Tail fuze No.17

[pzgr40]

Cutaway model of a British tail fuze No.17. This fuze is a chemical time fuze with an anti removal device. The fuze is used in the 250 and 500 Lb GP bombs. With the old type of these bombs the fuze was placed in the tail cone of the bomb, causing many duds as the fuze bend during impact. With later types of these bombs the fuze was placed insuide the bomb body, leading to less duds.

The tail fuyze No.17 was a very complicated type of fuze , designed in such a way that it was safe until impact. It could not explode in the bomb bay of the plane , even if the glass bottle with aceton is broken, as there is no spring force on the firing pin. It could also be removed from the bomb body if the bomb was brought back to homebase (with a normal landing).
During WW2 however, with major losses to bombers en route -or back- to the target, this safety issue became less important, and the -complex and expensive to manufacture- tail fuze No.17 was replaced with the tail fuze No.37, that was less safe, but much easier and cheaper to manufacture.

Description and functioning of the fuze:

Parts:
1 - Inertia weight housing (brass)
2 - aft cover (brass)
3 - Inertia weight (steel)
4 - Spring (steel)
5 - resistance ring (red copper)
6 - Piston housing (brass)
7 - Piston (steel)
8 - piston (brass)
9 - ampoule housing (brass)
10 - ampoule (glass ampoule filled with aceton)
11 - Sift ring (hardened steel)
12 - distance piece (brass)
13 - filler ring (steel)
14 - celluloid disc (celluloid)
15 - release mechanism housing (brass)
16 - release piston (steel)
17 - spring (steel)
18 - filler ring (steel)
19 - balls (6x , steel)
20 - firing pin housing (brass)
21 - connection piece (steel)
22 - firing pin (steel)
23 - firing pin spring (steel)
24 - firing pin spring tensioning piston (red copper)
25 - radial fixating sleeve (brass)
26 - radial fixating sleeve inertia weight (brass)
27 - fixating screw (brass)

In safe position, the inertia weight (3) is fixated by the nut on the propeller (missing here) over the protruding threaded rod of the weight, kept in backward position. The red copper spring tensioning piston (24) is kept in backward position by the firing pin spring (23), which is not tensioned in the safe position. In this picture it is in armed position. The piston (24) also keeps the inertia weight (26) -connected to the radial fixating sleeve (25) by means of a fixating screw (27)- in backward position.

Functioning of the fuze.:
Upon release from the bomb rack, the safety wire of the propeller is withdrawn, allowing the propeller to unwind in the airstream. When the propeller falls away, the inertia weight (3) is free to move forward upon impact.

Upon impact three things happen simultanoiusly:
A – the inertia weight is swung forward against it’s spring (4),bending inward the red copper resistance ring (5), pushing the steel (7) and brass (8) piston forward, breaking the ampoule (10). The aceton starts dissolving the celluloid disc (14).
B – The firing pin tensioning piston (24) is swung forward by inertia, tensioning the firing pin spring (23). A small edge of the forward end of the piston is widened as it is forced over the chamfered edge on the back of the connection piece (21), fixating it in it’s forward position, keeping the firing pin spring tensioned.
C – The radial fixating sleeve inertai weight (26) moves forward , together with the radial fixarting sleeve (25), allowing the release mechanism housing (15) to rotate against the firing pin housing(20). This activates the anti removal device.

The aceton (10) starts to dissolve the celluloid disc (14) up to the moment it becomes so soft that the spring (17) inside the release piston (16) moves the piston upward, allowing the balls (19) to move outward, releasing the firing pin (20) to move downward into the firing cap of the detonator, exploding the bomb.

If any attempt is made to unscew the fuze from the bomb, unscrewing the release mechanism housing (15) against the firing pin housing (20), the spring (17) between the firing pin (22) and the release piston (16) will move the piston upward , allowing the balls (19) to move outward, releasing the firing pin, detonating the bomb.

This specific fuze has the marking “1/2” stamped in the firing pin housing, so I think it has a 0,5 hours delay.

Regards, DJH.

