Missiles go extremely fast. Like stupid fast. Most ABM systems today use hit-to-kill, which means that you actually have to make contact with the missile to kill it. Physically hitting something that small and fast from a long distance away is hard even if you know exactly where it's going to be. Terminal guidance needs to be very accurate, since in the last second the missiles are a mile apart. With an update rate of about 10 hZ, you have to make your final correction that is accurate to about two feet (the diameter of the missile) over about 1/10th of a mile. That's tough.

Most other missile systems (like surface-to-air or air-to-air systems designed to kill planes) have proximity fuses, and don't need direct contact, they just blow up nearby and throw shrapnel into the plane. That's enough to kill a plane, but doesn't work very well to knock a ballistic missile off it's trajectory.

To add to this, the closing speeds are so fast that you would need to detonate explosives far in advance for anything to happen anywhere near the target. If you detonate at the usual anti-aircraft distances (a few meters), the time it takes for the detonator to fire and for the shock wave/chemical reaction to propagate though the explosive mass would leave the shrapnel cloud way behind the target.

Wow, I hadn't actually thought of this. So, I assume that's why you go for hit-to-kill kinetic warheads, then, and just solve the accuracy issue rather than accuracy plus tricky timing issues?

That's part of it. There's also a point where the kinetic energy of impact surpasses the chemical energy of explosives for equivalent mass. This is around Mach 10 as a huge ballpark figure. So with intercepting high mach and ablatively shielded targets like ballistic missiles, it might be better to budget your projectile mass to increasing likelihood of a hit vs attempting to damage via a proximity explosion.

The UK Starstreak [0] missile seems to be trying for a compromise with a combination of relatively high speed (Mach 4.0) kinetic tungsten penetrator with a very small (again, relatively) 450g high explosive charge. It's an odd looking thing, because the sub-munitions aren't faired over at launch so you get this normal looking tube shaped missile with three tiny missiles stuck on the front. It seems they rely on proximity fuzing with time delay, so once the penetrator has breached the aircraft skin, causing a reasonable amount of damage, it waits until it knows its inside and then detonates for maximum effect. I guess three of the penetrators hitting your plane at M4.0 within a ~1.5m group is enough to make you have a very bad day, even if the explosive warheads turn out to be all duds!

0. https://en.wikipedia.org/wiki/Starstreak

Starstreak is a really clever design. They use a neat clutched rotation mechanism with skewed fins to get 2d attitude control via a 1d clutch signal. The US adopted a similar mechanism to build a smart fuse that can be used with existing dumb artillery shells to give them gps guidance at low cost.

I wouldn't think so. Hit to kill is going to have the same but worse timing issues as a proximity hit since the size is much smaller.

I would guess the real reason is that if the target is ballistic, you need to actually "kill" it, not just wing it like you can with an airplane. If you wing it the warhead will still fall in roughly the same place with a probably intact warhead. To be safe you want to actually destroy the warhead. Worse case it goes off at point of impact, which is better than on the target.

Yes, Scuds go Mach 5 (3800 mph) which is indeed stupid fast. That Patriot PAC-3 only goes Mach 4.1 but then it's not chasing its target down.

Can you explain why the update rate is 10 Hz? It seems orders of magnitude away from current tech, and even from tech of 20 years ago.

Perhaps I misunderstood what the update rate was, but I interpreted as the rate at which the "chaser" can update its trajectory or take action (kind of like the main loop running 10 times a second).

It's only "about" 10 hZ, on that order of magnitude. Indvidually you can have faster components and even though the process is pipelined, the last guidance correction that matters takes the sum total of all component times in the system. The seeker has to see the target is off-axis, then the computer has to calculate the proportional control input, and then the servos actuate the fins.

You also might be overestimating what "current tech" means in military hardware. The PAC-3 Patriots were entered into service from 1995-2000, meaning they were designed with late 1980's technology. The Saudis don't get to have the true top end hardware, the US won't sell their "best stuff" until they have "new best stuff" to beat it.

Whats our current "best stuff" in missile tech?

