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I'm working on a military sci-fi piece that involves a lot of exo-skeleton enhanced, well, parkour -- for lack of a better term -- and was wondering how to deal with descents from great heights.

The characters want to traverse a city while remaining as high above the street as possible to avoid hostile creatures lurking below, but do inevitably come across gaps between buildings that cannot be jumped even with exoskeletons. They can't just jump down, because even plate armor won't protect you from deceleration injuries and I'm trying to avoid more handwaving than I already have for other components of the setting.

I'm trying to come up with a faster, more dynamic way for them to make descents to street-level than just taking the stairs every time. I had them using a material-mode of their protective suits called Sharkskin that essentially coats them in sandpaper. In my head, while discover-writing this as a poor student of physics, I thought the friction would allow them to stay on the wall while slowing their descent. However, with more thought I just realized this would probably result in a tilting effect, with the part of them not clinging to the wall descending faster than the part that was -- leading them to simply fall to their death.

They don't have grappling hooks or jetpacks or climbing gear.

Am I right in that an incredibly abrasive material would not be adequate for this? If so, is there a way they could distribute their weight to make such a vertical descent possible such as staying flat against the wall? What other sort of materials or suit-attached solutions could be used to descend vertical walls quickly? I don't want them to have more gear than their weapon and a utility tool thingamajig they have.

But all creative and engaging solutions are welcome.

NOTE: It should be mentioned that I submitted a chapter of this to an online writing group of about 30 people and nobody called bs on this, and it did not seem to break the story for anyone. It was just me, working on another chapter involving this mechanic, that the thought occurred to. But I'd still like to take a closer look at this before potentially disseminating it to a wider audience.

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    $\begingroup$ I'll point out that if you have sufficient friction to stop your descent, you also have enough friction to climb back up. This type of friction-based system won't just let you descend quickly, it'll also let you scale a building like Spider-Man, which may or may not be a desirable possibility for your story. You do need a way to "stick" to the building, though. $\endgroup$ – Nuclear Wang May 5 at 13:36
  • $\begingroup$ To be honest, I would think some kind of climbing gear would be standard! I mean if you're going to devise a machine that can help its wearer leap and run and climb and so forth, some means of descent, some means of dealing with random "hostile creatures" and some means of ascent after would be an integral part of the planning of this machine! $\endgroup$ – elemtilas May 5 at 13:43
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    $\begingroup$ And if friction is just slowing but not stopping descent, you have a lot of heat to move off in a very short amount of time. $\endgroup$ – R.. GitHub STOP HELPING ICE May 5 at 21:18
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    $\begingroup$ Friction does NOT HELP AT ALL - there is zero force towards the wall. $\endgroup$ – Fattie May 6 at 18:55
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    $\begingroup$ I have to support Fattie here, wall friction will not help you slow, it will just push you away from the wall. in order for friction to work you need a force pushing you against the wall, otherwise friction almost instantly falls to zero. If you want to slow a fall you need air drag, that is some form of parachuting. $\endgroup$ – John May 7 at 15:36

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Always be yourself, unless you can be Batman, then always be Batman

Batman has a good few tricks up his sleaves for rapid descent, the two that are most interesting to us are the wingsuit and his ropes.

Your average modern wingsuit isn't really designed for this stunt, it's more for entertaining yourself on much bigger falls, before deploying a full parachute. Batman's wingsuit has much larger wings which are utterly useless for anything other than looking good ahem which are more suitable for slowing short to medium falls.

Rope descenders along the lines of an autobelay are a more realistic option, (apart from the fact that they're rather bulky, require a harness attachment and you need to know in advance the maxiumum fall distance), you just hook them onto something solid, clip in, and jump. Just letting go at the top of a wall takes some getting used to.

Frankly you're better off doing what special forces currently do and abseiling.

Your current plan requiring high friction suits wouldn't work, due to the lack of a force between jumper and building (there's nothing holding them onto the wall), but a parkour style leap from balcony to balcony, or even window ledge to window ledge could be done. Simply going down the stairs may be quicker and would certainly have a lower casualty rate.

