The design and manufacture of a pressure vessel for aerospace application is a major undertaking requiring significant amount of resources. The large pool of talents needed to carry out these tasks include stress analysts, design engineers, manufacturing engineers, planners, tool engineers, NC programmers, test engineers, and highly skilled workers such as machinists, welders, heat treat technicians, inspectors, and test technicians, plus program managers to put all the different pieces of the puzzle together. These people have to perform design analyses, create drawings for parts and assemblies, write process procedures, generate NC programs, produce tool drawings, build manufacturing tooling, plan manufacturing sequences, generate shop travelers, write test procedures, and produce test reports. Suffice it to say the non-recurring cost to design and manufacture a pressure vessel can be high.One of the ways Pressure Systems, Inc. (PSI) and its customers use to keep this cost down is to develop derivative tanks based on existing, qualified tank designs. This approach utilizes, as much as possible, analyses previously performed, documents previously generated, and tooling previously manufactured. The approach has been in practice at PSI for over three decades. It accounts for nearly 80% of the over 350 qualified tank designs developed by PSI. Over the years this approach has saved the government and our commercial customers tens of millions of dollars. Additional benefits derived from this approach include accelerated program schedule, qualification status, and flight heritage. This paper provides some examples of PSI's long and extensive history in the development of derivative tanks.