Blimps, keel airships and dirigibles are all kinds of airships. Blimps are also called non-rigid airships. They have big bags sewn like a cigar that is filled with helium gas. Keel airships are also called semi-rigid airships. They have one big long gondola shaped like a pole (keel), and are a hybrid between blimps and dirigibles. Dirigibles are also called rigid airships. They have a rigid framework made of longitudinal girders and cross rings.