Anti-aircraft artillery & firepower alternatives, 1930-1950 (1 Viewer)

[Shortround6]

TANSTAAFL

A projectile that needs a small propelling charge may be able to carry that charge inside the projectile.
But if you are looking for high velocity you need a certain amount of propellent to get the projectile (of a certain weight) up to that speed inside the barrel.
Splitting the propellent in two parts, one in the cartridge case and one in the projectile does not make things more efficient. You want 20% more velocity? you need 40% (or more) propellent, how you add that is the question. British 2pdr HV shells lost about 14% of their weight to help get the extra velocity.
Using rockets outside of the barrel solves the problem of higher pressure inside the barrel, doesn't really solve the problem of the needed amount propulsion (energy needed to get X amount of mass up to desired velocity). A small charge in the base of a shell is not going to turn an 88mm/56 into an 88mm/71. That 25% increase (820 to 1000ms) required a 100 increase in propellent and the longer barrel. If you want to use a rocket motor to get rid of the extra barrel length OK, but now you need around 2.5 kg of propellent in the back of the 88mm shell and you need either a longer shell or a much reduce HE charge. Much longer shell may require a different rifling twist in order to stabilize.

Rocket propelled or rocket assisted artillery shells have always had an accuracy problem. Rocket assist was a really big thing in the 1970s/80s with huge gains it range from existing 155mm guns/howitzer and even 105mm howitzers. These worked by delaying the rocket firing for a number of seconds after the projectile left the barrel. The rocket motor fired after the shell had lost some of it's initial velocity and boosted it back up and/or helped maintain velocity. And the rocket fired in thinner air so you need a bit less propellent to fight air resistance. However accuracy was usually poor. The rocket motors did not all fire at the exact same point in the trajectory which is curved so large variations in range. All shells wobble a bit in flight. That is the nose is making a small circle as the shell rotates. When the rocket motor fires the shells tended to take off in a new direction, which in a shell that was still thousands of meters from it's target doesn't have to be a big deviation.
Artillery customers finally gave up and just bought guns with longer barrels and more streamlined projectiles. And that is with 1970s-80s-90s machining capabilities and design and aerodynamics.

Better shaped projectiles could have done a lot for better performance. German 30mm MK 108 ammo shows some of the difference. The Ausf C was still moving at 370ms at 600meters while the Ausf C shell was doing 370ms at 300 meters when both were fired at 500ms. Ausf C shell arrived at 600 meters about 0.25 seconds sooner. Maybe the Ausf A would have the same time of flight if it was fired at 600ms instead of the 500ms but that would need a lot more propellent and a longer barrel.
Few AA shells were as badly shaped (blunt nose) as the MK 108 Ausf A unless they were WW I left overs.

 

[tomo pauk]

A projectile that needs a small propelling charge may be able to carry that charge inside the projectile.
But if you are looking for high velocity you need a certain amount of propellent to get the projectile (of a certain weight) up to that speed inside the barrel.
Splitting the propellent in two parts, one in the cartridge case and one in the projectile does not make things more efficient. You want 20% more velocity? you need 40% (or more) propellent, how you add that is the question. British 2pdr HV shells lost about 14% of their weight to help get the extra velocity.
Using rockets outside of the barrel solves the problem of higher pressure inside the barrel, doesn't really solve the problem of the needed amount propulsion (energy needed to get X amount of mass up to desired velocity). A small charge in the base of a shell is not going to turn an 88mm/56 into an 88mm/71. That 25% increase (820 to 1000ms) required a 100 increase in propellent and the longer barrel. If you want to use a rocket motor to get rid of the extra barrel length OK, but now you need around 2.5 kg of propellent in the back of the 88mm shell and you need either a longer shell or a much reduce HE charge. Much longer shell may require a different rifling twist in order to stabilize.

On a heavy AA shell, even a small rocket engine in the shell bottom might help to push through the thickest air in the low altitudes. Say, 3 sec duration - even when fired at SL could mean that by 2.5km the heavy shell still has the same velocity as it had when it left the barrel. That indeed will not make the L56 as good as the L71, but it might've split the difference in dealing with the targets flying above 20000 ft.

With the lethality business - aircraft hit by the 2pdr shell that has the part of HE content sacrificed for the rocket motor is still likely to go down.

Rocket propelled or rocket assisted artillery shells have always had an accuracy problem. Rocket assist was a really big thing in the 1970s/80s with huge gains it range from existing 155mm guns/howitzer and even 105mm howitzers. These worked by delaying the rocket firing for a number of seconds after the projectile left the barrel. The rocket motor fired after the shell had lost some of it's initial velocity and boosted it back up and/or helped maintain velocity. And the rocket fired in thinner air so you need a bit less propellent to fight air resistance. However accuracy was usually poor. The rocket motors did not all fire at the exact same point in the trajectory which is curved so large variations in range. All shells wobble a bit in flight. That is the nose is making a small circle as the shell rotates. When the rocket motor fires the shells tended to take off in a new direction, which in a shell that was still thousands of meters from it's target doesn't have to be a big deviation.

