The SDO Propulsion Subsystem

Abstract

The Solar Dynamics Observatory’s (SDO) propulsion subsystem was the first bipropellant system designed, built, and tested at the Goddard Space Flight Center (GSFC). This paper describes the development and performance of this system as well as some of the technical challenges that were overcome during subsystem integration and flight operations. SDO was launched via an Atlas V on February 11, 2010 from Cape Canaveral Air Force Station. The launch vehicle released SDO in a Geosynchronous Transfer Orbit (GTO) where the spacecraft’s propulsion subsystem raised its orbit to a Geosynchronous Orbit (GEO) through several apogee engine firings. GEO provides constant instrument visibility of the Sun, while allowing for near continuous contact with a single ground station. The propulsion subsystem’s primary functions are to provide orbit transfer delta-V, three-axis control during delta-V, momentum unloading, and station keeping delta-V. These functions are achieved by eight Attitude Control System (ACS) thrusters and a single main engine. All thrusters are located on the aft end of the spacecraft, opposite the science instruments. Two tanks containing Propellant Management Devices (PMDs) were loaded with 1,409 kg of monomethylhydrazine fuel and nitrogen tetroxide oxidizer (containing 3% nitric oxide), which provides enough propellant for a five-year mission. The pressurant delivery system is pressure regulated, and was isolated via a pyrovalve once the spacecraft reached its final orbit. On-orbit maneuvers are performed using the ACS thrusters in a blow-down mode. Several challenges were overcome during the integration, test, and orbit transfer phases of the mission. Some of the technical challenges included titanium line welding and repairing a leaking fill and drain valve. The spacecraft also experienced some difficulty getting to GEO due to propellant slosh when firing the main engine.