Shaped Charge Warhead Basics

The shaped charge warhead of an antitank projectile is based on the technique of directional control of the energy generated during a high explosive detonation, Instead of allowing the gases to expand in a radial direction, they are focused into a narrow jet. This may be compared to a light being focused by a conical reflector into a sharp beam,

In a shaped charge warhead the fuze fires a primer which initiates the detonator which in turn sets off the main charge. The detonation wave of the formed charge reaches the apex of the liner producing very high pressure, causing the liner wall to collapse. The Inner portion of the liner cone forms a molten jet travelling at a high speed along the charge axis. The velocity of this gas-metal jet approaches the burning rate of the high explosive used (i.e. 5000-8000 m/sec). Such a jet may penetrate armor to a depth twice that of the liner diameter, The jet-metal performance inside the target Is further accentuated by the fragments of the armor from the penetration hole.

For the best performance the liner is brazed to the warhead body around its entire circumference, The joint is then carefully inspected and tested under hydrostatic pressure and very rigid controls. After the liner-warhead assembly passes the Inspection, the standoff shield and the conductive hood (if the piezo fuze is in the projectile nose) Is crimped and/or brazed to the warhead, After a final waterproofing with a silicone-llke adhesive the warhead Is ready for filling with the explosive charge.

The explosives used In the warhead are high energy materials, The final choice is dictated by the availability of such an explosive, the method of filling and forming the charge and the urgency to transfer the weapon into the end user's hands.

The 2.36 inch warhead (HEAT - M6) held approximately 450 grams of Pentollte, a mixture of PETN and TNT in 50/50 proportion, The 3.5 Inch warhead (HEAT -M28A2) contained 850 grams of Composition B. The Panzerfaust 150 warhead was charged with approximately 900 grams of mixture of RDX and TNT In 50/50 proportion. Both, the PETN (Pentaerythritol tetranitrate) and RDX (Trimethylentrinitraamln) are excellent high explosives - powerful, relatively safe

Pentaerythritol Tetranitrate

2.36inch heat rocket m6a5

warhead

Shaped Charge Proportion

3.5 inch heat m28a2 rocket[

warhead!

in handling and storage, stable, and produced from readily available raw materials. Their one main drawback is in their physical form, They are fine powders and are not castable.

During filling and loading operations the PETN and RDX are usually blended with a small percentage of a binder, then compacted into small pellets to prevent dusting. Only then are these explosives deposited into the projectile (or mine) and compressed in a hydraulic press. Shaped charge warheads with thin and light walls are not readily suited for such compression of explosive filler. Thus PETN and RDX are added in loose powder form to molten TNT and blended into a thin porridge-like slurry, Such liquid is then poured into the warhead body and solidified by cooling.

This operation is a very closely controlled one. The metal parts of the warhead are first preheated to about five degrees Centigrade higher than the temperature of the poured explosive.

While the explosive is being poured, the warheads are vibrated to such a degree that a slight movement of the molten filler may be observed. The amount of the poured explosive exceeds slightly the required quantity for each charge. This, like in casting metals, eliminates the voids during shrinkage at cooling and assures a full and uniform warhead charge.

The overall performance of a shaped charge warhead depends on the following factors:

1 - Type of high explosive used in the main charge - the more powerful the better.

2 -Standoff distance - the distance between the base of the liner and the target surface, The target penetration increases with the increase of the standoff distance until the standoff equals about 3 times the liner diameter.

Any further increase in standoff will decrease the penetration.

3 - Liner diameter - an armor penetration is about 1 V2 times the liner diameter.

4 - Liner cavity form - a hollow cone of 42° produces optimum performance.

Other forms tried were ellipsoidal, paraboloid, etc.

5 - Liner material: of all materials tried, the best was copper, then steel; alumi num was not very effective.

6 - Liner walls : tapering of the liner thickness improved the performance.

7 - Projectile rotation: high speed rotation (such as artillery shells) markedly decreases the shaped charge effect.

If the shaped charge and the target were stationary (like a cratering charge) the selection of the best condition would be relatively simple, However, the target and the projectile are mostly moving during the impact. The standoff distance then becomes a variable, depending on the impact angle of the warhead and the strength of the standoff shield, Furthermore, the projectile continues moving towards the target after the impact and before the detonator sets off the charge. In the early models of the Bazooka, Panzerfaust and the PG-2. the detonator was fired mechanically by impact of a striker against a percussion primer. This Induced a long delay,

Present day warheads use mostly piezo-electric fuzes and electric bridge-wire detonators, This assures a practically Instant detonator action.

Typical high explosive mixes used in the shaped charge warheads are:

composition (by weight) Pentolite PETN 50% TNT 50%

This material has a density of: ±1.0 gram/ml (granular form) ±1.65 gram/ml ( in cast form)

Detonation rate: 5500 m/sec (granulated)

Composition B RDX 60 % TNT 40 %

Octol HMX 75%

Pentolite, grade I, specification JAN-P-408 was used in the 2,36 inch M6A3 rocket. Although powerful, it is not as effective as the Composition B or Octol. Furthermore, in wet stage it is slightly corrosive to Copper, Zinc and brass. The composition B with a detonation velocity of 7800 rn/sec is used in a wide variety of explosive ammunition, It is powerful, stable, readily available and relatively easy to melt load. Octol, used in the M72 rockets has a detonation velocity of 8400 m/sec, Warheads with Octol filler have penetration and damage effect about 20% higher than the same warheads filled with composition B. Octol also allows a shorter standoff of shaped charges. However, at present the cost of Octol Is about 3 times that of composition B, Both explosives are usually melt loaded at 90 ±3°C,

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