In general, when calculating caloric requirements for animals, we can apply what's known as Kleiber's law. Kleiber's law states that the food requirements of various animals scale by the mass of the animal to the $\frac{3}{4}$ power. This value is then multiplied by a constant that's dependent on the type of animal.
To compute a dragon the size of Smaug's caloric intake, then, we can start by computing his weight. According to this source, Smaug was around 60 meters long. A tyrannosaurus rex, by comparison, is a little over 12 meters long. Smaug had a long tail, though. (About 2/3rds of his length is tail), while t-rexes look to only be about half tail. If we compute weight based on body length, we'd then have Smaug's 20 meter body compared to about 6 meters for our t-rex. If we assume weight scales with the cube of body length, we can then figure Smaug's weight to be around $\frac{20}{6}^3 = 37$ times the weight of a t-rex. The more bird-like assumptions about dinosaurs put the weight of a t-rex at about 4.5 tonnes. Since Smaug flies, we can assume that his weight is probably similarly proportioned to his mass, perhaps a bit less. Since 37*9 is 166.5, we'll round down to a respectable 150 tonnes for Smaug. This is comparable to the weight of a blue whale (190 tonnes), which, while only about half as long as our dragon, is significantly more densely built.
Now let's apply Kleiber's law. First, let's get some base data on how much animals eat
- A 290 gram barn owl needs the eat about 72.9g of small mammals per day. (About 120 kCal)
- A 20kg crocodile needs to eat about 2kg of meat per week. (About 470 kCal/day)
These numbers give us estimates for about how much reptiles that are fairly stationary (our dragons sleep a lot) and flying birds (dragons fly around to catch food) need to eat. We can scale these numbers up to get some estimates on how much our dragons need to eat per day. The data I found is specified per day (or per week) rather than per hour, but we can extrapolate based on the activities of our sample animals what sort of activities these things correspond to.
Bird-like dragons
Our dragons weigh as much as around 500,000 owls. Based on Kleiber's law, they should then eat about 20,000 times as much food, for a total of 2.3 million Calories/day. Smaug ate ponies in the lord of the rings, so we can say that this is about 8 ponies worth of meat, for comparison. This activity level corresponds to a bird that hunts quite a bit, so this is probably a fair estimate for the activity level of a dragon while scouting or hunting.
Reptile-like dragons
If their caloric requirements are closer to those of crocodiles, they'll need a bit less. A dragon weighs 7500 times more than a 20kg croc, and so should need about 806 times the food as one. This comes out to a mere 380,000 Calories per day, or a bit more than one pony per day. This is more like a dragon that's sitting around snoozing on some gold or schmoozing with other dragons and the odd invisible hobbit.
Final estimate
In summary, if our dragons eat about as much as owls (relative to weight) while they hunt, but only hunt once a week and snooze like crocs the rest of the time, they'll need an average of around 655000 Calories per day. That's about two ponies per day, so once a week a dragon will need to go out and eat a small herd of them before going back to snooze on its pile of gold.
23Do you want that in Calories or princesses? – None – 2015-02-11T21:01:26.773
6I'd prefer calories, since no respectable dragon would eat a princess, given their typical ransom price. I would assume a princess could be ransomed for a large number of cattle or beasts of burden. But depending on the princess, I suppose... – corsiKa – 2015-02-11T21:16:53.310
Sidenote -- neither the size you give, nor the quoted length for Smaug himself, are that large in the grand scheme of flying things -- Smaug is about as long and heavy as a lightly loaded A330-200, and the much smaller dragon in your OP is roughly the size of a mid-sized business jet. – Shalvenay – 2015-02-12T00:50:36.330
4"(assume the firebreathing is a chemical reaction that does not draw substantially from their caloric requirements)" Two points here, 1) chemical reactions do have caloric requirements in a creatures body. We know this because 2) *all* activity in an animals body are in fact merely chemical reactions, and they all have caloric requirements. So unless you want to fiat it as Magic, you've got to take it into account, but, then you could invoke Magic for all of these requirements, thus making any answer possible, – RBarryYoung – 2015-02-12T04:21:26.213
The key word being substantially. Yes it will have minor requirements, but not much more than just pushing the chemicals around the body, and the muscles needed to open and close the flaps that mix the chemicals. At least in the theoretical model I've built of how the dragons breath fire, which is very much still in development. – corsiKa – 2015-02-12T15:23:26.117
2but what produces the chemicals for the reaction? – Mr.Mindor – 2015-02-12T16:51:53.047
They're ingested. A cow contains about 3kg of Sulfur, and for particularly warmongering dragons they take supplements just like we take vitamins. – corsiKa – 2015-02-12T16:52:51.920
No, you're not quite getting it. Ingesting it is not nearly sufficient because it's not just about what chemicals are in a material, it's largely about how they are combined and how much energy their chemical bonds contain, and how much energy it takes to change/release it. Consider that dirt has all of the elements needed for gas, but unlike oil, we cannot make gas from dirt because we would have to put in more energy than we would get from gas(oil already has the energy). Likewise, a dragon could make gas/napalm from normal carbs (as some bacteria do), but only with enough energy/calories. – RBarryYoung – 2015-02-13T17:52:10.273
I can make an entire oil field light up with a single match. Same thing. I'm planning to have something like that, a hidden tongue in mouth that acts like a flint against the dragon's teeth, and spewing out a flammable fluid through the sparks. Basically. – corsiKa – 2015-02-13T17:55:50.367
So for instance, we can estimate food as having about 7000 kCalories per gallon, whereas gasoline has 31,000 kCal/gal. Therefore, a dragon would need to ingest at least 4.5 gallons of food for every gallon of gas/napalm/whatever-a-dragon-breathes. Taking into account the natural inefficiencies of any such process, they would need at least 8 times as much food (and some amount of water) as gas produced. At that is in addition to whatever else they need to live and fly and whatever. – RBarryYoung – 2015-02-13T17:58:52.547
2No, you still aren't getting it. Food has caloric energy. So does Gasoline. *You cannot create energy out of nothing, it has to come from somewhere.* To make gasoline from food, you need to ingest as many (more actually) calories as you produce. There's no free lunch here. – RBarryYoung – 2015-02-13T18:01:09.773
sigh I, as a human being, am capable of mining a piece of sulfur from the ground, striking it with a rock to light a fire, and watch the rest of it burn. Now, imagine the same thing, except i've chewed it in my mouth first and spit it out as a fluid. Still flammible (assuming the spit doesn't extinguish the flame, that is - it would not be hard to have a spit that doesn't significantly hinder combustion). Same thing as the dragon. There are naturally occurring flammable substances - the dragon ingests those substances and stores them in his cheek, and lights it as he spits it out. – corsiKa – 2015-02-13T19:52:04.103
2*sigh* and that sulfur has a caloric value that is equal to or greater than the energy it produces. There's still no free lunch here. If you are saying that your dragons mine the chemicals that they use to breathe fire and then ingest those chemicals, that is fine (odd, but OK). They still have to ingest enough to account for what they breathe out. Sulfur, for instance, only has about 1/5th the energy per pound as gasoline, so it is not providing much more energy than food such as say sheep would. And it's probably easier for your dragons to find sheep than to find and mine sulfur. – RBarryYoung – 2015-02-14T12:53:04.263