  The petrologic and geochemical diversity of meteorites is a function of the
bulk composition of their parent bodies, but also the result of how and when
internal differentiation took place. Here we focus on this second aspect
considering the two principal parameters involved: size and accretion time of
the body. We discuss the interplay of the various time scales related to
heating, cooling and drainage of silicate liquids. Based on two phase flow
modelling in 1-D spherical geometry, we show that drainage time is proportional
to two independent parameters: $\mu_m/R^2$, the ratio of the matrix viscosity
to the square of the body radius and $\mu_f/a^2$, the ratio of the liquid
viscosity to the square of the matrix grain size. We review the dependence of
these properties on temperature, thermal history and degree of melting,
demonstrating that they vary by several orders of magnitude during thermal
evolution. These variations call into question the results of two phase flow
modelling of small body differentiation that assume constant properties.For
example, the idea that liquid migration was efficient enough to remove
$^{26}$Al heat sources from the interior of bodies and dampen their melting
(e.g. Moskovitz and Gaidos, 2011; Neumann et al., 2012) relies on percolation
rates of silicate liquids overestimated by six to eight orders of magnitude. In
bodies accreted during the first few million years of solar-system history, we
conclude that drainage cannot prevent the occurrence of a global magma ocean.
These conditions seem ideal to explain the generation of the parent-bodies of
iron meteorites. A map of the different evolutionary scenarios of small bodies
as a function of size and accretion time is proposed.
