Solid composite propellants based on copolyurethanes comprised of hydroxy-terminated polybutadiene (HTPB) and poly(12-hydroxystearic acid-co-TMP) ester polyol (PEP, as binders and ammonium perchlorate (AP) as solid oxidizer, have been prepared and characterized. The mechanical properties, such as tensile strength, initial modulus, elongation at break and hardness of the propellant samples having varied compositions of copolyurethanes, have been measured experimentally at various solid loadings. A semi-empirical model has been developed to evaluate the modulus and % elongation at break of the solid propellants, from the binder prepolymer characteristics. A good agreement observed between the calculated and experimental values validates the soundness of the approach.