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I'm working on a steampunk-inspired low-fantasy setting where combat is primarily conducted via WWI-style artillery. Airships are employed extensively in ship-to-ship and ship-to-ground warfare, and are countered by fixed ground batteries that are more powerful and longer range than those employed on airships. (Airship weight problems are not in scope here.) These ground batteries cannot move and must be connected to centralized power stations. Infantry are used to disable and capture ground positions, including disabling these batteries, which are usually some distance from the area that they are meant to defend.

This means that if a city is under attack and its defensive batteries are destroyed or disabled, the attacking fleet has effectively won, since aerial bombardment could presumably flatten a city once the fleet is in range. I was originally going to introduce mages that work effectively as sci-fi bubble shield generators, but that solution feels a little too contrived and tailored to this specific situation. My current idea is to have cities defended by short-range flak towers, which will attempt to detonate shells mid-air before they reach their target. Is this a feasible approach to countering high-altitude mobile artillery, and are there any other counters that could be possible with Industrial Revolution–WWI technology?

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    $\begingroup$ It seems like if your position has been overrun to the point where your fixed defense batteries are neutralized by opposing infantry, you have lost even before the airships show up. $\endgroup$
    – Willk
    Commented May 29, 2022 at 13:19
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    $\begingroup$ @willk Although it does say that the batteries would be taken by infantry, i think it would be safe to assume that saboteurs and spies would likely be in charge of disabling the batteries. Alternatively, maybe there is some reason the batteries are kept away from inside the city, where it would be easiest to defend. Perhaps there is something about them that makes them dangerous to be around people, and therefore, they are moved outside the city a short distance. Therefore, infantry could take them. Still, not a great choice for something required for defense, but hey, this is fiction. $\endgroup$
    – Chuck
    Commented May 29, 2022 at 14:52
  • $\begingroup$ In the first book of the World War series by Harry Turtledove we can see exactly how your scenario played out when an alien race - with far superior technology - failed to detonate some shells from the German gun Dora. The sheer ruggedness of an artillery shell completely defeated the alien's counterstrikes. $\endgroup$
    – Vorbis
    Commented May 30, 2022 at 7:50
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    $\begingroup$ Something worth remembering, many artillery shells in WWI were not explosive. They didn't "detonate" on impact, they simply shattered due to the force of impact. I have no trouble believing that such an event would be terrifying to experience. But the consequence is that you can throw all the flack in the world in front of the shell and at best you'd only deflect it (and probably not that substantially). $\endgroup$
    – JBH
    Commented May 30, 2022 at 20:48
  • $\begingroup$ @Vorbis The scene you're thinking of is SAMs vs very heavy artillery--building the shell tough enough to withstand the firing means it's tough enough to ignore a SAM unless it's skin-to-skin or shrapnel takes out the fuse. Flak will have exactly the same problem pitted against incoming shells. In Desert Storm we saw that shrapnel is pretty much ineffective against warheads even without a very tough layer on the outside. $\endgroup$ Commented May 31, 2022 at 1:19

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Interception of large high velocity artillery shells would require;

  1. The existence of exceptionally sensitive Radar or Ladar detection systems In real life artillery fire can be detected by ground based radar units with enough accuracy that the trajectory of the shells can be calculated and then back tracked to the firing positions allowing an opponent to call in 'counter batter fire' on the guns which fired the shells. But what your requiring is at least 10 orders of magnitude more difficult.

  2. Advanced high speed calculating machines capable of readings from the detectors, in real time and processing it (again in real time) to determine an interception point. These machines in turn would need accurate, real time data on local atmospheric conditions at the time of firing including e.g air pressure, temperature, wind velocity & direction etc.

  3. High speed/accurate clocks and precise maps (to assist in making the relevant calculations)

  4. Automated gun systems linked to and directly controlled by the calculating machines

In short your looking for a super hyped up version of something like Israel's 'Iron Dome' system or some kind of Phalanx CIWS. Which is, I suppose 'doable' in a Steam Punk setting but really? It's still veering more into conventional Hard SF.

On the plus side, as is the case with the two systems named above your guns don't have to be long ranged. The idea is to hit incoming shells before they impact so rapid, accurate, short range fire is the key not hits at long range. (Lasers if you have them?)

The issue you have though (if you insist on a WW1 setting) is that in real life there were no WW1 or even WW2 technological systems capable of identifying artillery shells on the fly and plotting intercept courses. Artillery fire control calculators were bulky electrical/mechanical devices and that initially relied on the Mark 1 eyeball for targeting information then later during WW2 on radar. No system then in existence was capable of plotting an intercept for an incoming artillery shell in real time.

