# The Giant Roc and Pulse Jet propulsion

I am currently in the process of making a world, and am trying to ground it in scientific fact as much as possible. The world is a hard magic setting, and while fantastical creatures exist, they are usually rare or exaggerated re-tellings. Dragons, for instance, do exist, and can fly, but only short distances due to their weight.

While Dragons rule the ground and the air around it, the Roc is king of the sky. I am not particularly scientifically minded myself, but have been looking at various means of making a creature that could have conceivably been created by evolution (and if necessary helped along somewhere by an incredibly powerful and rather insane wizard).

Would it be possible to create a creature that does the following?

1. The Roc would be able to fly at high altitudes, up to 15,000 feet
2. The Roc would be able to maintain altitude for long periods of time, rarely returning to the Earth to land.
3. The ability to take off again, given that it is supposed to be massive in wingspan and would most likely be heavier than a typical bird by quite a lot. (I have thought about some sort of pulse jet assisted takeoff that would double as a means to escape more dangerous prey near the ground, where they are slower and more vulnerable. Perhaps using Nitrogen collected in flight, which is then ignited by powerful electric shock, generated in a similar way to an Electric Eel?)

If necessary, some of this can be hand-waved as magic, although I am interested in limiting magical involvement in/with the Roc.

Thank you in advance, and may you have a wonderful day.

• Can you clarify what your Rocs would look like? Because the ones from 1001 nights I remember are MASSIVE. Them flying overhead would blot out the sun. At one point a group of sailors mistake a Roc's egg for a building. The text describes it as being about 50 paces to walk around. Assuming a pace of 75cm (average walking), that puts the circumference at ~37.5m, so the diameter is 11-12m. Here is a depiction of of Sinbad's tale in a movie and how large the Roc is there (not sure why the two heads, though). Basically, this seems TOO big.
– VLAZ
Jul 12, 2019 at 6:59
• Nitrogen is not flammable. If it were, the entire atmosphere would have ignited the first time lightning struck. Jul 12, 2019 at 14:47

That jet takeoff is going to be difficult to sustain.

I think somebody else mentioned it can't be nitrogen. Maybe digestive gasses including hydrogen and methane? I'm not sure how efficient such a jet can be made. There will be refinements in the "nozzle" as it were. Jet engines are far more complicated than just burning stuff through a small hole. You need some way to build up a lot of pressure and push the result out the back. Maybe that means Roc's need to have some kind of very special lined container that can sustain this. Conceivably this lining is ablative, and needs to be regrown after use.

Also there need to be improvements in ways of releasing large amounts of gas very quickly. Possibly it needs enzymes and special food to be able to pull this trick. Roc's might be desperate to obtain certain foods. Could be good drama there.

The main problem is going to be a structure that can handle the forces of being pushed by a jet in combination with wings that can sustain long energy efficient flight. Folding their wings in (as most birds can already do) is not enough. Where-ever the jet pushes on your creature has to have a solid and strong structure that can sustain the force. You've got two very different competing requirements. The jet takeoff needs a strong skeleton to take that force. The long flight on big soaring wings needs light construction with lots of surface area. If you don't engineer that carefully you either get broken bones and internal damage, or you get something too heavy to fly for long.

Possibly the wings can be folded in such a way that the long flight bones "weave" in some way to provide a temporary support for the rest of the creature. I'm having difficulty visualizing that. But imagine the critter is fairly long and skinny except for the flight muscles. And it can fold it's wings in to effectively make a cage that supports it's body in the direction of the jet. Maybe internal bones along its body are adapted to interlock with the wing bones. It would have very little ability to maneuver during the jet thrust. Probably pretty vulnerable to tumbling if it folds or unfolds its wings during flight.

Possibly it can wield the jet in low-thrust or no-thrust mode as a weapon. Possibly it can emit the gas from both ends, breathing fire when it wants to attack. Enough energy to push the critter into flight will probably be enough to toast any threatening critters that don't have extremely tough hide.

The evolutionary path to such a critter is tough to visualize. Maybe it started as the no-thrust weapon thing only? Then one learned to get a little thrust so it could expand its ability to land and take off in confined spaces. Or from difficult terrain. Maybe it started as just a gas sack storing small amounts of flammable. Then it started to develop chemicals to rapidly release the gas from some solid or liquid form.

The Roc would be able to fly at high altitudes, up to 15,000 feet

Many birds do this already. I remember reading that a flock of I think geese was seen by the crew of a commercial flight at cruise height, and they go 24000 feet and above.

