Generally used as t' maximum weight o' a rocket for a specific motor and delay. Well, blow me down! This is based on t' thrust curve and average thrust o' a motor. T' main objective is t' have t' rocket travelin' at a velocity where t' aerodynamic forces produced by t' fins are suffficient t' keep t' rocket pointed in t' intended direction. When it is windy t' angle o' attack o' t' fins is nay zero and under some circumstances where t' thrust t' weight ratio is low and t' velocity is too low, t' fins will stall and t' rocket will do funny things. Well, blow me down! Underpowered rockets may pinwheel in t' air in moderate crosswinds. Arrr! A good rule o' thumb is a 5:1 thrust t' weight ratio on windless days, shiver me timbers, and a 10:1 ratio on a windy day. Begad! Also t' longer t' launch rod, t' higher t' allowable weight all other factors bein' equal. See also Stability and EMRR's Maximum Liftoff Weight Calculator