Let's talk about dragons for a bit. Giant, flying, deadly predators with teeth and claws and fire and scales. Unfortunately, "giant" and "flying" don't play together nicely in physics' playground.

Birds, as we know, are lighter than they look thanks to having hollow bones and air pockets throughout their bodies, but even birds top out long before they are big enough to carry a human. But how much lighter could an animal become if those air pockets were filled with a lighter-than-air gas like hydrogen instead?

Could this be used to mitigate the tyranny of the square-cube law and create a giant flying creature? Hydrogen could be generated through bioelectric hydrolysis, and could also be used to create fire (though hopefully, the fire-generating organs would be kept very far away from the internal hydrogen sacs).

How big could a hydrogen dragon get while still looking and acting like, well, a dragon? I'd like a creature at least big enough and strong enough to carry a fully-grown human and still remain agile enough to make a formidable aerial fighter. A giant beast the size of an elephant, capable of trampling a formation of medieval soldiers, would be ideal.

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    $\begingroup$ The Hydrogen Dragons of Capella Signi Alpha related but not a dupe, I don't see your answer there but there should be some pointers, at least for what size balloons you need for what mass, which makes me think the answer to your 'How big could a hydrogen dragon get while still looking and acting like a dragon?' question is something along the lines of 'not very' if you want either traditional western or oriental dragons :) $\endgroup$
    – Pelinore
    Commented Dec 8, 2019 at 19:42
  • $\begingroup$ I think that the volume needed would make the creature more like the netch from morrovind than dragon like giantbomb1.cbsistatic.com/uploads/scale_small/1/17172/… $\endgroup$
    – lijat
    Commented Dec 8, 2019 at 19:59
  • $\begingroup$ Here we go again, Mephisto time... $\endgroup$ Commented Dec 8, 2019 at 20:03
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    $\begingroup$ Discovery Channel did a dragon documentary where they looked at these questions. Determined that the same chemicals used for buoyancy are the same chemicals used for creating fire and that performing one task would limit the ability to perform the other task and that surface deposits of platinum would need to be consumed as the catalyst for the fire. I forget the name of the documentary but they basically pretend they find a frozen dragon corpse in the Carparthians and perform an autopsy. Useful your what your trying to study. $\endgroup$ Commented Dec 8, 2019 at 20:24
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    $\begingroup$ @AlexP I don't think OP intends the hydrogen to lift the dragon hot-air-balloon-style, rather to simply allow the dragon to be larger without weighing more so that it can generate its own lift with its wings. Still, your point about how much weight is displaced is likely important and I think is a major problem for OP. $\endgroup$
    – Loduwijk
    Commented Dec 9, 2019 at 21:51

3 Answers 3


How big could a hydrogen dragon get while still looking and acting like a dragon?

From comments & answers to this question 'not very', apparently a 1,000 ib race horse would need something like 377 m^3 of hydrogen to fly.

I'd like a creature at least big enough and strong enough to carry a fully-grown human.

One small problem with that .. the more lift your applying to the animal with (what are for all practical purposes) helium filled balloons, then the less agile it's going to be .. howsoever if being able to carry a person about is all you're really concerned about?

Then I offer you my answer to to this other question.

Specifically Quetzalcoatlus northropi.

enter image description here

Ok, so there is this (see below).

'But when it flew, the Earth spun a little faster (so gravity was effectively (ever so slightly) less), the atmosphere was also thicker (making flying easier), & the atmosphere had a higher oxygen content (making muscles more efficient) ... so Quetzalcoatlus might not be able to fly today.'

But those are relatively small quibbles & can probably be hand waved away with just a small amount of hydrogen unlikely to impact it's appearance too much, fill it's hollow bones with it, give it a few gas bladders elsewhere filled with the stuff & wave your hands while you tell the crowd you've nothing up your sleeve & the jobs a goodun :)

In Summary :

So really big arse balloons aside (which definitely impacts on it "looking and acting like a dragon").

