[info] [FoRK] Anti-munitions munitions

[Eugen Leitl]

----- Forwarded message from damien morton <dmorton at bitfurnace.com> -----

From: damien morton <dmorton at bitfurnace.com>
Date: Sat, 22 Nov 2008 13:13:33 +1100
To: Friends of Rohit Khare <fork at xent.com>
Subject: Re: [FoRK] Anti-munitions munitions
Reply-To: Friends of Rohit Khare <fork at xent.com>

On Sat, Nov 22, 2008 at 11:09 AM, Kevin Elliott <k-elliott at wiu.edu> wrote:
>
> On Nov 20, 2008, at 4:42 PM, damien morton wrote:
>
>> On Fri, Nov 21, 2008 at 10:46 AM, J. Andrew Rogers
>> <andrew at ceruleansystems.com> wrote:
>>>
>>> First, there is the problem of reducing collateral damage.  A massive
>>> spray
>>> of high-velocity metal slugs works swell out in the desert but it tends
>>> to
>>> raise issues if you use it in a city. Consequently, there is a strong
>>> political bias toward "one shot, one kill" defensive systems to reduce
>>> the
>>> number of incidental casualties caused by a defensive action.
>>
>> In both the Redback system and the missile system, a blast enhanced
>> warhead is used as the kill mechanism - not fragmentation. For exactly
>> the reason you state. It has nothing to do with missile vs gun.
>
> Your misunderstanding the problem.  If you use a gun it must be fast firing
> with a high power round.  You need to spray a lot of lead (so you saturate
> an area).
> You need to use a reasonably powerful round both for penetration and flight
> time to
> the missile.
>
> Your going to miss.  And the bullets that miss are going to keep going.  And
> Going.
> Lethal range of 7.62MM NATO is nearly a mile.

Thanks, but I dont think I am misunderstanding the problem.

A gun doesnt necessarily have to be fast firing, or with a high
powered round. If you look at the video Stephen posted, the point of
interception happens just a few meters before impact. The interceptor
itself isnt moving very fast at all, certainly subsonic, and maybe
only of the order of 100 m/s. At a range of a few meters or 10s of
meters, a gun can be a whole lot more accurate, throwing a much
heavier projectile at a much lower velocity, than it can when trying
to intercept incoming targets thousands of meters out. In fact, the
missile in Stephen's video is being launched out of a gun, only
vertically and with a very low velocity, before it turns itself around
and directs itself at the target.

You dont necessarily need to spray a lot of lead around - the Israeli
system uses a concussion warhead to destroy the incoming missile/rpg.
After all, those HEAT warheads are basically a hollow copper cone. One
way to disable it is to crush it from overpressure alone - you wouldnt
need to deform it much to drastically decrease its effectiveness.
Again, if you use a gun, your interceptor can be comprised of
explosive and fuse alone, reducing the fragmentation compared to a
missile with motor, casing, sensors, guidance, fuse and warhead.

> Do the math.

Ok then.

RPG muzzle velocty 112 m/s
Turret slew rate 700 degrees/sec
Launch distance, say, 100 m
40mm grenade warhead, normal fragmentation kill radius 5m, but lets
say we need 0.5 m to kill an RPG or anti-tank missile with blast alone
(we will do this for 0.1m as well, just to be sure)
40mm grenade muzzle velocity 240 m/s (high velocity), 75 m/s (low velocity)
[different formulae I found show an overpressure of 1000+psi at 1 foot
for 0.4lb charge of TNT - this should be enough to destroy a missile
warhead]

Lets say that we can pick up and recognise an RPG launch within 0.1
seconds. (we can probably be slewing towards a possible launch before
final confirmation), RPG is 90m from impact
Lets say we have to slew 180 degrees (this is the worst case), 0.25
seconds, RPG is 60m from impact
If we fire low-velocity interceptors (75m/s) they will intercept at
25m from impact
If we fire high-velocity interceptors (240 m/s) they will intercept at
40m from impact

At 25m, a 0.5m kill radius implies a necessary accuracy of 70 minutes-of-arc
At 25m, a 0.1m kill radius implies a necessary accuracy of 14 minutes-of-arc
At 40m, a 0.5m kill radius implies a necessary accuracy of 43 minutes-of-arc
At 40m, a 0.1m kill radius implies a necessary accuracy of 9 minutes-of-arc

In rifle shooting, getting under 1 minute-of-arc is considered highly
accurate. The interception distances shown above are also much further
out than the minimum 5m, at which even less accuracy is required.

At 5m, a 0.5m kill radius implies a necessary accuracy of 350 minutes-of-arc
At 5m, a 0.1m kill radius implies a necessary accuracy of 70 minutes-of-arc

For a 5m interception, with 180 degree slew required:
    with low-velocity interceptors, minimum launch distance is 52m
    with high-velocity interceptors, minimum launch distance is 47m

For a 5m interception, with 90 degree slew required:
    with low-velocity interceptors, minimum launch distance is 40m
    with high-velocity interceptors, minimum launch distance is 35m

For a 5m interception, with 45 degree slew required:
    with low-velocity interceptors, minimum launch distance is 34m
    with high-velocity interceptors, minimum launch distance is 29m

As you can see, the velocity of the interceptor is not as important as
the time to slew.

You could decrease the amount of slew required by having a turret able
to fire both backwards and forwards - cant see why the Redback system
couldnt have tubes that can do that. This would cut the worst case
slew to 90 degrees, and cut 12m off the minimum launch distance.
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