 

[blu97]

came out beauthiful thanks dj, and yes 1/2 is 30 minutes delay

 

[blu97]

Here a link to the actual bomb and fuze ( second bomb). http://www.bocn.co.uk/vbforum/threads/7573-500-pr-gp-found-in-holland

 

[Fusse2004]

Hello DJ! Super cut !!! I've been chasing this Fuze for 20 years! Will not get it in this life anymore.:tinysmile_cry_t4:

 

[pzgr40]

It's a difficult piece indeed. And to my regret I have to note that it' s not mine. I only "repaired and improved" it.
Regards, DJH

 

[Bellifortis]

Hi DJH,
thank you for another piece of beautiful work. I'm amazed at the good condition of the fuze, after more than 70 years in the sandy, wet soil of NL. I can only discern a little bit of surface corrosion with the steel Inertia weight that is in contact with the outsides. Was the acetone ampule still intact ? Could the reason for not functioning be established ? Were the celluloid discs still intact or did they show signs of crumbling after 70 years ? By the way, the standard german procedure of making safe bombs with the old (outside) model No.17, was the attachment of a 250gr explosive charge in the lower third part, sticking out. This would bend the whole fuzebody so much, that the fuze could not function any more. That is fast and easy and could be done by nearly anyone.
regards,
Bellifortis.

 

[blu97]

here a cutaway of the 500 Lb MK v.

 

[pzgr40]

@ Bellifortis, thanks
No the ampoule was broken, but I had a spare one. The celluloid disc was totally gone, I machined a disc from perspex. I have no idea why it did not function, but as I unscrewed the lower part (aimimg the firing pin on a wooden plank) I only had to unscrew appr. 30 degrees and the firing pin ended up in the plank like an arrow from a bow. Somehow the release piston had not moved upward, it was stuck in it's safe position. I have no clue why. I can only guess....... maybe too tight tolerances on the parts.
The condition of the parts was amazing indeed, the only part I hasd to sandblast was the inertia weight (3) of which the housing was completely hardened mud filled, the rest of the parts needed a brushing only.
Regards, DJH

 

[Fusse2004]

How should the built-in part corrode? It is almost completely installed in the bomb. Most English bombsheets look like this. As from the factory !!!!

 

[Bellifortis]

Hallo pzgr40,
in what depth was the bomb found ? Regarding "Fusse2004"'s belief, that corrosion inside the fuze was not possible : Did you find any signs of oxidation (green discoularation, green crystal) inside the fuze. You mentioned that the firing pin was stuck and only held by frictional forces. Steam can enter through the tiniest of cracks and for a mechanical fuze to work, it has to have small tolerances. There was a lots of moisture present to oxidise the inertia weight that much. If the bomb ended up deep in the ground, i would expect less corrosion, because there is near to constant temperature year round. Near the surface that is different.
Regards,
Bellifortis.

 

[blu97]

The bomb was about 3 metres deep in the ground and probably didn't function because one of the metal strips blocked the release piston.

 

[Fusse2004]

Was the bomb at the airport schiphol?

 

[blu97]

Yes it was.

 

[pzgr40]

On the Wk2ammo forum I placed an A1 format PDF showing the functioning of the fuze. You can zoom in to the finest detail.
Download it for your files.
Regards, DJH

 

[pzgr40]

I found a little mistake in my drawing: a locking ball is missing in section A-A. I have added thisball , and also changed the colour of sonme screws as I thought they were too dark. I have changed the name of the drawing by adding Z1 to it's name.
I will exchange the Original drawing for the Z1 version, so please chane it out in your files too.
Sorry for the trouble, Regards, DJH

 

[pzgr40]

Does anybody have an official drawing of the bomb or fuze with dimensions of the thread with the red square around it ? I am trying to find out what it is (Withworth, gas, either UNC or UNF) but I come only as far that the thread has 14 revolutions per inch (n=14) , but no external diameter fits the thread range charts. The outer thread of the fuze is the inner thread of the fuze pocket in the bomb.
Thanks in advance

 

[TimG]

It will be 14 tpi (Whitworth form) whatever the diameter is, which may or may not resolve to a sensible fraction of an inch or decimal. We used 14 tpi (Whitworth form) from the smallest to the largest ordnance. In respect of threads, Woolwich were not constrained by screw thread standards.

TimG

 

[pzgr40]

Ok, thanks for the awnser