  Ring vortices are efficient at transporting fluid. They are often produced by
ejecting a volume of fluid through a circular orifice. The impulse given to the
vortex rings results in a propulsive force on the generator. Propulsive vortex
rings have been widely studied and characterised. After four convective times,
the vortex moves faster than the shear layer it originates from, and separates.
This separation corresponds to a maximum in circulation and a maximal spread of
the vorticity, quantified by the non-dimensional energy. The simultaneity of
these three events obfuscates the causality between them. To analyse the
temporal evolution of the non-dimensional energy of ring vortices independent
of their separation, we analyse the development of vortices generated in the
wake of cones. Cones with different apertures and diameters were accelerated
from rest to produce a wide variety of vortex rings and their energy,
circulation, and velocity were extracted based on time-resolved velocity field
measurements. The rings that form behind the cones have a self-induced velocity
that causes them to follow the cone and they continue to grow as the cone
travels well beyond the limiting vortex formation times scales observed for
propulsive vortices. The non-dimensional circulation, based on the vortex
diameter, and the non-dimensional energy of the drag vortex rings converge
after three convective times to values comparable to their propulsive
counterparts. This result proves that vortex pinch-off does not cause the
non-dimensional energy to reach a minimum value. The limiting values of the
non-dimensional circulation and energy are mostly independent of the cone
geometry and translational velocity and fall within an interval of 10% around
the mean value. With only 6% of variation, the velocity of the vortex is the
most unifying quantity that governs the formation of vortex rings behind cones.
