This paper investigates the morphology of a flash-atomizing jet at conditions representative for rocket engine operations. Due to its relevance for the aerospace industry, both storable and cryogenic rocket engine propellants are considered, namely ethanol [as inert equivalent of monomethyl hydrazine (MMH)] and liquid oxygen (LOx). A comparison between the flashing behavior of these two fluids is conducted. Despite the differences in their physical properties, a close similarity in the spray characteristics is found in terms of spray shape, spreading angles, and evolution of the flashing regimes as a function of the initial superheat. Based on this similarity, the applicability of a novel, nucleation-based onset criterion (chi parameter) for the fully flashing regime is verified for cryogenic propellants, showing a satisfactory agreement. This result has important implications. First, it corroborates that jet disintegration at highly superheated conditions is mainly controlled by the kinetics of phase transitions (i.e., the nucleation rate). Second, it explains the differences in the degree of superheat (R-p) at onset of the fully flashing regime between ethanol and LOx sprays, respectively. The low operating temperatures in cryogenic systems result in a significant increase of the energy barrier to nucleation. Consequently, the inception of nucleate boiling can occur either at significantly higher R-p values (homogeneous process) or may be triggered by heterogeneous effects. The associated increase of the chi parameter at the onset provides a good indication of the transition to heterogeneous nucleate boiling.