This is pretty fun.
With this software you can investigate how a model rocket flies by changing the values of different design variables. If you are an experienced user of RocketModeler, you may prefer a slightly different version of RocketModeler. If you experience difficulties when using the sliders to change variables, simply click away from the slider and then back to it. If the arrows on the end of the sliders disappear, click in the areas where the left and right arrow images should appear, and they should reappear.
(via All Things Christie)