PDS_VERSION_ID = PDS3 RECORD_TYPE = "STREAM" LABEL_REVISION_NOTE = " 2009-Feb-11; Initial draft; Carol Neese. 2010-May-06; Updated in response to review liens; Carol Neese. 2016-Feb-11; Updated in response to review liens; J. Mafi. " OBJECT = INSTRUMENT_HOST INSTRUMENT_HOST_ID = "DAWN" OBJECT = INSTRUMENT_HOST_INFORMATION INSTRUMENT_HOST_NAME = "DAWN" INSTRUMENT_HOST_TYPE = "SPACECRAFT" INSTRUMENT_HOST_DESC = " Instrument Host Overview ======================== The Dawn spacecraft is an ion-propelled spacecraft capable of visiting and stopping at multiple targets in the Main Asteroid Belt. Most of the spacecraft design was provided by Orbital Sciences Corporation (Orbital), and the Jet Propulsion Laboratory (JPL) provided the ion propulsion system (IPS) and parts of the electrical power system and telecommunications system. Orbital was responsible for overall assembly, including integration of instruments, system-level tests, and launch operations. JPL is responsible for mission operations. The IPS is an expanded version of the system operated extensively on Deep Space 1 (DS1). The spacecraft design benefits from inheritance from previous projects at Orbital and the JPL. The mechanical design is based on Orbital's Star-2 series, and the avionics draw from the LEOStar-2 series. There is significant redundancy throughout the design, and the mission can be accomplished in the presence of most credible single faults. A complete description of the Dawn spacecraft may be found in Rayman et al. (2006), from which most of the information in this file was taken. System Description ================== Ion Propulsion System --------------------- The core of the structure is a graphite composite cylinder. The hydrazine and xenon (Xe) tanks mount inside the cylinder, which provides a load path directly to the launch vehicle interface. The Xe tank, titanium overwrapped with composite, has a capacity of 425 kg. To provide reliability in expending that much propellant, the spacecraft carries three ion thrusters. The thruster design is qualified for a throughput of about 150 kg. Each 30-cm diameter thruster is mounted to a two-axis gimbal to allow for migration of the flight system's center of mass during the mission and to allow the attitude control system (ACS) to use the IPS to control attitude when the IPS is thrusting. The IPS system includes two sets of interface and control electronics and two power processing units, although no more than one thruster will be operated at a time. Attitude Control System and Reaction Control System --------------------------------------------------- In its normal cruise mode, ACS estimates attitudes and rates with a star tracker. The estimation is augmented with mechanical gyros in some ACS modes, including during science data acquisition. The star trackers and the gyros are mounted on the +z panel to minimize mechanical alignment errors between the sensors and FC and VIR. Coarse Sun Sensors (CSS), providing 4-pi steradian coverage, are used for rough attitude determination and for fault protection. All attitude sensors are fully redundant. ACS controls the articulation of the solar arrays and the basebody to assure that the arrays are normal to the Sun-spacecraft line. ACS can be commanded to use the reaction control system (RCS) or reaction wheels to control the attitude. Either type of actuator can be used in combination with the IPS for attitude control during IPS thrusting. The hydrazine-based RCS is used by ACS for direct control of attitude or for desaturation of the reaction wheels. In addition, some hydrazine is allocated for a contingency orbit-control maneuver at the asteroids in the event there is insufficient time to achieve the required delta-v with the low-thrust IPS. To date, no scenarios for this contingency have been identified. The titanium hydrazine tank capacity is 45.5 kg. RCS has two independent strings, each with six 0.9 N thrusters. Each string has pairs of thrusters pointed in +x, -x, and -z, with the last pair used for the contingency trajectory control. Electrical Power System and Thermal Control System -------------------------------------------------- Because of Dawn's uniquely high delta-v requirements, much of the spacecraft design is driven by the resulting need to maximize the power available to the IPS. When the flight system is at