Systems and methods for controlling a fluid-based engineering system are disclosed. The systems and methods may include a model processor for generating a model output, the model processor including a set state module for setting dynamic states of the model processor, the dynamic states input to an open loop model based on the model operating mode. The model processor may further include an estimate state module for determining an estimated state of the model based on at least one of a prior state, current state derivatives, solver state errors, and synthesized parameters. The estimate state module determines an estimator gain associated with the current state derivatives and applies the estimator gain to determine the estimated state of the model.