Any operational system, in order to successfully accomplish its intended mission, must rely on logistic support that will be available when required. According to this, unlike traditional approaches to availability development, the paper indicates on the problem of integration between two systems - operational and its supporting system into one 'system of systems'. Moreover, in many systems undesired event (system failure) occurs later than components failure, and only if repair is not completed within a defi ned period of time. The time dependency is a convenient approach for defi nition of interactions between the above systems. Thus, the paper considers the time dependent system of systems where the system total task must be executed during constrained time resource. For the developed model, there are derived general equations for the evaluation of system of systems availability function and steady-state availability ratio. The model solution is obtained based on analytical method. Furthermore, the mathematical expressions for the mean availability ratio are derived when the all probability density functions are to be exponential.