Turbopumps with TSR designation are series connected two stage belt driven units. Cooling air is introduced into the pump through a slot between the two stages. In this way the second stage cools the first stage indirectly, allowing the pump to run at extremely high vacuum and low airflow without overheating. The turbopumps are equipped with thermal overload protection on the outboard bearing which will trip out when bearing temperature becomes excessive. A back flow valve must be optionally installed on the inlet side of the unit when several units are to be installed in parallel.