Pom pom with director Mk IV vs Bofors 40mm with Mk 51

Guns, torpedoes, mines, bombs, missiles, ammunition, fire control, radars, and electronic warfare.
wmh829386
Member
Posts: 189
Joined: Mon Oct 04, 2021 5:43 pm

Re: Pom pom with director Mk IV vs Bofors 40mm with Mk 51

Post by wmh829386 »

dunmunro wrote: Sat Feb 24, 2024 6:12 pm
1)The COFAS has a reticle for eye-shooting aim off. The GRU sight required that the target be kept centred in the crosshair to allow the gyro to compute the target rate.

2) With continuous radar ranging the GRUB (computer could (and must) calculate true target motion otherwise radar ranging would actually make predicted target position more inaccurate,

Is there a drawing for COFAS on HACS? The COFAS on Mk ll or Mk lll seems to be an open ring sight, which is completely different from Mk IV pom pom's control officer's optic.

I am sure Mk IV pom pom director does not have GRUB involved. The radar range and RF range are displayed to the range follower to put into the director setting. There are a few possibilities.

1. Range follower only input range --> unlikely, it makes RF range almost useless. A range clock would keep the range somewhat reasonable between cuts.

2. A simple Range clock --> director has a range clock allowing easier range keeping without Radar. But director doesn't use range rate for aim off/ superelevation calculation

3. Ragne rate calculation in aim-off and/or superelevation -> most complex, I will need a technical drawing/documentation to believe it.

HACS of course consider range rate, but that's another topic.

Even ignoring the range rate, range allows a better estimation of the most important ratio in aim off:
Average projectile velocity / target tangential velocity

I am sure this is implemented.

Range also allowed for superelevation to be added, but it will be most complicated, so I am not sure if it is added. It's a further question whether range rate is also considered. They could reasonably just zero individual guns for different ranges and ignore it from director.
User avatar
marcelo_malara
Senior Member
Posts: 1867
Joined: Sun Oct 02, 2005 11:14 pm
Location: buenos aires

Re: Pom pom with director Mk IV vs Bofors 40mm with Mk 51

Post by marcelo_malara »

dunmunro wrote: Tue Feb 27, 2024 1:46 am

Tachymetric analog computers could predict target location accurately given accurate inputs; that was the whole point of it all. Where the computers failed was when the TOF was too long and the target's predicted location would no longer match it's actual location, due to target manoeuvring.
Look at this graph. Two planes flying different courses and speeds as seen from the director. If the time interval between the planes passing thru the LOS is the same, the gyro unit would give the same angular speed, but the planes at the moment of passing the second LOS would be at different distance. To compute the positions accurately at the second LOS would need measuring range at intervals and computing a range rate, and then integrating continually that range rate to provide for a delta distance. And all this in the seconds that a plane close to the ship would be at the range of the guns. I doubt that this was actually done. For sure that a radar range would help to compute further deflection for the TOF of the shell, more accurately than the Mk14 doing the same with the span or length of the plane.


Image
dunmunro
Senior Member
Posts: 4408
Joined: Sat Oct 22, 2005 1:25 am
Location: Langley BC Canada

Re: Pom pom with director Mk IV vs Bofors 40mm with Mk 51

Post by dunmunro »

marcelo_malara wrote: Tue Feb 27, 2024 3:32 am
dunmunro wrote: Tue Feb 27, 2024 1:46 am

Tachymetric analog computers could predict target location accurately given accurate inputs; that was the whole point of it all. Where the computers failed was when the TOF was too long and the target's predicted location would no longer match it's actual location, due to target manoeuvring.
Look at this graph. Two planes flying different courses and speeds as seen from the director. If the time interval between the planes passing thru the LOS is the same, the gyro unit would give the same angular speed, but the planes at the moment of passing the second LOS would be at different distance. To compute the positions accurately at the second LOS would need measuring range at intervals and computing a range rate, and then integrating continually that range rate to provide for a delta distance. And all this in the seconds that a plane close to the ship would be at the range of the guns. I doubt that this was actually done. For sure that a radar range would help to compute further deflection for the TOF of the shell, more accurately than the Mk14 doing the same with the span or length of the plane.


Image
Take a look at the Kerrison Predictor:

https://en.wikipedia.org/wiki/Kerrison_Predictor

In your example the two predicted target locations would vary enough that no hits could be expected on the further target unless target's actual course was modelled accurately.
dunmunro
Senior Member
Posts: 4408
Joined: Sat Oct 22, 2005 1:25 am
Location: Langley BC Canada

Re: Pom pom with director Mk IV vs Bofors 40mm with Mk 51

Post by dunmunro »

wmh829386 wrote: Tue Feb 27, 2024 1:52 am
dunmunro wrote: Sat Feb 24, 2024 6:12 pm
1)The COFAS has a reticle for eye-shooting aim off. The GRU sight required that the target be kept centred in the crosshair to allow the gyro to compute the target rate.

2) With continuous radar ranging the GRUB (computer could (and must) calculate true target motion otherwise radar ranging would actually make predicted target position more inaccurate,

Is there a drawing for COFAS on HACS? The COFAS on Mk ll or Mk lll seems to be an open ring sight, which is completely different from Mk IV pom pom's control officer's optic.

I am sure Mk IV pom pom director does not have GRUB involved. The radar range and RF range are displayed to the range follower to put into the director setting. There are a few possibilities.

