Supershell kinematics for cluster 1447 in the galaxy NGC 6946. Left: case 1, stellar winds. Right: case 2, supernovae. The star formation efficiency is assumed to be 30 per cent in both cases. The timescale for the global evolution of the GC is chosen at the birth of a coeval first generation of stars. Within each kinematics plot, the upper diagram shows the bubble radius (solid black line) and the Rayleigh-Taylor scale (dash-dotted black line), with the red dashed line indicating the half-mass radius. The middle diagram displays the shell velocity (solid line) and the escape velocity at the current bubble radius (red dashed line). The acceleration is shown in the bottom diagram (positive: dotted black line, negative: solid green line), with the gravitational acceleration at the current radius shown as a red dashed line. Gas expulsion succeeds in both cases. Details in Sect. 4.1.1.