Do some experimenting and testing. Bigger or smaller motors, longer or shorter thrust duration, when to fire the motor etc.

Better shaped projectiles could have done a lot for better performance. German 30mm MK 108 ammo shows some of the difference. The Ausf C was still moving at 370ms at 600meters while the Ausf C shell was doing 370ms at 300 meters when both were fired at 500ms. Ausf C shell arrived at 600 meters about 0.25 seconds sooner. Maybe the Ausf A would have the same time of flight if it was fired at 600ms instead of the 500ms but that would need a lot more propellent and a longer barrel.
Few AA shells were as badly shaped (blunt nose) as the MK 108 Ausf A unless they were WW I left overs.

The MK 108 probably needed the to be with the less sedate MV to begin with. Like, use it's "own" Mine shell design, 250-270 g, so the MV can be more conductive to accurately shoot at the greater distance and against the targets that can maneuver well. Accept the loss in the singe-shell lethality and move on.

FWIW, some thought was given to the increase of the MK 108 RoF from 600 to 1200 (it was admitted that it will be quite a task to do it).

 

[tomo pauk]

FWIW, some thought was given to the increase of the MK 108 RoF from 600 to 1200 (it was admitted that it will be quite a task to do it).

... like this, in a post-war report:



Seems like the MK 108 also featured as an anti-aircraft gun as-is:

 

[Elan Vital]

The report that the Edgar Brandt company made on AA guns and ammunition (October 1940) is quite interesting as well.

Brandt argues that to expect to hit a modern aircraft, the flight time to the aircraft must be between 8 and 10s to minimize the volume that it can maneuver in and thus avoid missing a plane that already changed course. He also recommends increasing the HE ratio compared to the optimum for ground artillery, as the lower density at higher altitudes allows lighter fragments to retain their energy better.

To obtain said practical flight times, Brandt recommends increasing the muzzle velocity and improving the shape of the projectile to better retain this velocity. For a given gun, this may be obtained with a lighter projectile with an elongated ogive that goes as close to the base as possible, requiring a discarding guiding band (e.g a ring sabot near the middle of the projectile). A higher HE ratio is conducive to a lighter round, and a good ballistic cap works very well with a lighter round.
While a lighter round is normally worse at energy retention, the practical flight distances are much smaller than the trajectory's maximum flight distance/altitude.
The fuze may be designed to be lighter and better shaped.

For sufficiently high calibers (90 and up), using actual subcaliber projectiles is practical and beneficial because:
- they allow high velocities out of comparatively short barrels and low pressures, increasing thermodynamic efficiency
- the powder charge size and projectile weight and length are reduced, facilitating higher rates of fire
- the wear is also reduced, enabling higher rates of fire when wear patterns are a concern
- due to the low projectile momentum, it is possible to install a more powerful barrel firing only subcaliber projectiles on an existing mount, which is good as barrels are generally faster to develop than mounts.

Brandt for example suggests replacing the 90mm CA 39 firing piece (11kg round at 820 m/s and 260 MPa), with a 230 MPa 105mm gun firing 80mm 6kg at 1220 m/s. In spite of a 20% increase in power, the 19% lower momentum allows use of the existing mount.

Naturally, a lot of this seems to be made in the context of a gun seeking an aimed hit on a target, rather than firing dozens or hundreds of guns in a box of air. Brandt's concept probably works better with good fire control systems and VT fuzes that enable high hit probabilities for individual shots, and where a low time-of-flight projectile reduces the impact of aiming errors.

The tables given from page 12 onwards show that lightweight projectiles with discarding guiding bands and a lightweight fuse with BC turn obsolete 75mm 570 m/s guns into near-75mm Mle 1928 (715 m/s) analogues.
For 75mm Mle 1928-pattern guns (CA 32), the lightweight concept saves 1.1 and 1.5 seconds respectively to 4 and 5km altitude (actually flight distances as the gun fires straight up at 90° here). Savings are even greater when the lightweight fuze with BC is used, and the small loss in HE payload with the first round is negligible with the lightweight fuze.
Even in ammo where the HE mass is reduced, the ratio to the total projectile mass is increased, to obtain more and lighter fragments.

The 90mm firing 80mm HE-DS outright reaches 1040 m/s using a projectile that is slightly heavier and has slightly more HE than a 75mm service round. Worth comparing to the hypothetical 105/80mm weapon.
Even the obsolete 105mm Mle 1913 firing 85mm HE-DS reaching 852 m/s instead of 550, has barely worse flight times than the considerably more powerful 90mm CA 39 with its service round.

 

[tomo pauk]

Brandt argues that to expect to hit a modern aircraft, the flight time to the aircraft must be between 8 and 10s to minimize the volume that it can maneuver in and thus avoid missing a plane that already changed course. He also recommends increasing the HE ratio compared to the optimum for ground artillery, as the lower density at higher altitudes allows lighter fragments to retain their energy better.

