The real problem with defence against drones

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A blog post about the real drone problem is overdue because I do not see ANY indication that the real problem is part of the public discourse. As a consequence, there's severe misallocation of resources.

 

Imagine you're driving a vehicle through a typical European landscape. This is a VERY different landscape than military training grounds, by the way. There are trees, bushes, ditches close to roads, buildings, fields with tall (more than 30 cm high) crops, enough garages to hide a SUV-mobile company per village, there's scrap lying around.

You pass by one of the gazillion trees, it's still 50 m away. Your vehicle is equipped with all kinds of drone detection sensors imaginable such as visual spectrum optornics, NIR, MWIR, LWIR, mmW radar, acoustic, broadband frequency analysers. A quadcopter is accompanying your vehicle at 100 m altitude, it pattern recognises threats and a buddy of yours in the vehicle checks the reports. Suddenly, you're dead.

What happened? A drone that attached itself to the backside of a tree heard your vehicle approaching, swung its sensor and EFP around the tree stem and fired an EFP warhead at you. No hard kill APS stands a chance of stopping that. The overhead drone saw nothing. 

source https://www.researchgate.net/publication/382231382_Crash-perching_on_vertical_poles_with_a_hugging-wing_robot

What "C-UAS" defences could have protected you? Some 70 mm rocket with semi active laser seeker? Some 30 or 35 mm autocannon with radar and thermal imager fire control? Some laser? Some microwave beam weapon? No, not even the kind of defence that I argued for since 2017 -a CUAS RCWS with machinegun or at most a 20 mm autocannon- could have saved you.

A lot of passive and reactive protection would easily have stopped that EFP attack, of course.

EFP attack; a shaped charge with 110° or more opening angle

 

Now suppose it wasn't one tree, wasn't one drone and it wasn't using an EFP. It was a patch of woodland or some bushes or a plowed field with drones hiding under soil. There were 30 drones with Faraday cage protection against microwave attack. They rise suddenly, overwhelm the APS with a series of attacks and hit the top of your vehicle with 70+ mm (500+ mm RHAeq) tandem shaped charge warheads.

Again, you're dead. There were more than 20 penetrations because no vehicle can be protected against this with enough passive and reactive armour.

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I believe I thought this through enough since 2017.

There's no line of sight defence against this.

We need to emphasise the thousands of years-old* concept of security effort

instead of staring at line of sight defences.

The traditional security effort would require to send men out, expose them to great risks to reduce the overall risk to the total force. That seems nonsensical now. We should send out drones as security effort, to search for, warn about and if possible neutralise hostile drones at a safe-enough distance.

Even a single car moving through hostile terrain would require a security ring around it, possibly at 200 to 2,000 m distance. Experiments will tell what the required radius is. Different terrain will require different security efforts and likely different sensors carried by the security force drones. So there would be a constant change of the security ring while that manned car is moving. The ring would become densified where needed, thinned out where that's acceptable, change of terrain from woodland to ploughed field would lead to a major change of what drones are scouting. The security force drones may even open and inspect buildings, weld shut manhole covers with thermite. The control software that controls the drones would report possible contacts, but it would also indicate the acceptable speed. It would need to be able to adapt to a change of route quickly. These security drones can look behind, under, into things. They can scan the ground - hostile drones may hide in loose soil such as mud and ploughed fields.

That's a LOT of effort for a single car. So mastering this AND having the ability to fend off a saturation attack of thousands of drones coming in one pulse (reminiscent of American carrier defence concepts of the late Cold War) is a huge effort. This leads to two conclusions:

1. A frontline with such a defence is much cheaper (and MUCH more reliably and more easily secured while static) than defending many individual areas all-round.
2. Any offensive movement in pincer-style or in form of a raid would have to be substantial. I estimate a brigade-sized effort may be debatable.

Conclusion #1 in particular would lead to a sizeable no man's land between human troops** and indeed between opposing high value hardware as well. The defender's advantage is slight when security drones fight security drones. Quality differences need to be substantial to matter much and even then they'd be unlikely to compensate for a factor three difference in production output. The party with the greater drone mass production capacity (and ability to supply the front with said drones) would win in a prolonged conflict. The peacetime inventories would matter for the first days, maybe weeks. The superior side could ruthlessly intensify the clash to quickly deplete the opposing drone forces. 

