This paper presents a novel extended modal series method for solving the infinite horizon optimal control problem of nonlinear interconnected largescale dynamic systems. In this method, the infinite horizon nonlinear largescale two-point boundary value problem (TPBVP), derived from Pontryagin’s maximum principle, is transformed into a sequence of linear time-invariant TPBVPs. Solving the latter problems in a recursive manner provides the optimal control law and the optimal trajectory in the form of a uniformly convergent series. Moreover, in special cases, the proposed procedure facilitates the application of parallel processing, which improves its computational efficiency. In this study, an iterative algorithm is also presented, which has a low computational complexity and a fast convergence rate. Just a few iterations are required to obtain a suboptimal trajectory-control pair. Finally, effectiveness of the proposed approach is verified by solving the optimal attitude control problem.

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