Any limit on how fast a Maglev can go?

[jaketodd]

When using magnetism to accelerate the Maglev, and neglecting the usual frictions, and also relativistic effects, is there any limit how fast it can accelerate to? Or is there any sort of increasing "drag" of any sort native to magnetism, which would get in the way of acceleration, as it goes faster and faster? Maybe if there's a speed limit to magnetic force? And maybe, if the train gets going super-fast, then the circuits that control the attraction in front of the train, and the repulsion behind the train, can't function fast enough to keep up with the train's increasing speed, and therefore produce drag, or at least a limit of speed?

Here's a good arxiv paper on superconductors and, in part, their relation to Maglevs:
https://arxiv.org/abs/2106.02825

For more basic descriptions, see here:
https://en.wikipedia.org/wiki/SCMaglev
https://en.wikipedia.org/wiki/Maglev

And here's a 2023 video that explores and depicts some of the fastest Maglevs:



Thanks,

Jake

 

[berkeman]

neglecting the usual frictions

What usual frictions? Like wind resistance? That's the main loss component unless you are in a big evacuated tube...

https://en.wikipedia.org/wiki/Hyperloop

 

[jaketodd]

What usual frictions? Like wind resistance? That's the main loss component unless you are in a big evacuated tube...

https://en.wikipedia.org/wiki/Hyperloop

Yes, I think the main friction would be atmospheric pressure.

And then there are other things like a bird hitting the windshield of the train.

Or a heavy guy like me sitting in my seat on one side of the train, and the train being offset by my weight! Unless you have adaptive magnetic forces that increase or decrease force on either side of the train, in order to compensate for that. I wonder if they already have that. Or is it not that far advanced yet?

But, yes, neglecting frictions like these, are there limits to how fast it can go? Oh, and of course neglecting relativistic effects.

Thanks,

Jake

 

[jaketodd]

I also suppose that when the train goes around a curve, if it's moving too fast, then it would fly off and crash, momentum. But just like there are banked roads for precisely this reason, I am thinking that could be adapted for by the mechanisms of the Maglev.

 

[berkeman]

But, yes, neglecting frictions like these, are there limits to how fast it can go?

I don't think you can neglect at least some air resistance. Have you looked into how fast the Hyperloop trains are supposed to be able to go, and what are their speed-limiting factors?

If you take all friction and air resistance out of it, you might as well be asking about railguns in space...

 

[Halc]

I also suppose that when the train goes around a curve, if it's moving too fast, then it would fly off and crash, momentum. But just like there are banked roads for precisely this reason, I am thinking that could be adapted for by the mechanisms of the Maglev.

The track curvature and banking needs to be rated for a specific speed. The faster you go, the larger the turn radius. The fastest (hyperloop probably) trains from say NY to LA required track that inverted the train in the Rockies, preventing the need for over-deep tunnels. Not quite orbital speed, but getting there...

One very limiting factor in the max speed is the distance between stops. The passengers can only withstand reasonable g forces (such as during a jet takeoff) and if the stops are close, deceleration is required before max theoretical speed can be achieved.

I don't think you can neglect at least some air resistance.

It wouldn't look like it does in the picture in the OP if air resistance wasn't an issue.
As for bird strikes, it probably needs to handle that. The military has the famous chicken cannon capable of firing a bird at supersonic speeds at a jet canopy, designed for exactly that purpose.

Even modern trains need to deal with large creatures such as moose, which tend to run on the track instead of to the side. They're not too bright. The trains can take it, but they have special countermeasures to convince the moose to take a left at Albuquerque.

No road crossings hopefully. There have already been about 6 tractor-trailer hits within a few km of me since I've lived here. Yea, we have one of the more dangerous crossings here.

 

[berkeman]

The fastest (hyperloop probably) trains from say NY to LA required track that inverted the train in the Rockies, preventing the need for over-deep tunnels.

Holy smokes, really? That's a nice relaxing commute for you...

 

[Halc]

Holy smokes, really? That's a nice relaxing commute for you...

Well that's assuming they ever build it. It's just paper at this point.

My actual commute was 120 miles each way, for 17 years. I put 4 cars in the knacker yard doing that.
No, I never did it every day. 1-2 of them a week, sometimes for 40 hours stints. Train would have been nice, but they never go where you need to be, and thus require secondary transit at both ends, and make it pretty uneconomical. The whole suburbia attitude really sucks compared to what I see in say the Netherlands where one can get by without owning a car at all.

 

[berkeman]

I guess if you're in a Hyperloop train car with no windows or dark outside, you might not even notice you're going inverted; you just feel postivie g's the whole time. They should have video screens for virtual windows, showing fields of cow whizzing by or something...

 

[jaketodd]

Forgive me; I am not educated well in this, but here are my questions:

If the Maglev ever got to relativistic speeds, would you need to increase the magnetism in order to support the increase in mass? Is there any limit to how much mass magnetism can support (in theory)?

