A front-to-back cooling system allows cooling of an apparatus containing two orthogonal sets of modules. Each set of modules is independently cooled. A vertical set of modules is cooled with vertical air flow across the modules that enters from a front of the apparatus and exhausts from a back of the apparatus. A horizontal set of modules is cooled with horizontal front-to-back air flow. When the horizontal set of modules is at the front of the apparatus, a plenum extending exterior to the vertical set of modules allows exhausting horizontally flowing air to the rear of the apparatus. When the horizontal set of modules is at the rear of the apparatus, a plenum extending exterior to the vertical set of modules allows moving air from the front of the apparatus to a chamber holding the horizontal modules.