  We present a comprehensive study of s-process nucleosynthesis in 15, 20, 25,
and 30 $\msun$ stellar models having solar-like initial composition. The stars
are evolved up to ignition of central neon with a 659 species network coupled
to the stellar models. In this way, the initial composition from one burning
phase to another is consistently determined, especially with respect to neutron
capture reactions. The aim of our calculations is to gain a full account of the
s-process yield from massive stars. In the present work, we focus primarily on
the s-process during central helium burning and illuminate some major
uncertainties affecting the calculations. We briefly show how advanced burning
can significantly affect the products of the core helium burning s-process and,
in particular, can greatly deplete $^{80}$Kr that was strongly overproduced in
the earlier core helium burning phase; however, we leave a complete analysis of
the s-process during the advanced evolutionary phases (especially in shell
carbon burning) to a subsequent paper. Our results can help to constrain the
yield of the s-process material from massive stars during their pre-supernova
evolution.
