Newsgroups: sci.military.moderated
From: "Paul F Austin" <paustin@digital.net>
Subject: Re: Can THAAD hit an aircraft?
Date: Tue, 6 Jul 1999 15:21:09 GMT

Larry Elmore wrote in message ...
>Matthew Saroff wrote in
>>
>> "Frank" wrote:
>>
>> >After all don't most SAMs use the 'shotgun' technique to get their
>target,
>> >where 'close' if good enough because the warhead is designed to fill a
>> >volume of air with shrapnel? If there any advantage in scoring a direct
>hit
>> >on an ordinary military aircraft as opposed to just shredding it with
>> >fragments?
>>
>> The Patriot Advanced Capability 3 (PAC-3) does that
>> already.  It is a smaller and slower missile than THAAD, and is
>> so more limited in intercepting tactical ballistic missiles, but
>> it has been tested as a conventional SAM, and it is hit to kill.
>> The Rapier has been hit to kill for years.
>
>The advantage for such missiles (or 'hittiles') is that a much smaller
>warhead is needed to destroy the target and therefore a smaller missile can
>be used to carry that warhead.


ABMs are hit-to-kill for an entirely different reason. Lethality at 10Km a
second impact velocity is the least of your problems.

Because the closing velocities can be a substantial fraction or even exceed
the speed of propagation of chemical explosives, a chemical fragmentation
warhead (no blast in a vacuum) is tough to make work. The fuzing has to time
the warhead initiation at just the right time to get the fragments out to
where the target is passing by when the target may be moving faster than the
fragments (relative to the interceptor).

In fact, some KE kill vehicles have "lethality enhancement devices" that
amount to a net with little tungsten balls located at the intersections of
the strands. The kill vehicle deploys the net as it nears the target so that
a near miss can be a kill.

The KE kill vehicles manuever by diverting sideways until the angle rates
with the target are zero. The kill vehicle doesn't rotate about the pitch or
yaw axis to manuever but instead moves in X- and Y-direction until the kill
vehicle velocity vector and that of the target are co-linear. The divert
motors won't work well in a dense atmosphere so even if the THAAD seeker
could detect an aircraft, the manuevering limits of the kill vehicle would
be very small.

Building an endo-atmospheric IR guided interceptor with Mach-4+ speed is
very tough because the window that the seeker looks out of gets heated by
the stagnation layer around the nose of the missile. That's why the SM-2
Block IV seeker has the IR window in the side of the airframe rather than in
the nose.


Conscience, that quiet voice that says "Someone may be watching"

Paul F Austin
paustin@digital.net

Newsgroups: sci.military.moderated
From: "Paul F Austin" <paustin@digital.net>
Subject: Re: Can THAAD hit an aircraft?
Date: Tue, 6 Jul 1999 15:21:06 GMT

Bertil Jonell wrote in message ...
>Paul J. Adam wrote:
>>Bertil Jonell writes
>>>  THAAD uses IR homing for the last part, it 'sees' the heat of the
>>>incomming warheads against the cold background of space. Thus it needs
>>>to be so high up that there isn't too much (warm, compared to space)
>>>air above it. So unless the aircraft is on the border between air
>>>and low orbit, THAAD won't see it.
>>
>>Don't buy it - there are _lots_ of imaging IR seekers around today, on
>>everything from Mistral updates to Standard SM-2 Block IIIA. IR-homing
>>missiles were guiding on aircraft skin friction back in 1982.
>
>  Possibly different IR bands: I doubt that a warhead would be above
>20 degrees C.

That's not the problem. A hotter black body will radiate more than a colder
one of equal area throughout the spectrum.

There are two problems that I discussed in an earlier post that disappeared
into the bit bucket. The first is window heating that Bertil refers to
below. Stagnation layer heating of any optical window in the nose of a Mach
4+ missile degrades IR seeker performance a lot. That's why SM-2's IR Seeker
window is in the airframe side. That puts it outside the stagnation layer.
There are some window cooling techniques but that adds complications to the
missile design.

The other and main reason THAAD isn't a dual role ABM/SAM is the way the
kill vehicle manuevers. Exo-KE kill vehicles manuever by diverting laterally
in X- and Y- directions until the interceptor velocity vector coincides with
that of the target. The kill vehicle doesn't use aero-manuevering at all so
it's limited by the total impulse of the divert motors which are small. The
booster delivers the KV into a fairly small basket along the line of flight
of the target. Because the total impulse is small and aero forces in a SAM
engagement are high, the ability to manuever is going to be small.
>
>  Another complication is that the seeker window of the THAAD is
.... generates skin friction around the
>kill vehicle that would overwhelm fainter signals.
>
>>Maybe the THAAD weapon logic looks for a minimum velocity as a
>>decoy-rejection measure,
>
>  Since it is slower than an incomming warhead it must intercept
>from the front, so the apparent velocities of the warheads will be
>low and pretty independent of their real velocities. The article
>claimed that the decoy-rejection was done by the radar and not by
>the kill vehicle.
>

In theory, an ABM can be any velocity at all, the difference being that the
higher the velocity, the farther out the engagement occurs and the more
shots you have at hitting. In practice, the optimum interceptor velocity is
about equal to the velocity of the target missile.

Bertil is right that all ABM intercepts will be front-hemisphere ones since
we're unlikely to produce an interceptor that can catch a ballistic missile
in a tail chase. The "apparent velocities" that Beril refers to are angle
rates. The closing velocities are very high, they can in fact be higher than
the propagation velocity of chemical explosives. That's the reason that ABM
kill vehicles are "hit-to-kill". Fuzing of a fragmentation warhead whose
fragments are slower than the target is too hard to attempt. Instead, some
Kinetic Energy kill vehicles have "Kill Enhancement Devices" that spread a
net loaded with heavy beads so that the kill vehicle has a larger effective
"diameter" for impact.

Conscience, that quiet voice that says "Someone may be watching"

Paul F Austin
paustin@digital.net