I think Quetzalcoatlus (with or without just a bit of hydrogen in play to assist the handwavium) is likely the best model there is for the largest a flying animal can plausibly be in Earth like conditions.

Some Other Links :

Quetzalcoatlus, see the section on flight.

I did look for a YouTube of Paul MacCready's 1984 model northropi ornithopter but wasn't able to find a good one, besides his was only a half size model, this one is full size but fixed wing & unfortunately it's more a 'making of video' than anything else with only few shots showing the thing really flying (the tether was also rather short so those shots there are don't look all that impressive).

Lifting Properties of Hydrogen :

A cubic foot of hydrogen lifts around 30.8 grams (68 lbs per 1,000 cubic feet).

Considering a hypothetical 7 foot long 2 foot wide 4 foot high helium filled bladder along the spine of northropi (dimensions that hopefully won't detract overly much from the dragon look & aerodynamics) as an initial back of the envelope calculation gives us 56 cubic feet .. so around 3.8 pounds of lift.

So even filling it's hollow bones with hydrogen you probably can't plausible get away with much more than 4 or 5 pounds of lift from hydrogen without seriously impacting the appearance of your dragon.

For Reference :

An adult human is 137 pounds, the Kori bustard 25 pounds & Argentavis was maybe 156.5 pounds.

Quetzalcoatlus we really have no idea but estimates have ranged anywhere from 150 to 550 pounds.

Person Carrying Potential :

The wingspan of Quetzalcoatlus is comparable to an ordinary hang gliders (often also around 10 m).

Tandem hang gliding is a thing.

So the idea a dragon the size of Quetzalcoatlus could swoop down in a shallow glide & snatch up someone from the ground is very plausible.

But that's with it already in the air & up to speed .. trying to get airborne from a standing start on flat terrain with the extra weight of a human body might not be possible & will be a serious struggle for it to achieve if it is (having a steep hill or cliff to jump off is another matter of course).

  • $\begingroup$ A thought here: would it be plausible to compress the hydrogen, putting it under pressure? Basically, get more lifting gas into the same space? Or would that just be impractical? $\endgroup$
    – Palarran
    Commented Dec 9, 2019 at 23:51
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    $\begingroup$ @Palarran : If you compress it you make it proportionately heavier & get less lift for the same volume, compress it enough & it's heavier than air, so no that doesn't help you ~ physics is not our friend here :) $\endgroup$
    – Pelinore
    Commented Dec 10, 2019 at 0:05

The short answer is


It takes one m^3 of hydrogen to lift up a kilogram. It is also flamable and would diffuse through the walls of the container.

The square-cube law only applies to things that have a similar shape. This means that with strong-enough materials and a major overhaul in design, a larger dragon is possible. You wouldn't want to make them that big, but you'd want to make them large, to combat air resistance.

Another problem is terrestrial locomotion. Dragons are in the postion where a huge chunk of their mass (25%) is only their pectoral muscles. However, this also means that if you were to pull a limpet teeth, and make the wingbones significantly stronger, then the dragons could use them as a very powerful weapon.

Dragons wouldn't really be able to rely on their legs directly in combat, as that requires more muscle, which means more dead weight in flight. The most likely weapons for them would use a lot of leverage. A spade at the end of their tail is a good "default" attack.

Hydrogen is light and not at all dense, it makes for a terrible breath weapon with an extremely short range. I'll get back to this later...

Maybe we could start at whale oil j,ust make sure to napalm/wild-fire a few whaling ships, so as to not invoke the whale spirits' wrath.

Making the thing into the WMD will require some inspiration, let's see how Ymir Fritz managed it enter image description here

I didn't want to sleep tonight anyway. So, if you don't have that titan laying around, you can always go the third route that is obliterating the enemy supply lines, and breaking their ranks via hit'n run and breath weapon.

You can also give a tough armor for better resistance, keen sight to be able to collect sweet intel and so on. If you can keep the agility along with some durability, it'd mean double troube: harder to hit with big weapons, and small ones would do nothing.

