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Imagine a medieval-style city, besieged by enemy army. It might be big and have wide river preventing besiegers from direct attack on the walls and launching projectiles significantly far into the city. Besiegers want to use hot air baloons to drop burning projectiles and biological weapons in form of infected meat etc. as well as drop some attackers during nighttime to open the gates of the city. Is it possible for hot air baloons to perform such missions?

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    $\begingroup$ Firstly, they're not exactly stealth craft, and secondly they're pretty easy to bring down if your enemy sees them coming. The other thing to bear in mind is that they travel with the wind, so the infected meat smell might be a giveaway as well. I'm suggesting a no... $\endgroup$ – Tim B II Mar 18 at 22:41
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    $\begingroup$ You have to ask yourself:Have the people in the besieged city seen balloon bombers before? $\endgroup$ – Spencer Mar 18 at 22:52
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    $\begingroup$ Of course it happened in real world (even if it wasn't very effective)! en.wikipedia.org/wiki/SMS_Vulcano $\endgroup$ – McTroopers Mar 18 at 22:59
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    $\begingroup$ @Mranderson I doubt they're stealth in nighttime either - guard 1; see that basket of flames up in the sky? guard 2; yeah, that can't possibly be a hot air balloon trying to sneak in during the night... Seriously though, the tech setting means that the flame is likely visible at night meaning that it's still not going to support surprise attacks. $\endgroup$ – Tim B II Mar 18 at 23:03
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    $\begingroup$ @Ynneadwraith Yes, all you need is a lightweight fabric with a weave tight enough to minimize air losses - you don't need to bring your fire with you if only having a short flight (and they would have used fairly light-weight charcoal anyway, not wood). $\endgroup$ – pluckedkiwi Mar 19 at 14:56

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Better for defense.

I pondered this scenario in 2009 and posted the same idea.

https://www.halfbakery.com/idea/Siegebreaker_20balloon#1249939283

The excellent comment by Bunsen Honeydew points out that the tethered balloon scheme has more advantages when carried out by the defenders within the city as opposed to those outside laying siege.

Consider: the besieged are at the center, and the besieging are at the circumference of a(n idealised) circle. The balloon is anchored in the city, and the anchor point is surrounded by all the infrastructure to support it - furnaces with hot air/gas hoses, ammunition supplies, bucket brigades, pipework, extra fuel, spare parts, winches, windlasses, mules, encamped technicians, support staff for the support staff, ancilliary cooks, batmen and wenches, and so forth. Should the balloon need to replenish itself of any vital supplies, it can be winched back in to the center.

Now consider a change in the winds: the balloon changes tack, but is still (given a few moments of belaying or withdrawing the tether) hovered above the enemy.

In the inverse case, where the circumferential siege-layers are attacking the central hold-fasts, a change of wind immediately places the weapons platform off-target. In order to reaquire the target, the anchor point - and all the associated infrastructure - must be moved a great distance across the landscape around the circumference, or one must wait until the wind is once more in one's favour.

Insert a river, a forest or other obstacle anywhere about the city and the case for the attacking forces becomes particularly dire.

I hold, then, sir/madam or other, that this system is inherently defensive, rather than offensive, in the siege situation.

— BunsenHoneydew, Aug 04 2009


Re size of balloon / @pluckedkiwi skepticism - consider sky lanterns as a smallish example.

sky lantern

You could tether a large one of these with a kite string. You could have a pull thread that dumps the fire payload when in position. After the dump you could probably reel in the lantern before it sinks. The balloon will not carry a cannonball. But a wad of flaming grease is more than enough to start a fire if it lands in the right place.

