5 ms·
Hydrogen is lighter than helium, easy to produce, and if you're talking smallish blimp with no people on board, it's not particularly dangerous.
by learc83 8y ago
Hydrogen 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.
- learc83 8y agoAlso worth nothing that a single kilogram of gasoline would release 4 times as much energy as the hypothetical hydrogen balloon.
- 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!"