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  • $\begingroup$ Note that if the starting building is taller than the gap, and you have sufficient width (which may require going back to a previous building), you may not even have to descend to street level - fix a line, go back the distance of the gap, then jump off and swing onto the next building. If the next roof is the same height, you don't even really have to worry about the landing - physics will put you (roughly) the same distance up. Usability depends on a number of factors, obviously. $\endgroup$ – Clockwork-Muse May 8 at 17:14
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Gummy rollers

This system relies on a sticky substance like tar or glue that could be used to slow a descent. I'm imagining something like paint rollers on the hands and feet, which would either be coated in a sticky substance, or slowly exude it from a reserve tank. As the user descends, the rollers peeling away from the wall will sap some of the downward kinetic energy. The rollers could be motorized to control the rate of descent, or even driven backwards to allow a user to slide up a building.

Something like flypaper might work, but a static sticky surface will become less effective over time as it accumulates dust or other contaminants. That's why I suggest a slowly oozing, thick liquid, which would constantly be refreshed and not lose sticking power over time. You might need to do some handwaving to get a futuristic nanofluid with sufficient sticking power, however.

As an added bonus, your characters will make cool slug trails as they descend buildings! Similar to how combat divers are informally referred to as Frogmen, this special unit might be referred to as Slugmen.

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    $\begingroup$ This was going to be my contribution. Not something I have ever seen in a military scifi, for sure! $\endgroup$ – Willk May 5 at 14:56
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    $\begingroup$ Nice idea, maybe with something like geckos use $\endgroup$ – Jason Goemaat May 5 at 22:33
  • $\begingroup$ Now that I'm reading this, I expect the handwaving part to be: "The slugooze, as they called it, is a highly sticky non-newtonian fluid. It feels like oobleck, if you can ignore the stickiness, but liquefies rapidly when in contact with a catalyst. Pumping a non-newtonian fluid through a nozzle is a problem, though, so it is mixed on demand from two components inside the rollers. As a third component, the catalyst is mixed in, determining the time until the slugooze liquefies again. Controlling the mixture ratio gives the Frogmen their ability to control the speed of their descent." $\endgroup$ – orithena May 8 at 11:23
  • $\begingroup$ I can't see an obvious reason for it to be non-newtonian. Authors like throwing terms like 'non-newtonian' around to handwave stuff really annoys people who understand the terms. You'd need something like the 'nano tape' you see advertised around (though I doubt there's much really 'nano' about that either! But yours could be, borrowing something like the gekko feet effect). $\endgroup$ – Dan W May 8 at 16:54
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Super pogo stick.

super pogo

https://www.pinterest.com/pin/554576141587551724/

Your soldiers carry pogo sticks on their backs. The unobtainium springs capture the energy of their descent, and when they are in a hurry give it right back; when they hit the ground they decelerate over a split second then bounce right back up nearly as high as they were. For long drops you need an exosuit on because it is still a lot of work for the quads.

The super pogo will be good for other things too. Of course you can move quickly on it although the exosuit itself is about as good; mostly this is useful if your exosuit is damaged or you don't have it, but you have the pogo. A changed setting will keep the spring coiled and after jumping down the soldier can stay on ground level; the pogo will gradually uncoil the spring if desired or (more dangerously) stay coiled until deployed. A soldier can gradually coil the spring by making a lot of little jumps and storing the energy. That can be used for an ascent, or punching open doors.

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    $\begingroup$ Neat idea. Instead of jumping all the way down to the ground and all the way back up, you could also use the pogo stick as a spring-powered launcher to move directly from rooftop to rooftop - just ratchet the spring down, point yourself where you want to go, and release. This would work on arbitrarily tall buildings, so long as they're sufficiently close, so the pogo wouldn't need to be as beefy as something that could absorb a very long fall. $\endgroup$ – Nuclear Wang May 5 at 15:30
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    $\begingroup$ @NuclearWang it wouldn't, actually. While the terminal velocity is your friend when you're trying to not splat against the ground - 30 meters of a stiff spring should provide a comfy deceleration of one second of 6G (which might not even knock you unconscious, assuming the rod doesn't slip, assuming you did your math right and assuming you can actually deploy the thing in time) - it's also your enemy when you're trying to recuperate that energy. One second of 240 km/h headwind doesn't sound like much, but it certainly is something. $\endgroup$ – John Dvorak May 6 at 6:32
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    $\begingroup$ So... 30 meters of a stiff spring coiled into something you can wear on your back? I know this is the far future, with unreasonably good materials and all, but ... I still wouldn't put that on me. Any containment failure would cut out a big circular hole through my torso and cause significant damage to surrounding property, which I'd have to pay. Also I'm not sure I like the idea of six times my weight up my ... whatever the attachment point is. I'll stick to a parachute, thank you. $\endgroup$ – John Dvorak May 6 at 6:37
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    $\begingroup$ @JohnDvorak I don't understand what you're getting at at all, or what part of my comment you're even referring to - what "wouldn't"? The size of the spring needed is related to the kinetic energy absorbed, which is proportional to the user's speed. Jumping from rooftop to rooftop will not require someone to be moving anywhere near terminal velocity - a person can jump a 20m gap moving at only 15m/s. A building has to be over 1500ft tall to hit TV. Why on earth would you need a 30m long spring? $\endgroup$ – Nuclear Wang May 6 at 12:43
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    $\begingroup$ For information, I've done the pogo stick's maths in this answer. $\endgroup$ – Blackhole May 8 at 21:01
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To have any noticeable amount of friction, you need a force that presses both surfaces together.
While it seems possible that your soldiers swing over the edge of a roof in such a fashion that some friction is created at the beginning, their fall, or glide, will mostly be guided by gravity, pulling them straight down, leaving (next to) no net force pushing them against the wall.

