Metal-clad airships are airships which have a very thin airtight metal envelope, rather than the usual fabric envelope. This shell may be either internally braced as with the designs of David Schwarz, or monocoque as in the ZMC-2. Only four ships of this type are known to have been built, and only two actually flew: Schwarz's aluminum ship of 1893 collapsed on inflation; Schwarz's second airship flew at Tempelhof, Berlin in 1897, landed but then collapsed; the ZMC-2 flew 752 flights between 1929 and scrapping in 1941; while the Slate City of Glendale, was built in 1929 but never flew. History Early designs One of the earliest proposals for a flying machine based on rational principles was Francesco Lana de Terzi's design for a vacuum airship, c.1670. He had measured the pressure of air at sea level and based on this he proposed the first scientifically credible lifting medium in the form of hollow metal spheres from which all the air had been pumped out. His proposed methods of controlling height are still widely used; carrying ballast which may be dropped overboard to gain height, and venting the lifting containers to lose height. In practice de Terzi's spheres would have collapsed under air pressure, and further developments had to wait for more practicable lifting gases. The concept of a metal-clad dirigible airships was again explored in the late 1800s by Russian rocket theorist Konstantin Eduardovich Tsiolkovsky. He wrote that since his teens (in the early 1870s) "the idea of the all-metal aerostat has never left my mind" and by 1891 he had produced detailed designs of a variable volume corrugated metal envelope airship that did not need ballonets. These were submitted to an Imperial department for aeronautics, which convened a conference to consider it. In 1891 they declined his request for a grant to produce a model, considering the idea "cannot have any considerable practical importance". In 1892 he published his designs as Aerostat Metallitscheski (the all-metal dirgible aerostat). At around the same time, in 1892 the Russian Imperial war ministry agreed to let Schwarz build his metal airship in St Petersburg, though at his own expense. Schwarz Schwarz's first aluminum ship of 1893 collapsed on inflation. His second airship flew at Tempelhof, Berlin in 1897, landed but then collapsed. Aircraft Development Corporation In 1926 the Aircraft Development Corporation announced in Detroit, USA, that they were planning to construct a