  We provide a new analytic blastwave solution which generalizes the
Blandford-McKee solution to arbitrary ejecta masses and Lorentz factors. Until
recently relativistic supernovae have been discovered only through their
association with long duration Gamma Ray Bursts (GRB). 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, as the ejecta mass is negligible in comparison to the swept
up mass. The recent discovery of the relativistic supernova SN 2009bb, without
a detected GRB, opens up 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 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 compute the synchrotron emission from
relativistic electrons in the shock amplified magnetic field. This models the
radio emission from the circumstellar interaction of a CEDEX. 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. We discuss the effect of baryon loading on the dynamics and
observational manifestations of a CEDEX. In particular, our predicted angular
size of SN 2009bb is consistent with VLBI upper limits on day 85, but is
presently resolvable on VLBI angular scales, since the relativistic ejecta is
still in the nearly free expansion phase.
