Gridded Ion Thrusters (NEXT-C)
NEXT Ion Engine Test Firing
|Dart Propulsion explainer package (played in DART Live Launch broadcast)|
NASA’s Evolutionary Xenon Thruster(NEXT) is a gridded-ion thruster. In an ion thruster, ions are accelerated by electrostatic forces. The electric fields used for acceleration are generated by electrodes positioned at the downstream end of the thruster. Each set of electrodes, called ion optics or grids, contains thousands of coaxial apertures. Each set of apertures acts as a lens that electrically focuses ions through the optics. NASA’s ion thrusters use a two-electrode system, where the upstream electrode (called the screen grid) is charged highly positive, and the downstream electrode (called the accelerator grid) is charged highly negative. Since the ions are generated in a region of high positive and the accelerator grid’s potential is negative, the ions are attracted toward the accelerator grid and are focused out of the discharge chamber through the apertures, creating thousands of ion jets. The stream of all the ion jets together is called the ion beam. The thrust force is the force that exists between the upstream ions and the accelerator grid. The exhaust velocity of the ions in the beam is based on the voltage applied to the optics. While a chemical rocket’s top speed is limited by the thermal capability of the rocket nozzle, the ion thruster’s top speed is limited by the voltage that is applied to the ion optics (which is theoretically unlimited).
NEXT is projected to be about three times as powerful as the NSTAR used on Dawn and Deep Space 1 spacecraft. NEXT affords larger delivered payloads, smaller launch vehicle size, and other mission enhancements compared to chemical and other electric propulsion technologies for Discovery, New Frontiers, Mars Exploration, and Flagship outer-planet exploration missions. Glenn Research Center manufactured the test engine’s core ionization chamber, and Aerojet Rocketdyne designed and built the ion acceleration assembly. The first two flight units will be available in early 2019.
The NEXT engine is a type of electric propulsion in which thruster systems use electricity to accelerate the xenon propellant to speeds of up to 90,000mph (145,000km/h or 40 km/s). NEXT can produce 6.9 kW thruster power and 236 mNthrust. It can be throttled down to 0.5kW power, and has a specific impulse of 4,190 seconds (compared to 3,120 for NSTAR).The NEXT thruster has demonstrated a total impulse of 17 MN·s; which is the highest total impulse ever demonstrated by an ion thruster. A beam extraction area 1.6 times that of NSTAR allows higher thruster input power while maintaining low voltages and ion current densities, thus maintaining thruster longevity.
In December 2009, the prototype had completed an 48,000 hour (5.5 years) test. Thruster performance characteristics, measured over the entire throttle range of the thruster, were within predictions and the engine showed little signs of degradation and is ready for mission opportunities.
The first two flight units will be available in early 2019, in time for possible use on a New Frontiers-4 mission. After that, the NEXT-C engine will be made commercially available for purchase by both NASA and Industry through Aerojet Rocketdyne.
Power Processing Unit
The Power Processing Unit on the NEXT-C Ion Thruster controls the power output to the thrusters. Two flight power processing units (PPUs) are being developed by the Aerojet Rocketdyne Corporation in Redmond, Washington and ZIN Technologies in Cleveland, Ohio, in support of the NEXT-C Project.
The power processing unit operates from two sources: a wide input 80 to 160 V high-power bus and a nominal 28 V low-power bus. The unit includes six power supplies. Four power supplies (beam, accelerator, discharge, and neutralizer keeper) are needed for steady state operation, while two cathode heater power supplies (neutralizer and discharge) are utilized during thruster startup. The unit in total delivers up to 7 kW of regulated power to a single gridded-ion thruster.
The project is presently in the prototype phase and preparing for qualification level environmental testing.
Ion propulsion has been studied since the early 1960’s. NASA Glenn Research Center has had a long history of leadership in the area of electric propulsion, and is currently NASA’s lead center for ion propulsion.
NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) Ion Thruster
The NASA Solar Technology Application Readiness (NSTAR) program provided a single string, primary IPS to the Deep Space 1 spacecraft. The 30-cm ion thruster operates over a 0.5 kW to 2.3 kW input power range providing thrust from 19 mN to 92 mN. The specific impulse ranges from 1900 s at 0.5 kW to 3100 s at 2.3 kW. The flight thruster and PPU design requirements were derived with the aid of about 50 development tests and a series of wear-tests at NASA GRC and JPL of 2000 hours, 1000 hours, and 8193 hours using engineering model thrusters. The flight-set masses for the thruster, PPU, and DCIU were 8.2 kg, 14.77 kg, and 2.51 kg, respectively. About 1.7 kg mass was added to the PPU top plate to satisfy the DS1 micrometeoroid requirements. The power cable between the thruster and PPU was comprised of two segments which were connected at a field junction. The thruster cable mass was 0.95 kg, and the PPU cable mass was 0.77 kg. The xenon storage and feed system dry mass was about 20.5 kg. A total of 82 kg of xenon was loaded for the flight. Thrusters and PPUs were manufactured for NASA GRC by Hughes, and the DCIU was built by Spectrum Astro, Inc. The feed system development was a collaborative effort between JPL and Moog, Inc.
The DS1 spacecraft was launched on October 24, 1998. In-space testing and the IPS technology demonstrations were completed within the next three months. By April 27, 1999, the primary thrusting of the NSTAR engine system, required to encounter the asteroid Braille, was completed. The thrusting time at the end of April was 1764 hours. Thruster input power levels were varied from 0.48 kW to 1.94 kW. On July 26, 1999 DS1 obtained spectrometer data and images of Braille fifteen minutes after the flyby.
The DS1 mission was extended to continue a thrusting profile until the encounter with the comet Borrelly in September 2001. By October 30, 2000 the ion engine had accumulated 6630 hours of thrusting. The NSTAR ion engine has already demonstrated a propellant throughput in excess of 30 kg. For comparison purposes, a SERT II ion engine expended about 9 kg of mercury. Propellant throughput is an approximate signature of total impulse capability. After the encounter with comet Borrelly, the ion engine will have operated for more than 10,000 hours.