Unobtanium

Thus the invention of high temperature fuels will enable us to consider cores which are radiation cooled; such cores would be surrounded by large tanks (cooled by water) which in turn would be penetrated by beam holes. The beam holes themselves would probably have to be made of "unobtanium". (This "unobtanium" is a word coined at Atomics International to describe the material needed to make reactor projects feasible.) In between the radiation cooled reactors and the water cooled reactors is a regime suitable for liquid metal (including boiling liquid) cooled reactors. Once again "unobtanium" is needed.

As early as 1969, during the initial Phase A studies, Lockheed had taken the initiative in proposing a two-stage fully-reusable design with both stages built of aluminum and using silica tiles for thermal protection. [...] Though that company indeed was in the forefront in developing such tiles, they were items for laboratory research. A design that specified their use had no more intrinsic credibility than one that proposed to use the miracle metals Unobtanium and Wishalloy.

It was this blend of genuine, authentic Bentley parts with his recreations of the unobtanium bits that had Patrick thinking that this ruse would actually work.

We might recognize, here, the blind pursuit of our own unobtaniums and the ways in which our ancestors forged ahead without care for the consequences of their actions to native peoples or to their environs.

[D]oes our device require any technologies that we're not likely to be able to develop, buy, or otherwise obtain? This unobtainable content is sometimes called unobtanium, and can take various forms: [...] Things that exist, but don't exist for us. If Acer needs a beautiful custom 15″ LCD panel that costs less than $50, they can get it, because they'll buy millions and millions of them. But for the vast majority of us, asking an LCD panel vendor for such a thing will yield little more than a chuckle.

People who think about space end up thinking about bullshit like Star Wars and Star Trek. [...] They have to invent unobtaniums, magic crystals that, for some reason, can't just be printed out by their transporter beams, or there's no story.