In my universe technology is advanced enough to have non-jet and non-rocket booster technology.

For a disc shaped UFO all of the technology is crammed in the ship and there was no more room left for any landing mechanism and so the designers put really strong ultra rare magnets all around the sides.

Using Lenz's law would make the UFO fall slowly through a tube coming to a slower stop at the bottom.

I want to make this as accurate as possible so I would like to know any potential problems with this setup.

for those who don't know - Lenz's law states that when an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such, that it produces an current that's magnetic field opposes the change which produces it. (https://www.electrical4u.com/lenz-law-of-electromagnetic-induction/)

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    $\begingroup$ Explain more about this tube. You are saying that there is a landing 'tube', and the disc shaped spacecraft maneuvers into the tube, then cuts power and lets the magnets lower it to the surface? Also, please replace 'UFO' with a more descriptive word like aircraft of spacecraft. $\endgroup$
    – kingledion
    Commented Nov 7, 2018 at 15:19
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    $\begingroup$ What Kingledion said on UFO's - if you built it, then the U in UFO loses its meaning. $\endgroup$ Commented Nov 7, 2018 at 15:22
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    $\begingroup$ If you have the ability to come to an accurate stop over the tube, and lots of advanced technology for your drive system, then why not use superconductors to create a diamagnetic landing pad to use with your supermagnets - you hover over it and stop, and the ship bobs in the air / vacuum, held there by magnetic repulsion. You can then replace your tall tube with a stack of several landing bays. $\endgroup$ Commented Nov 7, 2018 at 15:39
  • $\begingroup$ If you can accurately enter a landing tube at a slow speed as well as fly around the universe then surely all you need to do when landing is to avoid damage to the hull of the craft. There's not enough room for retractable legs so simply have an inverted tripod pointing up from the landing field and land on that with the tripod's legs docking into sockets in the base of the saucer. I don't see how a tube is necessary. $\endgroup$ Commented Nov 8, 2018 at 15:26

2 Answers 2


The main problem with this is that you are not actually dissipating the kinetic energy before landing (like any aero-braking would do), but simply turning it from kinetic energy to electrical energy, which still needs to be dissipated.

Since you state that space is cramped and there is no room for anything else, I assume you cannot afford having any dissipating device, so you are left with the problem of that excess energy.

Other than this, the same concept is used in the so called "magnetic brakes", which actually use this very principle (but have something to dissipate the generated electric energy).

  • $\begingroup$ But if magnets are on UFO and tube is metallic, then it's the tube that is getting hot, not UFO. and tube isn't cramped. $\endgroup$
    – Mołot
    Commented Nov 8, 2018 at 13:05
  • $\begingroup$ @Mołot, but once the tube is molten by induced heat there will be no more circuit to use Lenz law... $\endgroup$
    – L.Dutch
    Commented Nov 8, 2018 at 13:13
  • $\begingroup$ But why would tube be molten? UFO is cramped. There are no indications area around the tube can't support cooling infrastructure. There also is no length of tube given, so amount of heat per meter of tube can be arbitrary small if he will just make tube long enough. $\endgroup$
    – Mołot
    Commented Nov 8, 2018 at 13:25
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    $\begingroup$ The amount of heat generated depends greatly on the materials used. If excellent electrical conductors are used and this power is harvested at many points along the tube (preventing the fact that it is DC from being an issue), most of the transferred energy could be electrical, not thermal. Heck, add transformers at each of the sections to convert to AC, and that power could be put to good use on the ground or in assisting slowing the craft down as it nears the ground (placing a stronger electrical current in the coil closer to the ground). $\endgroup$ Commented Nov 9, 2018 at 11:27

There are three things needed for electromechanical systems:

  • a magnetic field,
  • an electric current,
  • relative motion between the first two.

If you have any two, you can create the third. You can use ultra rare magnets to induce a current in a tube through which they're moving, but unless the current is actually flowing you don't really get much deceleration. Even then, all that will happen is that you reach a lower terminal velocity faster. You could do that better and cheaper by filling your tube with a dense liquid and installing an air lock at the bottom.

Now, if you really want your rare earth magnets to slow down your craft, what you need is force acting upwards. Rather than Lenz's Law, you need Ambrose's Left Hand Rule.

Remember what I said earlier about 3 things needed for electromechanical systems? To get force acting upwards, you need a magnetic field and an electric current. Essentially, you turn your tube into a hollow cored solenoid, such that the polarity at the bottom of the solenoid is the same as that at the bottom of your ship. This will cause your ship to rapidly slow down. The coil also acts as a current multiplier so that you don't need to burn down the place supplying enough current to stop a several ton craft.

  • $\begingroup$ Actually I think he's trying to use eddy currents to break. Which is a thing $\endgroup$ Commented Nov 7, 2018 at 17:22
  • $\begingroup$ However, eddy current braking relies on the vehicle being braked moving along a t a fair clip - it will not work well for a slow moving vehicle - not enough induced current = not enough opposing magnetic field production = WHAMMO! $\endgroup$ Commented Nov 7, 2018 at 18:43
  • $\begingroup$ @ratchetfreak: No, I'm using eddy current braking. He's dropping a magnet down a conductive pipe. You can't generate sufficiently large counter currents unless you use a coil. And putting an existing current in the coil means you have a closed circuit that can absorb the induced current. It's the same principle as a dynamo. Better yet, use ac and you can actually use the current thus generated $\endgroup$
    – nzaman
    Commented Nov 8, 2018 at 12:20
  • $\begingroup$ @nzaman dropping a magnet down a conductive pipe will slow down the magnet. It's a really common classroom experiment to illustrate eddy currents. $\endgroup$ Commented Nov 8, 2018 at 12:34
  • $\begingroup$ @ratchetfreak: As I said in my answer, it only brings it to terminal velocity faster. The speed would still be enough to cause injury or death at the bottom. Also, consider the mass. You want continuous deceleration and you need the impedance of an opposing magnetic field for that. Getting energy back should be a secondary consideration only. $\endgroup$
    – nzaman
    Commented Nov 8, 2018 at 12:38

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