Unobtainium is a term used in fiction, engineering, and common situations for a material ideal for a particular application but impractically hard to get. Unobtainium originally referred to materials that do not exist at all, but can also be used to describe real materials that are unavailable due to extreme rarity or cost. Less commonly, it can mean a device with desirable engineering properties for an application that are exceedingly difficult or impossible to achieve. The properties of any particular example of unobtainium depend on the intended use. For example, a pulley made of unobtainium might be massless and frictionless. But for a nuclear rocket, unobtainium might have the needed qualities of lightness, strength at high temperatures, and resistance to radiation damage: A combination of all three qualities is impossible with today's materials. The concept of unobtainium is often applied hand-wavingly, flippantly, or humorously. The word "unobtainium" derives humorously from "unobtainable", with -ium, a suffix for chemical element names. It predates the similar-sounding systematic element names, such as ununennium. An alternate spelling, unobtanium, is sometimes used, perhaps based on the spelling of real elements like titanium and uranium. Engineering origin Since the late 1950s, aerospace engineers have used the term "unobtainium" when referring to unusual or costly materials, or when theoretically considering a material perfect for their needs in all respects, except that it does not exist. By the 1990s, the term was in wide use, even in formal engineering papers such as "Towards unobtainium [new composite materials for space applications]." The word "unobtainium" may well have been coined in the aerospace industry to refer to materials capable of withstanding the extreme temperatures expected in re-entry. Aerospace engineers are frequently tempted to design aircraft which require parts with strength or resilience beyond that of currently available materials. Later, 'unobtainium' became an engineering term for practical materials that really exist, but are difficult to get. For example, during the development of the SR-71 Blackbird spy plane, Lockheed engineers at the "Skunk Works" under Clarence "Kelly" Johnson used 'unobtainium' as a dysphemism for titanium. Titanium allowed a higher strength-to-weight ratio at the high temperatures the Blackbird would reach, but its availability was restricted because the Soviet Union controlled its supply. This created a problem for the U.S. during the Cold War because the Blackbird required huge amounts of titanium (and subsequent U.S. military aircraft such as the F-15, F-18, and F-22 fighters