While friction seems to be the least handwavey way of slowing them down, their suits must provide a way to press them against a vertical wall. Maybe thy could have some sort of overpowered vacuum cleaner in their gloves. After all, they must have some unobtanium battery or such to power their exoskeletons, so powering this super hoover should not be an issue. Magnets would work, too, if they could reliably expect ferromagnetic building materials.

But the hoover would of course have some massive benefits around the household...

Other than that, a wing suit or something similar to that might slow them enough. I don't know enough about wing suits, though.

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  • $\begingroup$ Is there a difference between the friction of an abrasive substance like sandpaper and adhesion created by a glue? What if the suit was able to create a 'sticky' surface or something like like millions of "micro-suction-cups?" $\endgroup$ – Delirium May 5 at 13:16
  • $\begingroup$ @Delirium - you could coat the buildings (and your machines) with velcro. $\endgroup$ – elemtilas May 5 at 13:46
  • $\begingroup$ @elemtilas where's my +funny mod when I need it? $\endgroup$ – Adrian Colomitchi May 5 at 14:08
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The Assassin's Creed series of games had one of its protagonists descend upon enemies using makeshift parachutes that fit inside his clothing. Players generally suspend their disbelief on that one.

Ezio Auditore, one of the protagonists for the Assassin's Creed series of games, jumping from a tower to a fortification while using a parachute to glide.

Your guys may not have climbing gear, but they may have base jumping gear. Base jumping is a lot like regular parachuting, except that since people usually leap from buildings you don't have a lot of time to do stunts before you open the chute.

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    $\begingroup$ A lot of suspension of disbelief required there! The concept of a "makeshift parachute"; the size of the "parachute"; holding it in your hands; holding it with bent arms; being able to steer it; the volume of the "parachute" fabric, rope and struts being greater than the volume of the character, never mind fitting in a backpack, and totally forget hiding it under clothes; and no doubt more I could think of with a little time. It's very useful if you have a shark you want to jump though - it totally succeeds on that... $\endgroup$ – Graham May 6 at 0:24
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    $\begingroup$ @Graham this is far future. Ultra-strong fabric that has no thickness and folds easily is quite low on the "unbelievable" list (GMO spiders for the win). I do suggest using wrist straps though. If you let loose while descending, you're going to plummet. $\endgroup$ – John Dvorak May 7 at 15:03
  • $\begingroup$ A bigger problem is getting the parachute to open, if your fall distance is less than about 300ft it likely will not open before you hit the ground. $\endgroup$ – John May 7 at 15:52
  • $\begingroup$ @John this is what base jumping is all about. It's a recognized sport. $\endgroup$ – Renan May 7 at 16:08
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    $\begingroup$ @Graham A guy successfully landed a 35 sq-ft parachute from an aircraft. Balled up, it looked small enough to fit an a fanny pack. It's on Youtube. Just search "smallest parachute." When you watch it, the thing on his back isn't it. That's his regular 'chute as backup. The small one is the balled up one his buddy tosses out behind him. $\endgroup$ – Random Driveby May 8 at 13:04
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GeckoPack

The GeckoPackTM Mk. 3 is a specialized descending device containing two elastic "arms" with a very low spring constant, enabling 10x expansion of each arm. In "descend mode", one arm with a Y-shaped terminator rotates up and over the wearer's shoulders, as both pads making contact with whatever surface the wearer is facing. After contact confirmation, the system sends a safe signal to the wearer's HUD, and they may push off from the surface, descending 10 m in a controlled fall. The wearer swings back to the surface in a smooth arc and the second arm rotates into position to make contact. Upon successful contact, the first arm detaches and the user is free to make another drop. When the pack's built-in IR proximity altimeter detects that the user is on a safe platform, both arms disengage and the user is free to continue on their journey.

