  The space-time structure of the multipion system created in central
relativistic heavy-ion collisions is investigated. Using the microscopic
transport model UrQMD we determine the freeze-out hypersurface from equation on
pion density n(t,r)=n_c. It turns out that for proper value of the critical
energy density \epsilon_c equation \epsilon(t,r)=\epsilon_c gives the same
freeze-out hypersurface. It is shown that for big enough collision energies
E_kin > 40A GeV/c (sqrt(s) > 8A GeV/c) the multipion system at a time moment
{\tau} ceases to be one connected unit but splits up into two separate spatial
parts (drops), which move in opposite directions from one another with
velocities which approach the speed of light with increase of collision energy.
This time {\tau} is approximately invariant of the collision energy, and the
corresponding \tau=const. hypersurface can serve as a benchmark for the
freeze-out time or the transition time from the hydrostage in hybrid models.
The properties of this hypersurface are discussed.
