  Some combination of binary interactions and accretion plausibly conspire to
produce the ubiquitous collimated outflows from planetary nebulae (PN) and
their presumed pre-planetary nebulae (PPN) precursors. But which accretion
engines are viable? The difficulty in observationally resolving the engines
warrants the pursuit of indirect constraints. We show how kinematic outflow
data for 19 PPN can be used to determine the minimum required accretion rates.
We consider main sequence (MS) and white dwarf (WD) accretors and five example
accretion rates inferred from published models to compare with the minima
derived from outflow momentum conservation. While our primary goal is to show
the method in anticipation of more data and better theoretical constraints,
taking the present results at face value already rule out modes of accretion:
Bondi-Hoyle Lyttleton (BHL) wind accretion and wind Roche lobe overflow
(M-WRLOF, based on Mira parameters) are too feeble for all 19/19 objects for a
MS accretor. For a WD accretor, BHL is ruled out for 18/19 objects and M-WRLOF
for 15/19 objects. Roche lobe overflow (RLOF) from the primary at the Red
Rectangle level can accommodate 7/19 objects, though RLOF modes with higher
accretion rates are not yet ruled out. Accretion modes operating from within
common envelope evolution can accommodate all 19 objects, if jet collimation
can be maintained. Overall, sub-Eddington rates for a MS accretor are
acceptable but 8/19 would require super-Eddington rates for a WD.
