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In the game Call of Duty: Advanced Warfare, characters wear exoskeletons with an ability called "land assist", which allows them to slow down their fall using a jet of compressed air (or some specific gas). Would such a device be possible in real life? If so, would it be efficient enough for slowing down a high fall (e.g: from hundreds of thousands of feet)?

For those who don't know the game, here is an example of this fictional technology in action: https://youtu.be/dOjyonpeXVk?t=368

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    $\begingroup$ Possible, yes. But not particularly useful in reality. Parachutes, simple ropes, and good tactics for their employment have done the same job well for a long time. Were my forces fighting soldiers so-equipped, we would stay out of sight and use lots more claymores and snipers from behind -- puncture the gas tank and poof that's a kill. $\endgroup$ – user535733 Feb 24 at 4:36
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    $\begingroup$ Do you mean "hundreds or thousands" and not "hundreds of thousands"? 100k feet is ~30 km, so more than about 300k feet up and you're asking about falling from space. $\endgroup$ – Salda007 Feb 24 at 6:19
  • $\begingroup$ @user535733 Unless you think the gas tank is going to be a larger and more vulnerable target than a soldier already is, that's not really much of a counter-strategy. A compressor pack like in Willk's answer doesn't add much to the soldier's profile except maybe from the side. $\endgroup$ – Cadence Feb 25 at 1:09
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    $\begingroup$ @Cadence many of those game-soldiers have magically featherweight super-armor. I really just want to watch them go pffffft like a deflating balloon...though I can understand that the OP might disagree. $\endgroup$ – user535733 Feb 25 at 1:59
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Sure. They have jet packs.

https://www.youtube.com/watch?v=EAJM5L9hhBs jet pack

Jet packs have been a sci fi staple since Flash Gordon. The land assist suits they have in Call of Duty are lifted straight from Heinlein's Starship Troopers, 1958.

I pictured the jump units in Starship Troopers as big, but the marines were wearing full exosuits and so were not slowed down by the weight.

Depicted is a screenshot from a pretty sweet video of a real jetpack from way back in 2018. It is not that big and the guy is flying around very fast and most awesomely. Over water, which is smart. It is a gas turbine which uses exhaust gases and a fan to provide thrust.

You could scale it down or maybe use a near future battery and just the fan. The guys in Call of Duty don't look like they are carrying that much - maybe they have on the grandson of this thing.

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    $\begingroup$ It could also be noted that terminal velocity for a human in air is around 200km/h. This is around the speed that jetpacks have hit. If a jetpack can take you from 0-200, surely it can take you from 200-0, right? $\endgroup$ – Zwuwdz Feb 24 at 2:37
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    $\begingroup$ @Zwuwdz this is not quite a coincidence; terminal velocity is the speed at which airflow provides the same force as your weight. Since a jetpack also needs to create enough force to counter your weight (so you can hover), it would also top out at around terminal velocity. $\endgroup$ – Skyler Feb 24 at 15:19
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    $\begingroup$ @Skyler That's not right; terminal velocity is the velocity (relative to the air) at which air resistance equals weight. Jetpacks work by a different principle: they propel a stream of air (or anything with mass - no physical law stops you from strapping a rocket to your back) downwards at high velocity, pushing you upwards. It's true that to ascend at terminal velocity, a jetpack would need to provide twice the force as it needs to hover, but there's nothing inherent in the design that says that this is the maximum force a jetpack could provide. $\endgroup$ – Milo Brandt Feb 24 at 17:23
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Apart from the other answers pointing out this is fairly possible with modern/future technology, I would like to point out another reason this is impractical and will probably not be developed further:

Balance

Iron man seems to hold his own when boosted from the bottom, balancing himself with boosters on his hands. But in reality balancing on boosters from your feet is extremely tricky. Imagine balancing yourself on two long flexible poles stood upright. Only with extreme balance and strength you will be able to, but one slight muscle twitch and you'll find yourself doing the splits. And when the boosters are no longer pushing from underneath you, they will push in whatever other direction, sending you flailing randomly and going very much splat. Imagine blowing up a balloon and letting it go without tying it. It will boost randomly anywhere because it isn't balanced. Especially the higher up you fall from (meaning you need to reduce more speed) the longer the boost, and the rate of failure will increase significantly.

