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  1. #1
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    20x129 Oerlikon HEI and SAPHEI

    Cutaway models of two cartridges for the Oerlikon 10ILa / 5TG anti aircraft gun, later renamed GAI-B01. The cartridge is also used in the 20 mm MEROKA (MEhr ROhr KAnone), a gun existing of 12 barrels (2 rows of 6 below another), as used by the Spanish navy, although it must be noted that the MEROKA normally uses APDS projectiles.
    Both these catridges are of Swiss manufacturing (Oerlikon). The cartridge is commonly known as the Oerlikon 20x129. This type of cartridge however has been largely overshadowed by the 20x139 series cartridges.
    The top cartridge is of the SAPHEI type (Semi Armour Piercing High Explosive Incendiary) and has the Swiss desination PSA.
    The projectile exists of a nose hardened steel body with a base fuze. This base fuze has a mechanical self destruct device.
    Functioning of the fuze:
    Two round weights (2) are connected to the lower side of the firing pin (1) by means of a radial rotation pin. Upon firing the weights swing outward. The firing pin is spring (3) pre-tensioned to move the firing pin into the detonator (8), however three pins (4) -at 120 degrees each- in way of the flanged part of the firing pin keep the firing pin in the lower position. The green (5) ring keeps the three pins in inward position. The green ring is part of a assembly, existing of two rings (green and blue) with an thin brass crush ring inbetween. Upon firing, The blue (6) ring moves down, crushing the brass intermediate piece, thus making the distance to the green ring smaller. The three pins have a 90 degrees point that now –being thrown outward- start to lift up the green ring by it’s inner chamfered edge, releasing the firing pin. The firing pin however is held in it’s lower position by the two outward thrown weights in the lower side of the firing pin which still overcome the force of the spring that wants to move the firing pin upward into the detonator. Upon impact, the firing pin is swung forward into the detonator, pulling the weights inward. If the target is missed and rotation decreaces the spring will finally overcome the centrifugal weights generated force of the weights in the groove, pulling the weights inward, causing self destruct of the projectile.
    A second safety is formed by a radialy spring loaded pin (7), placed through the hole the firing pin has to pass through into the detonator. Only after firing this pin is swung outward , enabeling the firing pin to move into the detonator.
    The lower cartridge is of the HEI type (High Explosive Incendiary) , and has the Swiss designation SBA (SLA with tracer).
    Functioning of the fuze:
    Two round weights are connected to the upper side of the firing pin by means of a radial rotation pin. Upon firing the weights swing outward. The firing pin is spring pre-tensioned to move the firing pin downward into the detonator, however three pins (at 120 degrees each) in way of the flanged part of the firing pin keep the firing pin in the lower position. The pins are placed in a 30 degree angle downward to the centerline of the projectile. These pins are held inward by a ring surroundig the three pins. The ring is held in upper position by means of a circular brass leaf spring with four inward lips blocking the ring . Upon firing the ring moves downward, bending away the four lips, releasing the three pins. However the 30 degree downward angle cause the pins to be thrown outward after acceleration decreased and the velocity is constant, only than releasing the firing pin. The firing pin however is held in it’s upper position by the two outward thrown weights in the upper part of the firing pin which still overcome the force of the spring that wants to move the firing pin downward into the detonator. Upon impact, the firing pin is hammered downward into the detonator, pulling the weights inward. If the target is missed and rotation decreaces the spring will finally overcome the centrifugal weights generated force of the weights in the groove, pulling the weights inward, causing the self destruct of the projectile.
    A second safety is formed by a radialy spring loaded pin, placed through the hole the firing pin has to pass through into the detonator. Only after firing this pin is swung outward , enabeling the firing pin to move into the detonator.
    More info about these shells can be found on this very nice Spanish site:
    http://www.municion.org/20mm/20x129.htm
    Regards, DJH
    Attached Images Attached Images
    Last edited by pzgr40; 17th November 2008 at 09:52 AM.

  2. #2
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    Interesting information pzgr40

  3. #3
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    In the documentation sent by Oerlikon to Spain when this caliber was adopted (in the 1960s), the designation of these sectioned rounds was as follows:

    APHEI type: PSBH/B (Panzer-Sprengbrandgranate Hochexplosive/Bodenzünder) (Base fuze)

    HEI type: SSB/K (Spitz-Sprengbrandgranate/Kopfzünder) (Ogive fuze).

    The types originally adopted by the Spanish army were:

    - HEI: MSB/K (Minen-Sprengbrandgranate/Kopfzünder).

    Spanish designation: PAE. INCD. (Proyectil Alto Explosivo Incendiario).

    Identification: Brown body, yellow band over rotating band, red band over yellow band.

    This Mine projectile had an increased explosive capacity over the normal HEI (18 grams of Hexal versus 12,2 grams).

    - APHEI: PSBH/B (Panzer-Sprengbrandgranate Hochexplosive/Bodenzünder).

    Spanish designation: PPAE. INCD. (Proyectil Perforante Alto Explosivo Incendiario).

    Identification: Brown body, yellow band over rotating band, light blue ogive.

    - TP-T: SUL (Spitz-Uebungsgeschoss Leuchtspur).

    Spanish designation: PT. INS. (Proyectil Trazador Instrucción).

    Identification: Brown body, white band below ogive, black ogive.

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  5. #4
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    Thanks for the info. I was told the Meroka used APDS cartridges for missile defence.
    Regards, DJH

  6. #5
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    Impressive work as always, thanks for the info

    the MEROKA system is weird, surprising design, i didn't know it

    regards

 

 

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