How to build the largest possible zeppelin with ancient tech?

Question

In my world lives a society with ancient technology. By ancient technology, I mean ancient roman-like tech, with full access to ancient greek-like knowledge (like the antikythera or Archimedes' war machines).

Now, I would like this society to develop a kind of military zeppelin, and I was wondering if it was possible.

The first hot air balloons were small and meant to be manned only by one person, or just a few persons. They would be made of paper sewn with cotton (and I know, that paper per se was not ancient roman tech, but it was ancient chinese tech, and it is not difficult for an ancient people to produce paper, so I'll handwave on this one).

However, I do not know if it is possible to build a zeppelin with paper. Or if they could make the balloon with a kind of fabric that would be accessible for a not-so-modern-people and still air-tight.

Other thing we need to consider is propulsion. Maybe these people could have perfected an engine with the same principle of Heron's aeolipile... but would it be sufficient to move a zeppelin? Or could we use another method of propulsion?

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All these things considered, the zeppelin I'm trying to create may not be as large as the zeppelins used on WW1... however, my questions are these:

Would it be possible for such an ancient people to produce zeppelins? And if so, how large could they be while retaining function?

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Note: If the problem is costs, remember that my people runs an empire with the same power and strength as Rome's. They would blow off cost-effectiveness just to have some war-machines (even just three or four) that would impose respect over their less advanced neighbors/tributaries

Answer 1

No, it would not be possible for ancient people to produce Zeppelins.

Rigid airships require aluminum and internal combustion engines to be even minimally feasible...and even so, their payloads are pitifully low. For safe operation, they also require navigation and reasonably good weather.

Aluminum requires electricity. An iron or steel airship will not lift off the ground. (Dry) wood will (barely) lift off the ground, and the meager payload won't be worthwhile - donkeys will be cheaper and more versatile.

Collecting large amounts of hydrogen also requires electricity. Ordinary hot air won't even get close to lifting the structure of a rigid airship off the ground. Not sure what available-to-ancients material can contain the hydrogen, so the gasbags might be a bit leaky. No smoking nearby!

Ancients are unlikely to discover any of the rare helium deposits while drilling for the petroleum they don't know about, and have no useful way to capture/store/transport the gas anyway.

Internal combustion requires fine steel, machine tooling, and petroleum. Steam engines are too heavy, too weak, consuming fuel (wood/coal) changes the buoyancy of the craft.

Navigation requires a sextant and accurate clock. Accurate navigation tools are essential - the Earth has a lot of clouds and fogs at inconvenient times and places. And mountains that look just like clouds at the wrong moment.

Weather information requires data transmission (telegraphy or better). Many airships were destroyed in rather ordinary storms, particularly at night.

Answer 2

Terminology

Flying machines are of three kinds: aerostats, aerodynes and rockets.

• Aerostats are flying machines which are kept aloft by buoyant gas; that's why they are said to be lighter than air.

• Aerodynes are heavier than air: they are kept aloft by the lift generated by the movement of air over one or more airfoils. Airplanes, helicopters and kites are examples of aerodynes. (For completeness: there exist flying machines which are kept aloft by a combination of buoyancy and lift; those are called hybrid airships.)

• Rockets are kept aloft by the thrust of an engine, with little or no help from aerodynamic lift.

Aerostats are of two kinds: balloons and airships.

• A balloon is a simple aerostat with no means of propulsion other than the wind.

• An airship is an aerostat with its own on-board means of propulsion. Airships are also called dirigibles, because they can be steered on a preset course. Airships are of three kinds:

  • Non-rigid airships or blimps which rely on the pressure of the lifting gas for maintaining their shape.

  • Semi-rigid airships have a rigid internal structure, but rely on the pressure of the lifting gas to maintain the shape of the envelope.

  • Rigid airships have a rigid framework covered by an outer skin or envelope. Once upon a time there were several manufactureres of rigid airships; one of those was

    • Luftschiffbau Zeppelin GmbH, a manufacturer of rigid airships founded by Count Ferdinand von Zeppelin.

    Nowadays, Luftschiffbau Zeppelin GmbH is a major shareholder in

    • ZLT Zeppelin Luftschifftechnik GmbH, a German company which makes the Zeppelin NT airships.

Ancient airships

I have written a long and detailed answer to a much more reasonable question asking for airships in the 18th century. Airships in the antiquity are several times more unlikely.

1. No lifting gas.

   The basic problem here is that the ancients had no idea of chemistry. None at all. Not even a wrong idea. They did not even have a word for "gas" -- that was coined by Flemish chemist J.B. van Helmont at the beginning of the 17th century. The only lifting gas which they knew how to make was hot air; hot air won't lift an airship -- but it may lift a balloon.

