1 a Roadmap for a Tentative Explanation of Bernd
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A ROADMAP FOR A TENTATIVE EXPLANATION OF BERND ROSEMEYER’S JANUARY 28, 1938 ACCIDENT (*) by Aldo Zana This document is a working draft focusing on technical and engineering issues. It has been written as a comprehensive background for future essays on the subject. Its value, if any, stays with the exclusive use of primary sources, i.e. Auto Union files, 1938 German newspaper articles, original photos, books by the relevant persons. While data, quotes, and statements are all taken from the above sources, explanations and comments are the author’s full responsibility. PROLOGUE November in Saxony: at Auto Union Rennwagenabteilung (Racing Car Department) in Zwickau, everybody is busy working on the new 3-liter Grand Prix cars for the next season. They just came back full of glory and pride from the October Rekordwoche (Record Week) on the Frankfurt- Darmstad Reichsautobahn. The 15 world and international records set by Bernd Rosemeyer were, at last, a most sought after reward after a season in which Mercedes-Benz regained its leading edge. On November 27, 1937 a newsflash hits the busy routine of the Department: Mercedes will go for record hunting the following day, on the Frankfurt- Darmstadt motorway. Eberan flies immediately to Frankfurt, only to witness an empty Autobahn: no Mercedes in sight. He also learns that they have asked permission to close the Autobahn for records in advance of the 1938 Berlin Auto Show, due to open on February 17. AU cannot stay idle when the gauntlet has been thrown onto the table. Eberan recaps the issue in plain words: they “couldn’t allow to see Mercedes- Benz win the records back from AU without any resistance, furthermore as the car, in the Rekordwoche, hadn’t yet reached its top speed, estimated at 415 km/h”. They switch the focus from the new Grand Prix cars to the improvement of the 1937 streamliner, following the plans already outlined since September together with Dr. Porsche. When Mercedes-Benz announces to be ready to go by the end of January 1938, AU managers feel obliged to share the racecourse. Adolf Huhnlein, the head of NSKK, the German Nazi Motorsports Guild, gives his approval to reserve the racecourse for both companies on Thursday, January 27, free of charge, courtesy of NSKK. In the late morning of that rainy and hazy January 27, 1937, the Messerschmitt Me-108 Taifun , mark D-IMXA, personally flown by Bernd Rosemeyer lands at Frankfurt airport. No record runs, today, but tomorrow the weather forecast points to a cold and clear day. The date is set. THE 1938 STROMLINIENWAGEN The February 11 internal memo by Eberan provides many details about 1938 Stromlinienwagen, listing the modifications from October 1937. It also clearly lists the main results of wind tunnel measurements. 1 The memo is well known, but from a German archive another version of the same document surfaced with handwritten brackets and the headnote “Dr. Porsche”. It could be a deliberate act of censorship: all bracketed words are related to proposals, designs, actions taken by dr. Porsche, the creator of the 1938 car, but no longer associated with it, as his contract with AU had expired at the end of 1937. The latter draft doesn’t have any signature, while the already known one is clearly signed by Eberan. The car selected for being transformed into the 1938 model is the one driven by Fagioli at Avus 1937 and by Rosemeyer in the June and October record runs. Its body, following Porsche’s advice, has been built sturdier and heavier than the first one. The car is a workhorse, extensively tested, reliable, and deeply familiar to the driver. Changes in engine, transmission, wheel and tyres Aiming for higher speed, engine capacity is enlarged to 6.5 l through a 78 mm bore (75 mm in the original 6 l engine). On December 27, on the test bed, the larger engine reaches 560 HP at 4,800-4,900 rpm, a significant improvement over the 525 HP of the 1937 engine. The new, optimal, transmission ratios don’t work: they have to select a 31:23 “never used before” and a 15:40 end ratio. Such a ratio selection, using 24” wheels translates into 456 km/h at 5,000 rpm. Continental produces new tyres and new 24” wheels: the wheel fairings on the 1937 Stromlinienwagen have, therefore, to be reshaped and ground clearance adapted. Stiffer torsion bars are mounted all around. Following new wind tunnel test with a model integrating all body changes, a slightly negative setting is selected to optimize downforce