Welcome to my fantasy, late-medieval world. For the sake of the question, we can divide it up in two major continents:

Continent A is considered the birthplace of my humans and features a solid presence of the other main race (winged humans, of which I talked in a previous question). The geopolitical landscape is divided in a human theocratic kingdom (ruling some of the mainland, the south and the underground cities), a federation of coastal merchant cities, and a somewhat loose union of winged-humans tribes controlling key-locations, mountain ranges, and the northern regions. Those factions seldom agree on something, and infact, have been mostly at war for the latest centuries.

Continent B has both humans and winged humans, some native of the place and some descendants of pioneers from A. Apart from some smaller kingdoms, B's much more uniform, the majority of the land is under the control of a feudal empire.

Now, all's fine and good.

A and B have trade routes via sea, the voyage being around two months long, a little less with good winds.

Example of airship: not the gas-filled, real world one, but the fantasy, sailed one. Source is FFXIV.

Suddenly, B invades A with airships. The airships are powered by a particular magic that flows in the air, but the focal point is that no one had been able to use that magic as a source of energy before.

A's kingdoms are dumbfounded. Nobody expected that. There were some signals that something was off (trade of some materials, mainly metals, somewhat diminished months before the invasion), but overall, nobody was expecting something like that.

If the kingdoms of A had spies at the court of B's emperor, they failed miserably.

So, How likely is that B has been able to develop a whole new mean of transport without most of A's noticing?

Additional infos and miscellanea:

  • The airships are not extremely large, the largest being able to transport up to 50 men.
  • Their main advantage is being able to fly (obv).
  • They are not heavily armed. The airships mostly attack with crossbowmen manning the deck, occasionally pouring burning pitch over the enemy's structures, and balistas for the largest ones.
  • Gunpowder doesn't exist in this world.
  • The airships can fly as high as the winged-humans can; winged-humans man some of the airships, being valued most as navigators.
  • B had never attempted to invade A before.

Clarification (as requested in the comments):

  • The first step in developing the airship is coming up with the engine (the component able to harness magical energy). The task was researched by a small team of engineers (around five) under the leadership of a keen-on-innovation noble. This took a considerable amount of time (around twenty years) with a long list of failed attempts. Note that the engine didn't need to be tested on actual, fully functional airship.
  • After the engine was built, the emperor took interest in the project, trying to bring everything under secrecy, informing the smallest number of people possible.
  • For the first working prototype, I expect having at least two high-ranking officials, 5 engineers, and somewhat ten unskilled workers (maybe soldiers).
  • For the mass-production of the fleet, I'm able to give you just an esteem, but I imagine we can expect five high-ranked officials to survey the whole operation, around 30 at least between engineers (including those from the previous team) and skilled artisans who need to have a larger view of the project.
  • Everyone else would be on a need-to-know basis, e.g. the carpenters working at the ships hulls, or whoever tasked with getting those materials. For those, we're talking around 1300 for a major Arsenal (took the Arsenal of Venice at the peak of its splendor for comparison, from Wiki: Venice-Arsenal)
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    $\begingroup$ Your continents are two month of travel apart. How much later medieval Europeans were aware of the events in India, for example? Even if these two continents' inhabitants belong to the same race, any potential spy will stick out like a sore thumb. $\endgroup$
    – Alexander
    Aug 30, 2018 at 20:21
  • $\begingroup$ Also, if you can provide us with how many people were involved in the development and construction of these airships, we can give you a mathematical estimation of how long it will take to break the secret. $\endgroup$
    – Green
    Aug 30, 2018 at 20:23
  • $\begingroup$ What's more likely is that B will repeat the mistakes of the English and Germans in WW2 where they have a great new secret weapon, test it in a limited way thus giving notice of the improvement. The enemy then creates a countermeasure to nullify the improvement. $\endgroup$
    – Green
    Aug 30, 2018 at 20:29
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    $\begingroup$ @Green The converse problem can also happen, as in the French mitrailleuse volley guns in the Franco-Prussian War, which were successfully kept secret - so secret, their own general staff often didn't know they existed. $\endgroup$
    – Cadence
    Aug 30, 2018 at 20:49
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    $\begingroup$ My recommendation: Watch "The Imitation Game." What you describe is right along the lines of what they explored in that movie, so it may provide some awesome prior art to work from. $\endgroup$
    – Cort Ammon
    Aug 30, 2018 at 22:18

