The thrust enable to lift of the rest of the rocket, but without enough fuel for going to orbit by itself

you need a lot of mass ejected to have a lot of thrust and the lower efficiency (specific impulse) isn't a problem at takeof, you don't have to carry this weight really high

opencascade may be a nice paramatric geometrical kernel but there is no opensource gui yet. freecad,exotk and other are far from being usable...