  Relativistic supernovae have been discovered until recently only through
their association with long duration Gamma Ray Bursts (GRB). As the ejecta mass
is negligible in comparison to the swept up mass, the blastwaves of such
explosions are well described by the Blandford-McKee (in the ultra relativistic
regime) and Sedov-Taylor (in the non-relativistic regime) solutions during
their afterglows. However, the recent discovery of the relativistic supernova
SN 2009bb, without a detected GRB, has indicated the possibility of highly
baryon loaded mildly relativistic outflows which remains in nearly free
expansion phase during the radio afterglow. In this work, we consider the
dynamics and emission from a massive, relativistic shell, launched by a Central
Engine Driven EXplosion (CEDEX), decelerating adiabatically due to its
collision with the pre-explosion circumstellar wind profile of the progenitor.
We show that this model explains the observed radio evolution of the
prototypical SN 2009bb and demonstrate that SN 2009bb had a highly baryon
loaded, mildly relativistic outflow.
