The Jagdpanzer 38 Starr
by William Auerbach | May 1982 AFV News
The word "Starr" in German means "rigid, or solid"
BACKGROUND: REQUIREMENTS FOR AN ALL-PURPOSE AFV
War experience clearly demonstrated the need for universal type AFVs that would replace all existing types, and could be produced on a scale never before contemplated. It was decided to do without a revolving turret and to mount the gun in the front glacis plate of the hull. Ever effort was made to ensure the maximum degree of simplicity, reliability, high mobility and robustness.
It was estimated that the man-hours required to produce a single Panther tank would be sufficient to produce three of the new vehicles, with the same firepower. It was argued that the lack of all-round turret traverse, was somewhat offset by the greater mobility, lower silhouette, and thicker frontal armor.
ORIGIN OF THE RIGID GUN MOUNTING
More than one German designer claims to have been the first to suggest eliminating the recoil mechanism from AFV gun mountings, but the first practical work was carried out in 1943 when Prof. Waninger of Rheinmetall-Borsig and Col. Schaede of Speer's Ministry both started to take an active interest. Most of the work was done under the direction of Prof. Waninger, first by Alkett and later by Rheinmetall. The principle advantages claimed for the elimination of the recoil system are: The main disadvantage was the difficulty of absorbing the shock of discharge. No attempts were made to mount the gun rigidly in a turret at this time.
DEVELOPMENT
The first practical experiment was a trial at Unterluss with a 12cm Russian mortar mounted with the barrel approximately horizontal and its breech piece supported against the center of a sloping plate. This plate was rigidly mounted on a PzKpfw.II chassis, said to be basically that of the "wespe". There was no traverse incorporated and the only elevation adjustment was bu shortening or lengthening the the member which supported the barrel near the muzzle.
10 to 20 rounds were fired, and the vehicle which weighed 11 tons, recoiled approximately 8" with the brakes off and 6" with them on. The crew were able to sit safely behind the plate while the mortar was being fired. Some surprise was felt that the equipment suffered no damage whatsoever. The shock of discharge was only 15% to 20% less than that of the 7.5cm Stu.K.40(L/48) and the experiment was considered conclusive proof that the idea of regid mounts was sound. The weight of the vehicle was the dampening element, and this type of mount could never be considered for field artillery or light vehicles.
FIRST ALKETT EXPERIMENTAL EQUIPMENT
At the beginning of 1944, Col. Schaede instructed Alkett to produce 2 experimental equipments consisting of a 7.5cm Pak 39(L/48) rigidly mounted in a PzJag.IV chassis. The gun was to have limited traverse and elevation, and suitable sighting arrangements were to be incorporated. The gun was offset approximately 6" to the right of center, being carried in a gimbal type mount, enclosed by a bell-shaped cast housing. The inner portion of the gimbal mounting consisted of a tubular cradle with an extension to the left rear. The gun barrel was rigidly secured in the cradle. The traversing and elevating gears were of conventional design and necessitated a heavy carrying bracket on the left side of the gun. This bracket, together with the gears, constituted a large asymetrical mass, which on firing, exerted an undesirable movement on the eccentric gun mounting. The traversing and elevating gears were by no means simple to produce and had to be robustly constructed if they were to stand up to the direct action of the accelerations of the glacis plate.
The sight mounting was similar to previous assault guns and traversed with the gun. The elevating motion of the gun was transmitted to the sight by a parallel linkage. This sight mounting necessitated a large opening in the hull roof, which was a bad feature, even though the opening was covered by an arcuate slide.
Electric firing was provided, but the breech had to be opened and closed by hand. The deflector guard for ejected cases also served as a loading tray, and was constructed in the form of a spring-loaded flap.
TRIALS OF THE FIRST ALKETT EQUIPMENT
This equipment was first tested at Kummersdorf in Sept/44 and proved itself able to stand up to rigid mounting. However, other serious problems soon made themselves apparent, these being mainly concerned with the traverse and elevating gears, and sighting arrangements. On firing, the gun layer was subject to acute discomfort. The shock transmitted to the traversing handwheel was so violent that he could not use his hand for about 2 days after each shoot, and the sight was frequently shaken from its mounting. Other fittings also broke away.
Accuracy was poor against targets at 500m and 1000m range and this was traced to the asymetrical mass of the traversing & elevating gears and the eccentric mounting of the gun. Damagae to the sighting telescope on firing was frequent and attempts were made to cure this defect by using resilient mountings. Finally, Zeiss developed an improved sight mounting which gave even more promise. Various shock absorbing devices for the traversing handwheels were tested, and a bit of improvement was obtained. Detailed reports were issued by Wa Pruf 4 and Wa Pruf 8 which showed that the greatest accelerations occurred in the least expected parts.
SECOND ALKETT EQUIPMENT: HETZER
The development of this project was being pushed forward on a high priority, and now Col. Schaede's procurement department gave an order for 100 specimens of this new equipment on the PzJag.38(t) "Hetzer" chassis, even though the defects had yet to be overcome. Improvements were to be incorporated into the new vehicles as trials proceeded. The gun was again mounted in a gimbal mounting, offset to the right and traverse and elevating gears were arranged in the old manner, thus weakening the plate. The sighting arrangements were altered but the equipment as a whole was, if anything, less satisfactory than before.
