Hybrid rockets using green propellants have been considered for launching nanosats into low Earth orbit (LEO). This work determines the preliminary mass distribution and sizing of hybrid rockets using 98 % H 2 O 2 and solid paraffin mixed with aluminum as propellants.An iterative process is used to calculate the rocket performance characteristics and to determine the inert mass fraction from given initial conditions. It is considered a mission to place a 20 kg payload into a 300 km circular equatorial orbit by air launched and ground launched hybrid rockets.