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I'm going to jump straight into a very small description of the world my first novel-thing is build around.

It's an Earth-like planet orbiting a binary star. Climate is slightly more extreme than on Earth (think colder winters, hotter summers) and the ~800 million inhabitants adapted quite well to it. Technology is on a late-medieval level, compare it to ~1400-1425. Magic IS a thing on this planet. Everybody can learn to use it to the same extent, though it requires a lot of studying and memorizing runes. The strongest incantation documented was cast by a famous scholar almost a thousand years ago and since then, people came close, but could never reach that level. What was the incantation? Well.. he summoned an orb of fire the size of what we on Earth know as a Hippopotamus. That's it. Magic can levitate small objects, too, but its nothing too frightening or too dangerous at any time.

My "Villain", the Protagonist, manufactured with some aspects of Peter's Evil Overlord List in mind, can summon a fireball the size of a Chihuaha. It takes about 8 hours to prepare and exhausts him. So he does not use it. At all.

He'd prefer something that works efficently with the technology that's available to him. Something that is relatively cheap to produce and doesn't require any hand-wavium to construct.

What for? To destroy the main fort of the opposing forces. Or at least damage it beyond repair. There's just one tiny problem: It's mostly made of Iron and Stone.

  • The fort stretches over an area of about 8km² and is surrounded entirely by walls that reach heights of 12m.
  • The walls are made of stone and have a small core structure made of iron inside of them (so: stone-iron-stone, 6m-2m-6m).
  • The buildings are mostly build with the same principle in mind. Build a small relatively small core of iron and "coat" it with stone.
  • There are roughly 4.000 men living in this fort, all armed with melee- and ranged weaponry.
  • The fort NEEDS to be destroyed, as it blocks a path to the Evil Overlords destination with no other way around it. Preferably in a matter of a couple years or less.

That should be enough (hopefully non-confusing) information. So the core question:

What weapon could my Evil Overlord build and use against this fort with the technology he has available, without using any hand-wavium and without magic?

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    $\begingroup$ If it was made out of iron, the best weapon he has is time. With a bit of time, the insane person that build a fort out of iron will either go bankrupt or kill himself in a kitchen accident or something. One prominent theme in fantasy is also that people feel the need to destroy and conquer any kind of fortifications when usually this has been done by year-long sieging. Why isn't that an option here? $\endgroup$ – Raditz_35 May 17 '17 at 12:56
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    $\begingroup$ And it continues (too few characters): Does he destroy the fort after he conquered it or during a siege? Further more: What is wrong with methods people used throughout history when conquering places? The earliest cities already had massive walls, there are so many examples from history. There have been soo many sieges you can draw from. Is this a one-man operation? $\endgroup$ – Raditz_35 May 17 '17 at 12:59
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    $\begingroup$ What do you mean by "iron"? Cast iron, steel, or just iron ore? 2 meter thick refined iron in a wall 12 meters high around a perimeter of a 8 km2 area would probably represent ALL OF THE IRON in the entire medieval world. Such a construction project would represent one of the wonders of the world. And it could be defeated just like any other fortification. Starve out the inhabitants, bribe someone to open a gate, or surmount the walls with siege engines/undermine them with mining. $\endgroup$ – Jason K May 17 '17 at 13:15
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    $\begingroup$ Two million tonnes of iron. In the middle ages. $\endgroup$ – AlexP May 17 '17 at 13:38
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    $\begingroup$ Here is what might help: How did the iron get there? Can't you just reverse that? Is it one solid block of iron or iron plates or what are we talking about? Other than that, have you checked out how people destroyed forts in the past? $\endgroup$ – Raditz_35 May 17 '17 at 14:13
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You could use immense, intense heat as was used against the vitrified forts

This image shows stones from one such fort. It looks like an igneous rock because of the bubbles, but this is in the UK. That rock partially melted. enter image description here

from archhighland.org.uk

A fort with an iron core in the wall would be extra susceptible to an intense heat attack. Once it gets hot enough the iron itself will burn. When that starts there is no stopping it. This was demonstrated most spectacularly in the 9/11 attacks. It is the reason insulation is sprayed on steel beams. enter image description here from http://www.sciremediation.com/Web%20Pages/Fireproofing.htm

How did the ancients vitrify the forts? It is an open question and a very interesting one. The vitrified forts are real and no-one knows how (or why!) they did it. In the book Merlin's Ring it was an attack by Atlantean swanships on their rebellious colonies.

