55th International Astronautical Congress 2004 - Vancouver, CanadaIAC-04-S.1.09 THE USE OF THE AEROJET MR-103H THRUSTER ON THE NEW HORIZONS MISSION TO PLUTO James Stratton The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA [email protected] The New Horizons spacecraft is designed to be the first mission to Pluto and its moon Charon, the last unexplored planetary system in our solar system. The planned New Horizons science payload drives the requirement for a 3-Axis stabilized spacecraft that has the ability to maintain tight pointing requirements with very low body rates. Due to the extreme power constraints of the mission, reaction wheels would not provide a viable control option, and thus 3-Axis attitude control should be maintained by thrusters. Similarly, mission lifetime and mass constraints drove the design to a blowdown hydrazine system that would serve as both the velocity control and as the only method of attitude control on board the spacecraft. The Aerojet MR-103H thruster was chosen as the New Horizons attitude control thruster for its minimum impulse bit performance, and for its heritage from the Voyager and Cassini missions. Spacecraft attitude control would be maintained by a total of 12 MR-103H thrusters, arranged in pairs to produce coupled torques about any given axis. Instrument performance and the subsequent science return would rely heavily on the spacecraft’s ability to accurately control spacecraft body rates, and thus performance matching of each of the sets of thrusters is of paramount importance to mission success. The requirements that drove the design of the liquid propulsion system and that led to the selection of the MR-103H thruster will be discussed in detail. Also, the measured performance of the New Horizons MR-103H thrusters will be analyzed, and the methods used to pair thrusters and meet spacecraft control requirements will be discussed. INTRODUCTION The New Horizons mission is funded under the NASA New Frontiers Program, and is being designed, built and operated by the Johns Hopkins University Applied Physics Laboratory (APL). New Horizons is scheduled to launch January 11, 2006 from Cape Canaveral Air Force Station aboard an Atlas V 551. During its planned 10-year mission, New Horizons would fly by Jupiter on its way to a rendezvous with the Pluto/Charon system in July 2015. The New Horizons spacecraft is designed to continue its mission past the Pluto/Charon encounter to investigate up to three Kuiper Belt Objects (KBOs). As designed, the New Horizons science payload consists of three core instruments, and four55th International Astronautical Congress 2004 - Vancouver, Canadasupplementary instruments. The core payload consists of the Alice UV Spectrograph, the RALPH visible and IR imager, and the REX radio experiment. The secondary payload consists of the LORRI long-range imager, the PEPSSI energetic particle spectrometer, the SWAP solar wind experiment, and the SDC dust counter that would count interstellar dust particles during the flight to Pluto and beyond. PROPULSION SYSTEM REQUIREMENTS The primary requirements for the New Horizons LPS are to provide velocity control and spin- and 3-Axis attitude control for the observatory. Some measure of velocity change would be required to correct the injection errors introduced by the Atlas Launch Vehicle and the Boeing 3rd Stage, and also to provide along-track and cross-track targeting adjustments for the Jupiter, Pluto/Charon and Kuiper Belt Object (KBO) flybys. However, as is shown in Table 1, the majority of the ∆V capability of the New Horizons observatory would be reserved for the large maneuver required to target KBOs after the Pluto/Charon encounter. 2006 (Primary Mission) 2007 (Backup Mission) Primary Mission 92 77 Primary Mission Margin 30 30 KBO Navigation 120 95 Total 242 202 Table 1: Proposed New Horizons Navigation ∆V Budget The ∆V requirements shown in Table 1, coupled with the attitude control propellant requirements of approximately 21.6 kg dictate that the New Horizons LPS carry a minimum usable hydrazine propellant load of 68.4 kg. Since the system must also meet the nutation time constant (NTC) requirements levied by the spinning Boeing 3rd Stage, significant testing was performed to measure the NTC of the system at various propellant loads and observatory/3rd Stage stack mass properties. It was found that to meet the 3rd Stage NTC requirements, the observatory could carry a maximum propellant load of 80 kg. To perform the majority of its observations, the New Horizons science payload would require either a 3-Axis stabilized bus, or that the observatory rotate about a certain axis at a specified rate. Due to the extreme power limitations of this outer planetary mission, reaction wheels did not pose a viable option for attitude control on the New Horizons observatory. Consequently, attitude control would have to be maintained by thrusters alone. In particular, the RALPH instrument requires that the observatory be capable of setting up and maintaining a scan rate of +/- 34 µrad/s about any given axis. This attitude control requirement, along with the ∆V budget shown above, is the primary driver for the design of the New Horizons LPS. The RALPH scan mode would require that the LPS be capable of delivering a minimum impulse bit no greater than 0.0066 N-s from each ACS thruster at the time of the planned Pluto/Charon encounter. This led to the selection of two candidate thrusters to meet the requirements of the mission: the Aerojet Minimum Impulse Thruster (MIT), and the Aerojet MR-103H thruster. At the time the program was proposed, the MIT thruster was still under development and awaiting qualification testing, whereas the MR-103H thruster was already qualified and had recently been produced for the Cassini and Deep Impact missions. Consequently, in order to minimize the total schedule and cost risk to the program, APL chose to baseline the MR-103H thrusters for use on the New Horizons observatory.55th International Astronautical Congress 2004 - Vancouver, CanadaPROPULSION SYSTEM DESCRIPTION The schematic for the planned New Horizons LPS is shown below in Figure 1. Spacecraft attitude control would be maintained using twelve Aerojet MR-103H 0.2 lbf thrusters, and spacecraft velocity change would be achieved primarily through the use of four Aerojet MR-111C 1.0 lbf thrusters. The