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Also, helium is for all practical purposes a nonrenewable resource. One drone wouldn't use a lot, but a few million refilling every day would put a dent in the
by dreamcompiler 8y ago
Also, helium is for all practical purposes a nonrenewable resource. One drone wouldn't use a lot, but a few million refilling every day would put a dent in the world's helium supply.
- learc83 8y agoHydrogen is lighter than helium, easy to produce, and if you're talking smallish blimp with no people on board, it's not particularly dangerous.
- neaden 8y agoI think having what essentially would be small robot controlled fire-bombs flying through the skies would be a tough sell. Even if you managed to make them as safe as possible it's not exactly the kind of thing people would be eager to approve.
- ProblemFactory 8y agoCurrent drone batteries are a much higher fire risk than a hydrogen blimp.
- penagwin 8y agoPlus assuming the drone is above the tree line and not next to a tall structure, the actual "fireball" should move fairly skyward and not pose much of a fire threat? I'd be more concerned about the falling drone/battery.
- caf 8y agoIt's not only direct contact with the fireball you have to worry about, a large amount of the energy in the explosion would be coupled into a pressure wave and infrared radiation.
- AlexCoventry 8y agoHow would you set one alight? A fire arrow seems much simpler.
- teraflop 8y agoIt's worth noting that hydrogen is orders of magnitude more energy-dense, by mass, than lithium-ion batteries. As a benchmark: the DJI Phantom weighs roughly 1kg, and its battery can store about 80kJ of electrical energy. I found a NASA study [1] which estimated that the energy released when a li-ion battery combusts is roughly 2x the usable energy capacity. So let's say that a battery fire would release 160kJ of energy. In contrast, a balloon capable of lifting a 1kg drone would need about 830 liters of hydrogen, with a mass of 75 grams. That amount of hydrogen, if burned in room-temperature air, would release about 9MJ -- roughly 50x as much energy as a battery failure. (Of course, most of the hydrogen would burn itself up quickly and harmlessly, assuming the balloon isn't flying near any flammable objects.) [1]: https://www.fire.tc.faa.gov/pdf/TC-15-40.pdf https://www.fire.tc.faa.gov/pdf/TC-15-40.pdf
- caf 8y ago9MJ is roughly 2kg of TNT. I'm not so sure that kind of energy release would be entirely harmless.
- learc83 8y agoA kg of gasoline is about 40MJ. The speed at which the energy is released is the important factor, not the total energy.
- caf 8y agoSure, and the speed at which a cloud of hydrogen-oxygen will release energy after ignition is very rapid indeed.
- learc83 8y agoThe theoretical maximum detonation velocity of a hydrogen /oxygen mixture is significantly less than TNT. You're also not going to get anywhere near the theoretical maximum detonation velocity if it ignites because of a leak. My point is comparing hydrogen to TNT is very misleading.
- 8y ago
- learc83 8y agoWhat do you think gasoline powered drones are? Agreed that hydrogen is perceived as being much more dangerous than it is because of the Hindenburg disaster. But (and I have no reason to think that this is true) if it turned out to be commercially viable, I'm sure that perception could be changed.
- dreamcompiler 8y agoAgreed. But there's a perception issue that arises from flammability that requires political skills I don't have. So hydrogen feels like a difficult solution to me. Example headline: "Flying Bombs, Oh My!"