  Many astrophysical flows occur in inhomogeneous media. We briefly discuss
some general properties of the adiabatic and radiative inhomogeneous systems
and discuss the relevance of those properties to the planetary nebulae systems.
We then focus on radiative hypersonic bullets and the applicability of this
model to planetary and protoplanetary systems such as CRL 618, NGC 6543, Hen
3-1475.
