The SpaceX Kestrel Engine was built around the same pintle architecture as the Merlin engine. It was designed to be a high efficiency, low pressure vacuum engine. It did not have a turbo-pump and was fed only by tank pressure. Kestrel was ablatively cooled in the chamber and throat and radiatively cooled in the nozzle, which was fabricated from a high strength niobium alloy. Thrust vector control was provided by electro-mechanical actuators on the engine dome for pitch and yaw. Roll control (and attitude control during coast phases) was provided by helium cold gas thrusters The engine had dual redundant torch igniters, tested in vacuum, to ensure a reliable engine start. Since the igniters use the same propellants as the main engine, they were capable of as many restarts as necessary for a particular mission. In a multi-manifested mission, this allowed for drop-off of payloads at different orbital altitudes and inclinations. SpaceX completed development in early 2005.