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Context

In the animé Attack on Titan, armed forces use a special device called 3D maneuver gear (also called vertical movement gear) to move through the air. The system is based on a pair of grappling hooks shot from the waist, and compressed gas. The gas is used both to shoot and reel back the wires, and to propel the user in multiple axes through a pair of exhausts on the back. The exhausts are mounted on gimbals to allow for rotation. The system is operated through a pair of wired controllers, one for each side of the user.

Here are some schematics:

Schematics part 1

Schematics part 2

And here is the thing in action:

Run, Eren, run!

I believe that such a system, in real life, would be useless for its intended purpose (military action, specially against giants). The main reason for its creation is a mix of plot necessity allied with the Rule of Cool. I think it is also anachronistic for its setting, and slightly Steampunk-esque.

However! Suppose that some billionaire from our real world wanted to build a functional 3D maneuver gear. Would it be possible?

Goal

To build a wearable device that would allow the user to swing among buildings, trees or other high/tall elements of their environments, by combining grappling hooks, steel wires and compressed air or some other gas.

The objective is to have one or more prototypes - mass production is beyond scope.

Edit: Achieving the velocities seen in the show is not necessary. Alternatives can allow for a leisure ascencion or descent, for example.

Constraints

The project will start with a technology level that we are expected to have on the next five years. It is ok (even expected) if new technologies are developed for and because of the project.

The real life 3D maneuver gear may be larger than the fictiinal one - a very large backpack woulf be accepted, but should not be heavier than 60 kg.

Money is not a constraint.

Motivation

This tool may be used by firefighters, search & rescue personnel, mountain climbers and window cleaners. Also the Rule of Cool.

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    $\begingroup$ Just as with Spiderman slinging around, and (he and Superman) snatching people falling at terminal velocity, I predict lots of shattered vertebrae. And energy densities are always astronomically higher in fiction than in real life. So... no. $\endgroup$ – RonJohn Oct 11 '18 at 16:42
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    $\begingroup$ Forget the gear, I want the knees of these guys who can land at those speeds on their feet and keep running without any injury. $\endgroup$ – SJuan76 Oct 11 '18 at 17:31
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    $\begingroup$ There's also the fact that at the velocities shown in the series, human response time is insufficient. It's like the speeder bike chase on Endor - at those velocities, people can't dodge unexpected obstacles. IRL, they'd just die. $\endgroup$ – jdunlop Oct 11 '18 at 17:36
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    $\begingroup$ Another crucial point: cables are always thicker and weaker IRL than fiction. (No, carbon nanotubes are nowhere near being anything but laboratory toys. It's why they're so great for fictional nonsense like space elevators). $\endgroup$ – RonJohn Oct 11 '18 at 18:23
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    $\begingroup$ And heavier. Cables are pretty darned heavy. $\endgroup$ – RonJohn Oct 11 '18 at 18:23
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Part one responds not to the technical difficulties of such a device, but a more fundamental conceptual issue.

Competitive Fencers are some of the fastest humans on Earth (both by self-selection into the sport and the Darwinian aspect that if you are able to win enough bouts to want to keep doing it...) and for us, the difference between you scoring a touch first or me scoring a touch first is set to 1/32 of a second, which is half of the lower end human response time of 1/16 of a second.

And I gotta tell you as a fencer, martial artist and a high performance motorcyclist - that's pretty amazingly quick response time.

So @jdunlop is onto a really solid point here: at the velocities indicated in the OP and the video, it's very unlikely a human swinger-operator could respond quickly enough to not end up pancaked against some built environment element or other.

It's also worth noting that humans "run out" of fast-twitch decisions pretty quickly, reaching "decision fatigue" when they are likely to make wild misjudgements or fail to decide - or more typically just... freeze.

In this scenario, either result is likely to be terminal!

Part two looks briefly at the physics (without maths)

Strength of materials is an immense issue with this concept, both for the device and its components and for the built & natural environment elements being used as hoist points for this idea.

The device:

Steel cabling would not be even close to strong enough or have enough longevity under the continually varying stressors applied, and would very rapidly reach metal fatigue and shear. Bear in mind we're not talking about the load of just the mass of the swinger-operator and the mass of their compressed gas canister and gas inside, we are also talking about the inherent load of the length of cabling itself, and more crucially, the force (simplistically Mass x Acceleration [more accurately see the vector equation below])being exerted along that cable (tension) by the moving mass of the swinger-operator and their equipment - and then subsequently massively changed in both amplitude and vector with each new connect-pull-swing-jet-disconnect event. Every hard stop event is a massive deceleration - placing immense strain on those cables - ask a crane operator about working load factors and braking stress - they ease those heavy loads down due to the huge impact sudden changes can have on hoist cables - and in fact the other massive failure mode for hoist cabling is sudden detensioning after overload - leads to a kind of unraveling / unweaving called "birdcaging" which then gets run through the various pulleys and blocks in the crane and the cable gets chewed to heck.

So the only material we know of at the moment which comes close to this level of tensile strength, strength-to-weight and resilience is natural spider silk.

enter image description here

where vector p is the momentum of the system, and vector F is the net (vector sum) force. If a body is in equilibrium, there is zero net force by definition (balanced forces may be present nevertheless). In contrast, the second law states that if there is an unbalanced force acting on an object it will result in the object's momentum changing over time.

