RDSim is a fast, accurate and flexible framework for the simulation of the radio emission of downgoing air showers and its detection by an arbitrary array, including showers initiated by neutrino interactions or tau-lepton decays. RDSim was build around speed and is based on simple and fast, yet still accurate, toymodel-like approaches. It models the radio emission using a superposition emission model that disentangles the Askaryan and geomagnetic components of the shower radio emission. It uses full ZHAireS simulations as an input to estimate the electric field at any position on the ground. A single input simulation can be scaled in energy and rotated in azimuth, 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. RDSim 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 and perform simple particle trigger simulations. Owing to the large statistics made possible by its speed, it can be used to investigate in detail events with a very low trigger probability and geometrical effects due to the array layout, making it specially suited to be used as a fast and accurate aperture calculator. In case more detailed studies of the radio emission and detector response are desired, 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.