In order to increase the useful flight range of the Lockheed P-38 Lightning for long distance missions, it was necessary to increase its fuel carrying capacity without appreciably decreasing its aerodynamic efficiency. Thus, a fuel container was needed that could be dispensed with when it ceased to have utility as a storage tank.
This project, which had been presented to Lockheed at the specific request of the Army Air Force, initiated an experimental program for designing a low-quantity production aluminum tank to determine functional performance. The wind tunnel tests and flight tests proved conclusively the feasibility of such a droppable tank, and Lockheed was given a medium-sized contract based on the experimental design.
This aluminum alloy tank incorporated a semi-monocoque structure of circular cross section and laminar flow contour. Since two tanks were to be hung under the wings, one in each bay between the fuselage and nacelle, it was possible to utilize the standard bomb shackles that were designed for P-38 center sections. The tank fittings which mated with those supports were manganese bronze castings, and were designed to tie in the two halves of the main supporting bulkheads. The original design called for split bulkheads to minimize final assembly operations. However, approximately 60% of all assembly time was spent after the two half shells were mated. This condition existed because of the limited access to the main bulkhead tie-ins mentioned above and also because of the excessive torch welding around the periphery to insure gas-tight seals.
Tools had been designed and tanks were being delivered then mass production requirements arose. And in addition to requiring over 200 tanks a day, the Air Force expressed a desire to have the tanks designed of a less strategic material than aluminum alloy.
At the time of that request the summer of 1942 Lockheed's engineering department had initiated its own critical material conversion program and was investigating the use of plastics, plywoods, and auto-body steels for just such secondary structural applications as this tank job. After preliminary investigations, from cost and production standpoints, plastics and plywoods were rejected in favor of auto-body steel construction. Of paramount importance to the decision of accepting steel was the basic material economy as well as the economic advantages of spot and seam welding, both of which are so readily adapted to this material.
The resulting tank is unique in "aircraft design," with all units fabricated out of low-carbon steel and almost 100 percent of all assembly work completed before the half shells are mated. The following is a brief summary of the design simplifications that were worked out with the full cooperation of the factory personnel which laid out the new plant set-up for handling the entire tank line. Probably the most spectacular of all improvements in this new "mass production tank" was the incorporation of a single half-shell stamping. These shells are now formed in a single operation, which is possible by utilizing the draw characteristics of deep-draw body steel. Of equal import in the minimizing of assembly time was the splitting of the main tank supports to allow complete half-shell assemblies. Those fittings are now welded low carbon steel units costing less than $4 per tank as compared with $34 for the manganese bronze castings.
Elimination of the castings inside the tank which had tied in the main bulkheads also allowed elimination of the final inspection holes. Standard aircraft tubing was replaced by commercial metal conduit, and four-piece built-up tail plugs were replaced by commercial pipe fittings. At the suggestion of factory personnel, the filler neck was redesigned to permit seam welding in one operation.
However, one of the most interesting production considerations initiated by the shop men was the development of semi-automatic roll-spot-welding machines for assembling five bulkheads simultaneously. With three operators, these machines are capable of putting out 400 spots per minute as compared with 15 to 30 single spots on the original tank. After final mating of the two halves, the outstanding flanges are seam welded around the entire periphery, thus eliminating all torch welding, except at the tail plug and pan intersections.
The factory coordinator who was assigned to this steel conversion job conferred with the manufacturing departments in the determination of all high-rate, low-cost production features, as those mentioned above. In addition, it was deemed necessary to set up a production line in some single location to take care of all fabrication and assembly problems from the first to the last operation, including painting and crating.
Lockheed 's available floor space, power and existing equipment were investigated, and the newly acquired plant at Maywood, CA, offered the best possibilities for the development of this project. After this decision was reached, a satisfactory layout was developed and built around a plan for using a continuous conveyor system. Much of the equipment that was set up on this line was secured from peacetime supplies, thus minimizing the need for new equipment.
This layout consisted of an overhead conveyor from which 36 cradles were suspended, each cradle holding one tank. All welding and assembly equipment was located along the line in accordance with the sequence of operations. Fundamentally, the layout embodied an assembling chain and a process shipping chain, both hung from overhead monorails. These two chains were synchronized to maintain a uniform flow of tanks through the slushing booth where the transfer from one line to another was made. The operations on the assembly conveyor consist of welding and mechanical assembly, with the cleaning, painting, and final inspection on the process line.
All of the above improvements, which were incorporated in the design and manufacture of the new tank, resulted in a cost saving of 1:4, plus a production time saving of 1:15. Such was possible only because of the close coordination and cooperation of all engineers and factory men.
This article was originally published in the June, 1943, issue of Aviation magazine, vol 42, no 6, pp 128-133.
The original article consists of the text above, one photo of a P-38 in flight, and 21 captioned photographs that illustrate various of the process steps described.
Photos are not specifically credited but are certainly from Lockheed.