Ceres at a heliocentric range of 3 AU, the electrical power system (EPS) has to provide sufficient power to operate the IPS. Therefore the spacecraft has two large solar arrays which together provide 10.3 kW at 1 AU and 1.3 kW at Dawn's maximum heliocentric distance of 3 AU (Ceres' aphelion). Each of the two solar arrays has 18 square meters of InGaP/InGaAs/Ge triple-junction cells. The arrays are articulated around their long axis. Because the IPS requires high-voltage power, the flight system has two power buses. The EPS delivers most of the IPS power at 80-140 V, and the rest of the flight system (including some IPS components) receives power at 22-35 V. A 35 Ah NiH2 battery, sized to provide energy during launch, supplements the low-voltage bus during IPS thrusting to cover transients in the flight system's power consumption. Apart from the IPS, the thermal control system (TCS) is the largest consumer of power, requiring 200 W during IPS thrusting at 3 AU. Ammonia based heat pipes embedded in the y panels aid in distributing the heat, and louvers reduce thermal power requirements under cold conditions. Because the solar arrays are always maintained normal to the Sun-spacecraft line, the sun will be in the plane of the y panels. Most components are mounted on these panels to take advantage of the relatively uniform thermal environment. Command and Data Handling System -------------------------------- The Command and Data Handling System (CDHS) is based on a RAD6000 processor running sofware written in C running in VxWorks. CDHS also provides 8 Gb of mass memory for engineering and scientific data. The system receives telemetry from all ACS sensors and includes drivers for all ACS actuators except the IPS thruster gimbals. Most components communicate over a Mil-Std-1553 data bus, although GRaND uses a RS-422 serial interface. The flight software incorporates virtual machine language (VML), currently in use on Mars Odyssey, Mars Reconnaissance Orbiter, and the Spitzer Space Telescope. VML allows conditionals and complex logic in command sequencing. VML is used for all commanding during the mission, although in order to rely on only the most basic functions, fault protection does not use it. CDHS supports uplink data rates from 7.8 b/s to 2.0 kb/s and downlink data rates from 10 b/s to 124 kb/s using a rate 1/6 turbo code, implemented as recommended by the Consultative Committee for Space Data Systems. Telecommunications System ------------------------- The telecommunications system (Telecom) operates in X-band for uplink and downlink. It is based on the Small Deep Space Transponder, validated on DS1 and used on the majority of NASA missions beyond the moon since Mars Odyssey. Radiating 100 W, the travelling wave tube amplifiers are identical to those in use on Mars Reconnaissance Orbiter. An unarticulated 1.52 m high gain antenna (HGA) is on the spacecraft's +x axis, and near-hemispherical (7 dBi) low gain antennas (LGAs) are aligned with +x, +z, and -z. The system is configured to always use only one of the four antennas at a time. Instrument Mounting Geometry ---------------------------- The figure below represents the Dawn spacecraft with the -y panel facing the viewer. GRaND and the FCs are mounted on the +z panel and VIR is mounted on the -x panel. All instruments are mounted so the centers of their fields of view are aligned with the spacecraft's +z axis. The high gain antenna is mounted to the +x panel. One IPS thruster is mounted at the -z panel and the other two are on their own panels, one canted towards -x and the other toward +x. The thrusters are about 48 degrees from -z. The two solar panels extend from the +y and -y panels. +X /\ High Gain Antenna ------------------- \ / \ / --------------------------/\ ___| | \ GRaND | | |_/_ Ion propulsion | | | | thrusters |__| | | Z _| o | | -Z FC |_| | | | |___| | | \ | | / -------------------------\/ VIR | | |_________| -X References ========== Rayman, M.D., T.C. Fraschetti, C.A. Raymond, and C.T. Russell, Dawn: A mission in development for exploration of main belt asteroids Vesta and Ceres, Acta Astronautica 58, 605-616, 2006. " END_OBJECT = INSTRUMENT_HOST_INFORMATION OBJECT = INSTRUMENT_HOST_REFERENCE_INFO REFERENCE_KEY_ID = "RAYMANETAL2006" END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO END_OBJECT = INSTRUMENT_HOST END