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For a fantasy race, or for bio-augmented humans, how durable would they have to be to walk away from a 1000ft free fall unscathed? And how would this logically translate to other forms of durability?

Would it make gunfire from calibers like 9mm or .45 entirely ineffectual? What about more powerful rounds like .308 or even .50cal?

Would the force from a bullet that powerful be enough to crack the skull of someone able to survive landing on their head if pushed off a skyscraper?

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    $\begingroup$ My parent's description of how hard my head was when I was a teen would suggest an age-based mechanism. Welcome to the site! When you get a chance, please take our tour. Thanks! $\endgroup$ – JBH Jan 21 at 1:57
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    $\begingroup$ Can I be bioaugmented to a weight of 1 kg? I am pretty sure that between that and my (considerable) surface area, terminal velocity would be pretty manageable. Maybe I could even glide around some, flying-squirrel style. I would stay inside when it was windy, or have my sidekick hold some kind of tether. $\endgroup$ – Willk Jan 21 at 1:58
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    $\begingroup$ It's not terminal velocity that kills, it's the sudden stop at the end. $\endgroup$ – pojo-guy Jan 21 at 3:07
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    $\begingroup$ If they survive, then clearly the velocity wasn't terminal. $\endgroup$ – Mark Jan 21 at 23:39
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    $\begingroup$ You would need to either be squishy enough (bones, organs, EVERYTHING) to absorb all of the impact without suffering massive internal bleeding or you need to be strong/heavy enough and deliver enough PSI to break through the surface which you land on in order to cushion your fall. There is a reason that they instruct martial arts students to punch through the board in order to avoid breaking their wrists. If you are squishy then bullets would deal less damage to you but if you chose the strong route then they would be better off aiming into your rib-cage instead of your skull. $\endgroup$ – MonkeyZeus Jan 22 at 14:35
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Normal human durability, and a lot of luck

People have survived terminal velocity falls. In 1972, Vesna Vulović fell over 33,330 ft without a parachute after the plane she was in exploded. She didn't exactly walk away from the fall, however. She spent days in a coma, and was hospitalized for months after that. But she did survive.

Vulović is not the only one to survive a fall that should have killed them. They all had varying degrees of injury, so perhaps it's not quite accurate to say that normal human durability is all that's required if you want to walk away afterwards. But the point is that there are a great many factors involved in surviving a fall, and the height and the body's structure are only two of them.

That said, here are some other suggestions of the types of people who would fall well:

Lighter people

From On Being the Right Size by J. B. S. Haldane:

To the mouse and any smaller animal [gravity] presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes. For the resistance presented to movement by the air is proportional to the surface of the moving object. Divide an animal’s length, breadth, and height each by ten; its weight is reduced to a thousandth, but its surface only to a hundredth. So the resistance to falling in the case of the small animal is relatively ten times greater than the driving force.

Terminal velocity is a function of (among other things) weight. The lighter you are, the lower the velocity at which you hit the ground, and the easier it is for you to survive.

Wider people

Wingsuit flying is a reasonably popular sport where flyers skydive in a suit with "wings" between the arms and body, and between the legs. These jumps typically end with a parachute, but in 2012, Gary Connery landed a 2,400 ft jump without a parachute, landing on a "runway" of cardboard boxes.

Also, having wings gives you more ability to steer towards softer ground, which is always a plus.

The best enhancements for falling durability are not going to make you bulletproof

Bullets kill by piercing - by applying a large amount of force in a small area, they are able to penetrate the protective layers of our bodies and apply that force to the vulnerable parts of our bodies. That's why bulletproof materials function by preventing the piercing effect and distributing the force over a larger area.

That sort of protection is not going to help against a fall, where the entire area of your body is experiencing the massive forces involved. Conversely, the cushioning and strengthening of the enhancements needed to protect against a fall are not going to do much to stop the piercing effects of a bullet.