To obtain said practical flight times, Brandt recommends increasing the muzzle velocity and improving the shape of the projectile to better retain this velocity. For a given gun, this may be obtained with a lighter projectile with an elongated ogive that goes as close to the base as possible, requiring a discarding guiding band (e.g a ring sabot near the middle of the projectile). A higher HE ratio is conducive to a lighter round, and a good ballistic cap works very well with a lighter round.
While a lighter round is normally worse at energy retention, the practical flight distances are much smaller than the trajectory's maximum flight distance/altitude.
The fuze may be designed to be lighter and better shaped.

Germans pretty much agree with his reasoning. The high-velocity 88mm gun was with the much greater ceiling and range than the 105mm gun, let alone the 'normal' 88mm gun. The main shortcoming of the long 88 was the low barrel life, several times lower than of the normal 88.
They were also suggesting several ways of improving the MV on the existing guns: under-caliber ammo*, going squeeze bore**, as well as well as loading the ammo hotter. If one does not have a fast and easy way to obtain the proximity-fused ammo, going to hit-to-kill will be much easier to do with the faster shells.

* this type of ammo is noted as reducing the barrel life on the field guns; however trading the barrel life for the greater lethality on the guns that are available in the thousands seems to me as a more economic way than making the much more expensive high-power guns that again have the much lower barrel life to begin with, while also using the much more expensive and powerful ammo
** again, lowers the barrel life, but same comment as above applies IMO

 

[Elan Vital]

* this type of ammo is noted as reducing the barrel life on the field guns; however trading the barrel life for the greater lethality on the guns that are available in the thousands seems to me as a more economic way than making the much more expensive high-power guns that again have the much lower barrel life to begin with, while also using the much more expensive and powerful ammo
** again, lowers the barrel life, but same comment as above applies IMO

The other consideration is that making ever more powerful guns firing conventional ammo leads to much heavier and bulkier systems than if you were using special ammo.

 

[tomo pauk]

The report that the Edgar Brandt company made on AA guns and ammunition (October 1940) is quite interesting as well.

Do you have that document saved? Seems like it disappeared from that site.

 

[Elan Vital]

Do you have that document saved? Seems like it disappeared from that site.

I fixed the link, it wanted to paste an unrelated discordapp link instead if the drive link.

Sad thing about this doc (other than page 8 being missing) is that only the lightweight ammo is shown (possibly not even those with the new fuze), but not the subcaliber ammo.

 

[tomo pauk]

Naval ammo was early adopter of the HE shells with aerodynamic caps, that looked like the APCBC ammo outwardly. That improved the length/diameter ratio, thus improving the aerodynamics without making the shell too heavy, or requiring the top-notch steel. Armies were slow to pick up on that, at least on the usual 15-15.5cm bracket. But here is the French 155mm shell, together with the usual shell for comparison (same shell was also used on the 155 GPF, while the similar shells were also available for the 145mm and the bigger guns):



Granted, this is something that might not be worthwhile to make in small calibers, as well as if the quick operation on the time fuses is required, but it will come in handy with the 75+mm guns when the hit-to-kill is wanted.

Also this:

 

[Shortround6]

Sometimes less radical solutions can work well.
Some shells were not very good to begin with a small changes can make significant differences without going for the last 10-15%.
A bit more attention to base of the shell (boat tail) can also pay dividends, but may be harder to manufacture. Exact taper and length of the boat tail may require some experimentation.
This is with 20/20 hindsight as it took quite awhile for many small arms bullet makers to either figure out (or publish) different ballistic co-efficients for different velocities for the same bullets.
Problem for AA shells in the larger calibers was getting access to the fuse.

Crewman inserted complete round in the fuse setter and the fuse setter twisted the fuse to the correct time as sent by the fire control equipment.
A large amount of error was built into the system.
The US would up (late in the war) with a system that set the fuse while the in the loading position. Crew dropped the round into the feed tray and the round was then held by arms while the fuse was set (needs locating tabs on the shell to get proper alignment) and then powered rollers moved in against the complete round and rammed it into the breech a fraction of a second after the fuse was set. This powered ramming also increased the rate of fire of the gun.
With the German method (everybody's method in 1930s and early 40s) there was a 2-3 second delay in getting the round out of the fuse setter, flipping it around 180 degrees and using shoulder/arm muscles to ram the round into the breech of a gun pointed up at 70-85 degrees.

Short time of flight was only part of the problem.
Perhaps more important with guns that had crappy range finding and crappy sights?

 

[tomo pauk]

It will also depend on a shell. A 75-90mm shell will be far weaker with the time fuse than the 120-130mm shell, and here (for the smaller shells) going for a hit-to-kill might've worked better than going for a timed explosion. The time fuse was probably a few times more labor-extensive and more expensive than the ordinary impact fuse - important to note if one looks to make hundreds of thousands, or even millions of the fuses.
Granted, with the impact fuse, there is no psychological effect of the heavy AA fire, and there is no such thing as damaging of the enemy aircraft by the nearby explosions.

One thing that might've improved the hit probability of the time-fused heavy shell is if the design of the shell includes the the pre-made fragments, that will go further ahead in a cone pattern, rather than to just go in the circle away from the trajectory. Sorta low-tech Ahead ammo. Just make sure to under-time the detonation.