A grappling hook, of some kind, is necessary when one wants to climb up the side of the building or obstacle but sees no obvious foothold to do so. An iconic symbol of Batman and other vigilantes, if one wants to scale walls, prevent death when falling or generally climb over obstacles one cannot ordinarily, it can be exceedingly difficult to do so without some kind of obvious climbing harness and a grappling hook. A grappling hook essentially is a line or rope attached to an object that can connect a hook on to whatever it is that is wished to be climbed. Traditionally, this implies multiple hook like devices connected together and propelled with a rope attached to hopefully "hook" on to something with an obtuse enough angle to attach, and then use this to support your weight and climb up. Unfortunately not all obstacles possess oddly-shaped things that jut out at precarious and desirable angles in just the right way to useful for a grappling hook, and landing a perfect hit with a grappling hook is relatively difficult even with a skilled user. The iconic view of a grappling hook wrapping around and tying around itself on an object perfectly at a moment's notice is more or less a work of fiction, which makes it virtually impossible to use in the way it's often presented. This is ignoring the weight issues, of which to propel a heavy hook a considerable distance one would need an extremely powerful propellant, be it gunpowder or an oversized air canister power, that likely would be much larger than what is often portrayed in films and comic books.
Therefore, a more practical device is preferred. Part of Batman or vigilante's appeal is their ability to get around places and move quickly, especially on roof tops, which would present a problem for most people or even police under ordinary circumstances. The ability to get on top of a building and drop down on unsuspecting bad guys, report information to the police or just continue the chase of a criminal is part of the appeal of such a character. The actual design, as a result, will be particularly important, not only to allow for something viable in the role, but also tactically efficient, something that can be deployed quickly, but also without the need for considerable skill to have the hook land in exactly the right spot to catch on to a surface. This would require a quick attaching mechanism, a lightweight launching mechanism that can be handheld, a lightweight rope, and a climbing harness that it can attach to in order to distribute the weight over the user. The idea of attaching a grappling hook to a belt is also a work of fiction, and is more likely to break the person's back and kill them than slow their descent; in fact, it likely would be a more agonizing death to stay stuck on a building stranded for hours with your back broken than it would be just to fall and die instantly.
Hook - Grappling Device
More idealistically, the "hook" would be powered by a small explosive charge of some sort, to embed the "hook" in to an object, so it can be designed to stick on to any surface with force, and then stay embedded within the object to support the weight of the user. A properly designed hook could shoot out and then latch on to the object, and then remained hooked into, and dug into supporting the weight by static friction and not just gravity. This hook could have multiple barbs, such as in a grappling hook, to allow for a broader distribution of the weight and reduce the chance of the hook releasing or breaking from the object it's embedded in, or any variety of things to allow it to stick into the object better. As long the material could support the weight of the user, it could theoretically be used. An advantage of explosive activated device would be that no matter the velocity of the grappling hook, it could still embed itself into the material. By changing around the shape of the device, it would be easier to latch on to whatever it embeds itself into, and be harder to remove. A barbed hook of some kind is therefore the ideal shape for a penetrating device. A similar proof-of-concept design was produced by the Mythbusters, for a familiar grappling hook design, although the design for use by a professional should be more aptly shaped (instead of just using a nail), more easily mass produced, and use a more powerful/optimized explosive. The basic concept of an impact detonated grappling hook is solid, however.
The alternative, or perhaps in combination with this for something more effective and less destructive, would be some kind of "sticky" material that immediately hooks on to any object it attaches to. Some kind of extremely powerful adhesive is an option, but runs the problem of not being powerful enough to adhere a human and their swinging wait to a building or other obstacle, not drying fast enough to latch on in a short amount of time, and also being virtually impossible to remove once it's stuck to the side of a building. Another option would be a material which replicates gecko-hair, or the tiny hairs on a gecko and other creature's feet which allow it to latch on to various surfaces, including glass. One would need approximately 1 square inch per 10 pounds, and while a human is only 200-250 pounds, extra material would be desired to carry multiple people, and resist the extra weight of swinging or movement which could dislodge the device. A sort of net that springs out that has the size of approximately 10 x 10 inches, or 100 square inches, is likely good enough for the job. In combination with or in replacement to the
Launcher - Grenade Launcher
More importantly than this, is determining a way to launch the supposed device and the desired line. The Plumett AL-52 is a compressed air pneumatic grappling hook launcher designed to send a 3mm rope line roughly 100 meters. It is relatively large, heavy, and cumbersome, with many details unknown about it, and being well over 15 pounds for a single use. At it's size, it would be somewhat impractical to carry around with a person everywhere they went, as a back up utility device. Air pressure is, simply, in a reasonably sized container, significantly weaker than gun powder and other potential propellants, reducing it's strength and therefore power significantly. Air pressure is on average 30 times weaker than gunpowder outright.
