User-friendly simulation of discontinuous trajectories with ESL

Continuous Systems Simulation Languages are often called upon to model discontinuous events in an otherwise continuous model. In the past few years mast CSSLs have added features to facilitate the solution of this kind of problem. These often take the form of a separate section or sections of the program which are executed only at the instant of the discontinuous event. This approach works well, particularly if tile size of the integration step is controlled automatically to ensure that the integration process synchronizes with the discontinuity. It can, however, lead to an unwieldy and relatively confusing programming style compared with the natural programming approach adopted by ESL. (ESL 1992) This approach is well illustrated by the ESL version of a familiar problem which simulates the trajectory of a vertically launched rocket which at a predetermined time experiences a step change in its trajectory.