To be honest, I did not think all such consequences through yet. 

It's probably not necessary anyway, for it's rather unlikely that war happens when neither side has a qualitative edge or either side has a known qualitative or quantitative superiority that cannot be overcome. Defences could be shifted towards becoming impossible to occupy for long in such a case, preserving freedom only rather than freedom and civilian hardware.

Conclusion #2 is probably mostly relevant for the first days when a frontline is not fully established or if the inferior side wants to win by invading quickly. Another possible period of relevance is after the opposing party's drone forces were largely depleted, offering the opportunity to strike with a manoeuvre force without becoming overwhelmed.

 

The legacy arms industry wants to make the big buck with the kind fo hardware it understands: Big, expensive sensors and weapons; lasers, microwave weapons, autocannons, radars, big thermal imagers. This approach can sweep the sky between 100f t and 15,000 ft altitude just fine. It's not going to be enough. It's not even enough to protect against the fibreoptic FPV drones that laid in ambush next to roads in Ukraine months ago already. The focus on attention on above treetop line of sight C-UAS is badly misguided in regard to the battlefield. It's at most suitable to reduce the effect of strategic attacks.

S O

defence_and_freedom@gmx.de

 

*: See Xenophon

**: Compare the movie "Screamers" https://defense-and-freedom.blogspot.com/2008/01/screamers.html  

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Optimal AAA calibres in WW2 and today

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Here's a shipload of hindsight: The optimal anti-air artillery calibres of World War Two.

7.62...7.92 mm machineguns were rather ineffective. There were some aircraft with thin armour plating during WW1 already. Such armour plating provided good protection at as little as 300 m altitude, for the bullets quickly lost energy by climbing against gravity and drag. The use of twin or quad mountings didn't help much. 

12.7...13.2 mm machineguns were little better. Their effective range was around 500 m, but they had the target in range for so little time that they depended rather on lucky hits to achieve much. The use of quad mountings didn't help much. It's irritating that the Soviets nevertheless introduced 14.5 mm anti-air machineguns based on their anti-tank rifle calibre after WW2, including multiple barrel mounts. Their usefulness was mostly limited to anti-helicopter work and the slightly bigger 23 mm twin gun (a Cold War design) was vastly better.

A single Czechoslovakian 15 mm gun design was of marginal relevance. It was too much gun and mounting for the modest effectiveness of the calibre.

The first really effective AAA calibre was 20 mm. It's widely recognised as the smallest calibre for a useful high explosive round with a proper fuse. 20 mm weapons weighed about double what 12.7 mm weapons weighed during WW2, but their weight was still bearable. 20 mm quad mounts were very heavy, but it was possible to devise a 20 mm light AAA pack gun. The calibre was also very effective in ground support and against merely bulletproofed AFVs (most early WW2 tanks and late in WW2 still the scout vehicles and halftracks). This was the best calibre to equip infantry regiments or even infantry battalions, but this was hardly ever done. The result was that the infantry was largely without proper air defences, but at least the infantry was fairly good at using camouflage, concealment and cover. This happens to have been the reason why 20 mm would have been sufficient for an infantry AAA capability: Its up to 2 km effective range covered approx. the distance at which aviators could still spot infantry targets save for road marches and field fortifications on open fields.

There were some 25 and 28 mm AA guns in France, Japan, Soviet Union and the U.S., but they did not earn good reputations. Japanese 25 mm suffered from sluggish mountings and a ceiling that permitted very accurate level bombing against anchored or moored ships from a mere 7,000 ft altitude.