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    $\begingroup$ But the attacker gains far more value from doing it than the defender. It doesn't matter that it's easier for the defender. $\endgroup$ – Loren Pechtel Mar 19 at 5:28
  • $\begingroup$ You don't need that much for a balloon - this isn't requiring a modern Zeppelin. Wind doesn't change that much moment to moment, and if it does just haul it around and try again. The bonus for the attackers is that they don't need to try to winch it back in against the wind (horrible prospect) - just collect it when it lands on the other side. $\endgroup$ – pluckedkiwi Mar 19 at 14:44
  • $\begingroup$ A central balloon would need a massive rope, of implausibly heavy scale, to give it the range to get out over the enemy. And said enemy can just move out from under the drifting balloon. I don't see how it could be plausible as a defensive weapon. $\endgroup$ – pluckedkiwi Mar 19 at 14:48
  • $\begingroup$ @pluckedkiwi - Enemy troops can move out from under. Enemy tents / wagons will be slower and vulnerable to surprise fire drop. $\endgroup$ – Willk Mar 19 at 15:29
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    $\begingroup$ Use 3 anchor points. If the attacker envelops the city and the balloon is still viable when supporting three times the anchor line the attacker can put the balloon anywhere over the city regardless of wind. $\endgroup$ – candied_orange Mar 19 at 19:39
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My initial thoughts are that hot air balloons would be very vulnerable to projectile weapons like cannons and arrows. As these balloons would be primitive, I am not sure how high they could fly or how well they could be maneuver over the target. Another issue is payload capacity. Most of the balloon's floor would be used to burn something in order to create hot gas, that will lift the balloon into the air. Some more space will be occupied by the pilot and perhaps the bombardier. There is a good chance that anything that was on fire and dropped from a great height, would extinguish itself before it stuck the target on the ground. That is assuming that objects could be reliably targeted to begin with. These biological agents would probably have just as much chance to infect the pilots as the targets on the ground. In order to drop of troops, said balloon would have to either land or fly super close to the ground. The light generated by the fire would give away the balloons presence. I think a trebuchet could accomplish most of want you want to do, with out having to actually leave the ground.

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    $\begingroup$ Balloon mounted artillerymen have the advantage of being able to see over the city walls and thus target better. A trebuchet just flings objects in a general direction and you kind of hope it hits something interesting. Perhaps the airmen could pour Greek fire on the besieged? That stuff probably won't go out. $\endgroup$ – elemtilas Mar 18 at 23:00
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    $\begingroup$ Perhaps spotters in balloons, could help better direct the trebuchet projectiles as they could see where they are landing? $\endgroup$ – SciFiGuy Mar 18 at 23:10
  • $\begingroup$ Good point! 1234 $\endgroup$ – elemtilas Mar 19 at 4:05
  • $\begingroup$ @elemtilas That is the advantage of an airial spotter, not artillerymen. Far easier to communicate "ok, a little to the left, and a bit further" than it is to carry a serious siege weapon into the sky... $\endgroup$ – Yakk Mar 19 at 14:25
  • $\begingroup$ @Yakk - I gathered as much! Mind you, "artillery" doesn't have to be huge or be of use. A small mechanical catapult could be used to cast flechettes or small incendiaries. $\endgroup$ – elemtilas Mar 19 at 15:40
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Yes, in a way.

We have plenty of questions here recently "could a medieval or classical or renaissance civilization X do Y?" The usual answer is "to do it properly, it wouldn't be medieval or classical or renaissance any more." A lone genius or well-informed time traveler trying to do things would run into the limitations of local industry and materials science and try to overcome them. In the process, the civilization would be selectively upgraded.

After this general answer, on to the specific question.

  • Paper hot air balloons are a party or educational gimmick. Party ballons come with a candle inside. (Before you do this at home, check fire hazards and air traffic laws.)
  • Making such a balloon with medieval technology will reduce the already low payload further since paper won't be the same uniform thickness (or rather, thinness). On the plus side, the candle can double as an incendiary.
  • Problem, the balloon will go down only after the flame is out. That might be helped if the glue comes apart in flight before burnout.
  • Targeting will be difficult.
  • To drop people, it would have to be roughly the size of the Montgolfier balloon and the landing site will be very visible, even at night.

Summarized, they can probably get something into the air. It will go down sooner rather than later. Payload will be limited. It will take the effort of many engineers to drop a single soldier.

With much later technology, but also with a greater range, Japan tried to balloon-bomb the United States in WWII. That did not work all that well, which is a sign that targeting will be just as problematic with lower technology.