As soon as possible, Alkett started trials at Hillersleben and it was soon seen that the vehicle was not ready for use. It was still impossible for the layer to hold the handwheels, and observation was out of the question because of the violent oscillation of the sight. The zone of dispersion at 500m range was 80cm high by 120cm wide. They had, however, been ordered to start serial production, since Hitler had fixed April 20, 1945 as the date they should enter service. However, once the trial results came out production was stopped. By that time about 100 pieces had been produced and what to do with them caused concern.
RHEINMETALL-BORSIG TAKES OVER
Since everything possible had been done to this point, the firm Rheinmetall-Borsig, and more specifically Prof. Waninger and his assistant Obering Mehlert were called in. Waninger was asked to advise whether these defects could ever be overcome. The two Alkett equipments were sent to Unterluss for study. Waninger left Baurat Elten in charge of the supply side and summoned Obering Krum to Unterluss to organize the work program.
Detailed measurements were taken using an electrical instrument developed for aircraft research. Extreme high local accelerations were recorded, reaching 380g in the handwheel and 400g in the sight bracket. The decision was to use larger masses. The acceleration of the barrel could only be reduced by increasing the portion of the AFV which was accelerated together with the barrel. With this end in view, the mantlet housing was to be made an accurate fit against the front plate. Previously, the abutting surfaces were left in rough condition, and this gave am air space of 3mm, which halved on firing. By welding both parts together, greater rigidity and more effective mass was obtained.
From an analysis of the results the following main conclusions were reached:
There was a possible solution by increasing the mass of the front plate, but this was ruled out by official requirements regarding economy of armor. No substantial reduction would occur unless the handwheels and sight were separated from the gimbal mounting. It was shown that the pannier sides showed the least vibration, along with the belly plate. Gun mount, sight mount and traverse gear all were redesigned and incorporated into the new Rheinmetall equipments.
THE RHEINMETALL DESIGNS
The first improvement, a combined screw-spindle traversing & elevating gear was produced and mounted in the second Alkett Hetzer chassis in Dec/44. It was remarkably successful. The accelerations of the handwheels were no greater than in normal field guns, and the lateral defelction at 500m was only 70cm now. The gear was arranged between a bracket on the floor (where vibration was low) and the bottom of the breech ring and central in the vehicle, thus reducing lateral acceleration to 50%.
The traversing spindle and shaft driving the elevating spindle through bevel gearing was arranged co-axially in the bracket on the floor and thus the 2mm - 3mm rearward movement of the gun on firing would merely cause the gear to pivot about the spindle and shaft. The available gun movement with this gear was 8 degrees traverse each side, and elevation from -8 degrees to +15 degrees, better than the normal Hetzer.
The next improvement was a new form of sight mounting and linkage which allowed the sighting telescope to be mounted in the roof, and separate from the gun mount. The acceleration measurements had proved the roof to be unsuitable, so the the pannier side plates were chosen, and the sight was mounted on a transverse bridge bolted to these plates, and a flexible element was introduced. The sight to be used was the latest German design, the W.Z.F. (Winkelzielfernrohr) which was to be the standard tank and SP gun sight. An ingenious linkage was designed for transmitting gun movement to the sight. The main feature of this linkage was the use of an arm or single main link which transmitted traversing motion bay a push-pull action and motion in elevation by acting as a lever. A specimen of this linkage had already been tried out with a normal recoiling 7.5cm Pak 39(L/48) and been officially approved by Oberleutnant Neumann of Wa Pruf 4. With this type of sight mounting, the large arcuate opening in the roof was eliminated.
By the end of 1944 the sight mounting was ready for firing trials, but once again the sighting telescope was damaged. The cause was the main link of the linkage, which in order to be stiff enough needed to be massive. It was now proposed they use the "Starrbau" (rigid structure) type of construction (developed by the firm Starrbau of Feldafing near Munich) for this link. A gas operated semi-automatic gear was also developed, the propellant gases being diverted thru a vent in the barrel, and showed great promise.
A further improvement was the substitution of a ball type mounting for the original Alkett-designed gimbal mounting. It was proposed to use this ball mounting for completely new and redesigned equipments consisting of the L/48 and L/70 guns rigidly mounted in the PzJag.38(d). This ball mounting had the advantages that the opening in the glacis plate could be reduced in size (820mm x 430mm instead of 1105mm x 715mm), and that the ball completely sealed the opening. This reduced size permitted the use of a smaller experimental mantlet shield.
TRANSFER OF DESIGN & MANUFACTURING TO PRAGUE
By March 1945 there were scarcely any facilities for producing this equipment in central Germany and fresh air raids on the Unterluss proving ground were expected daily. At the same time a specimen of the modified PzJag.38(t) had nearly been completed at the BMM plant in Czechoslovakia. The rigid gun was removed and shipped to Prague, and within 3 days of arrival it was installed in a modified PzJag.38(t). The equipment was to have been demonstrated in Kummersdorf on April 16 before Gen. Leeb and then shown to Hitler at the Chancellery in Berlin on April 17.
POSITION AT THE CLOSE OF HOSTILITIES
By the end of the war the following rigid gun mounts were in hand:-