A somewhat smaller, and more practical design could be based off of a grenade launcher. The M320 for example, despite launching a relatively heavy payload, can be fired like a regular weapon, potentially with one hand, is only about 3.3 pounds. With the high to low pressure system, the weapon can eliminate large recoil that could ordinarily be present from a high momentum device. The plumett AL-52 suggests that it produces roughly 2kj, or 2000 joules of energy with it's launcher, with a range of 100 meters. A grenade launcher uses an, approximately 180 gram round propelled at 76 m/s, that produces roughly 520 joules, with a 150 meter close range and a 400 meter maximum range if angled.
While not the first idea that pops in to the mind of someone wanting to be Batman (due to his aversion to firearms), a 40mm grenade luncher or 37mm flare launcher (a civilian equivalent) is considerably more practical due to it's availability, cheaper price, and higher power. Furthermore the use of specialty rounds, such as flashbangs, tear gas, or pepper spray can be an additional advantage, even if regular high explosive grenades are avoided (particularly for legal issues). The added versatility and power makes the grenade launcher an ideal choice, although the use of a grenade to fire a grappling hook will take some custom work. Notably a small hole will be necessary to feed the string through the grenade launcher itself, allowing it to be connected to the climbing harness at all times. The cable or rope will need to be attached to the suit, rather than the grenade launcher itself, with the grenade luncher and rope a permanent fixture to the suit so it's already connected to the grappling harness (instead of needing to do this in mid-flight or free-fall) and attached in some a way to stop releasing rope after the line has attached itself to an object. Finally, a mechanism to assist in pulling the user up, such as one found in U.S. Ranger's known as the "ascender" or by the mythbusters will allow a mechanism on the back to ascend without having to use their own strength.
It's conceivable that if only a little larger, more powerful, or using lighter weight materials the grenade launcher could provide a much smaller platform to produce the same general results. The primary limitation with the design will be weight, as the projectile, rope, and other materials need to be light weight enough to allow for a long enough distance of travel and for a high enough velocity to actually be able to get out to long ranges quickly.
Grappling line
A 40mm grenade launcher, which is only 3.5 pounds and can usually be used as a stand alone weapon, can propel a 180 gram round at 76 m/s to approximately 400 meters. At quadruple the mass, or 72 grams, the round would have approximately half the range; for E = (Mass)(Velocity)^2, 1040 = (.720)(38)^2, or approximately 38 m/s, which would be approximately half the velocity, or half the range. This means that at only half the range, you could theoretically propel a projectile four times the weight. Since the line's weight would increase as the round was propelled, the line gaining length through the air, the calculation would be different to determine it's exact weight and there would be a somewhat varying velocity in flight (mass essentially being gained in flight). The round would start off substantially faster and slow as the line, or rope, increased in length in flight, which would give it a longer range. For the purposes of these calculations, this will be ignored, but it means that the range of the projectile will be farther than what is actually listed. In addition to this, the weapon would have somewhat increased recoil due the heavier weight of the projectile; despite having the same level of energy, the impulse and momentum would be different due to a change in mass, therefore increasing recoil. In the case of the 9mm parabellum pistol round, when compared to the 5.7mm x 28mm, cartidges with approximately similar energies, the 9mm typically has around 30% more recoil, since the 5.7mm round is 2 grams while the 9mm is 7.5. Similarly, the .357 SIG round has comparable recoil yet a lighter weight round and higher energies than the .40S&W, which it was designed from, producing similar if not roughly the same recoil as it's parent round, the .40 S&W, which is much heavier. Increased mass, even at the same energy levels typically yields a somewhat higher recoil, and therefore the backlash and decreased control of the weapon (or the reverse, decreased mass at higher energy levels has approximately the same). However, due to the line being added in flight ("lengthening" as the projectile moves), and the only mild recoil increase even with a quadrupled mass (approximately 30-40%) this should only be a minor issue. Finally, higher pressure variants can be utilized which would increase the power of each round and thus it's range or the weight of the projectile (but also increase the recoil), but this depends specifically on the load-out and grenade launcher in question to determine if such an option is viable.
720 grams is approximately 1.6 pounds. Some form of device would be required to allow for the line to stick into the target, thus adding to the weight of the launched device. Determining the strength of the cable will be difficult, as the primary limitation is weight. A quarter inch steel cable (6.4mm) is somewhere around .11-.125 pounds per foot, has a safety load of approximately 1100-1300 pounds, and a breaking strength of around 6000 pounds, depending on the variant. While an industry standard, assuming a weight .125 pounds per foot, and 1100 pounds for the safety load, this would allow for only12.8 feet of line, without calculating if the rope has gained mass in mid flight (which would increase the range somewhat). This by itself is rather abysmal, and so such a steel cable would obviously not be viable for use in a grappling. A rope a quarter this size would have a safety load of approximately 275-340 pounds (and about quadruple this for a maximum range), and provide about 50-52 feet of range, or about 17 yards. This is actually fairly generous when climbing a short building is considered, but not capable of building to building-swinging like Batman or Spiderman are often shown to do.
