  Collisions of white dwarfs (WDs) have recently been invoked as a possible
mechanism for type Ia supernovae (SNIa). A pivotal feature for the viability of
WD collisions as SNIa progenitors is that a significant fraction of the mass is
highly compressed to the densities required for efficient $^{56}$Ni production
before the ignition of the detonation wave. Previous studies have predominantly
employed model WDs composed entirely of carbon-oxygen (CO), whereas WDs are
expected to have a non-negligible helium envelope. Given that helium is more
susceptible to explosive burning than CO under the conditions characteristic of
WD collision, a legitimate concern is whether or not early time He detonation
ignition can translate to early time CO detonation, thereby drastically
reducing $^{56}$Ni synthesis. We investigate the role of He in determining the
fate of WD collisions by performing a series of two-dimensional hydrodynamics
calculations. We find that a necessary condition for non-trivial reduction of
the CO ignition time is that the He detonation birthed in the contact region
successfully propagates into the unshocked shell. We determine the minimal He
shell mass as a function of the total WD mass that upholds this condition.
Although we utilize a simplified reaction network similar to those used in
previous studies, our findings are in good agreement with detailed
investigations concerning the impact of network size on He shell detonations.
This allows us to extend our results to the case with more realistic burning
physics. Based on the comparison of these findings against evolutionary
calculations of WD compositions, we conclude that most, if not all, WD
collisions will not be drastically impacted by their intrinsic He components.
