Nice choice for an environment. The stuff left behind at the end of the day can tell such an interesting story. My first reaction to your invisible faces is to assume that your tube object is only one face thick and that you have not capped the ends. Basically, polygons are typically invisible from the back(side facing away from the normals direction) unless you have double sided shading on. This is normal behavior and can be addressed by turning on double sided shading or creating some kind geometry (whatever shape your heart desires) to fill the hole.