Sonderfahrgestell auf Flak 41

Model by Mike Kendall

By: Mike Kendall


Toward the end of 1942 the allied air effort in World War II began to pay off with general air superiority above the western front. Germany's chief armaments officials continued to search in earnest for anti-aircraft guns in new and different forms to counter what had become the major threat to the land forces.

Krupp's fully tracked Sonderfahrgestell (special purpose built vehicle) was designed as just such a vehicle type. Completely new in concept, the Sonderfahrgestell provided a tracked undercarriage with armored cover for the driver and radio operator while offering a large, open gun platform. This concept was envisioned to replace the aging Panzer IV as a basis for many types of fully- tracked gun and personnel carriers. Unfortunately for Krupp, there was much competition for this medium tracked vehicle category. Eventually, the combined Panzer III/IV and Panther chassis won the general approval of the German High Command, due to the vehicles' proven track record and their lower production costs.

Throughout the field trials, the Luftwaffe showed the most interest in the Sonderfahrgestell, particularly as a mobile gun carriage for their famous 88 mm flak guns. The first gun to be mounted for trials in 1942 was the then "state of the art" Flak 37. Three were built and tested but found lacking in protection for their crew, as the sides of the vehicle had to be lowered for full traverse of the gun, leaving the crew vulnerable to small arms fire and shell splinters.

Krupp did not give up, however, and in early 1944 the brand new Flak 41 gun was mounted in place of the Flak 37. This new form of the 88 mm gun far surpassed the performance of its predecessor and was mounted on a lower crucifix wheeled mount. Maximum velocity and ceiling of the Flak 41 was 1,000 meters/sec and 14,700 meters, compared to 820 meters/sec and 10,600 meters for the Flak 37. In addition, the shelf was larger, contained more explosive, and was deadlier.

Once again, the lack of crew protection and the high production start-up costs doomed the vehicle and full scale production efforts were finally dropped by Krupp in the fall of 1944. Perhaps by that time no amount of anti-aircraft activity could have saved the crumbling Third Reich.


I have been intrigued with this vehicle since I first saw it a few years ago in a German tank book. Low slung, additional plate armor, lots of strange wheels, a BIG gun ... I was hooked! This model is easily my most ambitious building project to date for I have only recently taken the step from conversions to scratch building. The model is primarily scratch built with the exception of a few components listed later. In-depth reference material was very hard to find and to the best of my knowledge there are no accurate drawings of this vehicle. I used two different drawing sets, one from an old Scale AFV Series drawing by Tyson and the other from a German language armor encyclopedia, and made obvious corrections from the few available photographs I could round up from friends and museums.

Basically, the model is made of Evergreen styrene plastic sheet, rod and strip, held together with liquid cement and super glue. The hull, gun, and running gear were made as separate components and were combined at the end of the building process.

Construction began in earnest with the boxy hull; measuring the major pieces of the bottom, sides and front/rear plates, and then scribing, snapping and gluing them with liquid cement. After the main hull plates had set up the gaps and edges were puttied and sanded smooth, and probable weld beads were added at the estimated correct distance from each edge with thin strip, later worked with a light hand and a cutting head in a dremel tool.

In order to get the sexy curve of the front fenders the inside support plate was made as an extension of the hull side plate. The thin fender pieces were bent around a dowel in this approximate radius and then immersed in boiling water for a few seconds. Be careful and keep your fingers out of the water. The resulting bend was close enough so that when glued to the side plates they conformed well. The front fender flaps then were scribed on the ends and the additional hinge detail was added from small bits of wire and lead foil (from the wine bottle I drank later on, but I'm getting ahead of myself.

The rear engine cover screens are Dr. Microtools brass with strip plastic edges. The rivets found here and there are made from sheet plastic with a borrowed Waldron punch, then glued on with liquid cement. A pair of dividers was helpful to provide an even spacing between rivets.

The side and back plates are movable with hinges made from drilled out plastic rod and fine brass wire. The drilled plastic rod is glued to one piece and the wire passed through, bent at both ends and glued into holes drilled into the second piece. I have used this technique before were the hinges are of the heavy armor type and it seems to hold up to the test. The side and rear plates are double thickness and are sandwiched around a framework of strip to give the impression of spaced armor. More weld beads here.

Once the hull was mostly complete I began work on the gun. The barrel is made from a couple layers of telescoping plastic tube glued together to give the right overall thickness and then turned on my old drill press to make the taper. Ingenious folks could probably do this with a motor tool and a speed reduction box, but with the drill press I could chuck the tube directly and cut that section off later. The breech is a box built up around the end of the barrel and details were added as I could find them in the references.

The gun cradle, frame, associated controls, etc., were fashioned from sheet, rod and strip, with occasional brass rod used for bent pieces (such as the seat supports). I made the gun rotate on its platform using a screw and washer arrangement from below. The barrel elevates to the full 90 degrees of the prototype with the use of a gear made by filing teeth along a small piece of tubing and then slicing off a piece. The gear does not rotate because it acts as a support to keep the gun elevated at whatever position it is placed in.

The reference photos showed three seats for operators on the right side of the gun: two for elevation and one, near the shield, for traverse. On the left side of the gun is the fuze setter, capable of automatically setting the end cap fuzes of two shells at once for detonation time or height. The dial indicators are built up with a laminated layer system similar to the one airplane modelers use for instrument panels. The first sandwiched layer is clear plastic sheet taken from an old report folder. Behind that is a layer of sheet plastic with dial holes, and behind that are the dials, photo copy reductions from instrument dials used for RC planes. The sandwich is then cut to size, the edges wrapped with strip to simulate the casing, and tada, dial indicators! They look much better than those in the original Tamiya 88 kit.

