Galactic Patrol by E.E. Smith

Barely a month after his graduation, even before he had entirely completed the postgraduate tours of duty mentioned by Von Hohendorff, Kinnison was summoned to Prime Base by no less a personage than Port Admiral Haynes himself. There, in the admiral's private aëro, whose flaring lights cleared a path as though by magic through the swarming traffic, the novice and the veteran flew slowly over the vast establishment of the base.

Shops and factories, citylike barracks, landing fields stretching beyond the far horizon; flying craft ranging from tiny, one-man helicopters through small and large scouts, patrol ships and cruisers up to the immense, globular superdreadnaughts of space -- all these were observed and commented upon. Finally, the aëro landed beside a long, comparatively low building -- a structure heavily guarded, inside the base although it was -- within which Kinnison saw a thing that fairly snatched away his breath.