   To work around their inability to make hydrogen (or to mine helium) you must somehow make them discover and develop chemistry some 1500 to 2000 years ahead of time. Prepare for major impacts on their civilization; if they can make hydrogen they can also make advanced explosives, and sulphuric acid, and artificial fertilizers...

2. No adequate structural engineering.

   The ancients simply did not know how to make strong and lightweight trusses. Which is not a fault -- their engineering just did not require such structures. They didn't even have the mathematical tools to calculate such structures.

3. No engines.

   The ancients simply did not have any kind of motor. The aeolipile invented by Heron of Alexandria is a particularly inefficient model of a steam turbine showing that heat can be converted into mechanical power; but it had no practical applications.

Basically, a civilization which can make airships cannot possibly be similar to the Classical World. It must have chemistry, and advanced physics, and advanced mathematics, and advanced metallurgy, and good knowledge of the strength of materials, and internal combustion engines (or electric motors or some other kind of prime movers). Such a civilization would be so much in advance of the Classical World that they would have no rival. They would be the uncontested rulers of the world, no airships needed.

Historical framework

In the real history, airships enjoyed a very brief moment of glory: about 30 years, from around 1910 to around 1940. Once a civilization advances to the point where they can make airships they are also able to make airplanes; since airplanes are so much safer and faster than airships, and they can carry much more cargo than airships, the age of airships will be gone in a blink.

Answer 3

No (see other answers like AlexP's). BUT if you had to:

While highly unrealistic, let's see if we can get a basic model. Firstly, I'd throw out rigid airships. Romans didn't have the structural engineering skills to make the interiors, nor the gas needed to lift them (or a means of getting and transporting said gas). This means we're using hot air. If we assume the balloon is flying at 1000 feet up, and that the ambient air temperature is 15 degrees (F), then we can use this handy table to calculate how much lift we get from hot air. According to Wikipedia, hot air in balloons is 212 degrees F.

Balloon Gas ~= .057 lb/ft3, Ambient Gas ~= .085 lb/ft3

Therefore, hot air can lift ~0.028 lb/ft3 (or, after a quick hop to metric, we get that each cubic meter of hot air will lift .045kg)

Now we have to start making payload assumptions. Firstly, I'm assuming we're talking about what the Romans could have built, not what they had. Basically like if some Greek philosopher said "what if we used gears and pedals to move a propeller?" and now the Emperor orders that he try to build it. If we're allowing that, a VERY basic bicycle chain-like construct could be used to power a (rather weak) propeller that could go on the back of the airship. It would be hard to push much against the wind, but it's a start. In terms of pointing the propeller in the right direction, a simple light-weight rudder should do the trick (planes use them too). The balloon likely would have to be a sphere, as that puts the least pressure on the cloth. You mentioned hand-waving the cloth/paper was OK, so let's just say it's strong enough (though in reality this is probably what will kill you as the bigger the balloon, the more cloth, until the pressure on the cloth eventually bursts or it's so bit you can't heat the air inside)

So, all told (propeller man plus rations, propeller/rudder, wood/cloth structure, balloon, ropes, air heater/horrible fire hazard, etc), let's just pull a number from where the sun don't shine and say 1000 lbs. Not sure what you're using these balloons for so adjust as needed, but 1000 lbs will require a balloon volume of ~35000 cubic feet. Then double it for each subsequent thousand lbs.

This results in a balloon roughly 40.58 ft in diameter (google search "sphere calc: find d, V=" for other values). I'm no expert on ancient Roman cloth strength, but that doesn't sound that far beyond the realm of plausibility for the balloon to not rip...

Again, even with a pedal system propelling it, this would get blown all over the place. The fire needed to heat the air would probably result in a lot of Hinden-barbecues. The fire would need to be hot enough that fuel would be a massive weight, and range would be limited. Rain would probably soak into the cloth and weight you down, while hotter outside air would make you sink like a stone. And I'm probably heavily underestimating the weight of the payload. So as stated above, it probably wouldn't work. But if you need it to work, this is probably how you'd do it.

Answer 4

Successful wooden framed airships have been built (Schutte-Lanz, in WWI Germany) and hot-air blimps have been flown, both of which could have been approximated with Roman technology, but without an engine, which was decidedly beyond Roman abilities, both would only be free balloons with no use or need for their streamlined shapes. The Romans could conceivably have used hot air balloons for aerial spying during battles a la WWI (and even as far back as the US Civil War), dropping firebombs or flechettes on besieged positions (probably not as practical as just flinging them over the walls via ballista, but perhaps to great psychological effect).