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Comment by nkurz | Hacker News Reader

However, birds don't have screws and batteries.

This often gets mentioned (and is obviously true) but I'm doubtful (but not certain) that it makes that much difference. Things we think about as "soft" can be remarkably hard at high velocity. At 120 mph terminal velocity, water and concrete are similarly fatal: the incompressibility matters more than the hardness.

This isn't to say that drone strikes are not an issue, only that I don't think they are significantly more of an issue than bird strikes. And birds can do a lot of damage at several hundred miles per hour. When you are hit by one, whether it's filled with bones or metal probably isn't the biggest concern.

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> Things we think about as "soft" can be remarkably hard at high velocity. At 120 mph terminal velocity, water and concrete are similarly fatal: the incompressibility matters more than the hardness.

And yet planes fly through clouds and rain at several hundred mph without issue, though the same wouldn't be said about flying through concrete...

The issue here is relative hardness. Bone is relatively soft to titanium turbine blades, whereas screws and titanium turbine blades are in the same ballpark of hardness, so are catastrophic when they meet when spinning @ several thousand rpm...

I don't have any experimental data, but my mental models pull up:

1) chucking a chicken bone into an insinkerator vs a penny.

2) a hunk of wood vs a hunk of wood with a nail through a wood chipper.

Good examples, although I wonder if the relatively low speed makes a difference. The closest comparable that I can find is this: http://www.discovery.com/tv-shows/mythbusters/videos/chicken...

They shoot a frozen versus thawed chicken out of an air cannon, and conclude that the impact force is identical for both. I'm pretty sure that adding some metal bits inside both chickens wouldn't make much difference.

Density would make a big difference though. A basketball and a 1.75" lead ball have about the same mass, but the lead ball is going to do much more damage. But not because it's harder, but because it's dense and cohesive.

I'd guess that a drone of comparable weight to a goose is going to have a greater impact area, even though some of the internal bits are much harder, and I'd guess this is going to be more important for the level of damage.

Here's an article that supports that view that mass is the main factor: http://spectrum.ieee.org/automaton/robotics/drones/what-migh...

Here's more discussion (which I haven't read yet, and which may well contradict my instinct): http://aviation.stackexchange.com/questions/26223/would-a-dr...

> I'm doubtful (but not certain) that it makes that much difference. Things we think about as "soft" can be remarkably hard at high velocity ... I don't think they are significantly more of an issue than bird strikes.

Unlike birds, batteries by definition have a lot of stored chemical energy in them (1). in fact we want to put as much energy in as little weight as possible.

Violent disassembly by jet engine is not going to be a good combination with that.

1) https://www.google.com/search?q=hoverboard+catches+fire

https://www.google.com/search?q=laptop+catches+fire

https://www.google.com/search?q=phone+catches+fire

A GE90 jet engine like those used in a Boeing 777 can have a power output up to 75 MW. Let's take a fairly beefy phone battery -- 2500 mAh at 3.7 volts. Converting those units into something more useful we get 9.25 Wh. If we assume the entire chemical energy of the battery is converted into heat instantaneously, that's 33.3 KJ or .04% of the energy output of the engine per second.

It's possible that such a release could cause a catastrophic failure, but I have a hard time convincing myself that the battery poses a real risk.

I think the concern is more about the resultant chemical reaction of lithium: https://m.youtube.com/watch?v=gisdMQbtJqk than the total power of the pack.

Also, larger hobby aircraft (e.g. A 700-size heli) will be using battery packs on the order of 2x packs of 6 cells ("6s" aka 6 3.7v cells in series @4000-5000+ mah), so it's more like ~200-300Wh. Again though, doesn't matter, because even something like a 3s pack @2200mah can make quite a scary fire. This is why there are barrels of salt water at RC flying fields. An overheating lipo goes in the tank before it melts everything with hell fire.

Lipo fires are also why you can't take "hoverboards" on airplanes and why Amazon recalled them all: http://fusion.net/story/250086/airplane-hoverboard-ban-list/

http://www.theverge.com/2016/1/21/10806816/amazon-hoverboard...

Not sure how this is affected by being struck with a blade rotating at tens of thousands of RPMs at hundreds of miles per hour, but my point is that lipos are nasty beyond just their specific energy density :)

The fire in the video looks kind of wimpy against a jet engine that burns a litre a second or so of jet fuel. The heat from the fuel is probably 1000x that from the battery

The problem though is that the violently combusting battery is also a chunk of metal that, after being struck by the blades, is going to be moving at high speed roughly at right angles to the expected airflow through the jet engine. Damage could ensue, that is not equivalent to bits of bird flesh and bone.

The heat and flow in a jet engine is also carefully engineered and directed so that the flame front stays beyond a certain point in the combustion chamber. You don't want any of that in the compression stage, for example.

Maybe there is no real risk, but I did want to point out that it would be a mistake to assume that drone strike to a jet engine would be roughly equivalent to bird strike of the same weight. A drone is unlike a bird, a bird does not have lithium batteries, bird strike will not spray fragments of burning or exploding shrapnel through the jet engine.

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