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There are a lot of constraints and conditions that go into the design of propellant tanks, but we're only going to cover the basics for now. We can justify this because our team is currently testing from a stationary stand, so many sizing and loading constraints that an actual rocket would impose are not relevant here.
Tank Volume Sizing
Read Huzel & Huang, Section 8.2, “Shape and Size of Propellant Tank” subsection for more info.
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