Roman Balloons -> Earliest Blimp/Controlled airship

Michael Busch

Were Roman fabrics lightweight/impermeable enough to be used in a balloon? The Montgolfier brothers used taffeta, which was invented in the Renaissance OTL, so it may not be a coincidence that the first tethered balloons were used at that time.
 
Roman technology was up to the task. It would just have been incredibly labour-intensive, and you would have had to know what you were doing (which probably explains why the Romans didn't invent balloons - having stuff lying around and then coming up with an application is much easier than the other way around).

As to the question - if we assume that the rest of the tech tree stays the same (not likely, but let's go with it), about the same as OTL - from the mid-19th century on, reliably by the 1890s. The key is not the lift but the weight-to-power ratio of the propulsion.
 

Michael Busch

The key is not the lift but the weight-to-power ratio of the propulsion.

That's not quite the same: _controlled_ versus powered flight. If the fabrics were indeed suitable, then you could build a hang-glider - but that would require someone pulling a Da Vinci.
 
That's not quite the same: _controlled_ versus powered flight. If the fabrics were indeed suitable, then you could build a hang-glider - but that would require someone pulling a Da Vinci.

True ... maybe if the Romans got their hands on kite technology? They're likely to try some funny stuff with it. The Romans were not terribly practical people at all times, but they loved messing with technology that impressed them.
 
Uncontrolled lighter-than-air flight is so easy to see in nature (ashes rising in a fire) or create on a small scale by simply holding a paper sack over a fire that it is amazing that tethered or free hot-air balloons were not invented by a whole host of ancient civilizations. Perhaps the principle was known but no one really saw any value to it.

Regardless of how early free balloons are invented, it is hard to imagine efficient dirigibles much predating the 20th century, when light alloys and small internal combustion engines are invented. There may be other types of propulsion (human-powered propellors, or steam) which might be possible earlier, but it would take a very different evolution of technology than occurred in our world, where it took over 100 years of fanciful and unsuccessful dreaming of wind, oar, and animal powered airships before the first true dirigibles were possible.
 
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