  We investigate the early impact of single and binary supernova (SN)
explosions on dense gas clouds with three-dimensional, high-resolution,
hydrodynamic simulations. The effect of cloud structure, radiative cooling, and
ionising radiation from the progenitor stars on the net input of kinetic
energy, f_kin=E_kin/E_SN, thermal energy, f_therm=E_therm/E_SN, and gas
momentum f_P=P/P_SN to the interstellar medium (ISM) is tested. For clouds with
n=100 cm^{-3}, the momentum generating Sedov and pressure-driven snowplough
phases are terminated early (~ 0.01 Myr) and radiative cooling limits the
coupling to f_therm ~ 0.01, f_kin ~ 0.05, and f_P ~ 9, significantly lower than
for the case without cooling. For pre-ionised clouds these numbers are only
increased by ~ 50%, independent of the cloud structure. This only suffices to
accelerate ~ 5% of the cloud to radial velocities >30km/s. A second SN might
further enhance the coupling efficiencies if delayed past the Sedov phase of
the first explosion. Such very low coupling efficiencies cast doubts on many
galaxy-scale sub-resolution models for supernova feedback, most of which are
validated a posteriori by qualitative agreement of galaxy properties with
observations. Ionising radiation appears not to significantly enhance the
immediate coupling of SNe to the surrounding gas as it drives the ISM into
inert dense shells and cold clumps, a process which is unresolved in galaxy
scale simulations. Our results support previous conclusions that supernovae
might only drive a wind if a significant fraction explodes in low-density
environments or if they are supported by processes other than ionising
radiation.
