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Managing Momentum on the Dawn Low Thrust Mission. Brett A Managing Momentum on the Dawn Low Thrust Mission. Brett A. Smith, Charles A. Vanelli, and Edward R. Swenka Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 [email protected], [email protected], [email protected] Abstract—Dawn is low-thrust interplanetary spacecraft en- 1. INTRODUCTION route to the asteroids Vesta and Ceres in an effort to better un- Dawn is NASA’s ninth Discovery class mission on a journey derstand the early creation of the solar system. After launch to orbit two asteroids in the region between Mars and Jupiter. in September 2007, the spacecraft will flyby Mars in February Scientists believe the asteroid belt provides similar conditions 2009 before arriving at Vesta in summer of 2011 and Ceres in as those found during the formation of Earth. Dawn will in- early 2015. Three solar electric ion-propulsion engines are vestigate Vesta and Ceres, which are two of the larger objects used to provide the primary thrust for the Dawn spacecraft. in the region, and each provides a unique view into the forma- Ion engines produce a very small but very efficient force, and tion of planet like objects. The Dawn mission is also unique therefore must be thrusting almost continuously to realize the as it will be the first spacecraft to orbit two extraterrestrial necessary change in velocity to reach Vesta and Ceres. planetary bodies [1]. Momentum must be carefully managed to ensure the space- Launching on September 27, 2007, the Dawn spacecraft be- craft has enough control authority to perform necessary turns gan its 8-year journey. After a flyby of Mars it will begin and hold a fixed inertial attitude against external torques. mapping Vesta in 2011 and then Ceres in 2015. Built by Or- Along with torques from solar pressure and gravity-gradients, bital Sciences Corporation and operated by the Jet Propul- ion-propulsion engines produce a torque about the thrust axis sion Laboratory, the spacecraft has design heritage from both that must also be countered by the four reaction wheel assem- organizations. The mission is made possible by ion propul- blies (RWA). New constraints were placed on the 8-year mis- sion technology, successfully demonstrated by NASA’s Deep sion shortly prior to launch that required Dawn to minimize Space 1 (DS1) mission. Using an ion propulsion system on time spent in the sub-Elasto-Hydro-Dynamic (sub-EHD) re- Dawn provides a mission opportunity that would be too costly gion and minimize the total revolutions of all four RWAs. Ac- or massive with conventional propulsion, however a long tra- curate prediction of wheel speeds is the first step in develop- jectory is required to reach the asteroids. ing a strategy to minimize both wheel speeds and total revo- lutions. Due to schedule and staffing constraints, the ground tools needed to accomplish this momentum management pro- cess were developed post launch, in parallel with the missions initial checkout phase. This paper discusses the momentum management issues of ion-propulsion missions and specifically the Dawn space- craft. The discussion includes the tools and strategies devel- oped to manage the spacecraft momentum with the goal of preserving RWA health. TABLE OF CONTENTS 1 INTRODUCTION ................................... 1 2 SPACECRAFT CONFIGURATION ................... 2 3 EXTERNAL TORQUES ............................. 3 4 DAWN MOMENTUM MANAGEMENT TOOL ....... 4 5 CONCLUSIONS .................................... 7 ACKNOWLEDGEMENTS ........................... 8 REFERENCES ..................................... 8 BIOGRAPHY ....................................... 8 Figure 1. Dawn Mission Trajectory. The advantage of ion propulsion is the great increase in spe- 978-1-4244-2622-5/09/$25:00 c 2009 IEEE cific impulse, close to 10 times greater than chemical propul- IEEEAC Paper #1611, Version 0 Updated 31/10/2008. sion. Specific impulse is a measure of efficiency, it represents 1 attitude about the two axes perpendicular to the thrust vector. active wheel set is changed allowing a different wheel to be Only one IPS engine can be used at a time due to power con- turned off. straints. Since TVC can only control attitude about two axes, either RCS or RWA must be used to control the spacecraft Hydrazine thrusters provide a large control authority in com- about the thrust axis. parison to the RWA’s. For this reason they would be used as the fall back controller in safe mode or if multiple RWA’s Four RWAs are mounted in a pyramid orientation and can failed. Conserving hydrazine for use in desaturating the be seen in Figure 3. RWAs are the primary actuator for at- RWAs is another reason thrusters are not used as the default titude control when not using IPS. When under IPS thrust, controller. the wheels provide control about the thrust vector, with TVC providing control perpendicular to the thrust line. Control- Most of the mission is spent