1. Range follower only input range --> unlikely, it makes RF range almost useless. A range clock would keep the range somewhat reasonable between cuts.

2. A simple Range clock --> director has a range clock allowing easier range keeping without Radar. But director doesn't use range rate for aim off/ superelevation calculation

3. Ragne rate calculation in aim-off and/or superelevation -> most complex, I will need a technical drawing/documentation to believe it.

HACS of course consider range rate, but that's another topic.

Even ignoring the range rate, range allows a better estimation of the most important ratio in aim off:
Average projectile velocity / target tangential velocity

I am sure this is implemented.

Range also allowed for superelevation to be added, but it will be most complicated, so I am not sure if it is added. It's a further question whether range rate is also considered. They could reasonably just zero individual guns for different ranges and ignore it from director.
It's quite a simple matter to have an wide angle optical sight with a grid overlay reticule in the eyepiece. We can be fairly certain that if the Gunnery Pocket Book says that there is a COFAS that it actually existed.

See the previous post for info on a high speed tachymetric computer.
User avatar
marcelo_malara
Senior Member
Posts: 1867
Joined: Sun Oct 02, 2005 11:14 pm
Location: buenos aires

Re: Pom pom with director Mk IV vs Bofors 40mm with Mk 51

Post by marcelo_malara »

dunmunro wrote: Tue Feb 27, 2024 4:58 pm [

Take a look at the Kerrison Predictor:

https://en.wikipedia.org/wiki/Kerrison_Predictor

In your example the two predicted target locations would vary enough that no hits could be expected on the further target unless target's actual course was modelled accurately.
Didn´t know that device, thanks for bringing it to my attention. Basically another form of obtaining the angular rate, instead of using a gyro the operator would provide the rate difference between the generated rate and the real rate, till the real value is reached. There is nothing more than that. It will neither provide a solution for distinguishing two planes flying different courses and speeds if the angular rate of both is the same.

I said it before, These relative motion systems provide an approximate solution assuming certain simplifications to the problem. It is better than nothing, but do not provide an exact solution.

Regards
dunmunro
Senior Member
Posts: 4408
Joined: Sat Oct 22, 2005 1:25 am
Location: Langley BC Canada

Re: Pom pom with director Mk IV vs Bofors 40mm with Mk 51

Post by dunmunro »

marcelo_malara wrote: Tue Feb 27, 2024 6:50 pm
dunmunro wrote: Tue Feb 27, 2024 4:58 pm [

Take a look at the Kerrison Predictor:

https://en.wikipedia.org/wiki/Kerrison_Predictor

In your example the two predicted target locations would vary enough that no hits could be expected on the further target unless target's actual course was modelled accurately.
Didn´t know that device, thanks for bringing it to my attention. Basically another form of obtaining the angular rate, instead of using a gyro the operator would provide the rate difference between the generated rate and the real rate, till the real value is reached. There is nothing more than that. It will neither provide a solution for distinguishing two planes flying different courses and speeds if the angular rate of both is the same.

I said it before, These relative motion systems provide an approximate solution assuming certain simplifications to the problem. It is better than nothing, but do not provide an exact solution.

Regards
The range data (and the generated range rate) provides the needed inputs. This is for the USN Mk15/Mk63 director:

During tracking, the appropriate values of range and range rate are continually set into the sight. If no radar is provided, estimated ranges must be set in. In this case the range setter should determine whether the range is set correctly by observing the projectile tracers. For an incoming target, which is the usual case, the range should be set initially so that the tracers pass just behind or below the target. Tracers passing behind the target indicate that the lead angles are too small. This is the case when the range setting is less that the actual range, as a short range setting causes the restraining springs on the gyros to be stiff. It is readily apparent that, as the target approaches, the actual range will agree more closely with the set range, and the tracer stream will move toward the target. When the actual and set ranges are equal, the tracers should be hitting the target. As the actual range becomes less than the set range, the tracers will pass ahead of the target, because the generated lead angles are too large. Now the range setting must be decreased to make the tracers again pass behind the target.
https://www.eugeneleeslover.com/USNAVY/ ... -26-C.html
wmh829386
Member
Posts: 189
Joined: Mon Oct 04, 2021 5:43 pm

Re: Pom pom with director Mk IV vs Bofors 40mm with Mk 51

Post by wmh829386 »

dunmunro wrote: Tue Feb 27, 2024 5:15 pm
It's quite a simple matter to have an wide angle optical sight with a grid overlay reticule in the eyepiece. We can be fairly certain that if the Gunnery Pocket Book says that there is a COFAS that it actually existed.

See the previous post for info on a high speed tachymetric computer.
From everything I have read about COFAS, it is an eyeshooting sight. Since Pom pom mk IV director abandoned the eyeshooting principle, the sight should be different.

And it is not obvious who is responsible for correcting the fire. I am just curious about how that is actually implemented.

Kerrison device is like the Mk37 or surface gunnery system that use the disc-ball integrator to generate the exact straight line track. These analog computer can track a constant velocity target continuously with the correct setup. Hence the drift of the target in the sight provides feedback for correcting the setup.

I am certain that pom pom director do not have a such an analog computer built in, as target tracking seems to be a completely manual affair, and rates are pulled directly from GRU, which is just a better approach for a small and unstablised setup.
Post Reply