizure of the compressor system?



#416 gruntguru

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Posted 27 August 2013 - 00:58

I took a quick look at the "technical dissertation" linked.  And frankly many of the claims seem questionable.  One obvious issue is how will there be adequate heat transfer in a near vacuum to cool a several thousand horsepower electric propulsion system? Another issue is how will the vehicle deal with the torque reactions resulting from a seizure of the compressor system?

The propusion system is mostly external to the vehicle. The compressor system is a few hundred kW only. This heat is transferred directly to the air in the tube (air pressure in the tube = 0.001 atm). Surface area of the tube is enormous > no problem dissipating this heat.

 

I didn't find any mention of constraining vehicle in the roll axis except for a diagram of the linear induction motor which seems to involve a blade and slot. If the roll axis is unconstrained and relys on "COG below roll axis" (self righting) I doubt that the transfer of angular momentum from a seized compressor would be sufficient to turn the vehicle into a fairground "Twister" ride.



#417 bigleagueslider

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Posted 28 August 2013 - 05:53

The propusion system is mostly external to the vehicle. The compressor system is a few hundred kW only. This heat is transferred directly to the air in the tube (air pressure in the tube = 0.001 atm). Surface area of the tube is enormous > no problem dissipating this heat.

 

I didn't find any mention of constraining vehicle in the roll axis except for a diagram of the linear induction motor which seems to involve a blade and slot. If the roll axis is unconstrained and relys on "COG below roll axis" (self righting) I doubt that the transfer of angular momentum from a seized compressor would be sufficient to turn the vehicle into a fairground "Twister" ride.

The propusion system is not mostly external to the vehicle.  The primary propulsion is a linear motor device, and the "rotor" is mounted to the capsule.  Since the linear motor is not 100% efficient there will be heat accumulated within the rotor mass.  It will be very difficult to transfer waste heat from the pod to the tunnel wall via convection due to the very low air density in the tube. The capsule is effectively thermally isolated from the tube wall by the very low air density within the tube (convetive heat transfer) and the lack of contact between the pod and tube wall (conductive heat transfer).  If you read the paper you linked, Mr. Musk proposes to adress this heat transfer problem by using an on-board total loss system of evaporative water cooling, but there was not a detailed explanation of the total effects of such a system.



#418 gruntguru

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Posted 28 August 2013 - 09:00

Using the figures in the linked paper of 60 MJ/passenger/35 minute journey at 70% occupancy yields a total average power of 571 kW per capsule. Although this is a total figure including tube evacuation etc, I have pessimistically assumed that all of this energy ends up as heat in the tube atmosphere and the water cooling system. Assuming 200kg of the 290kg of water on board is vaporised during the 35 minute journey, the average heat absorption rate is 190 kW. That leaves 391 kW of heat applied to the air. There is no doubt that this heat will transfer out of such a large tube with only a minimal temperature rise above ambient. I will do a guesstimate if anybody insists.

 

The cooling of the rotor is a different question. With on board compressors consuming 276 and 52 kW the linear induction motor could be applying up to 243 kW average per capsule. Convective heat transfer from the 13.5 square metre surface of the rotor, assuming a temperature of 220*C and a coefficient of 0.2 W/m2/K is only 540 W.

 

Radiation loss at the same temperature and an emissivity of 0.1 is better at 3406 W. Total cooling - about 4 kW. So if the losses to the rotor are more than about 2% there could be a problem. Does anyone out there know what these losses would typically be?

 

Of course some of the total loss cooling (190 kW) could be applied to cooling the rotor, but the paper only talks about cooling the compressed air used to feed the air bearings.