On reading up I found this metalsmith who was mad that 911 conspiracy theorist were asserting that steel doesn't burn and so made a video. http://www.mediaite.com/online/angry-metal-worker-makes-video-debunking-jet-fuel-cant-melt-steel-beams-911-meme/

One would not think you could burn down an iron and steel wall but a smith would know. You can have the idea come from your blacksmith. You could do it with a big pile of coal or charcoal against the wall. You could have a tunnel underneath the pile leading a short distance away - the heat of the fire would pull air up thru the tunnel (strongly!) and that fire would burn really hot. The defenders would not be able to get near it. Once it was hot enough to ignite the iron inside the wall that would propagate and the whole wall would go from the inside out. It would glow.

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  • $\begingroup$ This is an interesting concept, with enough coal or similar fuels you could probably do this in a reasonable amount of time with a couple hundred men. $\endgroup$ – Olaf Klausson May 17 '17 at 14:13
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    $\begingroup$ With one little addition by myself, and a good 30 minutes trying to figure out how to explain it in-universe, I'm going to pick this answer. Make it Greek Fire. Or atleast something very similar to it. That is definitely something that the combined knowledge of a talented blacksmith and a couple scientists could produce. $\endgroup$ – Olaf Klausson May 17 '17 at 14:29
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    $\begingroup$ And there is no evidence that the steel in the Trade Center towers melted, let alone burned. What happened is that they softened and weakened. And the idea of causing iron to catch fire with coal or charcoal is unbelievably bogus. Smiths never burned iron, so how could you have an idea to do this from them? $\endgroup$ – WhatRoughBeast May 17 '17 at 17:15
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    $\begingroup$ @Will - What is being discussed on that forum is combustion (primarily) of the included carbon, with a small amount of iron involvement. It in no way suggests the bulk combustion of iron which was asserted by your answer, "the whole wall would go from the inside out" for the very good reason that it doesn't occur. Plus, blacksmiths work with a lot of fuel and air for a fairly small piece of iron, not remotely like the situation you were addressing. Enlightening and inspiring are all very well, but not at the expense of reality. $\endgroup$ – WhatRoughBeast May 17 '17 at 18:52
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    $\begingroup$ The problem is not that you can't light steel on fire with charcoal if you really want to. Carbon definitely burns hot enough to do the job if you can get enough air to it. The problem is that an iron wall is going to conduct the heat away pretty quickly and you're going to have a hard time getting it hot enough in any one place to get it going. Also, due to the rapid heat conduction, even if you do get the iron burning, it will have a difficult time producing heat faster than it is dispersed, so the fire will likely not be self-sustaining. $\endgroup$ – Perkins May 17 '17 at 18:58
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Sappers and... pigs

Just (well, it's not that easy, but still the easiest way IMO) dig tunnels under your walls, put a few dozen oiled fat pigs, set them on fire and watch the wall crumble under its own weight after the prolonged intense heat weakened the structure !

https://en.wikipedia.org/wiki/Early_thermal_weapons#Animal_renderings_and_parts

At the 1215 siege of Rochester Castle, King John ordered that fat from 40 pigs be used to set fire to the mines beneath the keep, which caused it to collapse, a cheap and effective technique in place of the more complicated mixture of sulfur, tallow, gum, pitch and quicksilver he had used in France the previous year.[81] Animal fat was not uncommon as an accelerant; in the 13th century French sortie-parties would often be equipped with animal fat, straw and flax to use as fuel when setting fires amongst enemy siege engines.

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    $\begingroup$ You don't weaken stone and iron, you weaken the foundations beneath it. Of course, this is based on the assumption that the wall is not anchored too deep in the soil. $\endgroup$ – Keelhaul May 17 '17 at 14:06
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    $\begingroup$ Fire was one of my main ideas as well in the beginning, but stupid as i am i never once thought about combining it with the digging. $\endgroup$ – Olaf Klausson May 17 '17 at 14:14
  • $\begingroup$ Would you have an estimate of the energy produced by burning a "dozen oiled fat pigs", versus the energy necessary to weaken a few steel beams? $\endgroup$ – Nicolas Raoul May 18 '17 at 3:53
  • $\begingroup$ As I said, the fire don't weaken the steel beams, it weaken whatever soil it is build upon. No matter how strong the wall is, it will fall under its own weight. $\endgroup$ – Keelhaul May 18 '17 at 6:21
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    $\begingroup$ Though to be really effective, reinforced concrete must have tension applied, otherwise as soon as you get cracks the concrete is just a heavy weight tearing the steel. OP did not state what kind of metalworking is known in his world. Dumping a pile of iron ore definitely won't have the same effect as smelting steal and making reinforced concrete with pre-tensioning. In between those extremes, is cast iron available? steel? $\endgroup$ – spectras May 18 '17 at 15:42
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Sorry, but this is not a medieval fortress. It's too damned big and it takes too much iron.

