A squid-like organism uses gravity powered flight but relies on jet propulsion for launch, pouncing, and evasive tactics.

It’s aerodynamically based on the way a true squid flies however it has evolved atmospheric jet propulsion to replace the constricting hydraulic jets.

flying squid

It’s tentacles and fins have become much larger for sustained flight, but it’s arms remain articulate for grappling and marine locomotion.

The jet deflagration chamber is the scavenged nearly perfectly flared cylindrical exoskeleton of another mollusk, with a diameter acoustically tuned to an OH deflagration cycle. This squid binds it to its body, having an intake on the nose and exhaust from the posterior of the animal. The material is unlikely keratin or calcium carbonate, it needs to have good thermal insulating properties and structural integrity.

The jet fuel is twofold: H gas is a natural fermentation byproduct stored in bladders; a symbiotic algae lives in a layer under its translucent skin, consuming its respiratory exhaust and producing O$_2$ which collects in another bladder. The two gasses are injected into the unvalved deflagration chamber for propulsion.

The jet only needs to run for 2-second bursts of possibly 8 - 12 pulses. It’s purpose is to simply gain enough momentum to get airborne, or evade a predator while in flight.

Assume the animal’s total gross weight is 5kg, and it can accelerate at 8 ms$^{-2}$ for two seconds.

Given the heat generated by this reaction,

What lightweight cooling method can be used to prevent heat damage to the combustion chamber and O$_2$ & H fuel jets?

The cooling method of course should include materials suited for heat transfer and possibly heat removal.

I don’t know how much fuel this activity would consume however assume it has evolved 40% efficiency in producing thrust, so 60% is converted to heat, but 80% of that heat is exhausted.

If H$_2$ doesn’t have sufficient energy density to generate the required thrust, I’m open to other organically derived fuel options.

  • $\begingroup$ How does gravity powered flight work? $\endgroup$
    – L.Dutch
    Oct 26, 2019 at 15:13
  • $\begingroup$ @L.Dutch gliding, one presumes. $\endgroup$ Oct 26, 2019 at 15:14
  • 2
    $\begingroup$ Yes. It’s called gliding. The pulse jet gets it to altitude and gives it momentum, then it glides. $\endgroup$
    – Vogon Poet
    Oct 26, 2019 at 15:18

2 Answers 2


So, ignoring drag (too much like hard work), a 5kg mass accelerated from stationary at 8m/s2 for 2s has a kinetic energy of 640J. With your 40% efficiency, that means it needs a 1.6kJ burn producing 960J of heat of which 192J needs to be disposed of by active cooling.

The specific heat capacity of water is about 4.2kJ/l. 10ml of water would be heated by slightly less than 23K if it fully absorbed that heat energy. The simple solution therefore, is to use a small water bladder to force seawater through cooling ducts around the rocket chamber.

The issue of what sort of rocket you should be using is complicated enough that you should ask about it in a separate question. Oxygen is a perfectly reasonable oxidiser, but storing hydrogen is sufficiently awkward that you may as well use some lightweight volatile hydrocarbon instead. You do lose out on Isp, but your fuel density goes through the roof (comparatively) and your chamber temperature is likely to go down which will help with materials engineering (you'll also get a nifty orange rocket flame, rather than an invisible one).

You have not addressed the problem of how your rocket is actually ignited. The fuel/oxidiser combination you've suggested is obviously not hypergolic, so you'll need something else (perhaps taking a leaf out of the bombardier beetle's chemistry book).