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    $\begingroup$ I think you are not using orders of magnitude properly if you really mean 10. $\endgroup$
    – o.m.
    Commented May 29, 2022 at 15:04
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    $\begingroup$ I was about to comment that @o.m. might be wrong when I found data for missiles vs. artillery. Your average ICBM's velocity is 7km/s while your average artillery shell is a scant 0.7km/s. A single order of magnitude ... in the wrong direction. Given today's tech, it would be easier to track and intercept an artillery shell - based purely on the issue of velocity. Perhaps the bigger problem is time-in-the-air, which would be several orders of magnitude in favor of ICBMs. You have comparatively no time to intercept a shell. $\endgroup$
    – JBH
    Commented May 29, 2022 at 21:11
  • $\begingroup$ No I meant 1000 times more difficult. A typical artillery shell (say the 155mm) has a diameter of (just under) 155 mm and a length of about 600 mm. That's the 'target'. Modern counter battery radars can detect shells that size but the target is an area say 50 x 50 meters from which the shell was fired. Not the shell itself. That's what the radar directs its own guns to fire at. What the author really needs is a point defense system, which is different. (Albeit counter battery radar would be used to give you early warning of an incoming attack and the vector? It was fired from.) $\endgroup$
    – Mon
    Commented May 30, 2022 at 1:17
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    $\begingroup$ @Mon that'd be three orders of magnitude then. Ten orders of magnitude is ten billion times. $\endgroup$
    – hobbs
    Commented May 30, 2022 at 3:26
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    $\begingroup$ 10 binary orders of magnitude are 1024, i.e. roughly 1000. $\endgroup$
    – Jan
    Commented May 30, 2022 at 9:54
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Frame Challenge -- Blimps are Squishy

enter image description here

You want to balance Blimp vs City combat. You are worried the blimps are too powerful. The solution is to realise that blimps are also hyper vulnerable bags of air.

It is much easier to make a hole in a big bag of air than to make a hole in a ground-based gun emplacement.

The blimp's only defense is being high above the ground. This protects it from smaller guns. On the other hand it can drop bombs from as high as it needs since they fall due to gravity.

To balance the combat simply impose a flight ceiling to the blimps. If they fly too high they will be blown around by the weather or they cannot aim their bombs correctly. This is WW1-era so the bombs are unguided.

The best thing about this solution is you do not need to decide the exact ceiling. Too high makes the blimp invincible. Too low gets the blimp snagged on radio antennae and goat horns. Somewhere in between lies the ideal height for your plot. Purposefully leave the details unexplained.

With the blimps a safe 2000ft or so, it is much easier to shoot them down. You don't need big mortar emplacements. You just need something one stage up from a soldier's rifle. Think a Vickers but built as a big rifle rather than a big machine gun.

enter image description here

Those things can easily be moved around by hand. Heck, even a team of riflemen can have reasonable success with sustained fire at a blimp.

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    $\begingroup$ Do bear in mind, these aren't blimps, but rather airships. If they're meant for combat, it's unlikely that they're just a bag full of air. Chances are the gas that is used to keep them in the air is kept behind armor and in compartments, kind of like with regular water-based warships. That way if one gas compartment is damaged it can still fly. So, I doubt a machine gun could actually shoot it down, but it would have success against the crew. Because you have to armor the gas tanks, and you still have to stay very lightweight, i doubt there would be a lot of armor around the crew areas. $\endgroup$
    – Chuck
    Commented May 29, 2022 at 14:46
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    $\begingroup$ I guess technically. I was thinking more like giant (and decently armored) air tanks with a frame built around them (the airship part). The skin of the airship is still just like old zeppelins, definitely not immune to machinegun fire, but the tanks themselves are armored. So you could hurt the ship with machine guns, but probably not bring it down. $\endgroup$
    – Chuck
    Commented May 29, 2022 at 14:54
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    $\begingroup$ Edited to make clear Frenchie is shooting at blimps $\endgroup$
    – Willk
    Commented May 29, 2022 at 15:20
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    $\begingroup$ Couple of thoughts on this: Armoured airships don't really work, unless they are filled with handwavium. The first tanks had 6mm steel armour. That's 47kg/m². Since surface scales with r² while volume (and thus lift) scales with r³, that means, using helium, you will need to have an airship with a volume of at least 5.5 million m³ volume, if the whole thing weighs 0g other than the hull. That's more than 27x the volume of the Hindenburg. Adding to that structures, motors, bombs, personnel and you quickly have an airship that's larger than a small city. $\endgroup$
    – Dakkaron
    Commented May 30, 2022 at 11:34
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    $\begingroup$ Next one, WW1 airships have a really low service ceiling. Most of them couldn't get higher than 1000m, the best I could find could go up to 3000m. That's well within the range of most regular guns, no special artillery needed. For example, I found a common Flak gun from WW1, that has a rated vertical range of 7000m. For lighter-than-air aircraft, the service ceiling is a direct function of their lift-to-weight ratio, which gets worse the higher the aircraft flies (because the air is thinner up there). I made my above calculation with surface air pressure. $\endgroup$
    – Dakkaron
    Commented May 30, 2022 at 11:37
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Short answer: no.

