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NASA-Funded Teams Prepare For Key Space Electric Propulsion Tests

Graham Warwick 

Ground tests of two different high-power electric propulsion systems are planned for November in a key milestone in NASA’s plans for the exploration and commercialization of space beyond Earth’s orbit.

Under NASA’s NextSTEP program, a 50:50 cost-sharing effort, Ad Astra Rocket and Aerojet Rocketdyne (AR) will conduct tests in which their thrusters are planned to run for 100 hr. at a power level of 100 kW. The tests will demonstrate electric propulsion technology for long-duration, deep-space transportation missions.

Ad Astra will demo its Variable Specific Impulse Magnetoplasma Rocket (Vasimr), while the Aerojet Rocketdyne team will test its XR-100 Nested Hall Thruster. A third contractor, MSNW, has dropped out after failing to meet system performance goals with its Electrodeless Lorentz Force thruster.

·         High-power electric propulsion aimed at space transportation

·         Ad Astra’s Vasimr can vary specific impulse and thrust

·         Aerojet Rocketdyne’s XR-100 uses nested Hall thrusters

The objective is to demonstrate an electric thruster generating more than 5 newtons of thrust, with a specific impulse (Isp) of 2,000-5,000 sec., a system efficiency greater than 60% and a specific mass less than 5 kg/kW. Demonstrating thermal steady-state operation for 100 hr. at 100 kW will take the two thruster designs to a technology readiness level of 5 and pave the way for a technology demonstration mission in space.

Ad Astra says potential missions that would be enabled by high-power solar-electric propulsion systems include in-space logistics, satellite servicing, orbital debris removal, space resource recovery and faster deep-space robotic and human missions.

The VX-200SS reached 100 kW during testing in 2017, using argon as the propellant. Credit: Ad Astra Rocket

As the three-year NextSTEP program approaches its final demonstrations, the rival teams provided an update on progress with their thrusters at the American Institute of Aeronautics and Astronautics’ Propulsion & Energy conference in Cincinnati in early July. The Vasimr is a high-power electric plasma rocket. The engine has three linked magnetic chambers. First is the ionizer, which produces low-temperature plasma from a neutral gas—argon has been used so far. Second is the radio-frequency heater, or booster, which heats the plasma with radio waves to a very high temperature. The last chamber is an open magnetic nozzle where the heated plasma accelerates in an expanding magnetic field to produce thrust. The Vasimr can vary exhaust velocity (specific impulse) and thrust without changing the power setting of the engine. If more thrust is required, more of the power is directed to the ionizer and less to the heater, to make more plasma. A denser, but cooler exhaust generates more thrust. Alternatively, by shifting more of the power to the heater, less plasma is generated, producing less thrust, but the exhaust is faster and more fuel-efficient.

The plasma steady-state VX-200 was developed by Ad Astra with private funds. Credit: Ad Astra Rocket

The VX-200SS test engine incorporates improvements made over more than three decades of Vasimr development. These include using helicon discharge to efficiently produce plasma in the ionizer, and ion cyclotron resonance heating to provide a single-pass RF boost to accelerate the ions. Ad Astra developed the initial 200-kW VX-200 on private funds, demonstrating greater than 73% efficiency and 5,000-sec. Isp. Redesigned to produce long-duration pulses, the thermal steady-state VX-200SS uses a superconducting magnet. Efficiency, at better than 60%, is limited by the existing magnet, says Ad Astra. The next magnet will be a high-temperature superconductor, leading to a smaller, lighter and more thermally robust design for the first-flight engine, the company says. Ad Astra is also moving to a quadrupole configuration, from a dipole design, to eliminate torquing in the magnetic field. This 150-kW TC-1Q (Thruster Core-1, Quadrupole) system is aimed at a technology demonstration mission, if funded by NASA.

X3 is installed for risk-reduction testing in the VF5 vacuum chamber at NASA Glenn. Credit: NASA Glenn Research Center

The XR-100 electric propulsion system is being developed by Aerojet Rocketdyne in partnership with NASA Glenn Research Center, the University of Michigan (UoM) and Jet Propulsion Laboratory (JPL). It is based on the NASA/UoM-developed X3, a three-channel 200-kW Nested Hall Thruster, combined with AR-developed modular power processing units and mass flow controllers. A Hall-effect thruster traps electrons in a radial magnetic field, then uses them to ionize propellant and accelerate the ions to produce thrust. A neutral gas, typically xenon, is fed through the anode into an annular channel, where the atoms are ionized by collisions with circulating high-energy electrons. The xenon ions are then accelerated by the electric field between the inner anode and external cathode.

The three-channel X3 Nested Hall Thruster has been operated at 102 kW in testing. Credit: Aerojet Rocketdyne

Single-channel Hall thrusters are flying at 4.5 kW, with systems up to 14 kW in design, says AR, but to reach high power levels the X3 uses three concentric channels. This allows increased throttling and redundancy, each channel firing individually, all concurrently or in any combination of two. It also provides higher power density: the 80-cm-dia. X3 producing the same power as a 150-cm.-dia. single-channel thruster. The passively cooled X3 is designed to throttle from 2-200 kW, and 1,600-3,200 sec. specific impulse when using xenon, while maintaining greater than 60% efficiency. The full XR-100 system, to be tested in November incorporates several design improvements, including a third-generation JPL-developed hollow cathode to reduce erosion for increased life and a redesigned segmented insulator ring to avoid arcing at high temperatures.