Technology

While the exact details of GeckoPackTM Mk. 3 are classified, patents indicate they utilize Van der Waal's forces between nano-textured setae on fractal-structured contact pads and the desired surface. This possibility was first demonstrated by the lowly Squamata Gekkota order, commonly known as "geckos". Note that the pads do not require any consumable adhesives and are, in fact, self-cleaning.

Limitations

GeckoPackTM Mk. 3 is suitable for descending any smooth, non-porous structure. Structures with rough surfaces may also be descended, but no guarantee is provided as to the safety of such operation. As always read the User Manual completely before using this product. If the contact pads are not able to generate adequate adhesion force, a "weak contact" alarm will be sent to the user's HUD. THIS WARNING MUST NOT BE IGNORED!!! Please note that the GeckoPackTM Mk. 3 logs all uses and indicator signals for later telemetry recovery. This information is admissible in a court of law should the user misuse this product.

Thank you for purchasing your GeckoPackTM Mk. 3, and happy rapid descents!

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    $\begingroup$ If I not mistaken I believe this is an actuality today. $\endgroup$ – Frank May 6 at 5:40
  • $\begingroup$ There is this: sciencemag.org/news/2014/11/…. Just not fully integrated into a climbing/rappelling backpack. $\endgroup$ – Lawnmower Man May 6 at 6:16
  • $\begingroup$ the problem is as soon as the first pad makes contact the person rotates away from the wall, tumbling ass over teakettle until they hit the ground. Contact will only push you away from the wall. gecko feet require static contact to work. its a great way to climb down but it will not help you in a fall. $\endgroup$ – John May 7 at 15:49
  • $\begingroup$ @John Well, I didn't draw a picture, but the idea is that the device is worn like a backpack + climbing harness, and the "legs" attached to the "feet" are highly elastic. The pads act like an anchor for a very heavy rubber band. As you push off, the leg stretches, but with increasing force, so that it brings you back to the surface after several meters. You get two legs so that one can establish contact at the lower point while the upper leg is anchoring you. $\endgroup$ – Lawnmower Man May 7 at 20:10
  • $\begingroup$ That makes more sense, doing it that way the second leg is basically irrelevant. Note however the pads will need to be gigantic, it has to be far larger than all the pads on a gecko climbing rig, and you have to pray the wall can take the lateral loading. otherwise you are still falling just have a mass of brickwork or a giant pane of glass falling above you. $\endgroup$ – John May 8 at 2:24
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Why not have them just wall-jump back and forth between the building they're on an another nearby building?

Boing! Boing!

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  • $\begingroup$ For buildings which are extremely close together, that would work. They would need to be so close that you can touch both walls though. $\endgroup$ – Graham May 6 at 7:37
  • $\begingroup$ @Graham Not necessarily. The motion of travelling toward one wall and pushing off of it gives you a very small moment of friction on the wall, to slow a bit and jump back in the other direction. Repeat. The building would have to be close enough to jump back and forth, but still far enough away that jumping from one to the top of the other is too far. $\endgroup$ – Steve May 6 at 21:11
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    $\begingroup$ @Steve Sorry, I didn't make that clear. For the amount of push you can get to do this repeatedly, the furthest you could travel in each direction is not much further than that. $\endgroup$ – Graham May 7 at 9:47
  • $\begingroup$ @Graham even if you are in an exoskeleton enhancing your parkour abilities? I'm imagining of course strength enhancements, but possibly also icepick-like toe or finger attachments for creating your own hand/footholds in the concrete of this monster-infested city, if they would help $\endgroup$ – Syndic May 8 at 8:14
  • $\begingroup$ @Syndic I'd say that any exoskeleton that allows you to jump from top1 to wall2 to wall1 to top2 also allows you to directly jump from top1 to top2. (Unless top1 is so slippery that you don't have enough friction to gain enough speed in the horizontal direction, of course.) $\endgroup$ – orithena May 8 at 11:38
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The rubber on climbing shoes is surprisingly sticky. So long as you're exerting SOME force into the wall, they will probably stick. With that in mind...