But there are a few other ways to spin this, especially if you look towards reality.

Airships didn't play the role you envision them for a few reasons:

  • They are slow. A defender has lots of time to prepare.
  • Low service ceiling

WW1 means no pressurised hulls, which imposes a pretty hard limit on the service ceiling. Also, the higher you get, the lower the lighter-than-air lift. All WW1 airships I could find, have a service ceiling <3000m, most <1000m. That makes them vulnerable to a lot of weapons that couldn't hit a plane.

  • They are huge and thus easy to hit
  • They are literally air balloons filled with explosive gas, so one hit can take them down.
  • They can't carry a lot of bomb weight (max I could find for WW1 was around 300kg)

Initially, the airship was considered a very dangerous bombing weapon. But soon the men in charge noticed, that they were to easy and soft targets for frontline warfare.

So how can you use this in your setting?

First, how to make airships effective?

  • Use lots of (possibly leash-guided) decoy ships to draw fire away from the real ships.
  • airships should fly their combat missions as close to the service ceiling as possible (even if the crew might occasionally pass out). This will limit their payload, so if the skies are safe, they can fly lower and carry more.
  • Camouflage the airship. Maybe use dazzle paint?

How to counter this?

  • FLAKs can easily shoot down airships. I randomly googled WW1 FLAKs and found some with a vertical range of almost 7km, which is more than double what the airships can fly to.
  • Counter balloons with balloons: the city people could "drop" small hydrogen balloons with proximity trigger shrapnel grenades, which would float up and try to explode when they get close to the airship. These would need to be brought out in carpet style, but that should be possible.

Bonus: Why do lighter-than-air vehicles fly so low?

The main issue is that a lighter-than-air vehicle needs to be actually lighter (meaning less dense) than the surrounding air to fly. Air density decreases the farther up you get. That means, if you want to fly higher, the aircraft needs to be less dense. Less dense means less weapons, lighter (less stable) structure and overall less payload.

Combine that with the very limited choice of low-density-high-strength work materials available during WW1 and you end up with airships that can either fly low and actually carry something to make them worth being used, or fly high and empty.

Also, if you fly higher, you also need a pressurized cabin for the crew. This adds a lot of weight and wasn't too feasible in WW1 era at all.

Bonus: Why are armoured lighter-than-air vehicles not a thing?

The oldest tank I could find used 6mm steel armor. 6mm is not a lot. It can stop regular gunfire, but it won't work wonders for bigger guns or even explosive/armour piercing rounds. Also, an airship has large, almost unsupported surfaces. A round hitting this armour will create shockwaves and all in all 6mm of steel armour will probably not help much. But, for argument's sake, let's say it's good enough.

Also, let's say, our aircraft should be able to fly at 3000m height. That's about as high as the best lighter-than-air aircraft in WW1.

Also, let's assume, the crew, payload, engines, fuel, internal structure and everything else weighs nothing.

This means, our aircraft would have to have 15.7 million m³ of volume. If the ship has the same radius of the massive Hindenburg, it would have to be 11.7 km long.

If it's supposed to fly at 7000m height, the volume would be 57.5 million m³ of volume, and the length would be 42.7 km. That's roughly the diameter of London.

Edit: all the calculations are for the perfect shape to reduce surface area while increasing volume: a sphere. So if you actually want to build this in the shape of an airship, it's going to be much longer than the above mentioned numbers.

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Barrage balloons

barrage balloon

https://en.wikipedia.org/wiki/Barrage_balloon

The problem with flak is that the area of denial is so fleeting - you have to catch the shell as it goes through the sphere of flak.

Your world has airship tech. Use that. Barrage balloons were used to hang cables down and prevent aircraft from approaching. Nets of cables could intercept some shells. Between the cables that come with the ballons are a web of ad hoc planks and scrap wood (wood because it is light) tied along ropes (also light) in hopes of increasing the area of denial provided by the balloon. Some of the wood is unmodified - chairs, church pews, wagon wheels, tree limbs.

For the hanging cables there could be very high balloons with long cables. If this is an anime there needs to be a scene where an enemy airship gets fouled in the cables. Defenders climb up the cables and commandeer the airship, turning its air artillery around and firing on other airships.

Balloons themselves could be used as a flying shield. Surfaces facing the expected approach of incoming shells will be armored and shells will spend themselves against the aircraft.

For armored balloons I like the idea of many small balloons rather than a few large airships. For one if a hit balloon is destroyed it is less of a loss. Possibly lightweight armored balloons will be thrown sideways but not ruptured by the shell.

