Discovery One

Here we clearly see the hexagonal structure of the Discovery One spacecraft from 2001: A Space Odyssey. The engine housings as well as the connective structure between the engines and the habitation spheroid have an overtly hexagonal form to them.

The hexagon is widely considered the most futuristic of all polygons, of course, and its use here is reflective of the larger trend towards hexagonal motifs in modern science fiction. One can only assume, in this particular context, that the hexagonal nozzle housings offer superior resilience to the stresses induced by the gas core nuclear reactors.