What I'm imagining is sticky rubber pads on the hands and feet of your exoskeleton. When you want to descend, you jump at the wall of the building opposite where you are standing. When you reach that building (at a lower altitude), the force of your landing pushes into that wall, causing the rubber pads to stick.

Of course you can't stay there without some sort of attachment or you'll just fall off. So you push off again, do a half turn as you cross the street, and are now pressing the rubber pads into the wall of the building you started from, again lower down. Repeat until you reach street level.

You can't STOP, you have to keep switching between buildings, but you can go down, up, stay at a level (bouncing back and forth), reverse direction (down to up, or up to down), or enter the building via your window of choice at any elevation along the building.

As a bonus, it looks cool.

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  • $\begingroup$ This is the only way friction with the wall can slow you down, but you need to be able to generate enough force to stop completely at each point of impact, otherwise you can't generate enough force to get static friction and just end up tumbling head over heels. that you need to be able to reach both walls at once. $\endgroup$ – John May 7 at 15:57
  • $\begingroup$ The force to stop comes from the lateral (side to side/building to building) kinetic energy. The question is based around using exoskeletons capable of generating enough power for rooftop to rooftop jumps, so they should have no problem generating the lateral KE necessary. Once you reach building two, you have to push off and return back to the first one, otherwise yes, you will end up tumbling down the side of the building. No reaching both walls at once required, just lateral kinetic energy. $\endgroup$ – Mark May 7 at 22:09
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The idea of springs or pogo sticks, already proposed by Willk and Darrel Hoffman is interesting. Let's do the maths.

The main problem is the sudden deceleration: it's not the fall that kills you, it's the landing. We can limit this deceleration using two methods.

  1. Minimizing the terminal velocity: the skydiver should be in a belly-to-earth, face down position. That should not be a problem for trained soldiers, and the terminal velocity is then limited to $v=50\ m/s$ [ref].
  2. Maximizing the landing duration. Fortunately, contrary to spine-parallel accelerations, humans can tolerate pretty high spine-perpendicular accelerations [ref]. With a bit of training, $a=20\ g$ doesn't seems to be a problem. The landing duration is then $\Delta t=v/a=0.255\ s$.

From the free fall to the stop, the skydiver goes through a distance $d=\frac{1}{2}at^2=6.13\ m$. Good, we need a six meters spring: not totally absurd! The force generated by such a spring on the diver would be $F=ma=20\ kN$ if we take $m=100\ kg$ for a fully equipped soldier. If we suppose that this force is perfectly distributed on one side of the torso, which typically has an area of $S=0.4\ m^2$ [ref,ref], that's a pressure of $P=F/S=50\ kPa$, or $0.5\ bars$, which is almost the same pressure as a weight of $10\ kg$ on the palm of the hand: not a problem for such a short duration.

The spring in itself would have to be non-linear (for the deceleration to be constant), but that's doable, even today. It could be deployed while in the air, and locked just after the landing, almost immediately ready for another jump.

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Since the main issues raised seem to be:

  • lack of climbing gear
  • surfeit of gravity's knock on effects (such as, e.g., catastrophically landing)
  • lack of sufficient attractive force

Let's consider a way to assist the friction element of the exoskeleton device. We basically want the wearer to be able to "stick" to the side of a building, which would slow the descent. Ideally we'd like them to be able to ascend the building after dealing with a variety of hostiles.

I'd recommend:

Acme Industries' EXKELERATOR MARK-VII! --- this is a piece of add-on technology, compatible with a variety of manned and unmanned exoskeletal type devices that allows for (relatively) safe vertical descent and ascent.

Drawing power from the device's onboard power packs, the EXK Mark VII delivers a bimodal system of vacuum and blower capabilities designed to offset the vertical surface effect. That is, this system allows the wearer to make use of natural friction between device and surface to slow descent and allow ascent.

V-Mode -- Powerful suction ports in the gloves, elbows, torso, knees and ankles are fully controllable to allow for strong adhesion with considerable stopping power all the way to minimal suction. The fully controllable system allows the user to move the selected limb from location to location, facilitating a smooth vertical climb.

B-Mode -- Powerful blower ports acting in tandem with the suction system assist in acting against the tendency for a person to be pushed away from a vertical surface, either by localised wind or even by their own attempts to "grab hold" of the surface.

The automated bimodal systems are integrated with the exoskeletal device's command functionality and its own on-board orientation sensor array. This system allows for the user to quite naturally "stick to" a wall whether climbing up or down. It also allows for certain emergency procedures, such as breaking a fall with one's back to the wall, allowing one to seamlessly flip onto one's stomach for a more effective manoeuvre.


