  We present a model of a hypersonic, collimated, "single pulse" outflow,
produced by an event with an ejection velocity that first grows, reaches a
peak, and then decreases again to zero velocity in a finite time
(simultaneously, the ejection density can have an arbitrary time-variability).
We obtain a flow with a leading "head" and a trailing "tail" that for times
greater than the width of the pulse develops a linear, "Hubble law" velocity
vs. position. We present an analytic model for a simple pulse with a parabolic
ejection velocity vs. time and time-independent mass-loss rate, and compare it
to an axisymmetric gasdynamic simulation with parameters appropriate for fast
knots in planetary nebulae. This "head/tail plasmon" flow might be applicable
to other high-velocity clumps with "Hubble law" tails.