Assume for the moment that it is circular. Then it has a diameter of about 3 km. Total wall length is 10 km. With an iron core 2 meters thick and 10 meters high, it uses 200,000 cubic meters of iron. or about 1.6 million tons. Given early iron production techniques, this is simply not feasible. It's hard to be precise, but by 1500 (end of the medieval period), total iron production for all of Europe was on the order of 60,000 tons, so the fortress represents total European production for 27 years - all of cased in stone and unusable for tools or weapons.

Furthermore, if the core is to be any good, it must be formed into a solid structure. Just piling a bunch of iron plates or ingots between 2 stone walls will provide no more protection than a bunch of rocks. This, in turn, is beyond the capabilities of any medieval technology.

And speaking of weapons, the existence this much iron (and it must be cheap to use so much) suggests that iron (and steel) must be widespread, and there's no way steel cannon are not in common use. Once you've got these bad boys running around, high walls are simply irrelevant - unless you can make your impossible iron structure. Even so, the stone facing is a waste of time - it will fall off in sheets as soon as it comes under concentrated fire, exposing the iron core.

So, sorry to be a spoilsport, but your fortress is itself inconsistent with medieval technology, so limiting the attackers to medieval technology makes no sense.

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    $\begingroup$ There currently is a background on how-on-notearth this construction was possible. Is that information that should've been in the question? The main concern never was "how do i build this fort" but how can someone reliably take it down, everything else up to this point is explained in more or less detail $\endgroup$ – Olaf Klausson May 17 '17 at 14:06
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    $\begingroup$ @OlafKlausson - My point is that the existence of the fort invalidates the limits on the solution. The construction of such forts (with iron cores) makes a mockery of limiting the attackers to medieval technology. $\endgroup$ – WhatRoughBeast May 17 '17 at 14:29
  • $\begingroup$ @WhatRoughBeast I did get that point. My answer still stands: there is an in-universe explanation for it. $\endgroup$ – Olaf Klausson May 17 '17 at 14:31
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    $\begingroup$ @WhatRoughBeast The fortress can be an ancient construction of a past civilization. And about the stone coating, I can almost hear the discussion: king: "This is a gift from the gods! Look! This fortress even has iron walls! We settle here". queen: "But that iron-look is so cold... I'll buy 10+km of wallpaper to decorate it". "WHAT??? No way". "But dear, all my friends live in beautiful stone castles...". "ok, let it be stone.." $\endgroup$ – frarugi87 May 17 '17 at 15:12
  • $\begingroup$ I'm not sure I understand your point about the cannons. Gunpowder weapons need a whole lot more than just an abundance of steel. As for the iron production figures, couldn't it be that the OP's culture has an advanced knowledge of metallurgy in a world filled with metals? $\endgroup$ – Kys May 17 '17 at 15:19
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Acid

Midieval chemistry; though most often referred to as alchemy, was already capable of producing several strong acids.

Put this in a container that breaks upon impact but doesn't react with the acid (glass containers) and watch how these acid filled container not just destroy, but dissolve the iron fortifications.

Raw materialistic damages aside, this weapon would be feared as the damage to soldiers and their armor would be substantial as well if they get doused in acid. So you're not only destroying the iron buildings, but it would break the morale of the soldiers as well when they see the buildings dissolve and their comrades get burned straight through their armor.

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  • $\begingroup$ Acid sounds like a great idea! Though can it dissolve the stone as well? I suppose it could flow right through small cracks and then hit the iron? $\endgroup$ – Olaf Klausson May 17 '17 at 13:30
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    $\begingroup$ Have you ever worked with acid? This would take so long and require so much acid ... There are many problems with that approach, to few to mention. $\endgroup$ – Raditz_35 May 17 '17 at 13:36
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    $\begingroup$ If one can accept a medieval city having 2 million tonnes of iron one can also accept them having ten thousand tonnes of nitric acid. $\endgroup$ – AlexP May 17 '17 at 13:37
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    $\begingroup$ @AlexP That were just lying around, they no longer need and then just dump the waste in the river? I argue that's even a step further. As I mentioned before, the people here must be completely insane. If I was that overlord, I'd go another way tbo $\endgroup$ – Raditz_35 May 17 '17 at 13:39
  • $\begingroup$ It is not about dissolving every single gram of the iron, it's about destroying the walls, and buildings if need be, to an extent that disables the forts functionality enough and enables the marching of an army (not just men) through the fort without problems. But I see, that even acid doesn't quite cut it $\endgroup$ – Olaf Klausson May 17 '17 at 13:47
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Fire and smoke...and animal carcasses

But you are not going to target the walls.