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    $\begingroup$ This is a great answer. The only thing I can think to improve would be mentions of how a humanoid could be more resilient to a high-speed impact. Maybe things like faster reflexes to enable better control of impact angle, more cushioning around vital organs (like the brain), and stronger bones, tendons, muscles, and blood vessels. These improvements could allow for good survival odds even with a terminal velocity impact against solid rock. (Severe injury would still be likely in such a situation.) $\endgroup$ – Kyle A Jan 21 at 4:34
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    $\begingroup$ The tricky thing about stronger bones is that all else being equal they tend to be heavier. So maybe all else is not equal, and they're made of a material that's stronger by weight than mammalian bones. $\endgroup$ – nasch Jan 21 at 5:28
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    $\begingroup$ In fact, the absolute best you can do is to have thick enough bones and fill them with helium. Or hydrogen, since that one is easier to get. $\endgroup$ – John Dvorak Jan 21 at 6:10
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    $\begingroup$ "They all had varying degrees of injury, so perhaps it's not quite accurate to say that normal human durability is all that's required if you want to walk away afterwards." One of them, Nicholas Alkemade, only suffered a sprained leg. That might make walking away unpleasant, but you can do it. $\endgroup$ – Ray Jan 21 at 21:17
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    $\begingroup$ Vulović didn't exactly survive a freefall in the strictest sense. She was in the tail section of the plane when it broke up, and the stabilizers on the tail likely slowed the fall, in addition to the body of the plane section protecting her from some of the damage. If she had been falling by herself without the protection of a piece of the plane, she would surely not have survived. $\endgroup$ – Darrel Hoffman Jan 22 at 17:41
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Bullet resistance and falling resistance are different from each other. A bullet that impacts some pierce-resistant skin will cause a shockwave to propogate through the body, which will bruise the flesh underneath and potentially cause bloodvessles to break and organs to be damage enough to die off. If you fall from a great height (or are in a carcrash, or have your body accelerated by a car/truck/whatever hitting you), all your organs will suddenly decellerate (or accelerate) and need to be slowed down over as long a distance as possible without ripping the nerves and bloodvessles.

Against bullet, extra fat would help as it would increase the distance the shockwave needs to propagate through before it reaches something vital. Against falling extra fat would mean more velocity for all your organs counteracting the extra distance the organs can now slow down over and increasing the chance the organs will rip out of their place, it's going to kill you faster!

One thing missing in @ArcanistLupus his answer about falling (https://en.wikipedia.org/wiki/Free_fall#Surviving_falls) is that almost all these falls had something break their fall slightly. They didn't really hit the ground with terminal velocity even though they reached terminal velocity at some point in their fall. It's like saying a parachutist survived a terminal velocity fall when he hit the ground.

This leaves 3 answers:

  • Make the person lighter without making them smaller. How much I can't say, but this will reduce their terminal velocity and increase their chance of survival.

  • Increase the person's surface area. This does mean he's going to need something along the lines of a parachute of surface area somewhere on his body to survive a fall.

  • Redesign their internal organs to have more room for the nerves and bloodvessles to follow the organs during the crash. Additionally the organs would need to be suspended with ligaments to allow them to move more and slow down over a larger distance. Possibly the organs could be molded into a bunch of smaller pieces that will each individually be slowed down. This will likely not be enough alone to help you survive.

  • as a last option: All of the above.

For a few other options check this similar question I made: Creating a scientifically semi-valid super-soldier, part 3: Physical shock resistance

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    $\begingroup$ On the subject of bullet-resistance. I like to remind people that while you can strap a brick of steel to your chest and technically the bullet won't get through, the impact energy will still be transferred and break your ribs, possibly even kill you. Bullet-proof vests work by spreading the impact energy across your entire torso and allowing it to propagate around you to the backplate of the armour. $\endgroup$ – Ruadhan Jan 21 at 15:04
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Cats frequently survives high falls. How durable are cats?

The cat example suggest it is not the durability (because I guess cats are quite similar to humans in this respect) but how you handle the fall. I would imagine an experienced parachute jumper that had trained on landing without parachute would have quite good chances of surviving (but seriously injured).