Writer's PS: I'm guessing that no one called BS on you because "mechanico-exoskeletal assisted movement is an assumed thing in the SF genre. We simply assume that some technowizardry allows the wearer to descend without crashing and ascend without falling again. Though kudos to you for sorting the details!

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  • $\begingroup$ I think there was a question about these jump jets here recently... $\endgroup$ – Willk May 5 at 14:55
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Bungee

Instead of using ropes and harness for a controlled descent, Mr. Gadget can pull out a thin super-stretchy cord, quickly lash it to something and jump, holding the cord. It's more like freefall, but at the end the cord stretches until he just touches the ground. His guesstimation of what length of cord is needed would be handwaved/ignored I suppose.

Electromagnet on a rope

Kind of a tech version of spiderman, Mr. Gadget can have a (again stretchy) cord with a powerful electromagnet on the end. He can throw the cord towards something metal (street lamp, girder, building) and turn the magnet on, allow him to swing (and the cord to stretch a little) and thus greatly reduce his downward momentum.

Friction won't work

  • Pressing against a wall will push you away from the wall.
  • In a narrow alley, pressing against both walls will create enough friction to slow you down.
  • BUT if you're going from freefall to stationary, that's a lot of friction, which turns your gravitational potential energy into sound and heat. It's likely to be very loud, and definitely will be hot enough to burn you (think sliding down a rope but holding the rope with your hands). It will also tear up the fabric (if the fabric is too tough to tear, then it won't have enough friction to stop you).
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As many other answers have pointed out, the problem with friction is that you need a normal force pushing into the wall. If your suit has one or more wings (or an array of tiny airfoils) that deploy during the fall in the direction of descent it will create a normal force into the wall - like aerodynamic lift but horizontal. If the coefficient of friction can be very high, the system could be designed such that the soldiers reach a constant descent velocity low enough that they would survive impact.

It would also be interesting if the suits have nano-generators to harness and store the energy lost in the descent. Since the wings don't require any additional power to work (other than that used for deployment) the suit could actually net some energy through this process.

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Leg springs

You could do something like the long fall boots used by the protagonist in the Portal games: enter image description here These look a little too simple for the job, of course. The way they attach to the legs seems highly insufficient. But you could modify this concept by attaching them to the back of some sturdy boots. (I don't recommend using them barefoot as in the picture.) You could replace the springs with pneumatic shock absorbers for added cushioning. Depending on the length of your fall, you might extend the springs' "feet" several feet below your actual feet when falling. (I could not find a way to say that without using the word "feet" three times in one sentence.) It would certainly take some practice to land on these without then falling over on your face, but these people are parkour experts, I'm sure they'd be able to handle it.

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It is possible with a piece of equipment. Excuse my ASCII art:

   _______
\o| n  n  |
 I|       |
  | n  n  |
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Our guy would have a hard wing made out of some material that can be folded and expanded. In order to scale down the building, he will just jump close to the surface. This requires your sandpaper like system for friction too. Now the problem with the friction descent is that in microscopic level grains in the sandpaper will push you slightly away from the building, removing the friction almost instantly. But the wing being angled will push you back to the wall. This will significantly decrease your terminal velocity to a point where the exoskeleton would be able to handle it.

Failing that, you could also make all your buildings angled. You don't need 45° or anything, even few degrees can help. One more idea popped into my mind while reading other answers. You can also require all buildings to have a quick descent gap, which is a 1m² inset on the side of a building where your cops can use both sides to descend safely. Have some sort of wheels on exoskeleton which uses magnetic breaks to slow down the descent.

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  • $\begingroup$ Doesn't need to be passive, they are in an exosuit so using a fan like these cars do might be an option - youtube.com/watch?v=ChOt_yXJ9f4&t=180 $\endgroup$ – Pete Kirkham May 6 at 11:33
  • $\begingroup$ Of course if your wing is large enough to generate that much force, you can just use it to parachute. $\endgroup$ – John May 7 at 15:42
  • $\begingroup$ @John The required surface for parachute compared to the wing that is needed to push you slightly to the surface are not the same. Also you cannot use parachute in the middle of the city and land safely on the ground. $\endgroup$ – Cem Kalyoncu May 7 at 18:39
  • $\begingroup$ @CemKalyoncu I think you underestimate how much force you need to generate to slow down, you need to be generating a significant fraction of the person weight in lateral force, if the wing can generate that force, it would work as a rigid parachute. $\endgroup$ – John May 7 at 19:51
  • $\begingroup$ It depends on the surface material. For instance, if Velcro could stand up to the forces involved, it will require much less to keep sticking. But obviously moving through Velcro will damage it. A self healing nano material could have similar properties. All you would need is a gentle push to ensure you won't disconnect or roll. $\endgroup$ – Cem Kalyoncu May 9 at 5:26
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Gravity manipulation

