Cutaway model of a FMU-7A/B all-ways impact fuze with an AN M23A1 white phosphorous igniter.
This combination was used to ignite the M77 series Napalm bombs.
The AN M23A1 white phosphorous housing with the FMU-7A/B fuze is screwed into a flange in the nose and the tail cone of the sheet aluminium bomb housing (see pict 04).
The complete fuze set consists of the Pull wire-firing and pin activated battery, the FMU-7A/B all ways fuze , the electric cable(s) connecting both, and the AN M23A1 White Phosphorous igniter bushing.
Description and functioning of the Pull wire-firing and pin activated battery (Pict.01);
The assembly consists of a machined aluminium body (01), with a hexagon head in top (02), used to tighten the assembly in place in the napalm tank. In the base, a Bakelite insulation plug (03) is screwed, housing the brass electric connector plug (04). The battery (05) is placed above the Bakelite insulation plug.
The spring loaded firing pin (06) is fixated by a steel ball (07), which is kept in inward position by a small steel pin (08) with a chamfered lower side. This pin is kept in fixated position by an aluminium disc (09) , connected to a steel wire (10). The disc is fixated by three indentations at 120 degrees each around the disc in top of the battery housing, so force has to be applied to pull the disc out. The steel wire on the aluminium disc is connected to the bomb rack of the airplane with a ring at the other end of the wire.
Upon release of the napalm tank, the aluminium disc is pulled out, allowing the ball to move outward, pushing the pin upward, releasing the firing pin. The firing pin hits the firing cap (11) in top of the battery, igniting a small charge that activates the battery to deliver an electric current. This current is lead to the FMU-7A/B fuze with an electric cable through the napalm tank.
The Fuze FMU-7A/B consists of a machined Zamac fuze housing. The body (11) is Hexagonal and threaded on the outside on both top and bottom. At the base it receives the middle piece (12) that houses the all-ways fuze mechanism. To the base of this middle part the booster (13) is screwed, containing 12 grams of teryl (yellow).
Both the base of the fuze housing and the lower part of the hole in the middle piece are machined conical.
To the side of the hexagonal housing an electric connector (14) is placed to receive the electric signal of the battery, used to activate the arming mechanism of the fuze.
If the Napalm tank is released from the bomb rack , the battery is activated. The battery ignites the explosive bellow (15) , which expands, pushing a piston (16) downward. This piston locks up a horizontally placed camlock pin (17, drawing yellow pin) that –as soon as the piston has released the pin to rotate- is rotated by the spring loaded Arming pin (18) that moves upward, allowing the two balls (19) to move inward. The fuze is now fully armed.
After the arming pin (18) is retracted, both the firing pin housing (20) and the firing cap housing (21) are kept apart by a weak spring (22).
Upon impact of the napalm canister with the ground, the firing pin in the firing pin housing (20) will be driven into the firing cap (red) in the firing cap housing (21), indifferent of the position in which the napalm bomb will hit the ground. If head or tail on, the firing cap is driven into the firing cap or visa versa, when sideways, both the firing pin housing and firing cap housing will climb the conical slopes, forced so by inertia, forcing the firing pin into the firing cap. The firing cap ignites the detonator (23), which detonates the Tetryl booster. This will ignite the 0,567 kg charge of white phosphorous, that ignites the Napalm from the ruptured container.
The AN M23A1 igniter is a sheet steel pressed housing (23) to which a pressed sheet steel fuze well (24) is brassed. In the base a threaded filler hole is brassed, closed by a plug screw (25). The lower part of the housing is threaded (26) so it can be screwed into the front and back flange of the BLU-27 Napalm fire bomb.
Maximum diameter : 100mm over the thread
Height AN M23A1 housing : 96mm
Total height with fuze : 131mm
Length of FMU-7A/B fuze : 123 mm
Pict.01:
Length of firing pin activated battery (incl. connector pin): 101mm
Key width hexagon head : 45mm
The mechanical version of this fuze is also described in this forum with the cutaway chapter under the name “Napalm bomb fuzing, Fuze AN M173A1 with AN M23A1 white phosporous igniter”
When searching for the term “napalm bomb Vietnam” on you tube one can clearly see the ignition of the phosphorous ignited by the white smoke stripes shooting away from the impact point.
