On Non-Supersymmetric String Model Building

We study a class of heterotic string compactifications possessing $SO(10)$ ``observable" gauge symmetry and spontaneously broken supersymmetry via the Scherk--Schwartz mechanism. We formulate a set of phenomenological criteria and identify models that comply with all requirements. This approach allows us to address and propose solutions to some of the open problems in non-supersymmetric string model building including the vacuum instability due to the appearance of tachyons and dilaton tadpoles, the presence of chiral fermions and the existence of Standard Model breaking light Higgs fields.