I’m not sure how Hard SF you’re going for, but since this isn’t tagged science-based, why not fuck with gravity? I figure you have one of two fun options:

Reduce gravitational pull

The lighter you can make them, the more they can depend on air resistance to slow them down, and the less energy they need to shed when they hit the ground. If they could reduce their effective weight — even for only dozens of seconds at a time — they could easily drop down from the tops of buildings without fear. As long as the effect isn’t too strong, they shouldn’t be able to exploit it to get back up.

‘Tilt’ gravitational pull

The problem with friction, as you noted, is that it doesn’t work at 90 degree angles. So if your characters have a technology that allows them to slightly offset the direction of gravity acting on them, say, by fifteen or twenty degrees, then friction is back on the menu.

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    $\begingroup$ Being able to reduce gravity will have worldbuilding impacts far beyond commandos being able to descend walls quickly. $\endgroup$ – Mark May 5 at 21:40
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    $\begingroup$ Very true. But that’s up to the author to integrate. $\endgroup$ – Daniel B May 5 at 22:05
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While falling their suits are constantly monitoring their immediate surroundings (small radar/lidar/camera systems built in), their vertical speed, and their height.

This system fires a grapple into the nearest wall when it detects the operator is nearing the ground (or running out of valid grappling surfaces). The grapple then slows the user down as they near the floor, cancelling out their vertical momentum in a 'suicide burn' of sorts.

This lets your people jump from the buildings without any extra gear past their suit, but still has some tension elements. They need to stay close enough to the wall that they don't slam into it when the grapple deploys (not everyone can use these, they can't be used in all situations), the system could fail or be tampered with etc.

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I was thinking some sort of technology in the exoskeleton so that when it gets air coming upwards through it (from falling) it distributes the wind out the back, kind of like a way of propulsion, and that the exoskeleton also has equipment kind of like ice pickaxes/climbing axes either seperate or built in to the forearms or some area that could be utilised easily. This way, when you leap from the building you get that extra propulsion to reach the other building and could perform a self arrest and then continue to climb back to the top of the building (or even climb back down). The self arrest would be much easier to pull off because you still have the wind pushing you towards the wall, keeping you on it.

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Expanding foam

An aerosol of highly expanding foam, which they squirt at the ground below them as they fall. They therefore land into a column of foam which dissipates their energy as they land.

They can squirt more or less depending on how much speed they need to loose.

The foam would be pretty unstable – it only needs to last a couple of seconds – and dissipate quickly after they've landed.

May provide a fun mechanic for your world?

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Three Things

Gravity Engine:

Considering that this is all fictional, make the exoskeleton an armor-like suit, in its core, place a gravity engine/plasma reactor, which will power the suit. This engine could be connected up to every part of the suit. For the higher-ranking officials, throw ina gravity gun too, which could function as a portal gun too - manipulating gravity to create controlled wormholes.

Wingsuit:

This wingsuit should be designed in tandem with the gravity manipulator/engine, this combination will allow the user to glide more efficiently. The wingsuit should be made out of a material with high tensile strength and surface area, this will make it function more efficiently, allowing for precise tactical insertions. It needs to be collapsible, preferably able to fold away into the suit. Here's an idea: Wingsuit Drawing

Either way, it must be reusable and able to fold back into the suit.

HUD and AI

A HUD and AI are imperative for this system to work, the HUD will allow the user to view their surroundings and their altitude, target, velocity, and so on. The AI which will be wired to the user's brain will allow the gravity manipulator and wingsuit to work in perfect harmony with pinpoint precision.