However, some form of ultra-light weight, high strength material could be used instead of steel which would be ideal for the role. Kevlar for instance is approximately 5 times stronger than steel in terms of weight. With a rough estimate, this would a kevlar cable of the same weight have a 250 foot range, or about an 85 yard range, in comparison to the steel, which obviously is considerably farther and much closer to the desired estimate. This is more or less adequate for most realistic applications, although Kevlar has issues with stretching, UV and heat issues, and problems with absorbing quick sudden shocks, making the material wear out quickly, and be worse on a fall. However, the line would probably hold for a few uses. Kevlar makes a great high strength fiber, and generally has a much higher tensile strength than steel, although other properties need to be factored in to have a completely accurate calculation.
M5 fiber on the other hand, offers even more promising mechanical properties. At approximately 10 times the strength of steel, or twice as strong as kevlar, elastic, resistant to water, UV, heat, friction, and possessing many other desirable qualities in a line, M5 fiber could have an approximately 170 yard range. Assuming similar strengths to steel, this would be a 275-340 pound safety range, and significantly higher for it's breaking strength. Elastic, somewhat more resistant to damage than Kevlar, and less prone to damage upon stretching, this material would make a somewhat more ideal line for bearing weight than Kevlar or steel, and yet be significantly stronger. Dyneema is another option, widely used as a fishing cable and 1.6 times stronger than kevlar, being far more common than M5 fiber currently is and also possessing more desirable mechanical properties than Kevlar, although not being as elastic as M5 fiber. Spider silk is another theoretical option, but one that is also not widely available.
Although, the weight of the grappling hook needs to be added. This could vary depending on the design, material construction, strength of the explosive, design of the impact etc., although it stands to reason it wouldn't be much more than a 100 grams. This would put the range of the launcher at around 150 meters, far below it's breaking strength, and at around it's working strength, giving it potential to be even longer (theoretically 5 times longer, if the breaking strength is used, which would be more dangerous).
Other Design specs
In order for the design to be more practical, several considerations need to be taken. How would one propel themselves up the line, how would one make sure the harness doesn't cut too much into them when they fall etc., these things can mostly be alleviated by simple design considerations. The multiple point harness suit should have rubber and other energy absorbing material underneath it, potentially impact gel, as well as soft padding. Ideally it's put on over the body armor, so the body armor can help distribute the force over an even greater area and be softer on the body on a rough fall.
Probably, the best way to store the line, is to have it already attached to your suit, before it is even fired. A multiple point harness suit, which could spread out the load of the rope, over the legs, arms, chest, torso, stomach, and other parts of the body (avoiding places where it could become dangerously, damaging, or painfully tight), rather than putting it all in on area, would make it easier to support the weight compared to just being attached to one place, such as the belt (which could actually kill the user). The tether or rope should then be attached to the suit before hand as well, similarly spread out, to allow the energy to be distributed over the whole body instead of on a specific area. When the 40mm grenade shell is loaded into the grenade launcher, presumably with the grappling hook, the wire should be attached to the suit through a hole in the grenade launcher, already being a permanent fixture to the suit. In this way, you would not shoot out the grappling hook and hope your arms are strong enough to hold on to it with your bare hands and with no safety harness (and possibly pull your shoulder out of your socket when you fall), but already have it attached to the suit and be more readily be absorbed over the body. This same rope would also be connected to an ascension mechanism, to help you climb automatically, rather than being connected to the launcher. This would limit the number of grappling hooks you could have, while simultaneously making it's use much faster and intuitive.
Additional factors to consider are the use of a glider suit to slow the descent of the user, a rubberized connection to the harness to spread out the energy of the fall given that the rope (the Kevlar or M5 fiber) will be more taught and not as springy as a bungee-jumping rope, and thus will more quickly decelerate the user and potentially injure them. Further energy absorbing materials, such as the body armor, helmet and impact gel should be utilized to help prevent the user from A well trained paratrooper can fall approximately 24 feet without injury, and so with proper training and materials an equivalent fall of 50 feet, on top of the rope slowing the user down and the glider suit should allow for falls that might ordinarily injure or kill a normal human. A combination of different factors should dramatically impact the performance of the grappling hook, and be used in conjunction with it. In short, such a device would not be the product of one, lone gun firing a single grappling hook, but rather a system, with the grenade launcher, rope, and ascending device all separate from each other. Some forms of equipment can be used for multiple purposes, such as when falling, or absorbing bullets or punches and kicks like with the body armor, which makes the system somewhat synergistic when all the factors are combined together. Some elements might be ignored all together, such as the ascender, while others are crucial for function (such as the grenade launcher and rope). In the end this is one small piece of equipment to the overall batman suit, but it is nonetheless one that must be planned considerably ahead of time to be practical.
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