Resizing injector holes: check the manifold pressure order across one-way seals, not just stiffness

When a liquid rocket injector has fuel and oxidizer grooves separated by a single face seal that only seals from one side (for example a spring-energised PTFE seal pressurised from the fuel side), the design quietly depends on the fuel groove staying above the oxidizer groove. Each groove sits at chamber pressure plus that circuit injector drop, and drop scales roughly as 1/d^4 at fixed flow. Enlarging only the fuel holes (common when they are too small to machine) can flip the order. In one case, fuel holes going from 0.37 to 0.42 mm took the fuel groove from +52 psi above the oxidizer groove to -56 psi below it, while both drops still looked healthy (51% and 71% of chamber pressure). The fix is to resize the pair: enlarging the oxidizer holes too (0.65 to 0.74 mm) restored +25 psi, kept both drops above the facility minimum, and lowered the supply pressure needed by about 80 to 100 psi. The alternative is a seal rated for pressure from both sides. Recheck mixing and momentum ratio after any pair change, and the oxidizer orifice L/D against the hydraulic-flip band (SP-8089 cites L/D 4 to 6).