thrusting with the IPS engines, ling spacecraft attitude with RWAs is based on the principle which can be operated in 4 configurations: RWA control of conservation of angular momentum. Altering the wheel (with IPS seen as an external torque), hydrazine thruster con- speed of one RWA changes the momentum of that RWA. The trol (with IPS seen as an external torque), TVC-RWA, TVC- spacecraft rotates to counteract this change in wheel speed. JET (i.e. with hydrazine thrusters for control about the thrust At least three RWAs are necessary for controlling all three axis). The first two configurations are rarely used as any- independent spacecraft axes. Over prolonged times external thing other than a transition to one of the TVC configurations, forces can add more momentum to the system than the reac- because momentum grows fast without TVC. TVC actively tion wheels can accommodate. Hydrazine thrusters must be aligns the thrust axis through the center of mass (c.m.): when used periodically to remove this momentum. TVC is not on, any torques from the engine misalignment must be compensated for by another system. TVC-RWA is the preferred operational mode primarily for the pointing ac- curacy but also to conserve hydrazine and to minimize unde- sirable delta-V. Although TVC is controlling two axes, exter- nal torques still build momentum about the third axis 3. EXTERNAL TORQUES There are three significant sources of external torque that the Dawn spacecraft encounters: solar pressure, gravity gradi- ent, and IPS swirl torque. Solar pressure is inversely propor- tional to the square of the solar range, if the center of pres- sure (c.p.) is offset from the center of mass (c.m.) a torque is produced. Since the solar panels are always kept normal to the sun, the spacecraft is relatively symmetric about the c.p., however Figure 2 shows location of the large high gain Figure 3. RWA Configuration. antenna which causes the c.p. to move away from the c.m.. The hydrazine thruster system consists of two redundant sets Gravity gradient torque is the torque created on the space- of 6 thrusters that can be used for attitude control, or to ad- craft by nearby bodies of mass. This torque is inversely pro- just the momentum of the RWAs. Not all of the hydrazine portional to the cube of the distance between the body and thrusters are coupled, and multiple thrusters must be fired to the spacecraft. During cruise this distance is insignificant and produce a torque about any axis other than the z-axis. Ev- thus gravity gradient torque isn’t considered. However, grav- ery time the uncoupled thrusters are fired a small delta-V is ity gradient torque will be a factor during certain phases of imparted to the spacecraft. the mission, most importantly during Mars flyby and at Ceres and Vesta. ACS Operational Modes The Dawn spacecraft operates under several hardware con- The ion engines produce a rotational torque about the thrust figurations based on the task. When not thrusting with the axis called swirl torque, the most significant torque measured IPS engines, the attitude may be controlled either by RWA’s on Dawn. See Brophy et. al. for more on swirl torque [3] or the hydrazine thrusters. Because they provide better point- (referred to as roll torque) . Charged ions pass through two ing stability, RWA’s are used as the primary actuator while plates with a grid of holes while exiting the engine. Very not thrusting. All four RWA’s can be used with a speed bias, small rotational misalignments of these plates is thought to to keep the zero momentum state out of the sub-EHD region. produce the swirl torque observed on Dawn and DS1. Gim- Although the four wheel configuration allows flexibility in bal actuators which can also translationally move the thrust momentum management it is generally not used. The Dawn vector may contribute as well. Components of torque in the project has chosen to operate in the three-wheel configuration yaw and pitch axis of the IPS engine can be removed by TVC, to reduce the overall use of each wheel. Every six months the but the remaining component results in roll/swirl torque. The 3 magnitude of the swirl torque is a function of the ion engine • Total revolutions throttle level, and for Dawn this swirl torque is generally an Just prior to launch a new constraint was placed on the total order of magnitude larger than solar pressure torque. revolutions for each RWA. A failure scenario was identified that was associated with total revolutions of the RWA. Dawn 4. DAWN MOMENTUM MANAGEMENT TOOL has taken the conservative approach, constraining the total revolutions to prevent any RWA from exceeding the new limit Constraints on the momentum and wheel speed made accu- before the end of the mission. Since the mission and space- rate and efficient prediction a necessity for activity develop- craft were not designed with this constraint, the result has a ment early on in the mission.
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