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Comment by dhx | Hacker News Reader

Perun (Australian military procurement geek) presented the maths for missile defence vs. attack and it's heavily weighted in favour of the attacker.[1] He also previously analysed the Ukrainian truck-full-of-drones attack against a Russian airbase and it was in this video (sorry no link I can't find it quickly) I think he investigated whether the cost-benefit exists to bury military and critical infrastructure underground (or use heavily hardened buildings).

Both topics are very relevant to the current conflicts of eastern Europe and the middle east. Perhaps I took away the wrong message but I don't think Perun would propose to stockpile interceptor missiles any time soon.

[1] https://www.youtube.com/watch?v=CpFhNXecrb4&t=2382s

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I think Perun would agree that it's better not to be attacked but unfortunately you don't always get a choice. Since you don't get that choice, you have to stockpile interceptors anyway, even though they're not nearly as effective as a good attack.

Part of this is the sheer expense of hardening everything. If you're trying to protect a single airplane, putting it in a hardened hanger is a lot cheaper than trying to get enough interceptors in place that you're going to protect it for sure. However, if you protect that one airplane, they'll just attack the one next to it and they'll get reasonable effects onto that. Or if you protect the whole airport, fine, they will just go attack something else.

The defense equation is not about the cost of the interceptor versus the cost of the missile. The equation is the cost of whatever the missile is going to hit versus the cost of the interceptor. Thus, although interceptors are very expensive, they tend to be cost effective anyway when you consider that they can protect a lot of different unhardened things.

It doesn’t matter how valuable the thing is that’s being protected. If the interceptor costs more than the missile it shoots down, then a peer adversary can afford to overwhelm your defenses.

We did this during the Cold War where the things being defended were entire cities. Obviously an interceptor that costs a few million dollars is worth orders of magnitude less than an entire city. But if they cost more than an ICBM then the enemy can just out-build you and they don’t work.

I think the political fight around SDI/Star Wars gave people the impression that shooting down ICBMs was just not technologically possible, or it was but evil politicians decided that having guns to each other’s heads was preferable to defense. Fact is, there were technologically viable interceptors in the 1970s. The US deployed a system for about six months to defend missile fields in the northwest. The Soviets deployed one defending Moscow that still exists to this day. The British made it their mission to ensure one of their submarines could overwhelm those defenses and ensure the destruction of Moscow and they succeeded. The British! The US didn’t even have to do anything special.

SDI/Star Wars wasn’t about figuring out how to shoot down ICBMs. That was already figured out. It was (ironically, given the costs) about figuring out how to do it cheaply so that it could actually stop a peer enemy willing to outbuild your defenses.

The US has defenses today that can take down Chinese or Russian nuclear missiles. There are only a few dozen interceptors, while those nations have hundreds of missiles. The interceptors cost a lot less than a city, so isn’t it worth building more? No, because they cost more than the enemy’s missiles, so if we add more, they can add more too, and always be able to overwhelm them. Hence the system exists to defend against rogue launches or rogue states, not an all-out attack.

This assessment works because in the Cold War, the goal isn't to intercept enemy missiles, it's to prevent your cities and civilization from being destroyed or being overly coerced - which is something that you could achieve much cheaper via mutual deterrence.

Once you can no longer achieve mutual deterrence (perhaps because you are a nation who has a streak/role of engaging in interventionist wars of choice, or maybe you are planning for the case when deterrence fails, and you are now under attack), you have to decide how to protect your assets, if even in a limited way.

An airbase which gets overwhelmed and lost after 5 days of bombardment is an airbase that hopefully was able to contribute 5 days of value to your effort. It's true that the costs will not always work out in your favor.

I think nuclear deterrence/MAD are actually downstream of the cause here.

Once sufficiently effective long distance military tools (bombs, missiles) could be produced at large scales, the defense equation was fucked. That happened before widespread nuclear weapon availability. If your peer adversary had the ability to spend debt on missiles, it was always an unaffordable, asymmetric proposition to counter that with a spend on missile defense. Sure, you spent some money on defense, but eventually the reality of the balance sheet became clear: damage at a scale that previously required your adversary to mobilize big chunks of their population in armed forces now just … cost money for missiles. And it would cost you much, much more than they spent to even semi-effectively defend against that. The opportunity cost of offensive action got massively reduced, IOW.

