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52 pointsarchgoon17 comments

Cool sci-fi story hook, but the "Gen 2" magnetic levitation idea is insane.

> For the elevated end portion, the design considers magnetic levitation to be relatively less expensive than alternatives for elevating a launch tube of a mass driver (tethered balloons,[19] compressive or inflated aerospace-material megastructures).[20] A 280-megaamp current in ground cables creates a magnetic field of 30 Gauss strength at 22 kilometres (14 mi) above sea level (somewhat less above local terrain depending on site choice), while cables on the elevated final portion of the tube carry 14 megaamps in the opposite direction, generating a repulsive force of 4 tons per meter; it is claimed that this would keep the 2-ton/meter structure strongly pressing up on its angled tethers, a tensile structure on grand scale.[3] In the example of niobium-titanium superconductor carrying 2 × 105 amps per cm2, the levitated platform would have 7 cables, each 23 cm2 (3.6 sq in) of conductor cross-section when including copper stabilizer.[4]

> A SPESIF 2010 presentation stated that Gen-1 could be completed by the year 2020+ if funding began in 2010, Gen-2 by 2030+.[1]

Even assuming these numbers are right, I disbelieve that we'll have the technology, let alone the resources, to magnetically levitate a megastructure 22 kilometers above the Earth, using 280 million amps of current, in the next 10 years. I don't think we even have a material strong enough to create 22km-long cables which can hold their own weight, let alone be useful as tethers.

At that point you probably start doing weird global scale things like screwing up all the bird migration patterns too

Yeah, we have enough problems with MRI machines generating scary magnetic fields inside a building, imagine the same thing but affecting everything within a 50km x 100+km swath of terrain.

What problems exactly? The fringe field of an MRI is only a few meters outside the bore before it drops off to 1 Gauss. Especially with all the shielding around the machine there is not really a problem with it with normal use. The power consumption of the MRI machine itself are also pretty modest. To keep the magnet on field and superconducting you'll only need to power the helium pump.

The biggest power consumer in an MRI system is the gradient system, which is only necessary for generating images. These need powerful amplifiers to generate the RF pulses.

Maybe "problems" was the wrong word. We need to take enough precautions with the magnetic field generated by an MRI, which drops to 1Gs in "a few meters". This proposed system generates a field of 30Gs at 22 kilometers distance. That's an unfathomable difference in field strength compared with some of the strongest magnets we use right now.

Ah okay, yeah I see what you mean. You are definitely right. RIP everyone with a pacemaker or another type of implant within more than 22km of that system. Implants are usually safe up to 5Gauss so that would be beyond 22km, I guess.

I looked it up BTW, and the maximum extent of the fringe field for a 1.5T MRI scanner drops to 1Gauss at about 4 meters from isocentre. Fun fact, the field strength of an MRI scanner refers to the strength at isocentre, whereas the maximum field strength generated by a 3T scanner is beyond 11 Tesla.

This concept is like someone in the 1800s suggesting that transatlantic travel would be best accomplished by building floating railroad tracks across the ocean...instead of aircraft.

Perhaps something like a launch loop would be easier than Gen 2: https://en.wikipedia.org/wiki/Launch_loop

I think most options are more feasible than Gen 2, from "just use Falcon Heavy" all the way through to building a space elevator.

Maglev trains are cool! There's actually one here on earth: https://en.m.wikipedia.org/wiki/Shanghai_maglev_train

There’s more than just one. The Shanghai one is the fastest, though.

There's a faster one in Japan, although it's not open to the public yet: https://www.bbc.co.uk/news/world-asia-32391020

Japanese trains are crazy. I rode the Shinkansen for a few hours. I actually wondered at the time if it was maglev because the ride was so smooth, quiet, and fast.

It is really incredible, so smooth and quiet - and so fast! What surprised me the most was how the track was banked in the curves like a race track.

Makes sense to have mass drivers like this on the moon or asteroids for sending cargo (mined materials) back to Earth orbit. Reduces the need for rocket fuel.

This is covered in-depth by Robert Heinlein in "The Moon is a Harsh Mistress" ([1], [2])

1. https://archive.org/stream/TheMoonIsAHarshMistress_201701/Th...

2. https://en.wikipedia.org/wiki/The_Moon_Is_a_Harsh_Mistress

Spaceline is another concept that makes access to space cheaper and easier to achieve. It's a space elevator dangling from the Moon's surface into Earth's gravity well.

Previous discussion: https://news.ycombinator.com/item?id=20895443

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