BDV wrote: ↑
03 Oct 2018 12:11
Are there any obvious reasons why the 88mm gun of Tiger I was not ported to tank destroyers?
It was shorter than the 75mm L70 by 32 cm, and that could've helped with the nose heavy problems of Jagdpanzer IV; while providing a bigger HE.
One aspect of a KwK 36 superiority over KwK 42 was barrel life. Based on "Datenblätter für Heeres-Waffen, -Fahrzeuge und -Gerät" (1944) [Reprint] the 88 had a barrel life of ca. 6000 and the 75 ca. 2000 rounds; Probably on all-Pz.Gr.39 diet. Thus despite a RM 12 000 purchase cost against KwK 42’s RM 18 000, KwK 36’s lifetime costs could in fact have been lower. There would probably also have been scope for KwK 36 purchase cost reduction since e.g. according to Datenblätter, 8,8 cm KwK 43 (L/71) required 4421 kg steel in its manufacture as opposed to the 5550 kg for the lighter KwK 36.
There is an odd thing in the ”Merkblatt für das waffentechnische Personal der Panzer-Einheiten" considering the weights of the two guns. Although the KwK 42 had a considerably lower barrel and breech weight of 1000 kg against 1330; KwK 36 had the lower complete gun weight of 1760 kg against 1860 kg. This might mean that KwK 42 had higher recoil stresses than KwK 36 and required a sturdier mounting. Unfortunately good recoil/average braking force figures are hard to come by (e.g. Spielberger figures for KwK 43 and Pak 43/3 & 43/4 appear especially unbelievable). However according to p.110 of ”Germany's Panther Tank" by Thomas Jentz & Hilary Doyle, the recoil force of 7,5 cm KwK 44/1; a KwK 42 derivative; was 18 tons without and 12 tons with a muzzle brake.
Now in order to approximate KwK 36 recoil let us check KwK 42 recoil against the above figures using the recoil velocity formula V = ( b*v + c*p ) / W from http://kwk.us/recoil.html
where b is the bullet's [projectile] weight, v the muzzle velocity, c the charge weight, p the average velocity of the escaping propellant gases, and W is the rifle's [gun’s recoiling parts’] weight, using 4700 fps or ca. 1433 m/s for p.
V,42 = (6,8 x 925 + 4,0 x 1433) / 1000 ≈ 12 m/s
Recoil force in kg: R = (1/2*W*V^2)/(g*s) where s = recoil length
R,42 = (0,5 x 1000 x 12^2)/(9,80655 x 0,4) ≈ 18 355 kg ≈ 18 tons; Seems like a good ballpark formula.
Checking against Rheinmetall-Borsig's BK 5 Datenblatt at http://www.deutscheluftwaffe.com/archiv ... 5%20cm.pdf
V,BK5 = (1,52 x 965 + 0,83 x 1433) / 259 ≈ 10,3 m/s
R,BK5 = (0,5 x 259 x 10,3^2)/(9,80655 x 0,285) ≈ 4 916 kg.
Given the high mv and massive muzzle brake let us assume a 40% recoil reduction
R,BK5,MB = 0,6 * 4916 ≈ 2950 kg which is close to the 3000 kg RMB figure.
Even with 1/3 recoil reduction as with KwK 44 muzzle brake the figure would be ca. 3280 kg; The formula appears to again give adequate approximations.
Now for KwK 36:
V,36 = (10,2 x 773 + 2,5 x 1433) / 1330 ≈ 8,62 m/s
R,36 = (0,5 x 1330 x 8,62^2)/(9,80655 x 0,55) ≈ 9 160 kg; A figure appreciably lower than KwK 42 without (or with) muzzle brake.
How about with the same recoil length as KwK 42:
R,36,0,4 = (0,55/0,4) x 9160 kg ≈ 12 600 kg Thus a muzzle-brakeless KwK 36 with KwK 42 recoil length would have similar recoil force as KwK 42 with muzzle brake. Given the Tiger’s extra weight and large turret ring a muzzle brake appears to have been less vital.
Add a muzzle brake: Let us assume just a 25% recoil reduction due to the lower mv;
In a Tiger E:
R,36,MB = 0,75 x 9160 = 6 870 kg, ca. 58% of KwK 42 recoil.
With KwK 42 recoil length:
R,36,0,4,MB = 0,75 x 12 600 ≈ 9 450 kg, ca. 80% of KwK 42 recoil.
In short KwK 36 compared to a KwK 42 had an almost as good penetration, greater after armour effects, better HE performance, longer barrel life, lighter recoil and probably lower lifetime costs. Despite the KwK 36 being a better general purpose gun the Germans complicated logistics by using the KwK 42 in the Panther. Despite the heavy recoil they also tried to shoehorn KwK 42 in StuG III, Pz. & StuG IV etc. Still sane fringe occasionally prevailed and the Tiger with KwK 42 came to nothing much more than a mock-up.