Nuclear weapons and other mutual-deterrence strategies emerged from that essential reality of modern mechanized warfare, they didn’t cause it. Once countries could no longer affordably defend against common attacks, they inevitably started looking for ways to deter attacks via incentive. The Cold War was the result.

> It doesn’t matter how valuable the thing is that’s being protected. If the interceptor costs more than the missile it shoots down, then a peer adversary can afford to overwhelm your defenses.

That is way oversimplified. There are many more factors. Few countries are peer adversaries in the fist place. When your economy is larger you can sustain losses. Interceptors can be scaled up (if they work)...

Also even if interceptors don't work perfectly, they can be good enough. Russia is spending a lot more on drones now that the original Shaheds - Ukraine's interceptors for the first generation is working near perfect and they have the economic advantage over Russia's attempt to overwhelm air defenses. (resulting in higher costs missiles, which Ukraine cannot intercept - but even there Ukraine is working on interceptors because they have no other option - the missiles are coming either way and will do even more economic damage than the cost of an interceptor.

The point is that interceptors don’t become worth the cost based on the value of what’s being protected. Cost of attack versus defense is very much what matters, and then that feeds into things like relative capacity to build attackers/defenders.

Also to move up the first step of the ladder: if you can reliably intercept (eg) a dozen missiles, then an adversary must launch more than that.

But a few dozen launches removes pretenses like “limited strikes”, which may allow for easier on-ramps. Missile defense can ensure the first step is a full step.

> but unfortunately you don't always get a choice

Arguably USA does have this choice, by virtue of sitting across the ocean.

It's an illusory choice. The USA has tried isolationism a couple times before and that didn't work, either. While I do favor a less interventionist foreign policy it's naive to think that the oceans will protect us. Hostile powers including North Korea and Iran either already have the capability to strike the US homeland — or will have it soon.

Remind me again why Iran and North Korea have beef with the US?

There's no historical reason for it like fomenting revolution, waging a war of choice that killed a double digit percentage of the population, extrajudicial assassinations, or any other shenanigans like that, right? The US hasn't been actively waging economic war against these countries for the better part of a century have they? These countries just have it in for the US, and no choices were made that led to that?

That's a non sequitur. I won't attempt to defend the foreign policy errors which put us in this situation but we have to deal with the situation as it obtains today, regardless of the cause. There is a very real possibility of missile and drone attacks on the US homeland as well as critical overseas facilities and allies.

Its not a non-sequitur. You were saying that isolationism doesn't work, my argument is that non-isolationism is at the root of US problems with MANY countries.

Sure the US needs to prevent that sort of attack, but one way to prevent it is to avoid the situation in the first place. I don't need to worry about Iran or North Korea nuking me because I live in a country that hasn't had a war of choice with either of them. The price of installing the Shah is still being paid in the need to defend from Iranian retaliation.

When did the USA try isolationism and have it not work. If you refer to WWI and WWII, the USA was hardly isolationist in either of those conflicts. Between the embargoing of Japan in WWII (which was reasonable, and Japan's attack on pearl harbor very stupid), and US ships following German U-boats and broadcasting locations, they were hardly impartial. In WWI the Lusitania was actually found to be carrying weapons. The US has not done real isolationism.

Pearl Harbor, 9/11. If you'd argue limiting oil exports to the empire of Japan or arming the Mujahideen (1) aren't isolationist policies and (2) caused those attacks, consider that your definition of isolationism might require the US to give up its sovereignty.

What sovereignty exactly did we get from messing around in the middle east?

And there it is, the "no true isolationist" argument. It's always disappointing to see arguments about major policy issues on HN descend into this type of low-effort "gotcha" while entirely missing the main point.

If everything short of boots on the ground is Isolationism, then nothing is. These are definitionally non-isolationist things that were being done despite the populace at the time being isolationist.

I agree. This is US strongest defensive "weapon". I always compare it to playing Age of Empires II while having isolated Island for yourself with all resources imaginable.

It gets even better: the US sell and profit a lot from expensive interceptors

Except that interceptors are in demand, and in turn others are not starting to enter that market. Which in turn drives interceptor costs down...

Distance didn't stop 9/11 or Pearl Harbor. And now there's GPS and other COTS tech that bridge the distance for an adversary.

Of course. But the idea that you can arm yourself to the point where you can stop any and all attacks is an imperial folly.

> The equation is the cost of whatever the missile is going to hit versus the cost of the interceptor.

