Bubble wrap instead of a parachute

[iamthewalrus_3]

8 years ago this guy did a squirrel suit landing without a parachute, landing in a bunch of cardboard boxes. Estimating from the video, the boxes look like they might be 4-5 feet tall, and they are stacked 7 layers high.

Not exactly the same, but not that far off from what the OP is asking about.

[Tired_and_Cranky]

@Machine_Elf

My mental model of bubble wrap was that its spring behavior (force as a function of displacement) was due to compression of the trapped air, like the piston of an engine compressing the air in the combustion chamber: low force at first, and then very high force near the end of compression, with an exponential relationship. But of course bubble wrap can’t handle very much pressure before it ruptures, so my mental model is probably inaccurate. It seems likely that the spring force is more related to the mechanical properties of the plastic film that makes up the bubbles, and less due to the compression of the trapped air.

Thanks. That clears it up because in my mental model, bubbles are popping nearly until our skydiver has come to a full and complete stop (alive or dead). In my head, the spring force would only come into play near the end of the descent, and my thought was that the spring force wouldn’t get nearly high enough to be a risk because if it were, more bubbles would pop. I understand where our mental models diverged though.

I just googled “bubble wrap force versus distance,” and found this Wired article, in which the author explores the properties of bubble wrap while attempting to answer a similar question. So it turns out that the spring force is a linear function of deformation, much like a conventional steel coil spring. This does still mean that you start with infinitesimal decel at the moment your cocoon first makes contact with the ground, ramping up to a max decel level at the moment your speed reaches zero.

The author was more thorough with his math than I was upthread. he assumed the jumper jumped from the sixth floor (20 meters up, impact velocity 20 m/s), and he chose a G limit of 31 for the landing. He determined that you’d need 39 layers of the small-bubble wrap, with a total thickness of about 14.2 cm.

@Mangetout

Many of the things we routinely pack in bubble wrap are actually capable of surviving more G than the human body - we are actually protecting against localised blunt impact rather than overall G forces.

I get that but thank you.

[Telemark]

@iamthewalrus_3

8 years ago this guy did a squirrel suit landing without a parachute, landing in a bunch of cardboard boxes. Estimating from the video, the boxes look like they might be 4-5 feet tall, and they are stacked 7 layers high.

He also has a lot of linear distance to stretch out the force of impact, although you can’t really tell in that shot how much of that he used.

[Tim_T-Bonham.net]

Well, there is a maximum: the bubble-wrapped skydiver has to fit into (and out of) a plane.

So a limit of 13.5 ft (4.11m) wide to 14 ft (4.6m) high. Those are the size limits on the cargo space in the largest military cargo planes, the Lockheed C-5A or the Antonov An-225 Mriya. The Boeing 747-8 limit seems to be about 20 ft (6.1m). Even the Hughes H-4 (‘Spruce Goose’) only increased that a bit (cargo hold specs rather unclear). As a bubble-wrapped sphere, the minimum of any of those directions would seem to apply. So you’re talking about 6 ft (2m) of padding around a person, maximum.

There is also the problem of exiting the plane in-flight. These are largo cargo aircraft, designed to be loaded & unloaded on the ground. I’m not sure they could open the largest cargo hatches while in-flight, and continue flying.

Now both the 747 and the An-225 have carried much larger loads externally (remember the space shuttle piggybacked on a 747), but that was an airworthy vehicle, securely attached, and not leaving while in-flight. A skydiver in bubble-wrap would need some shield to withstand the slipstream of such a plane in takeoff & flight. Also a solid attachment device (a human couldn’t hold on tight enough), and a way to disengage that attachment while in-flight. And that secure attachment device better not break any of the bubble-wrap!

You might have to go to a bomber plane; then the skydiver would have to fit through the bomb bay doors. (And survive the windspeed at the exit, even at the slowest speed the bomber could fly.)

Perhaps the best ‘plane’ for this skydiver would be the Sikorsky/Erickson S-64 Skycrane helicopter, if that counts. A platform could be built as large as needed for the bubble-wrapped skydiver, and attached by cables to the helicopter. (Again, this would probably need an attachment device to hold the wrapped skydiver on the platform against the propwash of the helicopter rotors. And not breaking any of the bubbles.) Then when the appropriate height is reached, the skydiver could just roll off the platform and fall.

But any of these seem to imply a lot of work & expense to try this.

[Tired_and_Cranky]

Or she could use a hot air balloon.

