HY steels are preferred for rocket motor casings because any increase in engine structure mass requires more propellant, which increases the overall rocket thrust needed. Lighter engine structures are desirable so the maximum amount of the total vehicle consists of useful payload and propellant rather than deadweight supporting structures. HY steel is preferred for rocket motor casings because solid rocket motors require high internal temperatures and pressures for efficient combustion and thrust, and HY steel can withstand these demanding conditions.