The shell casings are solid brass rod of the right diameter (including the end flange, unless you want to glue on a disk), first shaped on the same old drill press with files and sand paper and then drilled out. This was easier than I expected and now opens up many possibilities for shell casings in the future. The trick is to get each shell to look about the same, so my crude method only works when you need just a few and they are not stacked next to each other.

On the other hand, the hardest part of constructing the Flak 41 gun was fabricating the gun shields. Having no drawings of these means trying to size the pieces by comparing them to nearby vehicle parts I did know the size of and then trying to get all the pieces to fit together at the right angles. Creating these angles was a matter of judging the cuts, trimming and testing, discarding and trying again, until the pieces looked about right. I initially wanted the side shields to fold as in the real thing so that the vehicle side armor could be completely folded up, but that was asking to much from me. I had to walk away from the project a couple of times during this stretch (hence the wine bottle) and return later the next day when I had calmed down enough to be trusted with sharp objects. Save the pieces that you screw up and use the plastic for the tiny bits of stuff needed here and there later (I had a lot).

The main wheels were resin cast (a first for me) from molds made from my own masters (modified Tamiya early Tiger I dished wheels- the old kit) and include two types, star ribbed and holed. My scarce photo references showed two types of ribbed wheels, I choose the ones with fewer ribs that looked easier to duplicate. The layout and drilling of the holes on the other wheel type was also tricky but I only needed one master, and I had all the wheels from the kit, so eventually I made one that looked right. The back sides of both wheels was hard to see on the finished model so I made them flat so they all could be produced in a one piece mold. I used RTV rubber for the molds and a urethane resin for the wheels, both purchased from a supply house that specializes in this kind of stuff. After calling the company and talking to a sales person I was led to the products that worked the best. Believe me, it is not hard to cast presentable small parts. I think the hard part is making great looking parts.

Continuing with the suspension, the drive sprocket is modified from a Tamiya Sturmgeschutz and the idler is from an ltaleri Panzer IV (Krupp also manufactured these vehicles). The idler is too big but I couldn't find one the right size and making one of these looked a little to daunting to me. Maybe someone else can suggest a smaller tubular idler.

The wheels are attached to the huff with trailing arms (the suspension was typical German torsion bar) made from tubing and thick strip glued together to look like the traditional drop forged originals. These were all aligned more or less even with the hull by marking their locations, supporting the hull on a block of wood the correct height from the table, and gluing the wheels on so they all touched the table and were in line. No easy feat! I used slow curing super glue and a length of tube running in the slot between the pairs of wheels to help get them aligned. The end result was that the wheels were lined up well enough so that when the track was added it traveled in a straight line without wiggles along the length (almost). Take your time with this process because continuously out of line wheels will throw off the appearance of the whole model (if/when you judge contests watch for this on scratch built models and you'll be surprised how many have problems).

The tank track is DML, from one of the first Nashorn kits before they were corrected. A problem here was that the tracks wanted to slew off in a circle as the line grew longer and I had to keep an eye on them to straighten them out as I glued them together, one by one. I do not know if this was a result of my building or the track link, but watch out. I HATE individual link track! There, I said it and now I feel better. So why use it? Since the major feature of tracked vehicles is their track, it follows that the track should look as realistic as possible. The older rubber band track often only has detail on one side and the rubber makes cleaning up the flash very difficult. You can also droop individual link track easier for great effect. The best solution to me is the type used by ESCI/AMT and others, longer lengths for top and bottom and individual links for the wrap around ends. Another possibility would be to make links that snap together and do not require glue. Someone should go into business.... The pioneer tools were scratch built or came from my spares box.

Speaking of scratch built, the other parts of the model not scratch built include one of the hand wheels on the gun carriage (I got tired of making hand wheels from sliced plastic tube), and the helmets on the packs, also Tamiya pieces. The packs themselves are tissue bundles soaked with white glue and then peeled off my fingers with tweezers.

Once the major sub-assemblies were completed they were airbrushed with slightly thinned Model Master and Tamiya acrylic paints and then weathered with washes and pastels. The idea behind the weathering was to represent the vehicle after a heavy day on the test range. One of these days I will learn to lighten up with my weathering; sometimes I get carried away with the pastels and have to remove more than I apply. The pieces were then assembled, the smaller detail pieces were added, and the monster began to look like the totally weird vehicle it was.

In conclusion I have to say that a lot of people helped on this project: from reference, to resin casting, to scrounging kit parts, to critiquing the finished product. Without their help and motivation I doubt if I would have attempted this project at all. You may have noticed by now that exact authenticity is not as important to me as learning new techniques and having fun. For me, the challenge for each new model is to try something new, to add new abilities to my modeling tool kit. I highly recommend attempting new feats with each model. It keeps the hobby exciting and new for me and continues to open new horizons of possibilities.

News Flash! I now understand that this vehicle has just been introduced in 1/35 scale by Accurate Armour in Great Britain, with the Flak 37. The Flak 41 with wheels and mount, likewise, has been released by a resin company in France. I sure would like to get hold of these and see how we differ. I do know one thing, it took me about two months, around $30 in materials, and a lot of sweat and frustration to make the Sonderfahrgestell. I guess you could buy these two super kits for about 10 times that much, but ... what about the sweat and frustration? Now let's see, what should the next project be?

© 1998, Mike Kendall

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