Throughout the years a number of novel aircraft have been developed. These craft for the most part provide an interesting, if not always significant, footnote to aviation history. One was the Navy’s “Tin Bubble.” Officially designated the ZMC-2, it was the world’s first, last, and only successful all-metal airship.
The idea of all-metal aircraft had suggested itself to aviation designers and engineers quite early. In 1897, David Schwarz constructed an all-metal airship in Germany, which consisted essentially of an aluminum framework covered with aluminum sheets. This craft was not successful. As early as 1915, however, successful all-metal airplanes were being built in Germany by Dr. Hugo Junkers. Subsequently the all-metal airplane idea was developed in other countries, until today practically all military and commercial airplanes are of all-metal construction. But to construct a successful all-metal, lighter-than-air craft was quite another problem—one which was not solved for many years.
Born of an attempt to build an airship which would withstand the ravages of the elements and of time, the ZMC-2 differed from conventional airships in that a metal hull replaced the usual fabric envelope. This hull, of some 200,000 cubic feet capacity, consisted of 12 circular Duralumin forms of varying diameter joined together by longitudinal members of the same metal, the whole covered with Duralumin sheets. These sheets, .0095 of an inch thick, were lap-jointed with three staggered rows of rivets, these placed a quarter of an inch apart. In order to place the 3,000,000 rivets required, an automatic riveting machine was designed and built.
The metal hull or envelope of the airship served a dual purpose as a gas container and as an essential part of the structure itself. Weight loads were transmitted directly to the rigid hull forms, while tensile and sheer stresses were taken by the skin.
Although possessing some of the structural characteristics of a rigid airship, where the shape of the hull is maintained by a rigid framework, in contrast to the non-rigid, wherein the shape is maintained by internal pressure, the ZMC-2 was actually a non-rigid, inasmuch as it depended upon internal pressure to maintain the rigidity and form of the hull while it was in flight.
The hull was 149 feet, five inches long, and 52 feet, eight inches in diameter. It was much shorter in length in proportion to diameter than conventional dirigibles, a fact which gave it a stubby, well-rounded, or roly-poly appearance. This fully rounded form caused some problems in stabilization. These were solved, at least to some extent, by using eight tail fins instead of the usual four. These eight fins were considerably smaller and were located farther forward on the hull than was the case with conventional airships.
A small Duralumin control car 24 feet long and some six feet wide was attached directly to the underside of the hull. This provided accommodations for a crew of three. A Wright J-5 200-horsepower air-cooled engine was mounted on outriggers on each side of the car. These engines gave the airship a cruising speed of 50 m.p.h. and a maximum speed of 62 miles per hour.
Delivered to the Navy in September 1929, the ZMC-2 was stationed at Lakehurst where it was used for training and experimental purposes. Costing the builders—the Metal- clad Airship Corporation, of Detroit, Michigan—$1,150,000, it was sold to the Navy for the conservative amount of $300,000, with the hope that the type would prove successful enough to ensure further development and further orders.
After six years of service, the ZMC-2 was deflated for overhaul and inspection in 1935 and the envelope was found to be in excellent condition. Up to the time of deflation, it had made 468 flights totalling 1,232 hours.
The Navy used the Tin Bubble for 12 years. At the end of that time the hull was still sound and still gas proof. Purely an experimental venture as far as the Navy was concerned, the ZMC-2 did not demonstrate any exceptional advantages over conventional types. Further, stabilization and directional control left something to be desired. Nevertheless, the idea itself of an all-metal dirigible did apparently have some value and merited some further study. To this end, contracts for the design of similar craft were made; however, no funds were ever appropriated for this purpose. In the meantime, the builders were drawing plans for larger ships, including a military model of 3,800,000 cubic feet capacity (larger than the rigid airship Los Angeles). This dirigible was to be powered with eight engines, totaling 5,000 horsepower. It was estimated that this craft would carry a crew of 40 and have a maximum speed of 100 m.p.h. However, nothing ever came of the plans to build this ship.
So, with the decomissioning of the ZMC-2, and her dismantling, the all-metal airship became just another historical footnote.