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  • $\begingroup$ The Japanese balloon bombs are relevant-but-not-relevant. They were only launched at such great distance because the Japanese had discovered the jet stream by that time. That wasn't known about in medieval times, but the intercontinental range wouldn't have been needed, anyway. Surprisingly, one of the Japanese balloons made it as far as Omaha! $\endgroup$ – David Richerby Mar 19 at 15:51
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    $\begingroup$ @DavidRicherby, my point is that large swarms of balloons were launched and then ended up all over the place. No accuracy at all. Edited the answer. $\endgroup$ – o.m. Mar 19 at 15:55
  • $\begingroup$ Sure but bombing another continent, thousands of miles away, is a very different proposition from bombing that city just over there. I just don't think that Japan's failure to perform a very difficult task with fairly advanced technology tells us much about whether one could perform a much less difficult task with much less advanced technology. Anyway, +1 for the rest of your answer. $\endgroup$ – David Richerby Mar 19 at 16:30
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Attacking balloons could be vulnerable to anti-balloon-ballons:

Consider unmanned, small, less expensive and numerous, balloon loaded with a paylod of burning oil and guided by tethers.

The attacking balloon's position, even given long tether(s), is to a large extent at the mercy of the wind. If a defensive tather point(s) could be maintained between the attacking balloon and the "sensitive" parts of the city - ie walls/defenses somewhat outside the "main" city walls, defensive balloon could be guided near the attackers and the burning oil (with could also be its lift power source), could be poured into the attacker, destroying it. The agility of the smaller balloon would conceivably exceed the attacker's sufficiently to make attack by balloon difficult.

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    $\begingroup$ To be fair, you wouldn't need a balloon to defend. Longbows, crossbows, and ballistae would be highly effective (particularly if you use fire arrows/bolts) and much easier to target. $\endgroup$ – anaximander Mar 19 at 14:40
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    $\begingroup$ @anaximander a quick search revealed that a longbow could shoot an arrow at about 53 m/s, which if fired vertically would reach a height of about 140m. Crossbows were at most 5% faster, call it 160m. That’s not very high for a balloon. I put it to you that balloons would have out of range of all available projectiles. $\endgroup$ – Bohemian Mar 19 at 17:06
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Arrows can launch at 280m/s which gives 500m up. I thought crossbows could do better but that's not what I'm finding. (It has been pointed out this is awfully fast--I suspect it should be f/s and someone made a unit error. That doesn't change the basic argument, though.)

If your bomber is appreciably higher than that (in case someone has some very strong bow) you're safe. Your bombing accuracy won't be very good, though. If the winds don't cooperate you'll have no choice but to land and haul the balloon around and try again.

I don't see how there's a big advantage over simply using a trebuchet to fling the same sorts of things into the city.

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    $\begingroup$ 500m up is maybe the max distance an arrow could go in the opposite direction of gravitational acceleration (I assume this is what you calculated) but you'll need to take wind and air resistance into account too. And you'd also need to find what is the velocity at which point an arrow is still dangerous. I doubt it would be dangerous to anything more than 100 meters up in the air. $\endgroup$ – Echox Mar 19 at 8:07
  • $\begingroup$ Also your archers would hit your own citizens and probably do more damage than the ballons :) $\endgroup$ – Fels Mar 19 at 8:24
  • $\begingroup$ A quick google search suggests crossbow bolts have a weight of 150 grains, while longbow arrows weigh 800 grains. A lighter projectile is much more affected by air resistance. In practice, longbowmen outranged the crossbowmen (the heavy longbow arrow is effective at the end of a high ballistic arc, while the much lighter crossbow bolt is not. On the other hand, at medium ranges the crossbow bolts have a much straighter trajectory). $\endgroup$ – Calin Ceteras Mar 19 at 8:57
  • $\begingroup$ If your longbow can't reach far enough, you can always just go larger and use a ballista. Up to a certain point, the energy they can impart to the projectile scales faster than the projectile's mass, so you'd be able to shoot a bit higher. Just be sure to do a little maths when installing them onto their swivel mounts, and install blocks to prevent them from aiming along trajectories that would drop bolts onto your own town. So, when the balloon is above the centre of town, you'd have a ballista on the walls shoot at an angle, rather than one in the centre of town shoot straight up... $\endgroup$ – anaximander Mar 19 at 14:43
  • $\begingroup$ 280 meters/sec? 920 fps? That's pretty good for a pistol bullet. I'm going to need to see a source for this, I'm afraid. 300 - 320 fps is a much more standard number, and that will give about 500 meters altitude. $\endgroup$ – WhatRoughBeast Mar 19 at 22:47
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Zeppelins were used for bombing, so yes. Clearly it was found worthwhile. They stopped being used because of heavier than air flying machines.