Regards, DJH.
This combination was used to ignite the M77 series Napalm bombs.
The AN M23A1 white phosphorous housing with the FMU-7A/B fuze is screwed into a flange in the nose and the tail cone of the sheet aluminium bomb housing (see pict 04).
The complete fuze set consists of the Pull wire-firing and pin activated battery, the FMU-7A/B all ways fuze , the electric cable(s) connecting both, and the AN M23A1 White Phosphorous igniter bushing.
Description and functioning of the Pull wire-firing and pin activated battery (Pict.01);
The assembly consists of a machined aluminium body (01), with a hexagon head in top (02), used to tighten the assembly in place in the napalm tank. In the base, a Bakelite insulation plug (03) is screwed, housing the brass electric connector plug (04). The battery (05) is placed above the Bakelite insulation plug.
The spring loaded firing pin (06) is fixated by a steel ball (07), which is kept in inward position by a small steel pin (08) with a chamfered lower side. This pin is kept in fixated position by an aluminium disc (09) , connected to a steel wire (10). The disc is fixated by three indentations at 120 degrees each around the disc in top of the battery housing, so force has to be applied to pull the disc out. The steel wire on the aluminium disc is connected to the bomb rack of the airplane with a ring at the other end of the wire.
Upon release of the napalm tank, the aluminium disc is pulled out, allowing the ball to move outward, pushing the pin upward, releasing the firing pin. The firing pin hits the firing cap (11) in top of the battery, igniting a small charge that activates the battery to deliver an electric current. This current is lead to the FMU-7A/B fuze with an electric cable through the napalm tank.
The Fuze FMU-7A/B consists of a machined Zamac fuze housing. The body (11) is Hexagonal and threaded on the outside on both top and bottom. At the base it receives the middle piece (12) that houses the all-ways fuze mechanism. To the base of this middle part the booster (13) is screwed, containing 12 grams of teryl (yellow).
Both the base of the fuze housing and the lower part of the hole in the middle piece are machined conical.
To the side of the hexagonal housing an electric connector (14) is placed to receive the electric signal of the battery, used to activate the arming mechanism of the fuze.
If the Napalm tank is released from the bomb rack , the battery is activated. The battery ignites the explosive bellow (15) , which expands, pushing a piston (16) downward. This piston locks up a horizontally placed camlock pin (17, drawing yellow pin) that –as soon as the piston has released the pin to rotate- is rotated by the spring loaded Arming pin (18) that moves upward, allowing the two balls (19) to move inward. The fuze is now fully armed.
After the arming pin (18) is retracted, both the firing pin housing (20) and the firing cap housing (21) are kept apart by a weak spring (22).
Upon impact of the napalm canister with the ground, the firing pin in the firing pin housing (20) will be driven into the firing cap (red) in the firing cap housing (21), indifferent of the position in which the napalm bomb will hit the ground. If head or tail on, the firing cap is driven into the firing cap or visa versa, when sideways, both the firing pin housing and firing cap housing will climb the conical slopes, forced so by inertia, forcing the firing pin into the firing cap. The firing cap ignites the detonator (23), which detonates the Tetryl booster. This will ignite the 0,567 kg charge of white phosphorous, that ignites the Napalm from the ruptured container.
The AN M23A1 igniter is a sheet steel pressed housing (23) to which a pressed sheet steel fuze well (24) is brassed. In the base a threaded filler hole is brassed, closed by a plug screw (25). The lower part of the housing is threaded (26) so it can be screwed into the front and back flange of the BLU-27 Napalm fire bomb.
Maximum diameter : 100mm over the thread
Height AN M23A1 housing : 96mm
Total height with fuze : 131mm
Length of FMU-7A/B fuze : 123 mm
Pict.01:
Length of firing pin activated battery (incl. connector pin): 101mm
Key width hexagon head : 45mm
The mechanical version of this fuze is also described in this forum with the cutaway chapter under the name “Napalm bomb fuzing, Fuze AN M173A1 with AN M23A1 white phosporous igniter”
When searching for the term “napalm bomb Vietnam” on you tube one can clearly see the ignition of the phosphorous ignited by the white smoke stripes shooting away from the impact point.
Regards, DJH.