Carrying stuff is the biggest hurdle as it'd require even more wing muscles. Gliding would be easier, and you could make it into a plot point that if a dragon lands with their rider, they ain't getting back up until they find a hill that's a-okay to take off from. Alternatively, the dragon flies up and tries finding a suitable take off point, while their rider follows them with an ODM gear (yes, I watched too much Attack on Titan).

  • $\begingroup$ I don't think the question was about the breath weapons .. just the size potential in relation to flight. $\endgroup$
    – Pelinore
    Commented Dec 8, 2019 at 20:55
  • $\begingroup$ you don't consider the hydrogen temperature. $\endgroup$ Commented Dec 9, 2019 at 11:22
  • $\begingroup$ @StefanoBalzarotti Even if you raise the temperature (you shouldn't) and miraculously get an almost perfect vacuum, it'd still be next to nothing. $\endgroup$ Commented Dec 9, 2019 at 11:48

It might let you move to something a little bigger, but the square-cube law is causing more than one problem here. Incidentally, I notice some people leaping to the idea that you need enough hydrogen to lift the dragon outright or at least cancel its weight fully (which would result in something closer to a hot air balloon than a dragon). Why? You would only need enough to put mass low enough that the wings would be able to lift it; you don't need to negate the mass entirely.

One problem is that as you get bigger, nerve pathways are going to get longer. As they get longer, signals take longer to travel. This introduces several complications, but one of them is that your reflexes are going to be slower, probably significantly so. Ambush one of these guys, and they're toast.

Another concern is that with larger wings comes a greater degree of force required to move them. Flapping those wings will put stress on the bones and flesh in them, stresses likely to be magnified as you go farther out along the wing (a small movement near the body translates to a huge movement out at the wingtip). At some point, something is going to give way if the wings are too big, and in a disastrous fashion. Whether it's the flesh that tears first or the bones snapping before that is an exercise for the reader, but it's not going to be pretty either way. This is just from the force of beating the wings, mind you: a T-rex with suitably scaled wings would snap them like twigs on the first wingbeat.

For softer restrictions, having a dragon with enough hydrogen in its body to be relevant as a lift/buoyancy force is going to cut into its agility. You could plausibly get away with air bladders in the back, likely beside or around the spine (I'm not sure if the added air resistance would outweigh the benefits from the hydrogen), but you're not getting any useful quantity of hydrogen in the wings without murdering the aerodynamics of those wings.

In summary: you're not going to get anything like Smaug from real-world physics, ultimately.

However, a more modest dragon at the lower end of your hopes (around the size of Quetzalcoatlus, as noted in Pelinore's answer, about 10m wingspan or smaller) might be viable. I'd advise you to focus on making your dragons fast. The greater part of lift is going to come from your airspeed; this is why airplanes get away with relatively short wings, and why hovering with wings isn't going to happen in anything much larger than a hummingbird. Quetzalcoatlus had to move fast to stay in the air, and it had the wing muscles to do that: it could hit or exceed 100km/hour (source here if you want to do some further reading), and I don't think it would take too much alteration/handwaving to let you have something that could support at least a lightweight human (no armor!) with, say, crossbow and ammunition. Just remember that it needs to keep moving: if it slows down too much, it's going to lose altitude.

  • $\begingroup$ 'a sizable underbelly filled mostly with hydrogen' will tend to unbalance it (it will tend to 'float' belly up) so will need a degree of active effort to stay upright, large helium bladders elsewhere may interfere with limb & muscle movement so I'd probably go for a line of bladders along the spine (should be out of the way of anything there), possible inflated from higher pressure bladders (so a bit like a fish's swim bladder). $\endgroup$
    – Pelinore
    Commented Dec 9, 2019 at 3:16
  • $\begingroup$ @Pelinore Makes sense. I'd mostly just been thinking where the hydrogen bladders wouldn't spoil flight entirely, and didn't stop to consider balance. I'll edit my answer accordingly. $\endgroup$
    – Palarran
    Commented Dec 9, 2019 at 11:05

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