[Melbourne]

@Xema

But a sphere 40+ ft in diameter will have a lot more drag than a human body, flailing or not. Unless it has a density comparable to the human body, it will have a lower terminal velocity.

And bubble wrap has notably low density.

The volume of a sphere is proportional to r cubed. The drag is proportional to r squared. Assuming that your bubble wrap floats on water (is less dense than a human), there will be a limit beyond which your gravitational force increases faster than your drag. This is why small particles of dust and small droplets of water and small spiders float – and large rocks don’t.

If you wrap yourself in bubble wrap, you will fall like a rock. If you use more bubble wrap, you will fall faster.

[Xema]

@Melbourne

If you wrap yourself in bubble wrap, you will fall like a rock. If you use more bubble wrap, you will fall faster.

So, you believe any use of bubble wrap will increase terminal velocity?

[Superdude]

@Tim_T-Bonham.net

Well, there is a maximum: the bubble-wrapped skydiver has to fit into (and out of) a plane.

So a limit of 13.5 ft (4.11m) wide to 14 ft (4.6m) high. Those are the size limits on the cargo space in the largest military cargo planes, the Lockheed C-5A or the Antonov An-225 Mriya. The Boeing 747-8 limit seems to be about 20 ft (6.1m). Even the Hughes H-4 (‘Spruce Goose’) only increased that a bit (cargo hold specs rather unclear). As a bubble-wrapped sphere, the minimum of any of those directions would seem to apply. So you’re talking about 6 ft (2m) of padding around a person, maximum.

There is also the problem of exiting the plane in-flight. These are largo cargo aircraft, designed to be loaded & unloaded on the ground. I’m not sure they could open the largest cargo hatches while in-flight, and continue flying.

Now both the 747 and the An-225 have carried much larger loads externally (remember the space shuttle piggybacked on a 747), but that was an airworthy vehicle, securely attached, and not leaving while in-flight. A skydiver in bubble-wrap would need some shield to withstand the slipstream of such a plane in takeoff & flight. Also a solid attachment device (a human couldn’t hold on tight enough), and a way to disengage that attachment while in-flight. And that secure attachment device better not break any of the bubble-wrap!

You might have to go to a bomber plane; then the skydiver would have to fit through the bomb bay doors. (And survive the windspeed at the exit, even at the slowest speed the bomber could fly.)

Perhaps the best ‘plane’ for this skydiver would be the Sikorsky/Erickson S-64 Skycrane helicopter, if that counts. A platform could be built as large as needed for the bubble-wrapped skydiver, and attached by cables to the helicopter. (Again, this would probably need an attachment device to hold the wrapped skydiver on the platform against the propwash of the helicopter rotors. And not breaking any of the bubbles.) Then when the appropriate height is reached, the skydiver could just roll off the platform and fall.

But any of these seem to imply a lot of work & expense to try this.

While I appreciate those details, as stated in the OP, the only facet I’m curious about is the bubble wrap and the fall. The type and size of plane are immaterial to my question.

[iamthewalrus_3]

@Xema

So, you believe any use of bubble wrap will increase terminal velocity?

That’s not what Melbourne said. They said there’s a limit beyond which adding more bubble wrap to a person will increase terminal velocity. Which is true! And that limit (assuming I’ve done the below math correctly) is about a 4m radius sphere.

I found This discussion of jumping off a building with bubble wrap, that says that the density of bubble wrap is 17 kg/m3.

The terminal velocity formula is

Vt = sqrt[ (2 * m * g) / (p * A * Cd) ]

p is 1.225 kg/m3 for air
g is 9.80665 m/s2
Cd is 0.5 for a sphere
m is 4/3pir317 for a sphere of radius r + lets say 80kg for the person, and
A is pir2.

Plugging that all into Wolfram Alpha, I get the minimal terminal velocity for a sphere of bubble wrap with a person inside it at a radius around 1.2m, and a terminal velocity roughly equal to 120mph around 4m radius.

ETA: Wolfram Alpha graph.

[Xema]

@iamthewalrus_3

… I get the minimal terminal velocity for a sphere of bubble wrap with a person inside it at a radius around 1.2m, and a terminal velocity roughly equal to 120mph around 4m radius.

Thanks - interesting, and possibly even useful.

Call me old-fashioned, but I think I’ll stick with the “nylon canopy deployed via ripcord” idea.

• Xema (Caterpillar Club member)