and drag. Engine, final transmission, wheels, tyres, and suspensions make up a totally new, untried system. Changes in the cooling system News on Eyston’s land speed record achieved without front air intake and with ice cooling, combined with data from wind tunnel tests, ease the way to the most striking change on the 1938 car: ice cooling, without the radiator. A new 40 kg heavy water tank, holding up to 180 l, replaces the fuel tank behind the driver. To shield him from the heat, a 10 mm asbestos plate is mounted on the front side of the tank. It’s a bulky thing, with stiffeners to let it go up to a steam pressure of 2 atm without blowing up. Water pipes to and from the engine are engaged directly to the tank. The front radiator is kept in place. Following wind tunnel data, a set of plates to close the front intake is readied: they could be put in place on L shaped brackets, bolted to the top and the bottom of the nose. The small holes visible in the body, above the intake, should have been drilled for this purpose. The car could run for up to 55 sec without overheating. Through the help of the near freezing temperature of that day, the cooling in Run 1 and 2 has always been nominal. The 6 air outlets on the floorpan are sealed, and three new outlets added in the same 2 mm dural plate. The driver could seal the three new outlets through a shutter, by pushing a pedal. The plate with all outlets is mounted using not less than 28 6 mm screws. 2 It’s bread and butter for today aerodynamicists that the inner flow in a hi- speed vehicle is more critical, in terms of overall stability and performance, than the outside flow. It’s a truism known since the early days of motoring, and routinerly applied in planes, yet it is something undeserving too much attention at the speeds attained by racing cars until late Thirties. Apparently, AU designers and Dr. Porsche put aside any thought on the changes in the inner aerodynamics of a vehicle travelling at 400-plus km/h. It’s a typical engineer’s approach to rely on the know-how gathered at lower speeds, say at 300 km/h (then already at the reach of a GP racer), and on safety margins built into the system. It could be a safe platform for extrapolating higher performances, if only the modelization was correct: in the 1938 streamliner, this was not the case, due to the lack of computational and measurement tools and to a science still in its infancy. In the real world, jumping from 300 to 400 km/h without an in-depth study of all forces and parameters, is like venturing into outer space on a sailboat. In Runs 1 and 2, the front air intake and the three air outlets were left open, thus preventing any perceivable ground effect because of the vortexes generated by the merging of two airflows at the same temperature, but with different velocity and pressure. Sealing the outlets in Run 3 altered profoundly the inner airflow. The air, though reduced by the partial closure of the intake, could now escape only through the gap around the head of the driver and the small openings in the engine bay. The closed outlets also made up the setting for effective downforce: maybe, it was the first time in motoring history in which it was exploited with a significant effect. Once again, the whole system was untried. Yet, AU pushed it into the unknown. Changes of the body The lower and upper fairings were added to the existing body in a modular way by bolting them in place: 20 screws for the upper one, 23 for the lower centre “miniskirt”, 12 for the lower rear fin, 5 for the lower front “shaper”. 10 mm steel studs, bolted to the inner side of the aluminum body, receive the same 6 mm screws used throughout the car. The upper fairings had to be taken away to load the car on the transporter. The lower ones were already in position when the car arrived on the Autobahn. Against any rule of good construction, the flush mounting of the upper fairings doesn’t use any cover of the junction, to make it smooth and sturdy enough to withstand aerodynamics forces. The upper fairings are shaped from 1 mm aluminum strengthned by an inner structure. They are reported by Eberan as “strong enough to carry the weight of a man either sitting or laying on them”. The windshield has an enveloping shape with a painted upper section. From the photos, it appears rather thin: about 3 mm. In October 1937, it worked, and, as such, it never came under scrutiny. THE WIND TUNNEL MEASUREMENTS Christmas vacations affect the wind tunnel measurement program, to be performed on a 1:2.5 scale model of the new body.