And with time dilation, riding a Maglev might be a great anti-aging effect! =)

And then there's length contraction. It would be funny to see a shrunk train zoom past you. And then when it goes around a curve, there would be more complex length contraction: Making the train skinnier, laterally.

Thanks for putting up with me,

Jake

 

[Ibix]

If the Maglev ever got to relativistic speeds, would you need to increase the magnetism in order to support the increase in mass?

The mass doesn't increase, popsci's love affair with the concept notwithstanding. The weight does increase, however, so yes, you'd need stronger magnets to support it.

Is there any limit to how much mass magnetism can support (in theory)?

Depends how strong your magnets are.

And with time dilation, riding a Maglev might be a great anti-aging effect! =)

You'd need around $10^{19}$J to accelerate a single human to the speed where the time dilation is a factor of two. That's about ten years of the electricity generation capacityof the UK, if I've done my mental arithmetic correctly. It does not include the energy needed to accelerate a vehicle, nor to maintain the maglev magnets.

And then there's length contraction. It would be funny to see a shrunk train zoom past you.

You don't see length contraction since the visual effect is nearly cancelled out by the significant variability of light travel time from different parts of the train. In fact you would see an effect called Terrell Rotation, which you can google.

And then when it goes around a curve, there would be more complex length contraction: Making the train skinnier, laterally.

No, its motion is still parallel to its long axis. Googling relativistic wheels will find you some of the peculiar effects one finds in bodies on curved trajectories at relativistic speeds.

 

[DaveC426913]

We have to consider the location for the OP's train, and how far its going.

Because if it's on Earth, or, really, any planet, you are going to be circumnavigating the planet in mere seconds long before you reach relativistic speeds. Which means not only is it inverted, but the forces pullnig it off the track will be impressive.

And if it's not on a planet, then you don't have to worry about air resistance. Or gravity. Or, frankly - a track.
All you need is a parallel cable from start to end to keep your "cable-ship on-course.

 

[Vanadium 50]

If the Maglev ever got to relativistic speeds

Oh boy.

Some questions for you. At, say half the speed of light, how long would it take a train to go from, say London to Beijing? Compare this number to the time it takes to load a train.

Another is, take the avergae weight of a train, and estimate the kinetic energy it takes to reach this speed. Compare this to the total amount of energy produced by people in a year.

Finally, estimate the acceleration needed to reach half the speed of light before our train gets to Beijing. Compare this to what a human being can stand before being turned into paste.

 

[jbriggs444]

Another is, take the avergae weight of a train, and estimate the kinetic energy it takes to reach this speed. Compare this to the total amount of energy produced by people in a year.

To be fair, with perfectly efficient regenerative braking and some Really Big Batteries capable of storing a large number of teratons of energy. We would only have to generate the requisite energy once.

Come to think of it, even without a perfectly efficient regenerative braking system, there would be no further need to generate the energy more than once.

 

[DaveC426913]

At some point, we have to acknowledge that what were dealing with here is a relativistic spaceship. A spaceship that the OP, for some inexplicable reason, wants to constrain to a few metres from some surface.

 

[jaketodd]

At some point, we have to acknowledge that what were dealing with here is a relativistic spaceship. A spaceship that the OP, for some inexplicable reason, wants to constrain to a few metres from some surface.

How about a purely imaginative Maglev going from the earth to the moon? =) Would that satisfy your need for a "spaceship," while still being a Maglev? Hmm. Would be more like a railgun, without the gravity on the earth's surface. I think you would need magnets all around the train, in this example, to prevent it from flying off the track.

 

[jaketodd]

And if it's not on a planet, then you don't have to worry about air resistance. Or gravity. Or, frankly - a track.
All you need is a parallel cable from start to end to keep your "cable-ship on-course.

But the cable would generate friction, and place an upper bound to the possible speed.

 

[jbriggs444]

But the cable would generate friction, and place an upper bound to the possible speed.

The "levitation" part of magnetic levitation means that there need be no physical contact between the car and the cables.

 

[DaveC426913]

How about a purely imaginative Maglev going from the earth to the moon?

That would be fun.

At +8gs to turnaround and -8gs to arrive stopped, ChatGPT thinks that would take about 2 minutes and 20 seconds. Add some gentle deflections in there and you've got yourself a heckuva rollercoaster. But it only tops out at 7km/s - so well below Earth escape velocity.

 

[jbriggs444]

ChatGPT thinks

No. It seriously does not. It also stinks at legal research.

Try a sanity check on the figures you quote. 400,000 kilometers at a maximum speed of 7 km/s. That would take over ten hours.

This should have been obvious from the moment you mentioned "below Earth escape velocity". That's the sort of speed we use for all of our actual moon missions.

 

[DaveC426913]

No. It seriously does not. It also stinks at legal research.

Try a sanity check on the figures you quote. 400,000 kilometers at a maximum speed of 7 km/s. That would take over an hour.

