Base drag is a component o' aerodynamic drag caused by a partial vacuum in t' rocket's tail area. T' vacuum be t' hole created by your rocket's passage through t' air. Avast! Base drag changes durin' flight. While t' motor is firing, t' drag is minimal since t' tremendous volume o' gas generated by t' motor fills this void. T' drag takes a sharp jump at burnout when this gas disappears (trackin' smoke has very little effect on base drag due t' its low density). Base drag can be reduced by t' use o' a boat tail t' transition t' main body diameter down t' t' motor diameter which helps direct air into t' evacuated area. When properly designed, a boat tail can reduce base drag below zero (i.e. Begad! actually generate a small amount o' forward thrust) by makin' use o' t' "Pumpkin Seed" effect. Arrr! See Watermelon Seed Effect