Yes, but how do you know what the missile is going to hit? Or if it’s going to hit? Or when? Or how many casualties it might cause? Might overspending on defense encourage bellicosity in rivals? Might underspending on defense risk empowering them via vacuum?

I think that is, in a nutshell, why defense budgeting is so hard. Because that’s not evaluating cost vs. cost; it’s evaluating cost vs. the probability of an event with a cost (a probability whose causal influences are deeply entangled with the military and economic decisions of the defender).

In that comparison mode it is very hard for even experts to pick the right spending thresholds, to say nothing of the public. People are not good at thinking about probabilities, and are especially bad at evaluating probabilities compared to concretes.

> you have to stockpile interceptors anyway, even though they're not nearly as effective as a good attack

Strange, I keep hearing from experts that attacking is 10x as costly as defending.

I believe that's for soldiers on the ground. One soldier can defend against ~10 attackers. For missiles, and drones, you can overwhelm air defence pretty cheaply, because the defence missiles cost more than the attack ones, as they need to be more precise.

But why is that? Soldiers move in 2D, missiles in 3D. Is the dimensionality what causes this flip in what's cheaper? Or the fact that soldiers are humans and need to be kept alive? What is the fundamental difference?

There are a lot of things and I don't pretend to know them all. The usual claim I've heard is 3x not 10x, but whatever. (edit - the claim I've heard is attackers take 3x the losses of defenders, re-reading a slightly different claim)

Defenders have a big advantage because they can choose a good position and hide. Often this means they can catch attackers by surprise, and so eliminate the entire attack.

Defenders have time to fortify their position - even if the attacker knows where they are, the attacker needs to run up to a fortified position to take it.

Defenders often know where others are and plan overlapping lanes of fire. If someone is attacking you, the next position will be firing at your attackers (and do the same for you).

Attackers are moving and so have a harder time identifying who is friend vs foe. Defenders know what is supposed to be there - if they see something unexpected they know it is the enemy.

That said the difference is a rule of thumb - but not a law of any sort. Well trained attackers with a good plan can greatly reduce the ratio. Defenders who don't take advantage of all their advantages also lower the ratio.

I suppose to be rigorous you must include time as a dimension. Stationary surface targets are easiest to find, moving surface targets are harder, and moving air targets are hardest. Especially fast-moving ones like missiles.

generally, you need to spend more than one defensive missile per attacking missile, because you might miss.

defensive missiles need to very fast and more maneuverable than attacking missiles, or the attacking missiles simply fly around them.

the fastest-accelerating vehicle ever built was a nuclear missile for intercepting nuclear missiles. they gave up https://en.wikipedia.org/wiki/Sprint_(missile)

> you need to spend more than one defensive missile per attacking missile, because you might miss

It's almost always the case that you'll expend more than one interceptor per incoming missile as interception is probabilistic. In general, you can estimate the number of missiles you'll need to achieve a certain probability of interception with the following formula: 1 - (1.0 - sspk)^n, where sspk is the single shot probability of kill (i.e., successful interception with a single interceptor) and n is the number of interceptors.

Shameless plug here, but I wrote up a post about this problem: https://smu160.github.io/posts/missile-defense-is-np-complet...

i remember that article, it's really good :) thanks for linking it here

The cost of intercepting a drone can be orders of magnitude higher than the cost of sending the drone. The cost of letting the drone get through can be even higher. If I send one drone that cost me $200, and you have to defend against it at $2,000 or $20,000 worth of munitions, and the target it would destroy is $200k or $20mm, then I win no matter what happens. This works at any scale. Iranian Shaheds are probably around $20k to $50k. They are being defended by patriots which cost ~$2mm-$5mm, and being aimed at US targets that cost $100mm. In this scenario, getting shot down is a win, and not getting shot down is a win.

With humans the math is reversed, being the defender is an advantage. Defenses can be setup pre-emptively and cheaply. Taking out a swarm of 20 Shahed drones is difficult and takes several missile batteries. Taking out a swarm of 20 infantrymen attacking a dug in position is light work for a matching force.

You are mixing up your facts. Patriots are primarily being use against ballistic missiles, not Shahed drones.

Shahed type drones are killed with APKWS missiles. A single fighter jet can carry ~40 of these missiles and the unit cost is similar to a Shahed. APKWS has evolved a lot during its 15 years in use and the new generations are expected to come in around ~$10k each, cheaper than the drones they are designed to destroy. You can put an APKWS on many different things. The US annual production rate is 35,000 but that could be increased several-fold if needed (it is based on a missile of which the US produces 200,000 per year). The Europeans are working on their own version of this.

