Animals using hydrogen to fly?

(checks copy of The Flight of Dragons) It doesn't actually say how the dragons ignited their gas, just that they found a way of making it work. That's one part the author couldn't quite figure out it seems. Regardless, one of the disadvantages of being filled with flammable gas is easy to guess. The book mentions an incident where a dragon was fed flammable substances and exploded. One of the reasons why such a system is unlikely to evolve.

Edited by Lymantria on Jun 14th 2019 at 6:03:13 AM

I don't think creatures with flammable substances in their bodies would be built like blimps for that exact reason, it would likely be protected by their skin, bone and muscle and things that could ignite them would have to go through that and would have killed them either way.

Edited by Kaiseror on Jun 14th 2019 at 10:46:36 AM

I generally agree: any event capable of igniting their stored hydrogen would be so injurious as to make them unable to fly anyway. The only exception I could think of is them ingesting a substance that is hypergolic when combined with it, such as a highly reactive oxidizer. Such substances would be highly toxic regardless of whether they made the creature explode.

Edited by Fighteer on Jun 14th 2019 at 11:33:25 AM

I imagine lighting the creature on fire would be similarly pointless, to ignite the gas bladders it would have to burn through the skin and muscle, which again, would have killed it either way.

Would biological rocket propulsion be remotely plausible for creatures with flammable gas in them?

Compressed gas propulsion is certainly possible, although not very efficient. Combustion-based propulsion... again, I don't see any plausible evolutionary path to an organism lighting stuff inside itself on fire.

The biggest problem is the ability to carry enough fuel. Rockets use several different kinds of propellant, but all engines are fundamentally based on pressure gradients. The air in a balloon is under higher pressure than the air outside the balloon. Create a path for it to escape and it attempts to equalize. The air is the propellant and the balloon provides the pressure differential.

Heat up the propellant and it will be under higher pressure, ergo it escapes faster, which provides more thrust. The rocket equation tells you how much total delta-V (change in velocity) you can get for any given propellant at any given mass flow rate.

Organic structures are not good at handling extremely high pressures and temperatures. An organism could expel gas from a bladder, sure. It wouldn't be able to store enough to get much total change in momentum. It could (somehow) light that gas on fire after it leaves the bladder, but the thrust this would generate would be tiny and the organism would be burned. Imagine using a flamethrower for propulsion. It doesn't work because the combustion is occurring outside the nozzle.

If you do want internal combustion to generate enough pressure to make for an efficient rocket, you need a system to pump fuel into a combustion chamber at high pressure, you need an oxidizer, and you need a source of ignition.

Edited by Fighteer on Jun 27th 2019 at 12:54:34 PM

It could just use rocket propulsion to take off but still uses wings to actually fly, perhaps as an energy saving method as getting off the ground is one of the biggest challenges for flying creatures.

Maybe if it's super light, like an insect. Otherwise the square-cube law works against you. The bigger the organism, the more propellant it would need relative to its size, and under higher pressure. The physics just don't work. It would require a lot more net energy to gain initial lift using that method than via jumping (for example).

Imagine a bird lifting off by farting. At Earth gravity, a 1 kg object would need to generate at least 9.8 N of thrust to hover. To lift off quickly, you need quite a bit more. I can't find any specific values for thrust of a typical rubber balloon, but this is what you'd need to compare it to.

You could do a science experiment. Inflate a rubber party balloon, attach it to a 1 kg mass, like a hardcover book, then see if it can propel it off the ground.

Edited by Fighteer on Jun 27th 2019 at 2:49:23 PM