I think its the THAAD https://en.wikipedia.org/wiki/Terminal_High_Altitude_Area_De...

I wonder if the laser weapon systems will go anywhere in the end. Certainly seems easier to target, and cheaper to shoot. But there's still a lot of energy that needs to go into a missile to be certain it's disabled.

https://en.m.wikipedia.org/wiki/Laser_Weapon_System

An ICBM warhead is going to be a tough target for that. Very fast so you don't have long to heat it and fitted with heat shielding for reentry already.

It depends on what kind of missile you want to hit. A scud is very different from an ICBM, which are both quite different from anti-ship and some of the area denial weapons.

In perspective: The Russian Satan 2 is indefensible, and also the older SS-N-27 Sizzler shot from very nearby submarines. Not enough time to react on those.

There's really no such thing as an indefensible missile, just like there's no such thing as an impenetrable computer network. Difficult to defend against, those exist. Missiles with an extremely low probability that we can physically intercept, those exist. But defending against a missile starts as soon as a possibility of launch occurs, and you can bet that a lot of people spend a lot of time figuring out how to defend against "indefensible." They aren't perfect, but they aren't going to bat .000 either.

Well, Russians claim otherwise and Wikipedia confirms. It's of course highly contra-productive to tell such things the American public.

To add, more specifically to ICBMs, missiles also have counter measures. A common one in nuclear missiles is multiple and decoy warheads. So you really want to hit the missile, and before the warheads deploy. Which really sets a timelimit on when you can hit it. After the warheads deploy, they spread out pretty fast, making it much harder to hit with an explosion instead of kinetic force. Really you only have a few minutes to detect the launch, track, launch, and hit your target (generally the sooner the better). That's a real time crunch, which is one of the reasons they go STUPID fast.

Tldr: kinetic force is definitely the way to go.

Ballistic missiles are fast, but satellites are faster. In 2008 the US showed that an SM-3 missile can destroy a satellite [1]. this is the same type of missile that is deployed in the Aegis ABM defense system.

[1] https://en.m.wikipedia.org/wiki/RIM-161_Standard_Missile_3#A...

So I understand that it is quite hard to catch a missile when you fly in the same direction as the missle, but couldn't you just intercept the missile by flying towards it and park you own missle in the tradjectory of the incoming missile.

Why is that not possible?

That's exactly how Patriot missiles work. They just get in the way. The entire trick is, to be in the right place at precisely the right time.

It's still a very small target going very fast. That doesn't become easier to hit if you fly against it's direction of flight and thus have even higher relative speeds.

Here is a video of the awesome Sprint ABM missile - (0 to Mach 10 in 5 seconds) - it includes a near miss of an ICBM RV and the RV makes the Sprint look slow:

https://www.youtube.com/watch?v=msXtgTVMcuA

Edit: Sprint was meant to defend against ICBMs - but the idea is the same, ballistic missile warheads are going very fast and really aren't that big.

My dad worked on Sprint. That near miss would have been a definite hit since Sprint was armed with an EMP warhead, a small nuclear device with a few kilotons of yield. Dunno how it would have faired against a MIRV.

It was cancelled after the ABM treaty.

You may know this already, but Vernor Vinge did a neat short story where an aged Sprint missile (or something very similar) plays an important role: "The Whirligig of Time".

Vernor Vinge. Best Sci-fi story I ever read that made me feel the VASTNESS of space was written by him: A Fire Upon The Deep.

I'm not sure I'd give it the title of the Best scifi story, but it's definitely very good, so I'll at least second the recommendation.

I did qualify my "Best" recommendation.

Fair enough - I read too quickly. I think I may have subconsciously inserted an "and" in place of "that".

Might have to bring that kind of stuff back, the way N Korea is going.

Sprint was amazing as a rather highly refined brute force bit of engineering - 100g launches, etc.

You know how a rocket is sometimes described as a 'controlled explosion'? The concussion from a Sprint launch was severe enough that a large area around the launch pad would be lethal to any unprotected persons present during a launch. It basically explodes out of the silo.