The Roc would be able to maintain altitude for long periods of time, rarely returning to the Earth to land.

Albatross do it routinely, they touch land only for reproducing, the rest of the time they soar the ocean

The ability to take off again, given that it is supposed to be massive in wingspan and would most likely be heavier than a typical bird by quite a lot.

If they can fly, they cannot be heavier than a bird. Better, their weight to wing lift ratio cannot be worse than the corresponding one for a bird. Nitrogen, under standard pressure and temperatures, doesn't react with oxygen. However, if you want to give them some assistance for lift off, which due to the large wing span might be needed, better use their guts: they use symbiotic bacteria to produce methane, which is then ignited using a spark on ejection from their intestine or an adapted section of it, providing some additional thrust.

• Commercial flights routinely cruise around 40,000 ft, and long-haul flights rarely cruise below 30,000 ft. It's not unheard of especially for short-haul commercial flights (where the time to climb and descend can be a large fraction of the total flight time) to cruise around 25,000 ft, but that's still well below the service ceiling of just about any jetliner, and most likely every modern turboprop as well.
– user
Jul 12, 2019 at 7:10

### Design:

The bigger and heavier the roc is, the larger the ratio of wings to body will be. If you want it to be huge, it might look like a small body suspended in between two massive sail-like wings made of an extremely thin, likely translucent membrane of flesh.

### Takeoff:

For take-off purposes, these giant wings could be collapsible, folding in multiple times like an accordion:

The landed roc would haul its folded wings up a tall tree or a clifftop, then jump down while expanding its sail-wings to gain lift. These wings would have a very low beat frequency, but the roc could have smaller, paddle-like wings to produce thrust. It would use these to slowly gain altitude.

The roc can also take off from a flat plain or desert using a large headwind: it just faces the wind and opens its wings, sending it backwards and upwards like a kite until it rises above that particular flow of air, then flies normally.

The roc will choose its landing sites carefully to ensure that these features are present; otherwise, it will have difficulty taking off. The roc is most vulnerable on the ground, because its talons are used for dragging the rest of its body, and can't be used for fighting.

If you think this is still implausible or just really want to add a combustion propulsion, you could think about using methane, which has 2 bonuses:

1. It's combustible: it can be released from under the roc, possibly in a mixture including a pyrophoric gas such as diborane so that it ignites on contact with air. This boosts the roc into the air on a large explosion below its wings for an emergency takeoff.
2. It's lighter than air: when not used for quick evasive maneuvers, the methane gas sits in large sacs attached to the body of the roc, working as a lifting gas to make long flights easier.

### Flight:

When high in the air, the roc would prey upon animals flying below it by partially folding its sail-wings and dropping down to grab the prey in the same talons it uses for climbing when on the ground. It then unfolds the sail-wings again and resumes its previous altitude.

Potentially, the methane sacs could be used to kill particularly durable prey by ejecting fire underneath the roc towards the prey held in the talons. A more controlled flow of gas could be used as a flamethrower.

Long, long ago, there was a creature now known as quetzalcoatlis Northropii. It wasn't the largest of the pterosaurs, but it's the most complete large pterosaur (at least as of the last time I checked in detail). It was primarily a soaring creature, as are all very large birds, but paleontologists believe it could launch itself from level ground by using a move combining a jump with a wing sweep. The same was likely true of the "Texas pterosaur" which had a wingspan of roundly ten meters (that's similar to the span of a 4-seat Cessna, though the pterosaur was much lighter).

As noted in another answer, any soaring bird is limited in altitude mainly by the need to breathe (and some of them apparently have a work around, likely akin to "supercharging" by compressing their air in their lungs momentarily before exhaling it). You can help by giving the Giant Roc at mechanism for locking its wings, so it needn't continually burn energy in its breast muscle to hold the wings in soaring position. This will lighten the bird.

Worth noting that without the need to use muscle to hold the wings in position, there's no reason a soaring bird (or pterosaur) couldn't be as large as a modern sailplane -- 15 meter span is very reasonably attainable. Bone strength, as I understand it, isn't the main limitation -- it's muscle power.

There are birds on Earth with long takeoff ratios. Examples include the bufflehead and the coot, who paddle and then run on the water to assist their wings. On land, turkeys run a few steps; many large predators jump from a treetop or a cliff to get going.

Remember - regardless of size, if a bird can fly it will have very low body density.