  We present RDSim, a fast and comprehensive framework for the simulation of
the radio emission and detection of downgoing air showers. It can handle any
downgoing shower that can be simulated with ZHAireS including those induced by
CC and NC neutrino interactions and $\tau$ decays. RDSim is based on a
superposition toymodel that disentangles the Askaryan and geomagnetic
components of the shower emission. By using full ZHAireS simulations as input,
it is able to estimate the full radio footprint on the ground. A single input
simulation at a given energy and arrival direction can be scaled in energy and
rotated in azimuth by taking into account all relevant effects. This makes it
possible to simulate a huge number of geometries and energies using just a few
ZHAireS input simulations. The framework takes into account the main
characteristics of the detector, such as trigger setups, thresholds and antenna
patterns. To accommodate arrays that use particle detectors for triggering,
such as the Auger RD extension, it also features a second toymodel to estimate
the muon density at ground level, which is used to perform simple particle
trigger simulations. It's speed makes it possible to investigate in detail
events with a very low trigger probability, as well as many geometrical effects
due to the array layout. In case more detailed studies of the radio detection
are needed, RDSim can also be used to sweep the phase-space for the efficient
creation of dedicated full simulation sets. This is particularly important in
the case of neutrino events, that have extra variables that greatly impact
shower characteristics, such as interaction or $\tau$ decay depth as well as
the type of interaction and it's fluctuations.