Reactive armor was not developed until after WW2 but I think the needed tech was available WW1 - it is essentially a counterexplosion triggered by the impact of a shell. One could have balloons with armor so light that the shell would pass through it, but the balloon provides a counterexplosion that diverts the shell (and destroys the balloon). Those would be fun for a fiction because maybe later in the story some of those reactive armor balloons are used for other purposes.

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With WW1 tech, probably not.

you would need highly sensitive detection equipment to detect incoming shells, like radar, then you would likely need a computer to calculate its speed and trajectory and all that good stuff, then you would need a very accurate weapon that is likely not controlled by a person but rather by that computer. And you would probably need proximity fuses for your shells which were not invented until ww2. Without them, you need an insane degree of accuracy.

Hypothetically though, maybe?

If the incoming artillery shells were extremely slow, like WW1 aircraft speed, and extremely bright, like from some kind of artillery shell tracer or the shells being highly reflective, and the flak dense enough, you could potentially spot and shoot down a shell. if the shells are moving at actual artillery speed, then there isn't any way you're going to shoot it down. Not only that, but you would likely need proximity fuses for your flak to shoot down the shells. Without them, you would need a direct hit on the shell, which a bullet hitting a larger bullet potentially hundreds of feet away while everything is moving at high speeds is very unlikely. But unfortunately for the defenders, proximity fuses were only really invented in WW2.

What could work

So its unlikely we can just shoot down incoming artillery (even if we could they could always just keep firing shells at us). Well, bring down the airships. We probably have aircraft available to us, like biplanes. We could bring the fight to the airships. They can probably just send another at us, but we can buy ourselves some time if we keep downing them. We could bring mobile artillery/AA weapons. We sally out of our cities and fight them before theyre in range of the city. Even if large caliber artillery is needed to consistently kill them, we can at least harass them or disable unarmored pieces of their ship like propellers and lighter armored crew areas.

we could even board them, which i think would make for great storytelling. We get our guys on their airship, either by having them drop from planes, or some ground based launch tech (its steampunk low-fantasy, im sure we can come up with something) with rifles, explosives, melee weapons, even flamethrowers (they would do very well in the close quarters of a ship). I doubt the enemy has any marines on their airships and even if they do, it would not take long for saboteurs to do plenty of damage. They would've either blasted huge chunks in your ship, set everything on fire, or just slaughtered tons of defenseless crewmen.

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  • $\begingroup$ This actually answers the original version of my question, which was just if flak was enough to strike the shell in mid-air (which as it turns it it is not!). Thanks! $\endgroup$
    – whlk
    Commented May 29, 2022 at 19:31
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For such close-in interception of artillery I would also have to wonder at the usefulness even if you succeed. Your counter-fire isn't going to remove the danger of the incoming round, only lessen it somewhat, all that material is still going to hit your position (assuming the trajectory was accurate to begin with).

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  • $\begingroup$ This reads more like a comment than an answer. The point of intercepting an incoming high explosive projectile is to disrupt the detonation mechanism. There is a vast difference in destructive power between an artillery shell that thumps into the ground - in one or more pieces - without detonating and the same shell that detonates, otherwise no one would bother with putting HE in shells. Note that the kind of explosive that can be loaded in artillery and fired clearly must be insensitive enough that it won't spontaneously detonate on being dropped. $\endgroup$ Commented May 29, 2022 at 13:10
  • $\begingroup$ One thing to consider here is that Flak shells don't need a direct hit to disable aircraft. Instead, they explode mid air (back then usually with timed fuses) which then shoot out a cloud of shrapnel to increase their target area. If you manage to shoot the incoming round in a way that it either doesn't explode at all, or explodes far away from the target, it will actually not do any damage at all. $\endgroup$
    – Dakkaron
    Commented May 30, 2022 at 11:09
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What you ask directly is impossible, but also not needed for your scenario

You want a defense for your cities. You mention ground emplacements, which also more than suffice, because the ground-based guns can outsize and therefore outrange airships considerably.

WW1 style airships have a very low service height, so the height-based height advantage is negligible when you consider what size of gun you can have on an airship in relevant numbers (plus ammunition) in regards to weight.

This means that airships simply cannot go near any city which had some time to prepare a defense cities near any dangerous border would likely have permanent gun emplacements, and all others have rail-based prepared fortifications, with anti-airship guns on standby somewhere in the country, to be ready within the hour if an enemy approach is spotted (don't forget that airships are slow as well). Once in place these ground-defenses could force enemy airships to keep multiple kilometers away, because one hit on an airship is enough to bring it down, while ground based bunker systems can easily take a hit or two.

Tl:dr: there is no need to intercept airship shells because they won't dare to get into range

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