8 Answers 8


The Fewer People, the Better

Objective: Maximize the effectiveness of the new aero-magic engine in the conquest of Continent A in the limited time available. Use of the aero-magic flying machines a terror weapon will only last so long. Countermeasures will inevitably be created.

The primary consideration for how long this secret will take to break will depend on the number of people who know about it. (Further reading of the original paper [remember that sci-hub is your friend]) As more people know about it, the likelier it is that the secret will leak. However, there are a few important factors that make it less likely that the secret will leak.

  • Loyalty to God, King and Country was stronger then. This secret will be a matter of great importance, so if keeping the secret is framed in terms of loyalty then the incentives to leak must be much stronger.

  • Greater social isolation of the nobility and learned classes from merchants and commoners. It's not hard in a society of nobility and servants to keep the innovative noble and his engineers/magi separate from the tongue wagging commoners. This kind of social isolation is built-in to feudal societies.

  • Great difficulty in describing what you've seen. The number of people who can read or write in the late medieval era is very small, maybe a few thousand people. Not many can draw either. Thus, any disclosure of information will need to be done by word of mouth (which brings in a host of information corruption effects that make it far more difficult to accurately describe the airship.) Anyone who isn't very sure what they are looking at will think they have seen a dragon or a spirit. Airships can be designed and painted to encourage these ideas. Rumors of dragons and flying men will be dismissed as easily as stories about fairies and other mythical creatures. (If dragons exist in this world then it will be some other flying monster that doesn't exist. Maybe make a new one.)

  • There's no such thing as science yet. The degree of critical sophistication in a common person about things outside their daily experience will be very low (this is still true of humans).

Counter-Intelligence: If the kingdoms of Continent A are made aware that the aero-magic engine is possible, they may seize on the knowledge and develop their own. Thus, it is of utmost importance to keep the aero-magic engine a complete secret. This will be accomplished in the following ways:

  • Keep the number of people who know about the aero-magic engine to as few people as possible. The more people there are, the more tongues there are to wag.
  • Make the airship designs as similar to existing watercraft as possible. This will be fairly easy as no one knows what an optimized airship looks like. Further, when carpenters and shipwrights are given the plans they will already know how to make water ships. From their perspective, these new ships will just look weird instead being really noteworthy.
  • Delay assembly of the various airship components for as long as possible. Design the airships so that final assembly only requires a fairly small team to finish.
  • Pay the highly privileged workers well and check them for grievances against the king or the nobleman. Do as much as possible to minimize the incentive for them to defect to enemy kingdoms.
  • Testing of the airships should happen against dummy targets in remote areas; ideally at night.
  • Navigation methods will need to be adapted from surface vessels to these new airships. This will take time and innovation. Because of the greater speed of the airships, better navigation methods may need to be developed.
  • $\begingroup$ A really good answer overall. I didn't think about the inherent social isolation of a feudal system and, more importantly, how difficult would be for commoners to report a possible airship sighting. $\endgroup$
    – Liquid
    Aug 31, 2018 at 7:34

An independent military leader with his own research group.