The next really convincing calibre was 30 mm. The German 30 mm HEIT (high explosive incendiary tracer) Minengeschosse were designed for a giant explosive charge with added incendiary side effect. One hit usually sufficed to shoot down a single engine aircraft. Even the rugged 4-engine B-17 was said to usually be doomed after four 30 mm HEIT hits. This was in part because of the huge blast effect an in part because the fuse was designed to explode just after penetrating the skin, so it would explode inside a wing or stabiliser. 30 mm wasn't only a sweet spot for its destructive power leap over 20 mm. It was also a sweet spot because the late WW2 Mk 103 showed that a huge rate of fire (for a light AAA at the time) and a high muzzle velocity was doable with this calibre without excessive weight of the whole system. The effective range was greater than 2 km in the AAA role with its muzzle velocity without needing a muzzle velocity at 900+ m/s, which would have worn out the barrel too quickly. So IMO a 3 cm AAA of about the Mk 103's weapon design (with a suitable mount) would have been the perfect light AAA for divisional air defence and SPAAGs. It would have reached higher than 2 cm guns, pushing the threats farther up to altitudes (8,000 ft and more) from where the rear elements of a division (esp. artillery positions) would be much less recognisable. Hardly any 3 cm AAA was used during WW2.

Next, there's the 37...40 mm group. These guns had vastly inferior rate of fire to 3 cm Mk 103, but the destructive power of late war HE-rich grenades left almost no chance to single engine aircraft with even only one hit and approached one-shot kill power against medium bombers. Moreover, they had increased reach than smaller calibres. The altitude of 10,000 ft was considered to be safe from them during WW2. Some of their fuse designs self-destructing at about that distance anyway, so greater theoretical ballistic ceilings didn't matter. This calibre was the best fit for naval guns. It pushed level bombers so high that they became too inaccurate even against anchored or moored individual ships. They made twin-engine skip and torpedo bombing inefficient or (if they got close) suicidal. It was a powerful-enough calibre for motor torpedo boats to fend off motor gun boats. The effective firepower against ground attack aircraft at short distances was certainly vastly inferior to the much higher practical rate of fire of 3 cm Mk 103, though. I'd like to add that the British 2 pounder went back to WW1 and this explains its poor muzzle velocity, which rendered it rather unimpressive against aircraft passing by. The 4 cm Bofors of the late 1920's was the best and earliest really good gun in this calibre and would still (even with 1980's munitions) be a very good AAA in face of the cheap cruise missiles used in the Russo-Ukrainian War.*

The 45...47 mm calibre group was a bust. The Soviets used it on ships, the Czechs had some developed, but these guns had a terrible rate of fire and marginal advantages over 4 cm.

The 50...57 mm group began with a German 50 mm gun meant to close a gap between 10,000 and 15,000 ft altitude at which 37 mm was ineffective and heavy AAA (88 mm) not yet effective-enough because of high angular speeds and rapid change of necessary time fuse settings. This was a sweet spot for level bombing during WW2 and also relevant over the Reich because of the poor service ceiling of the Short Stirling night bomber. Eventually, the 50 mm calibre proved to be too weak and Germany advanced to develop a 55 mm gun, but only completed prototypes. Its combination of calibre and muzzle velocity indicates that it would likely not have covered the 10,000 to 15,000 ft gap. 55 mm was considered to be capable of reliably one-hit kill even a 4engined bomber such as a B-17 or Lancaster.** A properly-developed 55 to 57 mm gun would have proved very useful for protecting railheads, railway stations, bridges and (against night bombers) cities during WW2. Basically, it would have been the perfect defence against light and medium bombers and heavy bombers used in typical medium bomber mission profiles (= level bombing mostly between 10,000 and 15,000 ft).

75...76.2 mm guns were often leftovers from the First World War, especially in France and the UK. Even the best guns in this calibre group lacked the ceiling to defend against American-style B-17/B-24 attacks due to the bombers' turbochargers permitting attack altitudes of about 27,000 ft. The calibre had such a low lethality with a time fused fragmentation shell that all the optical tracking, searchlight, radar rangefinding, computing and automated fuse-setting equipment of a heavy AAA battery would have been wasted on it. That is, unless one uses point detonation fuses and hopes for direct hits (then you need no automated fuse setters). There would still have been the ceiling issue, but the guns would have been deadly against large aircraft (medium bomber and bigger) within their reach at much lower expense than the next group. Everything changed for Western Allies' 76.2 mm AAA at about the end of WW2 when VT (proximity) fusing suddenly turned a 76.2 mm twin into a more effective counter-kamikaze defence than a quad-40 mm mount. 