In warfare it is generally best to strike the softest target. It does the most damage to the enemy and causes the smallest loss in your own troops.

According to your question you are willing to wait years for the siege to be completed.

So what to do with the items I listed.

Fire and smoke: These two obviously go together. Lob oil canisters over the walls with catapults or trebuchets, target everything, the goal is lots of fire and smoke, not necessarily major damage to the fortress. After all, its a nice fortress a place an evil overlord may want to set up camp later.

With the fire and smoke you could potentially create the fortress version of a beach head on the walls by making the defenders scurry away from the fire and smoke.

Carcasses: If you are the patient type of evil overlord you can simply kill the defenders the slow and painful way (as opposed to the fast and painful way) by lobbing dead animal carcasses over the walls until they all, or most at least end up succumbing to disease...

If you go this route it probably makes sense to keep the fire around for cleanup...

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Thermal shock can be your friend! Okay, not totally sure how to make it work on a very large scale, but here is how you might generate a breech. This should be done in the dead of winter.

Start with general bombardment, you want to slough off as much of the stone facade as possible, exposing the iron underneath. You want the iron exposed down low, near the base of the wall.

Protect your minions however you deem practical. Have them approach the base of the wall and build as hot a fire as possible right at the joint of stone and iron. Keep the fire going as long as you can and if possible, get the iron as close to red hot as possible. In the mean time have your forces build a trebuchet and a ballista. Also bring in some method to pump lots and lots of cold water. Get heavy iron arrows for the ballistae and load the trebuchet with blocks of ice.

Fuel the fire first with wood, then with coal, if possible. Your goal is to get the joint between iron and stone glowing red. If you can get the iron that hot, it will begin interfering with the defenders efforts to keep you away from the wall. Keep this going for weeks if you can. Don't actually hit the wall with anything yet.

At the same time, send minions in small parties around to other points in the wall. Near Every joint, have them drive spikes into the stone and pull them out, then fill the resulting hole with water, and retreat. Then, under cover of darkness, get as much water flowing underneath the walls as possible, using sappers and the like. As long as this place isn't firmly on bedrock, you should have some springs nearby that can be diverted to weaken the walls at the foundation.

The defenders will be trying to figure out what the fire is for and will likely ignore a lot of the other activity, since it will be small parties who look like they are running up, banging on the wall and running away.

Since this is winter, the water in all of those little holes is going to freeze and start cracking the stone. If your minions do it right, the aggregate damage is going to severely destabilize the walls. A few shots with a trebuchet will shatter the remaining stone facade. Then when you hit the iron core of the wall, you might be able to create a spalling effect inside, killing the defenders right up against the wall with stone shards.

Now it's showtime for the main assault. With the iron glowing nicely, begin bombardment of the hot spot with cold water, 500 lb iron arrows from the ballistae, and huge chunks of ice. Make a big show of it. If you are lucky, the resulting thermal shock and repeated hammering of heavy objects will tear the iron away from the stone all along the joint.

Meanwhile, after the bombardment of the hot iron wall begins, roll up trebuchet to start hitting areas that you have been driving holes in the stone to let the ice weaken it. If your sappers did their job properly I would predict that you will get a breech in one of those spots faster than your big show. This will have been the aim all along. If you can figure out a way to get a fairly small hole all the way through the iron wall and get a lot of water flowing through it to the gap between iron and inner stone walls You can create a lot of damage that way. it would make your sappers job a lot easier

If the thermal shock works, then you get a neat story to tell and your legend becomes that much greater. If not, you still knock some walls down and then get where you need to go. This should only really take a few months to do. Also, you have a place to keep warm in the winter (hah)

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  • $\begingroup$ Iron is a pretty good heat conductor. I doubt you'd be able to heat a part of such a huge iron wall to make it weak enough using medieval-ish technology. Indeed, the last thing you want to do is expose the wall - you'd want to isolate it from the environment as much as possible. $\endgroup$ – Luaan May 18 '17 at 13:32
  • $\begingroup$ @Luaan If I am trying to break through the wall, why would I want to leave the stone facade intact? Besides, the Sappers are the real threat here, the heated iron is more of a side show or distraction. $\endgroup$ – Paul TIKI May 18 '17 at 15:45
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    $\begingroup$ @Luaan, just realized what you mean, leave as much of the stone as an insulator. you can balance the assault timing to take advantage, but since thermal shock is only part of the general assault... $\endgroup$ – Paul TIKI May 18 '17 at 15:53
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Your'e not going to put much of a dent in 2 metres of iron encased in rock. So go over it or under it, or normal siege stuff like attack the gate or lob some diseased corpses over it. Or all of the above.