These have been used in the Middle East for a long time. Saudi Arabia, UAE, Qatar, and Israel all have orders for 10,000+ APKWS missiles each, so clearly they are effective.

The unit cost has been tipping toward the defender but the systems are not widely distributed yet. More effective defenses require more expensive drones e.g. you can't build an effective anti-ship missile (a type of jet-powered drone) for less than a couple million dollars because anything cheaper has too low of a probability of success.

The point isn't the specific weapon. It is the assymetry. I'm also not confusing things: Patriots are used against a variety of air targets, including Shaheds, planes, and - yes - ballistic missiles.

That said: how many APKWS missiles does it take to eliminate a full wave of Shaheds? If the answer is not 1:1 shot to kill ratio (it isn't), the match still works out in favor of the drones. If a single drone gets through it works out massively. If my defense platform is an F-15 or F-35 for cheap missiles, I also have to eat the cost and readiness loss of operating that jet ($30k per hour). We see this exact math in incidents like the Prince Sulatan Air Base attack which was defended by patriots, and where several planes were damaged or destroyed on the ground.

Its also worth pointing out that Patriots are reportedly getting used to defend against Shaheds in the gulf and Ukraine, especially the newer generations that are faster and higher flying.

Patriots are used when that's the only thing you have available. However, the world has already figured this out and we are ramping up production of much cheaper interceptors for the cheap drones very quickly. Patriots are not going to be using great quantity against small drones in the foreseeable future because everyone knows small drones are going to be a problem and so they're wrapping up their cheap drone defenses. Ukraine will sell you a drone interceptor and it's probably going to cost you less than the thing gets intercepting so long course as you try to intercept something you can get. Sending a cheap interceptor against a ballistic missile is obviously going to fail and then you need a patriot. But the ballistic missile is also vastly much more expensive I don't know numbers, but it wouldn't surprise me if the Patriot production is wrapping up and going to be cheaper than the ballistic missiles it's hitting in the near future.

The interceptor cost is only half the point.

If you can get any drones through at all, it still comes out assymetrical. If you shoot 9/10 of my drones down with magically free munitions, but I blow up a refinery or take out a fighter sitting on the ground I still win. Right now there just isn't a system that can guarantee 100% intercept rates over the long run.

Ukraine, Russia and Iran have all proved that there is no foolproof interceptor. Right now, the drone gets through.

> Soldiers move in 2D, missiles in 3D.

Infantry troops still operate in 3D as they can/will direct mortars, air assets, etc.

In general, what you laid out is a bit of an apples and oranges comparison. An infantry platoon applies fire and maneuver using all of its elements to "close with and destroy" a smaller element.

You emplace your weapons squad which has machine guns and other large weapons (e.g., mortars) that will keep your enemy's heads down. This allows your maneuver elements (i.e., assault squads) to maneuver to the enemy force.

In the case of missile defense, you have a limited set of complex and expensive bullets, and your enemy has cheap bullets. If you want to defend yourself, you're attempting to hit your opposition's bullets with your own bullets.

It depends on the objective. Given warring peers, if the attacker just wants to cause destruction, that’s incredibly cheap to achieve and expensive to defend against in the age of the long range unmanned weapon (missiles, drones, bombs).

OTOH, if the attacker wants a specific objective (control infrastructure, seize a resource, gain political power), that equation changes a ton in favor of the defender.

Lots of modern military strategy seems to be occupied with finding the appropriate balance between those tactics.

Physics. The incoming gets to play way easier games than your interceptor.

Harder to shoot an incoming bullet than it is to fire that bullet in the first place.

a) Attack missiles need to hit a stationary target, whereas the defenders need to hit a moving target. So defenders are more expensive.

b) A human defender can hunker down in a safe space and shoot at attackers. The attackers need to move and are more vulnerable

As a counter to point (a), Iranian missile salvos against Israel in recent years are stated by analysts to be militarily ineffective (or a failure) even if Iran has a 10:1 cost advantage. Reasons include:

1. US+Israel economies have a 100:1 size advantage over Iran's economy.

2. Israel is a very small area to defend, dramatically reducing the cost of anti-ballistic missile defence required. As little as 4 missile defence launch sites may cover the entire country.