85...90 mm guns were in another sweet spot that gave just enough ceiling (great ceiling if very high barrel wear was accepted) against almost every threat. The munition expenditure was terribly high per kill until late in war when Germany used impact fusing against large bombers*** and Americans created incomparably more expensive and sophisticated 90 mm proximity fuses that made the calibre effective against any target in range provided the fuse was not jammed and no ground features or waves triggered the fuses early. The calibre was used on some warships, but its usefulness against warships was limited to shooting illumination rounds at night. The shell was simply too weak against ships.

100...128 mm calibres were powerful with their fragmentation effect, but very expensive, bulky and heavy. The saving grace for this calibre group was the dual purpose potential; you could equip warships with these guns and plausibly use them both against aircraft and against any unarmoured ships at useful distances. Even 127 and 128 mm calibres were disappointing against unarmoured ships, but still accepted as barely good-enough for a naval DP gun. The Americans successfully used 127 mm guns as DP guns in the secondary artillery (against small ships and up to light cruisers) and tertiary artillery (heavy AAA) role, whereas Germany stuck with a 150 mm secondary and 105 mm tertiary/heavy AAA separation that yielded less guns for either role. The American 127 mm DP guns were unusually quick-loading and the first calibre to receive the new VT (radio proximity) fuses. Naval heavy AAA and DP guns were hardly worth their effort as AAA guns if not equipped with such VT fuses.

The Royal Navy used 133 mm (5.25 inch) guns as DP guns, but they were much less successful than the American 127 mm guns due to more sluggish mounts, smaller maximum elevation and lower rate of fire.

150 mm and bigger calibre heavy AAA and super-heavy AAA existed in marginal quantities with examples in Germany and in Japan. IMO such calibres should be limited to firing shrapnel rounds against incoming skip bombers and torpedo bombers from a super destroyer's or light cruiser's main guns (138...155 mm calibres, not real DP guns). These guns were so big, bulky and expensive that there was simply no realistic case for them being cost-efficient in a land-base AAA role. They were unsuitable for 85...90° maximum elevation naval turret mounting, so their AAA niche was a secondary capability of shooting at incoming aircraft within their elevation range (~30°) that was commonly provided for anti-ship fires. The USN introduced a class of light cruisers which had true 155 mm DP guns, but that proved to be inefficient and unsuccessful.

Surface-to-air missiles: Let's face it. The missile tech, guidance and fusing was simply not mature enough by WW2, but the experiments and the experience with the leap in aircraft speeds possible with turbojets showed that surface-to-air missiles would replace heavy AAA after WW2.

 

So all in all, I see for the WW2 period three different sweet spots for light AAA calibres (20, 30 and 37...40 mm), each dependent on tactical niches and two sweet spots for heavy AAA calibres (88...90 and mostly 127/128 mm with 100...105 mm being a very AAA-biased DP compromise), separated in land-based heavy AAA and naval heavy AAA/DP guns. Without hindsight, I'd likely have preferred 20 mm, 40 mm and 75 mm plus cruisers using their main batteries to shoot shrapnel at skip bombers and torpedo bombers at pre-set distances.

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Fast forward to today: 

Shahed 136-class cruise missiles can fly at 15,000 ft, above effective 40 mm gun ceiling. They are barely in reach of 57 mm guns. A modern 76.2 anti-aircraft gun based on the widely-employed naval gun and its munitions can easily reach Shahed 136 at 15,000 ft, but would struggle to reach and hit Shaheed 238 (more expensive, faster, higher, shorter-ranged) at their 30,000 ft service ceiling. Warships can self-defend against such cheap missiles even with small calibres (20...57 mm), but bigger calibres (76.2 up to 130 mm) are much more useful for protecting  escorted ships due to their greater effective range.

We need defences against flying munitions after land-based air defences obsessed about shooting at platforms (except in training) for generations. Something small calibre (up to 2 cm) makes sense for very short lines of sight (around treetop altitude), fairly compact autocannons can wipe clean the sky up to about 10000 ft. We need cheap guided missiles or the (unmanned) fighter approach to deal with higher-than-15,000 ft munitions without breaking the bank (which is what Germany did trying to defend against rugged bombers at 27,000 ft with 8.8, 10.5 and 12.8 cm guns).

