fragmentation effects

Discussion in 'Aviation' started by delcyros, Oct 28, 2007.

  1. delcyros

    delcyros Well-Known Member

    Mar 2, 2005
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    Berlin (Kreuzberg)
    I have searched a while for how projectiles do inflict damage and so far have come over different solutions for this. Heavier projectiles are thoroughly analysed by Nathan Okun in this regard. A cannon shell hitting an airframe will cause three effects:

    A) kinetic impact energy
    b) blast effects
    C) fragmentation

    The kinetic impact energy (A) depends on impact velocity and weight of the projectile and the obliquity. It is dealed with elsewhere and is critical when considering whether or not a projectile defeats armour, bulkheads or reeinforced skins.

    Once the fuze is set (usually instantious fuzes), the chemical effective charge will blew up the projectile causing primary blast effects(B) and secondary fragmentation (C). The projectile usuallly looses up to 33% of it´s weight for sideways fragmentation while the rest is attributed to larger body pieces (nose and base).

    The amount of airframe skin a cannon projectile can penetrate by sideways-blasted fragments from the middle body (not counting the heavy nose pieces or base plug) depends somewhat on the filler used and a lot on the percentage weight of explosive.
    Okun uses as approximation an "average" HE (circa TNT in power, 1.0). For our purposes it is probably better to adjust for the individual type of filler, which may range in between 0.9 to 1.1.
    The penetration ability for these main body sideways directed fragmentation can be extrapolated on his misc. armour penetration formula, hence it is possible to say that a normal 117gramms MG 151/20 API projectile with 3.1% filler (3.63 gramms) exploding 5 cal. next to a frame will produce fragmentation which will defeat up to 0.095 cal. STS-grade quality plates (full armour grade, but with ~10% less stopping power than the up to 1" US and german normal airplane armourplating), which has to be verified via the charge type. Thus, such a projectile may not pierce a 2 mm STS plate. With other words, it will defeat -roughly- 3mm dural with it´s fragments, only! If the skin of the plane is 3mm dural (which by all means is very thick, indeed!) the sideways fragmentation will cause a hole of approx. 7.8" diamter on it -if the projectile had no fuze delay. This linearly ramps up to 0.113 caliber of STS for the base-fuzed API shell in contact with the shell's middle side (0 cal distance).
    To make things more complicated, there also were HE and even mine rounds in use for 2030mm guns. To understand how they work, one must have to keep in mind that these mine rounds do produce MASSIVELY more fragmentation but also much lighter and smaller fragments, which will soak up the additional energy rapidly with distance. Nevertheless, the initial force by those sideways moving fragments is greatly improved compared to normal API projectiles. A usual 8% 20mm HE round (Hispano 20mm MK V with 10.4 gramms HE charge) will defeat about 0.11 cal. = 3.67mm dural. This linearly ramps up to 0.156 caliber of STS for the nose-fuzed HE shell in contact with the shell's middle side (0 cal. distance).
    A mine round has 22% filler charge (20.24 gramms), close to 5.6 times as much as the API round. At 5 cal. distance to the point of explosion, it will still defeat 0.135 cal. thickness (STS) = 4.5mm dural (typical fireproof bulkhead strength). This linearly ramps up to 0.185 caliber of STS for the nose-fuzed mine shells in contact with the projectile's middle side (0 cal. distance).

    The worst case is a 30mm mine round with body fragmentation to cause a 12" hole in a 6.75mm (0.267") dural skin (or- at 0 distance, it´s body fragments have reasonable chance to defeat a 5mm thick high grade armour plating) -Now imagine the plane does only has fabric or wood coverage!

    Going away from the target from the 5 caliber distance starting point, the penetration ability drops off with distance, as Okun showed. The heavier fragments of the API round having the least loss in penetration before they simply fall down to the ground and the smallest fragments of the mine rounds receiving most of the loss.

    Still, a projectile may be able to cause considerable fragmentation effects against the skin of an airframe (take notice of THIN SKINNED planes like the Zero) at up to 1000 cal. distance to the point of explosion altough probabilities are to low to be taken into account seriously. With respect to the concial spread of fragments, one could be able now to work out how large in diameter a typical hole by x projectile in an y-thicked skin would expected to be at z distance.
    I tried this with photographic evidence (on nose fuzed mine rounds) and it worked quite well in within 10%+- of error but would like to see more (photos comments) for comparison reasons.

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