High rotation speed is the key to enhanced power density in rotating machines. High speed, shock, and construction considerations argue for solid rotors. However, output transient voltage requirements cause concern that changes in the field current will not be registered quickly in the stator. This paper examines different techniques for estimating the machine time constant, including an analytic solution of a smooth air gap machine and a transfer relation solution based on time harmonic solutions. It discusses an empirical solution and an equivalent surface current approach for predicting the loss in the rotor. These solutions compare favorably to rotational transient finite-element solutions.