P-39L-1-BE 37mm cannon standard non armour piercing rounds

[--[---MAILMAN----]

I was testing the FOV relationship for being able to see the convergence point with the new tracer graphics.  I had to zoom in to a FOV of 60-70 degrees to be barely able to see the tracers converge at 280m (306 yds) with the wing mounted .30 & .50 caliber MG.

 

The 20mm and 37mm nose cannon and the nose .50 MG I could see converge at 300 yds at 100+ degrees FOV pretty easily.

 

When I was testing these while sitting on the runway I observed something I did not expect.  When firing the 37mm cannon on the P-39-1-BE the standard rounds exploded in mid air when the round traveled a particular distance like it had a proximity fuse.  I know they were Explosive rounds, but I thought they were supposed to explode on contact.  These rounds exploded in mid air without hitting anything like a flak round.  This did not occur when testing the Armour Piercing 37mm rounds available in the plane modifications list.

 

Would someone please explain exactly what type of HE round this is and why it explodes without coming in to contact with anything?  I just want to understand what type of round I am dealing with exactly.  Thanks in advance.

[ShamrockOneFive]

I don't know the details of which rounds have that feature but I do know that several types of rounds self destruct after a certain life time. In some cases that was done to limit collateral damage from undetonated high explosive shells landing in civilian areas. The MG-FF/M is the other one that comes to mind.

[Reggie_Mental]

3 hours ago, ShamrockOneFive said:

I don't know the details of which rounds have that feature but I do know that several types of rounds self destruct after a certain life time. In some cases that was done to limit collateral damage from undetonated high explosive shells landing in civilian areas. The MG-FF/M is the other one that comes to mind.

Mk108 ZZ1589B rounds had a self detonation mechanism based on centrifugal force decreasing as rotation and muzzle velocity reduce. 

'Due to the rifling of the barrel, the shell is given a high rotational speed during firing. This leads to centrifugal forces that push the six balls (7) outwards, forcing them in the collar ring and keeping the firing pin housing (4) which holds the firing pin (5) in upper position. Meanwhile the wrapped safety coil (6) rolls out, thereby opening the way for the firing pin housing to move downward forced by spring (8), however held back by the balls in the groove. At impact the firing pin housing with firing pin is simply hammered down in the duplex detonator (the balls are forced back into the firing pin housing), exploding the shell. If the rotational speed decreases too much (long time of flight, i.e. the shell missed target) the force of the spring will force the balls back into the firing pin housing after which it moves down due to the spring force. This forces the firing pin into the duplex detonator (10), exploding the shell. This description also applies to the ZZ 1589 A fuse.'

 

 

 

The P39's 37mm M54 round used a chemical, rather than mechanical self destruct method:

High-explosive shell, 37 mm, M54 standard

This shell used the M56 point detonating fuse. The complete round weighs 1.99 lb (900 g); as fired, the projectile weighs 1.34 lb (608 g). The 0.16 lb (70 g) charge of M2 powder is a Hercules NG formula of single perforated grains with 0.030 in (0.76 mm) web and gives the projectile the prescribed muzzle velocity of 2,000 ft/s (610 m/s).

The M54 used a shell-destroying tracer in addition to the point-detonating fuze. The tracer had a burning time of three seconds at the end of which it set off an igniting relay charge of 1.68 gr (0.109 g) of Grade A-5 Army Black Powder which ignited a relay pellet that detonated the charge, destroying the shell before ground impact.

 

I would have thought that both mechanical and chemical self destruct methods would have had a significant failure rate, too. ?

Edited September 29, 2021 by Reggie_Mental

[ShamrockOneFive]

See... I knew someone would know the exact details

[[APAF]VR_Spartan85]

Wow this is really cool, I always thought they just worked like dry motor model rockets that popped when they were out of fuel… lol