This study presents impact wave and spray properties for sheet breakup of rheologically complex gelled propellants using high-speed visualization and image processing. Two converging high-aspect ratio nozzles formed sheets for pressure drops in the range 414-828 kPa. Metalized gelled propellants are simulated using 0-20 wt% alumina nanoparticles suspended in a gelled rocket propellant. Spatio-temporal analysis of the impact waves formed on the sheet surface is presented for the first time and wave properties are compared against other liquids undergoing sheet breakup. Impact waves propagate in primary breakup region with constant velocity much less than jets. Unlike in Newtonian liquids, wave propagation velocity smaller than jet velocity delays the breakup of gelled propellants. Breakup length of the gelled propellant sheet varied with Weber number in a similar manner to impinging water jets. However, linear instability theories describing sheet breakup of Newtonian fluids could not describe the relationship between breakup length and Weber number. Bulk jets of 0.001 m diameter broke up into droplets of Sauter mean diameters 200-400 mu m. Smaller size in this range is observed in the sheet periphery and near to the impingement region. Larger size occurs in the middle regions of sheet towards downstream. Externally impinging jets must overcome the rheological resistance of gelled propellants by imparting higher kinetic energy to the sheet. This invariably results in higher velocities of the propellant jets for a given orifice diameter. Thus, a smaller injector parameter is obtained and results of this study suggest that this operational limitation could induce combustion instability.