  We develop a physical model for how galactic disks survive and/or are
destroyed in interactions. Based on dynamical arguments, we show gas primarily
loses angular momentum to internal torques in a merger. Gas within some
characteristic radius (a function of the orbital parameters, mass ratio, and
gas fraction of the merging galaxies), will quickly lose angular momentum to
the stars sharing the perturbed disk, fall to the center and be consumed in a
starburst. A similar analysis predicts where violent relaxation of the stellar
disks is efficient. Our model allows us to predict the stellar and gas content
that will survive to re-form a disk in the remnant, versus being violently
relaxed or contributing to a starburst. We test this in hydrodynamic
simulations and find good agreement as a function of mass ratio, orbital
parameters, and gas fraction, in simulations spanning a wide range in these
properties and others, including different prescriptions for gas physics and
feedback. In an immediate sense, the amount of disk that re-forms can be
understood in terms of well-understood gravitational physics, independent of
details of ISM gas physics or feedback. This allows us to explicitly quantify
the requirements for such feedback to (indirectly) enable disk survival, by
changing the pre-merger gas content and distribution. The efficiency of disk
destruction is a strong function of gas content: we show how and why
sufficiently gas-rich major mergers can, under general conditions, yield
systems with small bulges (B/T<0.2). We provide prescriptions for inclusion of
our results in semi-analytic models.