  • $\begingroup$ Oh yes, it’s borrowing a spark plug from an electric eel. Sorry. $\endgroup$
    – Vogon Poet
    Oct 26, 2019 at 15:35
  • $\begingroup$ @VogonPoet electric eels don't generate a high enough voltage to arc, do they? (also, aren't they freshwater species? you'll be wanting torpedo rays instead) Without a decent arc, I'm not sure they'd be able to do the job. $\endgroup$ Oct 26, 2019 at 15:39
  • $\begingroup$ @VogonPoet also the specialist electricity-generating muscles are quite big and heavy... not desirable things to cart around just for igniting a rocket. A small hypergolic starter seems like a better alternative if you can source one. $\endgroup$ Oct 26, 2019 at 15:44
  • $\begingroup$ With a small enough spark gap and a tiny ignition chamber an eel’s 860 v could spark, however this charge is focused on a much smaller area than the fish $\endgroup$
    – Vogon Poet
    Oct 26, 2019 at 15:45
  • $\begingroup$ @VogonPoet about 0.1mm in air; pretty tiny. Ignition seems like it'll be quite challenging, as will be handling electrode wear (and indeed the issue of forming electrodes in the first place). $\endgroup$ Oct 26, 2019 at 15:50

The squid has access to all the water it can use. It uses it in 2 ways.

  1. Water cooled.

The squid has access to much water. In addition to the H2 and O2 mixed in the chamber the squid adds water. The H2 and O2 reaction produces hot gaseous H2O, and once the phase change is accomplished, every degree over 100C is wasted. The squid adds H2O to absorb that extra heat. The squid adds just enough that much of the added water is itself converted to 100C gas (steam!) and so contributes to propulsion.

The nozzle itself rests within the internal reservoir used to supply the water - possibly the same one the squid uses for aquatic propulsion. Heat escaping through the outside of the shell is captured to preheat the water which will shortly be injected.

This halfbakery idea lays out the scheme as applied to an engine.


The heat lost by internal combustion engines is pure waste. If a deisel-type injector were programmed to inject just after ignition the exactly right amount of water to maintain a temperature of, say, 150C at the exhaust valve, the latent heat of evaporation would keep the engine at the right temperature, and the expansion ot the water droplets into steam would triple the work obtained from the fuel. The cylinders are insulated to conserve the heat, not cooled as currently done.

  1. Water for thrust.
    The propulsion conferred by the expanding gas is measured a F = mv2. The m here is the mass of the gas itself, thrown behind the squid. But the squid has additional cheap reaction mass - water. As it prepares to launch it takes in a quantity of water and uses the expansion of the gas to throw this water behind it. It is a water rocket - reaction mass of water propelled by pressurized gas, which here is the hot H2O from the reaction.

Home grown shells? I like the idea of a salvaged shell from another mollusk but that means evolutionary pressure on the squid cannot work on the shell, which in the flying squid serves a much different purpose than was the case for its original maker. Mollusks are very capable of making shells - an example being the doughty nautilus with an excellent shell. The shell would be specialized as a rocket nozzle because the squids which fly better live to reproduce. I am not sure cephalopods compete for mates but I like the idea of male squids showing off their flying prowess in a lek.

  • $\begingroup$ The idea behind the scavenged shell was to easily deal with the normal wear and tear of pulse jet stresses. A squid that has to grow it’s own combustion chamber is crippled if the chamber fractures. $\endgroup$
    – Vogon Poet
    Oct 26, 2019 at 17:15
  • $\begingroup$ The heat of the propellant is most assuredly not wasted. The hotter the gas, the more vigorously it expands, so the greater oomph you get out of your combustion chamber. If you want to make use of hot exhaust, do so via a separate bottoming cycle. $\endgroup$ Oct 26, 2019 at 17:26
  • $\begingroup$ Also, using water as reaction mass is only cheap whilst you can harvest it from the sea. You'll maximise the effectiveness of your jet by thrusting whilst clear of the sea, and you don't want to be carrying a load of heavy dead mass into the air if you don't have to. $\endgroup$ Oct 26, 2019 at 17:29
  • $\begingroup$ I like this idea for launching, it will deliver a much more powerful take-off. But how will the system perform in the air? Can he still fire the jet to avoid a predator, or dive bomb his prey? $\endgroup$
    – Vogon Poet
    Oct 26, 2019 at 19:14

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