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    $\begingroup$ A cat has a lower terminal velocity than a human. Look at this link from Arcanist Lupus for example: irl.cs.ucla.edu/papers/right-size.html. A human who trained on landing without parachute would still shatter his body. The best he could train for is to aim for something relatively soft to break his fall, say a tough and high bush or similar, and then pray for the heavens he lands just perfectly right and the bush is just soft enough but not too soft to cushion his fall without killing him anyway. $\endgroup$ – Demigan Jan 21 at 8:33
  • $\begingroup$ People do jump into water from quite high altitudes without severe injuries. Basically you go from full speed to standstill in a meter when jumping into water so if you exchanged water for the lower part of your body you will crush your legs, hips and so on but the more important upper part of your body will survive (although badly beaten). $\endgroup$ – EmLi Jan 21 at 8:47
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    $\begingroup$ That is pretty different. You dont reach terminal velocity (water acts like concrete when you hit it that fast) and your legs have a different rate of decelleration. Also shattered legs have a tendency to let pieces rip through your body, say the femur ripping off it's head, splintering and jamming that into your intestines. We are talking terminal velocity here, otherwise you might as well say "Hey you can survive if you jump from 10m so jumping from 1000m should be the same!" $\endgroup$ – Demigan Jan 21 at 9:15
  • $\begingroup$ @Demigan water isn't quite like concrete - but I agree it's still fatal as per Mythbusters youtube.com/watch?v=yGJqqDaKscQ $\endgroup$ – KerrAvon2055 Jan 21 at 9:19
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    $\begingroup$ There are very many highly experienced and trained parachute jumpers around the world. None of them can reliably survive a terminal velocity jump without a parachute. $\endgroup$ – reirab Jan 21 at 21:14
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You d have to look like this

This is a simulation of how you would need to be to resist 100mph car crashes. Free fall is essentially the same problem

enter image description here

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    $\begingroup$ "Free fall is essentially the same problem." No necessarily. Being big helps you in surviving the crash, but it does not help you in free-fall. A bulky rhinoceros has a much slimmer chance of surviving free-fall than a fluffy mouse. $\endgroup$ – tobias_k Jan 21 at 11:17
  • $\begingroup$ I forgot that render existed. Sontaran/Human trans-species hybrids - Just say no! $\endgroup$ – Ruadhan Jan 21 at 15:05
  • $\begingroup$ @tobias_k Of course, if you take that rhino's terminal velocity and slam it horizontally into a thick concrete wall at that speed, it also has a slimmer chance of surviving than if you do the same to a fluffy mouse. The biggest differences between a car crash and hitting the ground at terminal velocity are that your car's crumple zones will absorb a lot of the energy and slow you down over a greater distance and that the objects you hit with a car are typically less massive and more prone to compressing and/or moving out of your way than the object you hit at the end of a fall - the Earth. $\endgroup$ – reirab Jan 21 at 21:05
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    $\begingroup$ Would I need a beard to survive too? $\endgroup$ – Morgan Jan 22 at 2:08
  • $\begingroup$ @reirab the difference is that if you are in a car crash of for example 40km/h, being fatter will help. If you are falling then the extra fat will increase your terminal velocity and cancel out the advantage the Fat, stronger bones etc brings. $\endgroup$ – Demigan Jan 22 at 7:24
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The simple answer is no - the ability to survive a fall in no way implies the ability to shrug off a bullet. The answer provided by Arcanist Lupus is correct, but I think it can be simplified considerably.

What you have to realize is twofold: bullets travel much faster than human terminal velocity, and they provide a much smaller impact area. The two combine to make an enormous difference.

Human terminal velocity is not a precise number, but for adults at sea level it runs from about 120 mph (with the body horizontal) to about 200 mph (with the body vertical), or something like 180 to 300 fps. In fact, the vertical position is more survivable than the horizontal, since the legs provide a shock absorber effect, as well as what is called in automobiles a "crumple zone".

Furthermore, all of the energy dissipated in the landing is done so over the entire cross-sectional area of the body, which for a vertical position will be on the order of one to two square feet.

Bullets, on the other hand, travel roughly 4 to 10 times faster than terminal velocity (1000 fps to 3000 fps, roughly), and have an impact absorption area on the order of 2000 times smaller.

So a bullet, on impacting a body, will simply punch through flesh, and while it will eventually be absorbed, this will not happen until it has done localized damage far in excess of the amount done by a fall.

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okay so terminal velocity and being bullet proof have nothing to do with each other. the denser you are the more you weigh which increases your inertia. higher density could theoretically make you bullet proof but would increase your inertia, therefore a fall would be likely to be more lethal. higher bone density might help but the impact on organs would be worse. the lighter an object is the less gravity attracts it so the two problems would compound each other. maybe make them bullet proof with technology that helps them fly, or put some really good parachutes on them. (side note most readers would simply accept the being able to fall from heights if they were bullet proof so I would just ignore the lack of explanation on the former)

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The first thing to know is that fall-proof and bulletproof are largely unrelated. Sure, if you put enough "durability" on anything, you'll get both, but if you want to optimize your modifications, I'd focus on one or the other.

