  We explore the suitability of ultracold collisions between spin-polarized
SrF($^2\Sigma^+$) molecules and Rb($^2$S) atoms as elementary steps for the
sympathetic cooling of SrF($^2\Sigma^+$) molecules in a magnetic trap. To this
end, we carry out quantum mechanical scattering calculations on ultracold
Rb+SrF collisions in a magnetic field based on an accurate potential energy
surface for the triplet electronic state of Rb-SrF developed ab initio using a
spin-restricted coupler cluster method with single, double and noniterative
triple excitations [RCCSD(T)]. The Rb-SrF interaction has a global minimum with
a well depth of 3444 cm$^{-1}$ in a bent geometry and a shallow local minimum
in the linear geometry. Despite such a strong and anisotropic interaction, we
find that converged close-coupling scattering calculations on Rb+SrF collisions
in a magnetic field are still possible using rotational basis sets including up
to 125 closed rotational channels in the total angular momentum representation.
Our calculations show that electronic spin relaxation in fully spin-polarized
Rb-SrF collisions occurs much more slowly than elastic scattering over a wide
range of magnetic fields (1-1000 G) and collision energies ($10^{-5}-10^{-3}$
K) suggesting good prospects of sympathetic cooling of laser-cooled
SrF($^2\Sigma^+$) molecules with spin-polarized Rb($^2$S) atoms in a magnetic
trap. We show that incoming $p$-wave scattering plays a significant role in
ultracold collisions due to the large reduced mass of the Rb-SrF collision
pair. The calculated magnetic field dependence of the inelastic cross sections
at 1.4 $\mu$K displays a rich resonance structure including a low-field
$p$-wave resonance, which suggests that external magnetic fields can be used to
enhance the efficiency of sympathetic cooling in heavy atom-molecule mixtures.