A use of 155 mm (gun-)howitzers (which are equivalent to WW2 15 cm heavy anti-air artillery muzzle energy) for shooting down some munitions with special proximity-fused HE shells (normal PROX fuses won't work) should be possible, but this should be a mere secondary task for 155 mm guns.

S O

defence_and_freedom@gmx.de

*:  No better gun in the 37...40 mm calibre range was deployed after the Bofors other than its own evolution, the somewhat longer-barrelled 40 mm Bofors of the Cold War. 35 mm guns appeared around 1970 and excelled with much better rates of fire, but 40 mm remained the smallest calibre for proximity fusing until at least the 1970's. I don't know when 35 mm became viable for proximity fusing.

**: In the end, the Soviets brought a successful 57 mm gun into service soon after WW2. This gun has a much higher muzzle velocity than the German 55 mm prototypes. The Soviets quickly gave it radar and computer fire control and it's now again in wartime use in the Russo-Ukrainian War, presumably without the radar and computer fire control.  It's mostly used as artillery, but would no doubt prove highly effective against cruise missiles if equipped with functioning automatic fire control and proximity-fused munitions. The West uses a Swedish 57 mm gun in naval context.  

***:  Germany understood this very, very late in the war. All that fuse-setting equipment was wasted because 88 mm AAA was about triple as deadly against 4-engine bombers with impact fusing, 105 mm about double as deadly and the 128 mm calibre gained very little from it.

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Naval drones (USVs) in the news

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Naval "drones" and motherships appear to experience a political breakthrough in the UK, carrying the hope for an affordable navy. 

There are a couple problems with this, including an important one that I didn't mention back in 2009 and probably wasn't clear-enough about in 2018:

The fuel economy improves with size. It's almost impossible to build a boat-sized (shorter than about 60 m waterline) naval drone that could cruise across the Atlantic at 18 kts even without the drag of a towed sonar array.

So either the mothership is in large part a fuel replenishment ship (and then accordingly hardly looking like a destroyer any more) or a drone-enriched naval force would be rather short-legged, for European waters. The latter case raises the question why one wouldn't follow a land-based approach rather than a mothership-based one.

 

Another problem is the issue of fixed costs, and it doesn't go away by magically deleting the crew. Some surface boat drones would still need expensive equipment (expensive munitions, towed sonar or radar) to be of much use. This means it would be sensible to provide a modicum of survivability, regardless of crew size zero. This modicum of survivability includes soft kill defences, at least some autocannon, a tactical radar even for a sonar-centric USV and so on. In the end, this would be rather expensive. An analogy: Some mere flying drones such as Global Hawk ended up being obscenely expensive, close to manned combat aircraft or a FAC.

Truly cheap "drones" Could be little more than decoys, preferably drones that are being towed (by escorted transport ships) most of the time. They could launch infrared and radio frequency obscuration, feature cheap radio frequency jammers, trigger torpedoes and mines with their signatures. They could become freely-moving decoys during combat and be towed 99% of the time, solving the fuel issue. They would be an enrichment for a convoy, but not a replacement for a frigate or destroyer. Most importantly, they could be towed by a cargo ship rather than be tethered to some mother(war)ship.

 

I do insist on my two paths:  

1. A ship that deploys "USV" drones mostly as decoys and would rather resemble an LPD or helicopter cruiser than a FFG or DDG*
2. A self-defending convoy of armed merchantmen making use of modular naval equipment 

The alternative is to stay very traditional and have a GP frigate, BUT there's no way how NATO could compete with China in a naval arms race with such an approach. The Japanese might be able to pull that off, while the South Koreans better pay more attention to land power and air defence than a naval arms race.

 

(All this being said, a USV for picket duty (x-band radar and optronics) with the endurance (fuel) issue solved would be a valuable substitute for AEW in the quest to get early warning against sea-skimming threats.)

S O
defence_and_freedom@gmx.de

 

P.S.: I just found where I mentioned the fuel issue of boats before:

"convoys at sea would want to cruise at 15...25 kts, and I doubt that many small drones could achieve the necessary endurance to accompany a convoy at such cruise speeds. They would need to be recovered and refuelled all the time" 

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