So hitting them should be an easy task for a guided missle as long as it got enough time to reach it at all (which I think is not the problem here). So the only problem I can see is to disarm the missle during flight but am not sure if that is what is meant here.
Does anybody know why it should be hard to shoot down a ballistic missle?
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.
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.
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).
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.
Tldr: kinetic force is definitely the way to go.
[1] https://en.m.wikipedia.org/wiki/RIM-161_Standard_Missile_3#A...
Why is that not possible?
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.
It was cancelled after the ABM treaty.
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.
"It's like trying to hit a bullet, with a smaller bullet, while wearing a blindfold, riding a horse."
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...
Radiation hardening the warhead against this type of attack would make it less effective.
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.
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).
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.
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"...
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...
On the other hand, a system to protect the continental United States from incoming missiles from anywhere to anywhere was always fanciful.
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.
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.
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.
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.
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