Compressed gas has mass - and the more compressed it is, the denser, the higher the mass; in addition, pressure vessels are incredibly dense and have to be manufactured to pretty exacting specs - and they are heavy (ask a scuba diver) in the extreme. This adds immensely to the load on the putative steel cabling - and the more compressed your gas, the denser the pressure vessel has to be to constrain that gas. Moreover, to have enough gas to lift the swinger-operator through space and reverse vectors when necessary means your reservoir is staggeringly large. And of course, you have an ever-escalacting version of the rocketry lemma: more reactant = more mass to move = more reactant needed - but in this case with the additional constraint operating inside a 1G gravity reference frame.

Harness webbing / straps - the harness being worn to throw that human body around would need to be not only strong and tight fitting (with these forces, even the slightest slack will lead to immense impact damage from the straps slapping home on the swinger-operator's body with each event. The straps would need to be hecka wide in any place bearing significant load, and yet somehow conformable enough to body shape to not be incredibly uncomfortable or joint-de-articulating.

Swinger-operator - pretty much either isn't an unaugmented human to begin with or dies very quickly - even assuming we somehow handwavingly beat the reaction time point I raised above, the huge acceleration loads (3-5 gravities) in quick succession in varying directions would induce blackouts, redouts, broken limbs, de-articulated joints, broken spine, broken neck, decapitations...

Built or natural environment elements - each of the things being used as a point of purchase by the "grappling hooks" would be subject to both compressive and tensile loads in very quick succession in very small point-load sized areas, and in most cases we can assume things like concrete, stone, glass and other similar facade system materials - once in a while wood, and even less frequently steel or other metallic surfaces. With these kinds of impacts and loads, the grappling hooks will simply break through materials as load increases; as each grappling hook pops free (punching out concrete fragments from around its attachment point such that they become shrapnel moving towards our swinger-operator) suddenly plunging our swinger-operator out or down into space and shifting not just the remaining but now increased load onto other grappling hook connections - these too will catastrophically fail... I think you get the furry, flat and squidgy meaty pancake on the groundplane idea here.

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    $\begingroup$ Whether or not the gear is terminally unwise to use doesn’t change whether it can be built, surely? We come from a race that thought ‘Spandex with squirrel wings? I can jump off a cliff in that!’ $\endgroup$ – Joe Bloggs Oct 11 '18 at 19:18
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This is anime, so you already know the answer. Any one element of the task has a snowball's chance in hell of working. However, all of them working together... well. Let's just say good luck.

First, ditch the "pressurized gas container" that has far too low of energy density. Its a non-starter. At least start with a real power source, like burning hydrocarbons. A liter of gasoline goes a long way.

Second, it looks like he's throwing the grappling hooks. There's no way a human is going to be able to pull that off. The grappling hook falls with gravity at exactly the same rate as the climber. So if you want it to go much higher than you, you're going to have to fling it up really hard. Physically speaking, people dont have the muscle strength to do that. You're going to want to fire these grappling hooks from guns powered by that gasoline engine.

Third, darts glance off things. It's really hard to design a dart which embeds itself 100% of the time. A reasonable safety margin would involve making sure that the existing darts you have in place can keep you from becoming a splotchet on the ground before trying to fire another. Your aerial parkour practitioner is probably going to want 6-10 cables, and something really creative to keep them from entangling themselves.

Fourth, darts that are hard to embed are hard to take out. There's a very good chance that you're going to want to retract a cable in a direction it doesn't want to go. You cant just have the gas engine jerk on it, because that'll change your trajectory a bunch. You're going to need active darts which can get thinner on command to disengage. Generally speaking, that's opposed to the properties you'll need for a dart to penetrate in the first place.

Fifth, these wires would create some very brutal shocks. You need a static line to avoid stretching during use and maximize endurance, but you need some dynamic stretch as you start to lay into the line. This is actually the easiest to solve part of the puzzle. Each of the wire spools needs to be on its own little elastic mount. Still going to hurt, but better than what we see in the anime.

Then there's the human side, which many have mentioned. Training these perfect reactions so that you never once make a mistake, with your life on the line, is not going to be easy.

Speaking of which, do you know how many time the guy correctly threw the grappling hooks in the little clip you linked? Zero. Not a single one of them was correct. He actually became a bloody little splotchet multiple times over. You need to have a cable that's above you to oppose gravity. If all you do is launch them forward, you never gain altitude. He did manage to do a web-slinger like move once where he shot one to the side, but it was improperly balanced, so that just made him a bloody splotchet on the wall.

Oh, and the one throw that was useful also was the kind which would glance off the stone, so it probably was another red puddle on the ground as well. Tough angles.

Very hard to learn indeed.

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  • $\begingroup$ "Second, it looks like he's throwing the grappling hooks." In the animé, the hooks are shot from a muzzle on each side of the belt, powered by the gas. Anyway, great answer. $\endgroup$ – Renan Oct 11 '18 at 20:03
  • $\begingroup$ @Renan Ahh. It's hard to tell. The lines were laughably loose when they were launched, so it looked more like a throw. The lines really should be taut while being launched. $\endgroup$ – Cort Ammon Oct 11 '18 at 21:47

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