The gate or gates is assumedly a weak point. And that's why it was a focus in many sieges. Ideally you smash through the gate or climb the walls or use spies to open it from within. You don't expend too many resources trying to bash a hole in the wall.

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    $\begingroup$ It's funny how many people have the idea that when you encounter a barrier, you need to smash through it. For most barriers, going around is a lot more sensible option - you'll often find at least a weaker barrier :P $\endgroup$ – Luaan May 18 '17 at 13:33
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Infiltration

An unconquerable iron fort right on the path to your headquarters is bothersome when the enemy has it, but could be really useful if you had it. The thick iron walls of that fort would be incredibly hard to replicate. In fact, I'm pretty sure solid iron or steel structures of that size do not exist in our world now. So it would be a shame to destroy it.

You have some options:

Traitors - you could make people who inhabit it now come over to your cause. How you do it depends on what your cause is an who the inhabitants are. Once you get some of the high command to secretly support you, they can restructure the rest of the fort and come over to your cause in a relatively short time. If this is impossible, it still only takes a few people on the inside to open the gates and let you march right in.

Failing that you can still starve them out, then move in. Block access to their fort, and hit them with biological warfare. Block their food supply, poison their drinking water and try to infect them with diseases. Once there is nobody left in there who can resist your attack, get some ladders and go in.

Or you could conquer the fort through some conventional battle tactic.

If you absolutely have to destroy it, fire and/or undermining are your best bets.
But it would be a waste.

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Powdered Combustibles

Fuel-air explosives are relatively cheap and easy to make, safe to transport, and have been known about for a very long time. (Look up grain silo explosions on youtube.)

A cup of finely powdered flour dispersed in the air is roughly as potent as half a stick of dynamite.

Flour's not the only option. In a medieval world powdered coal or charcoal would be a much better choice since it would be a lot cheaper than burning up everyone's winter food supply. Of course, if you've broken into the fort and are using their food supply, the concern goes away.

Delivering the explosive is a bit tricky in combat conditions. You have to get your finely-powdered combustible mixed with the proper amount of air. During Sherman's battle for Atlanta, flour from the local mill was used to blow up buildings, but that strategic objectives well away from the active fighting. Modern fuel-air devices generally use liquid or gaseous combustibles sprayed from nozzles for reliability and ease of ignition, but building such a device at a medieval tech level would be a challenge.

Nevertheless, an enterprising artificer could probably come up with a way to throw a barrel of powdered charcoal from a catapult and have it burst open over the fort and disperse into a cloud. After that, you just need a way to ignite it before it dilutes enough to stop being explosive.

If you're planning to take the fort and then destroy it so it can't be re-occupied, then just transport the stuff in sacks. A bit of experimentation will let your men know how many sacks of charcoal to how many square feet of building are necessary to blow it off its foundations. Just powder it finely enough that you can shake it out of the sack and have it float in the air.

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  • $\begingroup$ What about an artillery (catapult or trebuchet) load that consists of a large quantity of fine coal powder encased in a relatively weak binding (animal skin, perhaps), and a bladder of pressurized air in the center? Upon impact, the air bladder should rupture and disperse the coal powder, and you can have archers standing by with flaming arrows who loose as soon as the coal "bomb" ruptures to ignite it (the flight time of the arrow should buy just enough time for the powder to disperse enough to burn nicely, but not so much that it's no longer sustainable). $\endgroup$ – Doktor J May 17 '17 at 19:27
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    $\begingroup$ @DoktorJ At that point, just start catapulting gunpowder over the walls. You may not make the walls crumble, but well, it surely will be a blast. $\endgroup$ – T. Sar May 18 '17 at 19:24
  • $\begingroup$ @TSar gunpowder requires sulphur and sodium or potassium nitrate. Much more expensive to obtain. (but easier to use) $\endgroup$ – Perkins May 18 '17 at 19:56
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    $\begingroup$ @DoktorJ Might work... Putting it in a barrel with a rope attached to pop the lid off when it hits the apex of its flight might work too... I can think of a number of different ways, but it's not exactly the kind of thing you can just go out and test without annoying the neighbours. $\endgroup$ – Perkins May 18 '17 at 19:58
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    $\begingroup$ I dunno, If my neighbor were doing stuff like that I'd want to go outside with a beer and watch. as long as it was aimed away from my house $\endgroup$ – Paul TIKI May 26 '17 at 13:47

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