Perun's video on the cost effectiveness of missile defence vs. attack is more focussed towards the economics of defence of an expansive country or region such as the United States, Saudi Arabia or the gulf region generally, where many missile defence sites would be required. Israel is a bit of an exception due to the small area needing to be defended.

Another minus specifically for Iran in the Iran/US+Israel war is being forced to build underground factories at much greater expense than an above-ground factory for Israeli interceptor missiles. I don't think analysts have been considering this long term cost in their cost comparison ratios. However, for both Iran and Israel--they're importing a lot of the components and materials from overseas anyway.

Attack missiles generally need to travel much farther which increases the cost. How it balances out it an open question and different for each system of missile/interceptor.

If your interceptors are good eventually the enemy is going to give up on that attack mode. - you have more incentive to defend than they do to conduct useless attacks.

Distance does have cost, but that's mostly a brute force solution of strapping more fuel on. The really expensive piece is the tracking and flight control systems that enable interception of fast targets.

If attackers are cheaper than defenders, the attacker might be able to bankrupt the defender, or at least temporarily deplete their supply of defenders. So not useless at all.

attackers are not cheap though.

Ukraine's interceptors for cheap drones are drones that are even cheaper and so advantage defender.

The consideration made in Perun's video, and I know it's quite different geographically for drones versus missiles, is a defender having to defend all important sites as if the attacker sent every drone or missile to a single target.

For a facility that Ukraine has decided would cause $10m in economic damage if hit by a Russian drone, how much should Ukraine spend on interception for that site to not make it worth Russia's expense to target it? Let's assume a Russian drone is $100k (current estimate in news for Geran-5 jet-powered drones). But Ukraine can build their Bars interceptor drone for half the cost at just $50k (low estimate from public rumours/reports). It wouldn't be worth Russia's expense to send more than 100 drones in a salvo at this target. For Ukraine, they should defend the site with $5m worth of Bars interceptor drones, assuming a very favourable 100% success rate (actual reported success rates are reported by media to be just 30%). Russia has now made every $10m facility in Ukraine cost $15m (+50% increase).

The defender is essentially forced to find a 10:1 or better cost advantage for their interceptors vs. incoming drones. This is hard to do when an interceptor and attack drone are substantially the same except for larger fuselage (cheap), larger fuel tank (cheap) and larger engine (possibly cheaper than making a small engine).

And perhaps that is the reason Russia and Ukraine are largely focussed on attack rather than defence in the hope of being first to cripple the other's economy and supply chain for munitions.

> Attack missiles generally need to travel much farther which increases the cost.

This is likely a a flattened curve after you enter ballistic missile territory.

Drones change the infantry picture quite a lot though. You could field 10 drones against each soldier. It's apples and oranges.

Depends on the situation.

An example is the https://en.wikipedia.org/wiki/Battle_of_the_Shangani where 5 guns defeated a force of 7000.

However when both sides had the same capabilities, attack became very costly.

The point is, depending on the type of drone, the sheer cost of fighting them off is prohibitive with current interceptor tech (ie anti air missles). In fairness they were designed to attack targets 10x their cost, and take years to build.

For the USA specifically, they have a bunch of bases within range of cheap drones. This means that you need radar/sensor controlled chain guns, or moveable nets to defend cheaply. Its almost time for barrage baloons.

Other way around.. a single large nuclear weapon is 2 orders of magnitude less expensive than defensive measures. Attacking wiht the intent to completely annhilate your target is vastly cheaper than trying to defend.

Under post-WWII US military doctrine, the air defense systems and munition stockpiles that we have now were never intended as a permanent shield. Planners intended those to (mostly) defeat the initial attacks only during the opening days of a conflict. Then offensive forces including ground troops were supposed to advance into enemy territory and destroy or occupy the air bases and other sites used for launching air attacks. But if that second step never happens then the entire doctrine around air defense has to change. Israel did that with the Iron Dome system and now the US (and other countries) will have to learn from their experience.

> presented the maths for missile defence vs. attack and it's heavily weighted in favour of the attacker.

Yes, and this has been known for a long time, since at least the 1950s: https://en.wikipedia.org/wiki/Cost-exchange_ratio

The problem is that a new generation of policy makers comes along every 30 years or so and thinks they know better than the people who came before them.

This is the "defender's dillema" and it has been true for a very long time.

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