Optimizing for Bullets

Bulletproofing is fairly straightforward, as it largely comes down to three basic ideas:

  1. Distributing kinetic energy across a wider area
  2. Absorbing kinetic energy (typically through plastic deformation)
  3. Preventing penetration

The first and second are reasonably interchangeable, where more capacity to distribute force largely obviates absorption capacity, and vice versa. If you can distribute the force of a bullet from an area less than a square centimeter in size (bullet cross-sectional area) to around 2,000 square centimeters (approximate torso cross-sectional area), you've reduced the kinetic energy applied per unit area by 2,000. If instead, you put a wall of concrete in front of you, that doesn't do much as far as distributing force, but it does wonders at absorbing kinetic energy through plastic deformation.

Preventing penetration is largely about putting fibrous materials that will "catch" a bullet before they enter the body, and these same types of fibrous materials (Kevlar, for example) tend to do distribution fairly well.

So some fibers for distribution and prevention, and some ceramics or other plastically deformables for absorption is the typical solution. Note that out of the fibers and deformables, the deformables will help somewhat with fall resistance, but the fibers will not. Which brings us to...

Optimizing for Falls

The first question to ask is what it is specifically that typically kills people in high-altitude falls. I'd imagine it's breaking of the spine, and the bones of the ribs being pushed through the organs. What causes both of these problems is the same two issues: things bending like they aren't supposed to, and jerk being applied to the body.

To stop things from bending, you can include a frame of some sort, either directly to the spine or to a frame outside the body. To stop the jerk is trickier, and comes down to either reducing the force (either absorption like with the bullets or decreasing speed of fall) or slowing down the impact (think a giant pillow, gently bringing you to rest upon impact). Here you have a few options:

  1. Add means of drag or decrease weight to decrease terminal velocity, for example, wings or a parachute
  2. Add plastically deformables (see bullet section for more on this) to absorb the force of an impact
  3. Add cushioning to slow an impact

Here your creativity is the limit. Plenty of real-life creatures can survive the force of an impact at terminal velocity. For example, most insects cannot be killed by falls from any height, because as you scale down objects, they tend to handle impacts far better. A human-sized ant would be only slightly more fall-proof than a human-sized human. So small size is your friend.

You can also add something like a biological parachute or wing, which can slow you down just before landing. Of course, this isn't as "cool" to a reader as an enhanced human who manages superhero landings.

Another interesting option is crumple zones, areas of your enhanced human specifically designed to smash, absorbing the force of the landing, perhaps quickly healing/regrowing after smashing. You can also try to capture the kinetic energy of the landing, with some electromagnetic damping or something similar "capturing" the force and generating electricity, glucose, fuel, or something else that can be used later.

Finally, cushioning. This one is tricky, because generally you need a lot of it. Crash pads for bouldering (low-altitude rock climbing) are typically around 13cm (5in) thick, and that's just for heights below 6m (20ft). From that height, you'd only be able to reach around 33km/h (20mph) at maximum, disregarding air resistance, well below the estimated 195km/h (122mph) terminal velocity of a skydiver. Basic physics dictates that kinetic energy, the real problem with falling, grows with the square of velocity. This means that if a 13cm (5in) mat can handle a fall at 33km/h (20mph), a fall of 195km/h (122mph) will pack not 6 times the punch, but 36 times the punch. If required mat thickness scales linearly with velocity (which it doesn't, it's probably even worse than that, so this is probably a best-case scenario), that means you'd need 78cm (31in) of pad to take an impact at terminal velocity. Again, this is only a best-case scenario, so you'd likely need much more than that, and 78cm (31in) is already a fairly prohibitively large amount of padding to add to your enhanced humans.

Conclusion

If you want to stop bullets, add fibers and plastically deformables. If you want to stop falls, slow down the fall, add plastically deformables, or slow down the impact with biological padding. Let me know what you come up with! As far as creativity goes, the sky's the limit. Then again, that's sort of the point :)

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