:humble nod: My bad. First time ChatGPT user. And I didn't even bother to do a mental sanity check. Won't make that mistake again.

Using SUVAT

Accel leg of journey:
s=192,200km
u=0
a= 8g (78.4m/s)
t=?

t= 2,214s. Double that to 4,428s for decel leg, so:
1.23 hours, topping out at turnaround at 173 km/s.
(And a max time dilation factor of 1.0000001665)

 

[jaketodd]

The "levitation" part of magnetic levitation means that there need be no physical contact between the car and the cables.

Then what use is the cable, if it doesn't interact with the Maglev at all? Seems to me that would have to be frictional, or completely superfluous.

Oh! you're saying the cables are magnetic! I thought it was meant as a guide, due to friction, that would keep the train on course.

 

[Ibix]

Oh! you're saying the cables are magnetic!

How else is your maglev going to accelerate without rails or a cable or something? It's not a rocket.

 

[DaveC426913]

I should have said "rail", or "maglev rail" instead of cable.

Of course, the "lev" part no longer applies... As you said, it's more of a railgun.

 

[jaketodd]

How else is your maglev going to accelerate without rails or a cable or something? It's not a rocket.

Like I said, I thought the cable was a frictional guide. I think it goes without saying, that even if that's the case, there would need to be magnets/magnetic cables all around the train, in order to accelerate it.

I think you would need magnets all around the train, in this example, to prevent it from flying off the track.

...especially with weak gravitation from the earth, sun and moon. Much less than a Maglev on the earth, but still significant, and adaptive, as it travels to the moon, and these gravitational forces change.

 

[Frabjous]

How about a purely imaginative Maglev going from the earth to the moon? =) Would that satisfy your need for a "spaceship," while still being a Maglev? Hmm. Would be more like a railgun, without the gravity on the earth's surface. I think you would need magnets all around the train, in this example, to prevent it from flying off the track.

It’s hard to build a track between objects that move relative to one other.

 

[jaketodd]

It’s hard to build a track between objects that move relative to one other.

Then let's build a Maglev that goes to the dark side of the moon. The dark side of the moon never shows itself, although the moon does move. So, ya point taken. But just imagine that the gravitating bodies involved are stationary. Or how about a Maglev track that has a lot of physical flexibility to it. Then it would remain intact even with the motions that occur between it and its destination.

 

[Frabjous]

Then let's build a Maglev that goes to the dark side of the moon. The dark side of the moon never shows itself, although the moon does move. So, ya point taken. But just imagine that the gravitating bodies involved are stationary. Or how about a Maglev track that has a lot of physical flexibility to it. Then it would remain intact even with the motions that occur between it and its destination.

Do you even recognize that what you are proposing is nonsense?

 

[jaketodd]

Do you even recognize that what you are proposing is nonsense?

Of course it's funny and impossible. But it's a thought experiment, and thought experiments are fun!

As the earth rotates, we would run into the problem of even a flexible Maglev getting severed. But how about this: A track origin on the earth that extends around the whole planet, and such that, as the earth rotates, it slides on that origin track, so that it never gets cut off by the earth's rotation. The earth also has "wobble," so we'd need to engineer it for that too. But the wobble, I'm pretty sure, is predictable, so it could be done... at least in our entertaining thought experiment here

 

[Vanadium 50]

Ha. Ha. Ha.

You got is good. We thought you were asking a serious, albeit grossly uninformed question, and took it seriously. But apparently you were just funning with us. You got us good.

 

[jaketodd]

Ha. Ha. Ha.

You got is good. We thought you were asking a serious, albeit grossly uninformed question, and took it seriously. But apparently you were just funning with us. You got us good.

Not trying to fool anyone. I think it's self-explanatory that a lot of my posts in this thread are hypothetical at best. But they are fun. And technically, theoretically possible. So, let's continue to stick to physics, with a sense of humor at the same time. When I first started this thread, I posed some really good questions, I think. So, it's not all a joke.

 

[Vanadium 50]

 

[DaveC426913]

Do you even recognize that what you are proposing is nonsense?

Fantastical, yes. Nonsense? (OK, well, not any more than it started out in the OP.)

Skyhooks are cables that reach down from orbit and can nab or deposit payloads that are moving at a significantly different velocity. They do this by not being rigid or straight, and both ends are independently maneuverable to some extent. Of course, the window for connection is limited by that disparity in velocity.

Since we're already talking about technology levels that can accelerate a payload to relativistic velocities, and can be long enough to each the moon...

Presumably, a similar principle could be applied to the rail-train. Not the skyhook part, but having a cable system that is flexible enough to join endpoints with significant relative motion.

There are a number of places its Earth-end could be situated to facilitate this. The poles are an obvious one, but various orbital locations could work too.

 

[jedishrfu]

It seems we are a good point to close this thread having answered and explored the OPs original question,

Thank you all for commenting here.

Thread now closed.