I don't recall the exact answer (and it looks like others have already supplied info) but I saw someone explain it in detail once and wrap it up with a quote from the Star Trek reboot:

"It's like trying to hit a bullet, with a smaller bullet, while wearing a blindfold, riding a horse."

They're going very fast and even small errors in the estimate of their position and velocity will be enough to miss. Also, the motion is less predictable if they're in the atmosphere.

Sure if you try to catch a bullet with a bullet you're making your life harder. Anyway to increase the interception surface/volume ?

> Any way to increase the interception surface/volume ?

Yes - the usual way is to fit the interceptor with a nuclear warhead (see Sprint, Spartan and Nike Zeus on the US side - Fakel 51T6, Novator 53T6 and S-300PS/S-300PM on the Soviet side)... But it has sort of fallen out of fashion...

These weapons used enhanced radiation warhead (neutron bomb). The mechanism was use of neutron flux to disable incoming nuclear warhead.

Radiation hardening the warhead against this type of attack would make it less effective.

Wouldn't the neutron flux mess with the properties of the nuclear payload? Shielding against a neutron flux from nuclear explosion probably not practical given the sizes warheads.

Light atoms are used to slow down fast neutrons. Thermal neutron absorbers (large neutron capture cross section) like boron, lithium or cadmium is used for shielding.

Li6H is perfect for shielding warhead. Li6H (Lithium-6 hydride) contains hydrogen to slow down neutrons and lithium to absorb them. It has hightest hydrogen content of any hydride, high melting point and can be casted.

The shielding takes some space and the warhead yield is smaller than without shielding. The goal is not to completely block neutron flux but to reduce it to the level where exploding countermeasure must be very close.

If I recall correctly the case of the re-entry vehicle tends to be stuff like depleted uranium, which is pretty good protection against neutron flux up until the point where it begins significant breeder contributions. You'd basically have the hit the vehicle anyway to get close enough for that.

Uranium or lead, heavy nuclei that shield well against gamma radiation would do little for neutrons.

Neutrons needs something with light atom like hydrogen - water or maybe polyethylene and lots of it to slow down. So a small nuclear explosion which emits a neutron pulse might work pretty well. It would hellish if it somehow exploded over a population center, though. Didn't get hit by the ICBM (good). Everyone's bones melted (bad).

Not really, I mean, your opponent is the one controlling the size of the thing you're trying to intercept and they're not going to be trying to make things easier on you. The only viable option given that is to go for area saturation and basically play the odds, but that's really expensive because not only do you need lots of interceptors in that case, but each one needs to be capable of getting a kinetic kill since there's no guarantee which one (if any) will actually hit. At some point I imagine the interceptors become a bigger danger to each other than they are to the thing you're trying to intercept.

I see. Were there research into non destructive interception ? a missile catching up another one and binding to it to divert it ?

That would be even harder because now you need to match position and velocity instead of just position.

I understand but I'm still curious..

Explode the interceptor?

This doesn't work well for ballistic missiles. A few ounces of shrapnel doesn't knock the missile very far from it's trajectory. You'd have to get lucky to break the fuse, and even then you'd have a big hunk of explosives heading toward the target and making a big crater. Plus a lot of old missiles have such simple fusing systems that you probably won't break them with just a bit of shrapnel anyway.

As I understand it, the only serious "explode the interceptor" proposal was to use nuclear interceptors against ICBMs. That provides a modest radius of non-projectile destruction, and more importantly provides a large radius of fuse/control-disrupting radiation.

So when it comes to "shooting down a Scud near the ground over a city", you're exactly right. Blowing up your interceptor only works when you've got a high-tech target and are willing to make quite a mess yourself to stop the thing.

Well detonating nuclear warheads (literally) near some important asset of yours is probably always some kind of last ditch effort.

I did a tour of the Nike missile site in the Marin Headlands / Golden Gate Park once.

They were very careful to "neither confirm nor deny" whether the site ever had nuclear payloads, but they _did_ make a point to tell everyone that the beautiful mechanical targeting computers had Sacramento programmed in as a target, "just in case"...

