Structural analysis of the hybrid rocket's N₂O oxidizer tank and endcaps at 750 psi MEOP. Sized the pressure vessel by hand, FEA-validated the tank wall and endcaps under combined loading, and confirmed margins of safety with a hydrostatic proof test at 1.5× operating pressure.
Size the tank wall and endcaps to contain 750 psi MEOP within required margins of safety
Validate against both yield (FoS 2.0) and ultimate (FoS 2.5)
FEA the endcaps under combined loading, internal pressure plus bolt preload
Correlate FEA results with closed-form hoop/longitudinal stress hand calculations
Started from first principles: hoop and longitudinal stress for the tank wall, bolt-circle and gasket loads for the endcaps. Sized wall thickness and endcap geometry in 6061-T6 aluminum to meet a factor of safety of 2.0 on yield and 2.5 on ultimate at 750 psi MEOP. These closed-form numbers set the baseline and the targets the FEA had to match.
Modeled the endcaps in ANSYS under combined loading, internal pressure plus bolt preload, to capture stress concentrations the hand calculations can't see (fillets, bolt holes, sealing surfaces). Checked peak von Mises stress against the material allowables and confirmed the endcaps held the required margins of safety.
Modeled the tank wall in ANSYS under the 750 psi MEOP pressure case, using a symmetry model to resolve the hoop and radial stress fields efficiently. Verified that the FEA results agreed with the closed-form hoop/longitudinal predictions, confirming the wall held the required margins of safety.
Validated the analysis on the real hardware with a hydrostatic proof test at 1.5× MEOP (1125 psi). The vessel held with zero leaks and no yielding, confirming the analysis and clearing the pressure vessel for the cold-flow and static fire campaigns.
Our endcap initially leaked, which forced us to rethink our O-rings and the groove geometry, the thickness, and the whole sealing approach. After remachining the grooves we reached a good compromise that resolved the leaking. But when we moved on to proof-testing the tank, a far worse failure appeared: the tank holes bored out completely, a catastrophic failure that threatened to push back our entire timeline. We traced the root cause to a mismatch in our calculation values. To solve it, we spent five days non-stop adding 8 holes to the endcap to add strength, cycling through remachining and many iterations. After a focused hydrostatic proof-testing campaign, we finally resolved the issue, a hard lesson in how the smallest mistakes can have catastrophic consequences on a project.
Both the tank and the endcaps were validated analytically, confirmed in FEA, and proof-tested to 1.5× operating pressure (1125 psi) with zero leaks and no permanent deformation. The hand calculations and FEA agreed, which is exactly what you want before trusting a pressure vessel with a high-pressure oxidizer. The work turned "we think it's strong enough" into a defensible margin of safety backed by two independent methods and a physical proof test, the standard real aerospace pressure vessels are held to.