As simple as that.

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    $\begingroup$ Zeppelins are not medieval technology. $\endgroup$ – Calin Ceteras Mar 19 at 9:00
  • $\begingroup$ No, but if they can (and were) historically used to bomb, that adds a lot of weight to the idea that a hot air balloon could usefully and practically do so - and that's the point. $\endgroup$ – Stilez Mar 19 at 9:24
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    $\begingroup$ Yes, but World War I era zeppelins were filled with hydrogen - a hot air balloon must be roughly 10 times larger in volume for a similar rising force. Creating enough hydrogen for a balloon is not possible in the midevil society. $\endgroup$ – Calin Ceteras Mar 19 at 10:48
  • $\begingroup$ A key difference between zeppelins and balloons is that zeppelins are a kind of dirigible. Dirigible means "able to be directed"; ie. you can steer them and go where you want. A balloon goes wherever the wind takes it... $\endgroup$ – anaximander Mar 19 at 14:45
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The big problem would be the material to make a big enough balloon - paper is out of the question, I saw a documentary on Da Vinci channel on creating a paper hot air balloon able to lift a man. In the end, the glue failed (by the way, the balloon was built into a former airship hangar, which is not quite medieval construction technique). The balloon was about 20m in diameter.
European medieval cloth would have been too heavy (I think), so the solution would be silk (primed with some air-proofing substance). However, in Medieval Europe silk was extremely expensive, being at the end of a 5000km transport chain.
The solution for "biological warfare" would be to move a mobile catapult (3-5 kg stone thrown) close enough to the castle in the dark, and shoot thingies by half-moon (or less) light. You'd need 6 hours to move it close enough to the walls - maybe on a raft - and a couple of hours to shoot it, then you could abandon it.

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    $\begingroup$ You do know that silk was produced by the Byzantine Empire since the fourth century? $\endgroup$ – Ville Niemi Mar 19 at 10:58
  • $\begingroup$ @VilleNiemi: I did not know that, thank you :) $\endgroup$ – Calin Ceteras Mar 19 at 11:02
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    $\begingroup$ If you look at the early manned balloons, such as those made by the brothers Montgolfier, they mostly seem to have been constructed from widely available cloth such as burlap or linen, lined with paper to keep the air in. So the expensive part would be getting lots of thin strong paper, not the fabric. In historic northern Europe this might limit you to the late medieval times, but in fiction it wouldn't be to far fetched for a king to acquire the technology during a crusade. $\endgroup$ – mlk Mar 19 at 14:25
  • $\begingroup$ The paper wouldn't have to be writing quality. You could pulp reeds and spread the pulp onto linen as it dried, perhaps. Alternatively, you might be able to use certain kinds of skins, or perhaps oilcloth (although beware the oil making the cloth more flammable; the balloon is enough of a fire hazard already). $\endgroup$ – anaximander Mar 19 at 14:46
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The driving force of innovation is necessity. Siege forces already had an effective method of providing the defending city with all the nasties they could fit in a bucket: trebuchets.

Why devise a new method for doing the exact same task? If someone did develop the siege hot air balloon, they would realize they take much longer to deliver the payload, need a return trip for the next payload & are vulnerable to the same city defenses as a traditional trebuchet (unless very high). After the idea was tested, it would be scrapped for more trebuchets.

If I was building a medieval army and still intent on investing in research, I'd look for a way to make existing trebuchets more mobile to avoid return fire. But even then, the offensive trebuchets need only aim at an entire city, the defensive trebuchets are attempting to hit one of the opposing trebuchets, or swaths of troops.