How it will work

First, the HUD will display the user's target and the most efficient path to get there. The user will jump off the building, using the gravity engine and wingsuit to quietly glide his way there. Here, if the gravity engine fails, the plasma reactor will go into overdrive, sending plasma down specially designed vents at velocities that will be calculated by the AI, to slow their fall enough that the armor will be able to withstand the fall if stealth is necessitated, then a grappling hook will allow to user to hang on to a point and slowly lower themselves. Next, the user will begin circling above the target, slowly losing altitude as the AI scans the area for hostiles and analyses the landing area. This will allow it to optimize the suit's stealth field generator for that area. Then, the user will take out their weapon and stay upright - toes down as the gravity engine decreases power, allowing the user to fall until they reach 5 feet above the ground, at which point, the gravity generator will kick in, slowing their speed to millimeters per second; when they touch down (in a 3-point landing, the gravity engine will kick into stealth mode, allowing the user to move without making any noise. The insertion is complete. In a pinch the user can overclock the gravity engine, creating an impulse that will allow them to glide to safety. The gravity engine will also allow the user to climb walls like Spider-man.

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Suits made of spongy, bouncy material. Ultra squishy polymer. Suction cup gloves/shoes for scaling walls.

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A safety cushion for emergencies in a backpack. You drop the backpack down and it inflates in one of these:

enter image description here

After that, the team can just jump down on it.

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  • $\begingroup$ you and the backpack will reach the ground at the same time. $\endgroup$ – John May 7 at 15:56
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    $\begingroup$ @John Not if you wait before you jump down $\endgroup$ – Art Krenn May 8 at 7:11
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Just pointing out that no one seems to be thinking about the possibility of climbing down (or up) the corner of a building instead of a flat wall. Still, without hooks of some sort it is not apparent how this would help.

I did have one idea, but it does require them to carry a very long rope (ultrathin, ultrastrong etc.). First, use the suit’s built-in laser to measure the distance to the ground, and the length and width of the roof. The rope has a loop in one end; they loop this over a structural feature on the corner of the roof. Next (playing out the rope as they go) they run to the far end of the side of less optimal length. They loop the rope around the outside of a structural feature here. (The rope has length markings on it.) Now — after turning 90° left or right (as applicable) — checking length markings as they go, they run an appropriate distance down the optimally long side, and jump off (holding tight to the rope, of course; this is important).

If they measured correctly, they will arrive near the ground with huge lateral speed but minimal vertical.

I’ve done my job; now it is your problem to work out how they can get rid of all that speed. ;) (Some physics expert out there can probably say if little wing things in their suits would help significantly or not.) (If they have little skateboards built into their suits, they could travel a fair distance… although again they would lose the rope that way. …Or loop back.) Maybe metal shoes? High-resistance skates?

As for the rope… all they have to do is unloop it from the two corners of the building (although this does require running around the building (except see below)).

One difficulty is that the typical building might be taller than it is wide. This is not a huge problem (as long as the buildings are not too tall). The protagonist must stand on the roof, at the chosen corner, playing out rope until they reach the right length. Then, they have to leap off and prepare for a sudden jerk when the free-play is exhausted. The real problem here is securing the second loop, given that it is not being pulled continuously. The remote release (as below) might work here. Failing that, the protagonist must hold the rope also at the immediate point, and let it slide away in a controlled way, to maintain the pull.

Of course, the idea scenario (from one point of view) is that they might be able to swing entirely to the next building. The problem now is getting the rope back. Amazing flicking skill might be all that is required to get it to release from the second loop… but then it will just hang down the side of the building, still hooked onto the far corner. All I have thought of here is a remotely-activated device that unclips the root loop. Having retrieved the rope, the protagonist manually clips it up again for next time.

One refinement is to have electro-elastic rope, but I am not sure that this would be of practicable use. (Actually, I am thinking that, with careful engineering and a great deal of practice, they could activate the elasticity milliseconds after they were going horizontally, and end up with half the speed and a shooort…ish drop. Conversely, this would probably work — if at all — only with a precisely measured scenario.)

The ideal would be to have a rope that worked as a bungee from (say) 80m from the loop. The difficulty here is that now we are not swinging… which negates the easy-off looping system (since the rope must either be secured or hang down somewhere such that it would not just fall off). Here, we rely on the remote release idea (or there might be some other arrangement that I have not thought of).

Refinement — I shall leave what I have already typed. Another difficulty is that the rope is held reasonably well while the protagonist is on the roof, but, when they are nearing the ground, the (second) loose loop that I have described would probably come off. I am thinking that they would thus have to attach the root loop diagonally opposite this point. Actually, that might make the bungee version more practicable.

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How about futuristic gloves? A "super-battery" not yet invented in a special pair of gloves could create such a strong localized magnetic field that the descender could literally insert his hand into the wall. With one or two hands in the wall, controlling these gloves would make the descent like sliding down a pole.

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