  • The military leader should be highly trusted by the Emperor, perhaps a sibling.
  • The military leader only shares the information on the technological advancement privately with the Emperor. Additionally he shows this technology will allow for a great military advantage such that they can extend the empire to the other continent, but only if used as a surprise attack.
  • Under tight operational security the airships are constructed, but the common soldier doesn't know about the new weapons until 1 month before the invasion.
  • One month prior to the invasion the plans for the coming invasion are shared with the court and the general populous on continent B. Large scale preparation and training of the military now begin.
  • This is too late for the spies to take the 2 month voyage in time to warn anyone on continent A (or send such a message).
  • $\begingroup$ I agree with your general timeline, even if I'm a bit skeptical about sharing the whole idea with the populace a month before. You're right about that they have to be informed for public support, sooner or later, as the invasion must be justified. $\endgroup$
    – Liquid
    Aug 30, 2018 at 21:23
  • $\begingroup$ It's because a large scale invasion doesn't happen without support from the populace, the details of the exact timing can be fluid depending on your story needs. $\endgroup$
    – Mathaddict
    Aug 30, 2018 at 21:50

It is most certainly possible to pull this off.

The entire prototype part can be done in complete secrecy in some random workshop in the middle of nowhere under tight guard, until it can be tested. Since the things aren't big, it wouldn't even require all that many workers and resources.

Now for mass production. This is trickier because large military projects are less likely to remain completely off radar. However this can be a solution:Closed cities

A sort of special place where one has access to skilled workers, materials and a spy can't just get into. These can be pretty well hidden too. The Americans needed satellites to find some of the Soviet ones.

You also have the advantage that with medieval communications it could take a month or two to get a message across.

Depending on how easy it is, maybe you could also make it look like you are building normal ships and then have them be quickly changed into airships.

  • 1
    $\begingroup$ Yep, the lower carpenters and unskilled labourers would be building a "normal ship", to the best of their knowledge; even if a sudden rush of ship-building could look suspicious to possible spies. The bit about closed cities is also nice. $\endgroup$
    – Liquid
    Aug 30, 2018 at 21:28

Dispersed manufacturing.

You can have each part produced by different companies in different areas. Nobody has any idea of what the whole is.

The only people who know is the group doing the final assembly


Develop your own Area 51.

I should think it is pretty likely this could be done.

The key you need is at this link, Distance to the Horizon. It provides a formula; $D=3.57\cdot\sqrt{h}$, where (on Earth) D is in kilometers, and h is 'height', the distance above the ground of the observer, in meters. Other sites will help you convert miles to meters.

So, for example, if your ships can rise to 1000 feet, about 305 meters, then $\sqrt{305}=17.47$, times 3.57=62.35, meaning the horizon is 62.35 kilometers away. That is 38.75 miles. Nobody more than 38.75 miles away can see your ship; it is below their horizon. Not to mention, seeing a small ship at 39 miles would be difficult to discern what it was; but I will presume your guys have telescopes, there is nothing very special in discovering optic tech.

This formula varies with the radius of your planet, obviously, but just adjust by the square root of the ratio. For example, the radius of Mercury is 1516 miles; the radius of Earth is 3959 miles, the ratio is $\frac{1516}{3959}=0.38292$, and $\sqrt{0.38292}=0.6188$, so the horizon on Mercury is 0.6188 as far away from the observer as it would be on Earth for the same $h$ (height) in the formula above.

Also, it could be much less: Your airship doesn't really have to test fly at 1000 feet, it could test fly at perhaps 200 feet (61 meters) as proof enough. Then your horizon is $3.57\cdot\sqrt{61}=27.9$ kilometers; or 17.4 miles.

On Earth I'd make it 40 miles for safety, and develop your airship there. That amount of empty land might be relatively easy to find in a late-medieval society, perhaps in a desert, or just invent a small handy-dandy uninhabited island 40+ miles offshore, where they decide to develop this technology, and the autocratic king can have a "coast guard" that sinks any ship that attempts to sail in that direction. Or obedient subjects that just don't attempt to go more than a mile offshore because the king has prohibited it. Or the king could mandate that every ship includes his emissary, and the emissary has orders (he doesn't know why) to prohibit such travel.