Of course, this will be different with jet packs, as they are more strapped to the centre of mass (back/sides). However this will still be tricky as you use your legs as stabilisers. The size of these compared to the body will be fairly big, meaning you will be able to carry significantly less gear. Even if an exoskeleton is used, the weight of the exoskeleton will add to the force needed to slow down, so the size will have to increase significantly to account for this.

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    $\begingroup$ If the exoskeleton can temporarily lock limb motion, a computer can vector thrust to keep balance even in an unstable configuration. Especially if the main objective is just soft landing and not fully flying around, this should be much easier. (Still very hard, but easier.) Also, if the boosters are one time use, they can be simpler in nature and also discarded, like a parachute can be, on landing. Great for high precision insertion of troops somewhere and much better than parachutes since you can go down fast and don't be exposed to fire during descent for as long as hanging under a chute. $\endgroup$ – Prof. Falken contract breached Feb 24 at 10:10
  • $\begingroup$ Agreed, a balancing computer in combination with an semi-autonomous moving exoskeleton would ease the descent and help with balance. However this still won't fully nullify the amount of strength it would take, because a fast descent like that would generate some G forces, and limbs are known to flop and flail under stress. Also I should point out that attacking anyone hanging from a parachute is a war crime under the Geneva convention since 1949. If your enemy has a shred of decency, being exposed to fire shouldn't have to be a concern. Being spotted however is a different matter. $\endgroup$ – Plutian Feb 24 at 10:16
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    $\begingroup$ @Prof.Falkencontractbreached It's possible it doesn't have to be very heavy. Say you want to slow a fall from terminal velocity (53 m/s) to normal parachute landing (5 m/s) for a 100kg weight. A calculator gives 125 kJ (35 Wh), while gasoline combustion gives 47 kJ/gram. Sounds implausibly low, but accelerating a one ton car to 5 m/s (18 km/h) shouldn't take much either (10 x the weight, 1/10th the speed) $\endgroup$ – JollyJoker Feb 24 at 14:40
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    $\begingroup$ If you look at old videos of rocket launches, you'll see this happen: The rocket would climb a few meters, then arc to one side and crash into the ground. BUT: This problem was basically solved ages ago. You need a closed-loop control of the rocket's thrust to stabilize the rocket. The same thing could be done here. $\endgroup$ – nikie Feb 24 at 14:46
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    $\begingroup$ ADD: This could even be used for steering, similar to a hoverboard $\endgroup$ – nikie Feb 24 at 14:50
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This is one of those ideas that is cool but extremely impractical at best, as you'll see in this thread on Portal's boots for some other ideas for achieving the same effect. The short version there is that it might be possible to find something, but it would be extremely hard to be effective in any kind of reasonably sized package.

While you're talking about thrust as a counter, this also has equally big problems. A jetpack like that shown above requires a great deal of strength to operate, and weighs enough that it would be extremely hard to carry on top of the hundred pounds of crap that soldiers already carry for such a limited role. Even with exoskeletons, it would be hard to make it work for such a niche role. I'm not actually sure the military is even seriously interested in the concept anymore.

With respect to the idea that soldiers would use such a system, there is another problem, that of stealth. The whole point of using infantry is that they are hard to detect, especially in urban environments. Instead of jumping down a building like that, take the stairs. If you must, rappel down the side instead. This is what actual paratroopers do when they are caught in trees.

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    $\begingroup$ If you are jumping off, you are probably more worried about speed than stealth... $\endgroup$ – Matthew Feb 24 at 17:38

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