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    $\begingroup$ Welcome to Worldbuilding.SE! Please have a look on the Tour and the Help Center! Please consider that the question was asking if it is possible to deploy such weapons, not if it is rational to do so. $\endgroup$ – DarthDonut Mar 19 at 13:53
  • $\begingroup$ I guess I failed to make clear my argument is that it is not possible because it is not rational. $\endgroup$ – Golden Ratio Mar 19 at 14:18
  • $\begingroup$ The question dismissed such siege weapons for lacking sufficient range - it would take modern engineering to get a projectile across a wide river and then still clear a wall of unknown height, and even then it is questionable. Medieval trebuchets only have a range of around 300 meters, so clearing a defensive wall on the other side of a wide river is unlikely and would at best barely make it into the city, where defenders would be on hand for exactly that reason. $\endgroup$ – pluckedkiwi Mar 20 at 13:46
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You're not going to produce anything like a modern hot-air balloon, but you don't need to. This doesn't need to sustain several hours of somewhat controlled flight without damage, just get up long enough to cross over the city.

A tightly-woven silk envelop (yes this will be expensive) will contain the air well enough to cross over the city so long as there is some reasonable breeze, so you don't need a constant source of heat (which would be heavier than it is worth). They will be completely silent (aside from the noise of the payload hitting the ground/roofs) and only visible as a dark spot obscuring any stars (no onboard flame so completely invisible on an overcast night).

Build something like a siege tower to conceal the chimney and obscures the large charcoal fire you are burning under it if you want some kind of subterfuge, else just build the scaffolding. Just as darkness falls, move the opening in the balloon envelope over the chimney (the fire should be roaring hot by now). It will fill with hot air, then you cinch the bottom and loose the tether. Keep the fires going all night and you could get a few off before it gets light enough for the enemy to see them coming.

You save a lot of weight by not needing a basket with a pilot (that saves you 100+ kilos of weight which will make the balloon far more plausible), so rig up some time-release mechanism (slow-burning match most likely but water draining through a pinhole will work too). Getting the timing just right will be excruciatingly tricky because of the unknown windspeed, but the bigger the city the wider your margin of error can be. You're not going to hit a specific target, but the city as a whole should be manageable.

A tricky bit is having a payload which will be effective in small quantities. A flea-ridden plague rat would be perfect, but how you would get one of those without your own army being stricken is another matter. Incendiaries would be good if you can figure out how to make them well.

When the balloon is crossing over the city, the timed-fuse burns out and releases the payload. Drop a rat (goes splat, you don't care, the fleas will survive and spread), or drop a string of hot coals at different times (most reliable given unreliable releases) in the hopes that one will fall into something flammable or even leave a string of fires.

Though the balloon will be cooling and falling, releasing the weight of the payload should give it a nice boost to altitude to see it get over the city and out into the fields on the other side of retrieval by morning. If they are lucky, nobody in the city noticed the balloon at all, but the plague will soon spread. If not lucky, the wind is too slow and the balloon doesn't make it all the way over, or worse yet the wind changes direction completely and blows it in a different direction.

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The Japanese attacked the U.S. with this concept during WWII. They used the prevailing ocean winds to blow over some balloons with bombs attached to them. The mechanism was timer based. So very hit and miss.

https://en.wikipedia.org/wiki/Fu-Go_balloon_bomb

Apperantly even Montgolfier suggested to do so 1792: https://en.wikipedia.org/wiki/Incendiary_balloon

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It depends on how much technology your "medieval" civilization has

Balloons were used for recon during the American Civil War, and that wasn't the first application. France used them as far back as the 1790s.

While the early 1800s are hardly medieval, the technology and manufacturing of that time is still very primitive by comparison to today.

If your army in question is the first to use this technology, it doesn't really matter if the enemy sees the balloon coming or not, as long as doesn't drop low enough to fall into projectile range. From there, it seems like it should be possible to drop something small but very hot, like Greek fire or burning phosphorous, which would be devastating in a town constructed of primarily flammable material.

On the other hand, if this had been effective, I suppose either of the real-world armies listed above would have done it.

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