  We present a detailed hydrodynamic model that matches the present structure
of the well-observed preplanetary nebula OH231.8+04.2. The purpose of the model
is to present a physically justified and coherent picture of its evolutionary
history from about 100 years of the start of the formation of its complex outer
structures to the present. We have adopted a set of initial conditions that are
heavily constrained by high-quality observations of its present structure and
kinematics. The shaping of the nebula occurs while the densities of the flows
are "light": less than the surrounding AGB-wind environment. The simulations
show that pairs of essentially coeval clumps and sprays of the same extent and
density but different outflow speeds sculpted both the pair of thin axial flow
"or spine" and the bulbs. The total ejected mass and momentum in the best fit
model are surprisingly large -- 3 M_sun and 2.2 x 10^41 gm cm s-1, respectively
-- however, these values are reduced by up to a factor of ten in other models
that fit the data almost as well. Our ultimate goal is to combine the present
model results of masses, momenta, flow speeds, and flow geometries for OH231
with those of other models to be published in the future in order to find
common attributes of their ejection histories.
