Two-phase thermal control systems are used for payloads that require temperature stability under high and fluctuating heat output. However, the complex interaction between the two phases in a flow makes it hard to predict the performance of such a system. The goal was to develop a numerical model able to predict the transient behaviour of such a system within a 30% error margin. An existing explicit one-dimensional finite-difference simulation was expanded using new correlations from literature. The performance of this model was validated using experimental measurements gathered with the NLR two-phase demonstrator setup. The resulting numerical simulation can predict the transient behaviour of the pressure drop and liquid level in the accumulator with a mean error of 7.7% and 1.3% respectively. The goal therefore has been reached.While room remains for further improvement and validation, this model can significantly shorten the development time of new two-phase systems.