Not necessarily, it would be going off pretty high. Much higher than in this video with people directly underneath.

https://www.youtube.com/watch?v=BlE1BdOAfVc

Yeah, probably not the sort of thing you go and do to stop a 5-kiloton conventional warhead.

having something deployed near encounter ... damaging flare/shrapnel, a flock of timed explosive ...

Years ago, in my high school Phsyics C class, the teacher gave us an exam which was just the process of calculating how to intercept an (IC)BM. It was a really great exam, leading you through all the steps required, and gave the answer really clearly.

It's incredibly hard. I won't repeat sibling comments about how difficult the actual interception is, but will add that you likely have seconds AT BEST for your own launch to take place. You can do guidance mid-air, but miss the window...

As with any X vs Y weapon, the scenario matters a whole lot. If you know where the missile is going and your interceptors are launched from that area you have plenty of time. This is the situation the USN is looking at with the new Chinese anti-ship ballistic missiles.

On the other hand, a system to protect the continental United States from incoming missiles from anywhere to anywhere was always fanciful.

There is always cone on uncertainty around the missile position and speed and interceptor position and speed.

Both the missile and interceptor must meet at the same place at the same time. The time window decreases with speed, so timing is everything.

Faster the incoming missile, harder it is to intercept. If the speed delta between incoming missile and interceptor is to high, interceptor can't correct it's course fast enough to hit the missile. Interceptor has inertia and it's deflectors or thrust vectoring can correct it's course only so fast.

Interceptor positioning on the ground is also important.

Atleast with an ICBM you are talking about velocities that are still close to suborbital, ie over a kilometer per second, Quora sets the terminal phase velocity at 30'000 kph (around Mach 30, pi by thumb, of by around 20%)

Hitting something that is moving at Mach 30 towards you is already hard. Especially considering this thing moving towards you is also burning up in the atmosphere, thus you only have that bright spot which will be larger than the actual missile core. However, if it's moving exactly (or almost) at you, it's not impossible but still hard.

If it is not moving towards you but rather along your view, you have to match a significant portion of it's speed, which basically means building a ICBM to hit another ICBM. And going that fast takes a while.

If it's moving away from you, you can always give up.

It's the velocity that makes things tricky. Imagine trying to hit a bullet with a bullet, you may know where it's being fired from and where it's headed, how fast it's going, the wind speed all other variables but hitting it would still be an impossible task.

One way to improve missile defense accuracy would be to use a small nuclear warhead but that's impractical when defending populated areas and anyway your adversary can make new missiles more quickly an cheaply than you can make nuclear bombs.

Modern missiles make things more complicated by using multiple re-entry vehicles, midflight maneuvering, creating a cloud around the warhead to prevent targetting etc.

Why is it so hard to throw a bullseye in a game of darts? You know the weight of the dart and the location of the target. Why don't we hit it every time?

Even if interception is conceptually simple, the speeds and distances involved make it a very challenging problem.

Additionally, you're starting from a false premise that ballistic missiles don't maneuver. Some of them, particularly MIRV ICBMs, are highly maneuverable.

Ballistic missiles move very fast. Repost: http://www-users.math.umn.edu/~arnold/disasters/patriot.html

Some ballistic missiles deploy counter measures. So it can become difficult to know what object is dangerous or not.

Decoy warheads, waste material to confuse tracking systems, etc. For every offense there is a defense.

It was described as trying to hit a bullet with another bullet, but in this case both of them are super fast ones. Complications I can think of:

1) ever seen a missile being fired and flowing? It isn't a smooth movement, rather wobbling in +- target direction. I presume this is due to turbulence in the air and resulting corrections by guiding software. Double the issue due to target and pursuer facing random turbulence on their own

2) some SCUDs have some sort of simple 'randomizer' in their flight for exactly these reasons - to confuse simpler defense (ie boat-based gatlink types which fires dumb explosive projectiles) - the target is still in aim, but there are random movements around axis of movement

3) sensors are/were not perfect, non-ideal conditions (clouds, night, cold, humidity, dust), bugs in software etc