Potential seal candidates for high-energy propellants

Five potential seal candidates (linear Tefzel, linear Halar, crosslinked Halar, Viton ECD-006, and phosphazine fluoroelastomer) were evaluated for the orbital maneuvering system of the space shuttle. Since this system employs nitrogen tetroxide (N2O4) and monomethyl hydrazine (MMH) as hypergolic propellants, the seal candidates were selected on the basis of projected chemical resistance as well as rheological behavior. Chemical resistance to these high-energy fuels was determined via conventional isothermal and thermal cycling immersion tests. Rheological measurements, however, were performed on O-rings molded from the subject seal candidates. Properties determined, such as cyclic work and hysteresis, stress relaxation, and indicated modulus, therefore, relate to the O-ring seals themselves.