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According to this site
Halifax Technical
LAPG started delivering (entry into service?) in Jan 1942 and Rootes in April of 1942.
Most sources on LAPG say they completed their first Halifax in Dec of 1941. Rolling out factory door and completing test flying and delivery to service unit might take a few weeks (or month).
London Aircraft Production (L.A.P.) | Explore 20th Century London
By the first site it appears that LAPG delivered 250 Halifaxes by July of 1943.
Rootes had delivered 150 by March of 1943 and was in the middle of a batch of 100 in July of 1943, took from April to Sept to build the 100?
Fairey may have delivered first Halifax in Aug of 1942. It appears they were working on at least two different sets of serial numbers (?) and the first 10 aircraft used a different set of letters to start the serial number. Second set starts "delivery/entry into service" in Nov 1942 (only 4-5 days late from the 27th Oct) and there were only 10 aircraft in the first set of numbers.
The official report into the Halifaxe's performance made in the autumn of 1942 is less than flattering. It said that,
"...deterioration in the performance of the Halifax II aircraft, consequent upon progressive application of external equipment, has augmented to such an extent that the aircraft has become incapable of meeting concurrently the operational requirements of both high loading and high altitude cruising."
Once the Mk III entered service the restriction on targets was removed. Which is why I tried to differentiate between the versions of the Halifax. The Mk III units generally had very similar losses to the Lanc, those with the Mk II did a lot worse
A Great diagram but the horizontal scale in yards distorts what should be a very pronounced ellipse over the gun.Actually the Heavy AA guns gained considerably with a reduction in height.
View attachment 281175
And that is for a plane flying directly over the gun, a similar diagram could be made for planes firing 5,000-10,000 yds to the side of the gun. Put the two together and the lower altitude can mean a lot more shells fired from the same number of guns.
The guns/projectiles are not laser beams. There is a certain amount of dispersion to the shells this goes up with the square of the range (that is ideal theory, in practice it is a bit worse) so the shells will be dispersing in an area about 60% larger at 19,000ft than 15,000ft.
You also have a shorter time of flight for the shell which helps in several ways ( less lead on the target is needed) and the since the fuse error is a percentage of the time of flight a shorter time of flight means less fuse timing error (more shells burst closer to the nominal altitude).
You also have the fact that even the range-finding equipment used by the AA gunners has errors expressed as a percentage of range. For example some Allied Air-borne radar in WW II had a possible error in range ( max distance or scale) of + or - 10%. at closer ranges it may be better but the longer the distance the more errors can creep into the firing solution. Put that together with the fact that the closer the bomber flies to the gun/s the more rounds can be fired and things get pretty bad real quick.
Missions to Germany would be likely to encounter similar and more heavy resistance than mine laying
At 15000 ft, the aircraft are just within the ceiling of the 3,7 cm Flak shells. Granted, the chances for a direct hit are slim, but the number of shells exploding, at around that altitude (due to the self-destruct mechanism) meant that there was quite a bit of fragments unleashed.
Fortunately for the bomber crews, the Germans skipped the chance on the more powerful rounds/guns for the 3,7-4 cm caliber range..