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    $\begingroup$ I didn't think about the distance to the horizon formula: it adds the nice touch of realism to the whole prototyping phase. An offshore island for test runs also looks good, they would just need to find a convenient isle far from trade routes. $\endgroup$
    – Liquid
    Aug 30, 2018 at 21:27

It's possible, after all governments do it all the time. So for the most part, it's easy. At first it is the noble and his team of scientists and you. You have the prototype engine, and now you need to build a ship.

First, split the work up between very few specialized crafts men. Some could work on the sails, some on the hull, and some on the oars etc. But the important part is don't let any of them have a full picture of what their doing. They will all know only their part, and nothing else, so none of them have a full picture of what your doing. This way, they will just think that you are building regular ship parts.

Second, once the pieces are complete, have a special team of your personal team put the airship together, along with the noble and his crew. Keep them all locked up at the castle, monitored at all times by your personal guard so none of them have any chance of spilling the beans.

Now here comes the hardest part - mass production. You have a 2 month window to get your armada in the sky, maybe even 1 as you don't want to give A any time to try to amass their army. You also have to factor in your own travel time to reach continent A. As you can see time is the biggest thing against you. Adding more people to your crew decreases time, but increases the chance of a leak, and vice versa, so you have to find a balance.

You should let 2 other of your most trusted nobles (so there are 3 in total) in the loop along with their crew, to give yourself a large enough force to build the ships, but while also minimizing the potential to leak information as much as possible. Also, increasing border security on ports, having extensive background checks, and forbidding all ships heading towards A from leaving port is will also slow down potential spies very much.

Once again, force all the people with knowledge of the project to live in the castle and never leave until the airships are build. Assuming each noble has a crew of 10-15 specialists, you will have 40-60 people working on the armada. This seems like a lot, but trust me, for inventing a new technology and preparing to invade an entire continent it's a pretty small crew. Remember that the parts can be allocated out to artisans to build individually, so you can make this process even more efficient. You will need to be very fast to beat the 2 month window to get your people off the ground, but I think it can be done.


  • Allocate parts to craftsmen without letting them know the full picture
  • Have a special team in the castle/fortress/palace to assemble the airships
  • Keep them locked up inside at all times
  • Include other nobles (3 total) to help with mass producing, having a team of 40-60 people
  • Your team locked up at the castle coupled with the unaware artisans should make process fast
  • Slow down potential spies by locking down on port security

I hope this helps, and happy world building.

  • $\begingroup$ I like your pretty detailed plan; i'm just worried that the numbers won't match my needs for mass-production (after all, my airships are pretty small and they need to carry troops as well). But I'll do some background checks about ships construction times in medieval eras. Thanks! $\endgroup$
    – Liquid
    Sep 2, 2018 at 8:35

Do it Manhattan project style. First you have a small group of researchers develop the technology and some skilled workers to make the prototype.

Once you get the prototype working, you send out orders for each part to different venders, having different craftsmen in different cities making the materials for the ships.

Any parts that you can't trust a civilian with are done by your in house craftsmen.

Finally, once all the parts are ready you bring call up your soldiers and have them help assemble all of these ships as quickly as possible. Researchers do delicate work or important details, while regular soldiers are your manpower.

The goal is to have as many as you can in the air in one month, so you can set out across the sea to continent A before a messenger would be able to get back, so even in the odd chance a spy did see something, the enemy can't prepare for it enough to prevent heavy loses.


I would suggest doing just like the Brits did in WWI with tanks : give it an inoffensive name, have very few people know the whole project, tell nothing to the manufacturers about the nature of the project, or just tell them it's something common and uninteresting, test in a deserted area.

You could pretend you are just trying to expand your merchant fleet or create better boats for fishermen. That would remove most suspicions about the military nature of the technology.

I would also suggest doing the magic stuff in the end, and in secret, to prevent people from knowing magic is exploitable.


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