Minimize Electric Propulsion Test Chamber

EP (Electric Propulsion) Test Chamber installed at The Aerospace Corporation

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July 7, 2020: Deep inside a laboratory at The Aerospace Corporation's El Segundo campus in California, scientists are recreating the vacuum of space here on Earth. Aerospace's electric propulsion lab specializes in testing electric thrusters in space-like conditions, and they recently installed a new vacuum chamber that will enable them to test the newer, high-powered thrusters needed for future space exploration. 1)

"This chamber adds not just to Aerospace's testing capability, but adds to the world's testing capability," said Rostislav Spektor, Laboratory Manager in Electric Propulsion and Plasma Science. "When it becomes operational, it will be the best electric propulsion testing facility in the world."

Why Electric Propulsion?

Everyone is familiar with the sight of fire and smoke pouring out of the bottom of a rocket using chemical propulsion.

Electric propulsion takes a different approach, harnessing electric energy to ionize gas into a plasma, which is accelerated out of the thruster through a combination of electric and magnetic forces.

Electric propulsion produces significantly less thrust than chemical propulsion but is much more efficient in terms of the amount of fuel used. It's too weak to launch rockets through the atmosphere, but once in space, the lack of gravity allows electric propulsion thrusters' true potential to shine.

"It's the Tortoise and the Hare. Electric propulsion is slow but steady and chemical propulsion starts very fast, but runs out of steam quickly," Spektor said.

Historically, electric propulsion has mostly been used for station-keeping of satellites. But its highly efficient nature opens up possibilities for long-distance space exploration missions with the small but constant thrust building up over time, accelerating the spacecraft to a very high velocity.

A Testing Powerhouse

In order to make those long journeys, however, scientists need to be able to trust that the thrusters will perform consistently and reliably over the duration of the mission.

That's where electric propulsion vacuum chambers come into play. These school bus-sized devices are outfitted with a series of cryopumps that make the chamber very cold. When the pumps run, the air in the chamber sticks to the chamber sides, similar to condensation on a cold glass of water on a hot day. With the air gone, the chamber simulates the vacuum of space, and the team can place electric thrusters inside for testing.

"Electric propulsion devices perform differently in space than they do on Earth. The relationship isn't linear, which can make predicting exactly how it will perform difficult," said Spektor. "The closer you are to test-as-you-fly conditions, the closer you are to measuring performance you would expect in orbit."

The new chamber, 14 ft (4.27 m) in diameter and 30 ft long (9.14 m), is considerably larger than the lab's older 8-foot (2.44 m) diameter chamber, which means it has more room for cryopumps. The Aerospace team considered buying commercial pumps, but in the end decided to design their own pumping system to ensure optimal performance.

The chamber body was delivered in four segments over the course of a week and then bolted together. The custom-designed cryopump system will be installed over the next six months, followed by the diagnostic system.

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Figure 1: The installation team maneuvers the first section of the vacuum chamber into place (image credit: The Aerospace Corporation)

End-to-End Electric Propulsion Testing

As a federally-funded research and development center (FFRDC), Aerospace is not allowed to produce flight hardware that could compete with commercial companies.

Instead, Aerospace provides end-to-end testing of electric propulsion thrusters, from measuring thrust, exhaust velocity and specific impulse to more advanced work like plume characterization, which helps quantify the risk of damage to other parts of the spacecraft. The lab also offers non-invasive testing using laser and optical diagnostics.

"We've carved ourselves a niche in electric propulsion as the testing warehouse. All the commercial companies come to us for unbiased testing and measurement," Spektor said. "We have probably the most comprehensive set of electric propulsion diagnostics that you could find anywhere in the world."

The recently installed vacuum chamber, with its increased pumping speed, is just the latest addition to this laboratory's arsenal of testing equipment. "As electric propulsion devices get larger and more powerful, higher pumping speeds are needed to maintain the proper pressure ratio and allow for accurate testing," said Spektor. "If there are any potential issues with the thruster, you might not see them at lower pumping speeds because you're not at the conditions you are in space."

Aerospace is equipped to test the latest large electric propulsion devices or smaller micro-thrusters that go on CubeSats, with tests spanning from just a few hours up to more than a year.

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Figure 2: The final section of the chamber is carefully attached, completing the 30-ft long piece of equipment (image credit: The Aerospace Corporation)

The Future of Electric Propulsion

When operational, the expanded testing facility will allow the lab to double its workload, providing testing services to military and civil customers, as well as a growing field of commercial manufacturers.

One of the first customers will be NASA, testing its 12.5 kW AEPS ( Advanced Electric Propulsion System) thruster, which is part of the Gateway mission to orbit the moon.

Other customers are also lining up to access the world-class chamber and benefit from Aerospace's expertise.

There are new potential applications developing with our national security space customers for high power electric propulsion," said Tom Curtiss, director of the Propulsion Science Department. "Watching those come to fruition will be a great thing, and we'll be able to help reduce the costs of qualifying and developing technology for the next 10 or 20 years."



 

Electric Propulsion Explained

Electric propulsion (EP) in spacecraft involves the application of electrical energy to a gas. The electrical energy breaks the gas down, producing ions and electrons, while the electric field of the thruster pulls these ions away from the thruster at a very high velocity, thereby creating propulsion. 2)

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Figure 3: An electrically powered spacecraft propulsion system uses electrical energy to change the velocity of a spacecraft (photo courtesy NASA)

Electric propulsion is gaining industry momentum as an important source of spacecraft thrust, and Aerospace has been at the forefront of research in the field of electric propulsion for decades, with an emphasis upon electric propulsion diagnostics.

Aerospace is renowned for applying advanced diagnostics to analyze thruster characterization, and has extended this approach to laser-based plume measurements and various devices used to determine electron temperature, density, and electric potential of plasma, e.g. plasma probes. Aerospace was the first to apply a number of diagnostic tools to the characterization of EP thrusters, most notably in the area of electromagnetic radiation measurements.

Electromagnetic Interference Test Facility

The Aerospace Electromagnetic Interference Test Facility is unique in the industry, and draws to Aerospace virtually all flight thrusters for testing to ensure that thruster operations will not interfere with satellite communications. Consequently, Aerospace maintains the most comprehensive database of EP thruster measurements anywhere.

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Figure 4: The NASA Evolutionary Xenon Thruster (NEXT), photo courtesy: NASA

The Aerospace Electromagnetic Interference Test Facility has also provided crucial mission assurance and anomaly resolution to the U.S. Air Force's Wideband Global Satellite Communications System (WGS) and Advanced Extremely High Frequency (AEHF) programs.

In addition, Aerospace is leading an effort to characterize facility effects upon Hall-effect thruster operation (a type of ion thruster with propellant accelerated by an electric field), in which the ability to reliably predict on-orbit performance using ground-based test and qualification data has been severely hampered.

Recent collaborations with the NASA/GRC ( Glenn Research Center) have been critical in advancing NASA's NEXT (Evolutionary Xenon Thruster) 7 kW ion engine to flight, as well as developing future higher-power annular ion engine designs. Aerospace has also developed novel diagnostics for the characterization of miniature electrospray thrusters developed at the Massachusetts Institute of Technology for use on CubeSats.



1) "New Electric Propulsion Chamber Explores the Future of Space Travel," Aerospace, 7 July 2020, URL: https://aerospace.org/article
/new-electric-propulsion-chamber-explores-future-space-travel

2) "Propelling Electric Propulsion Forward," Aerospace, 2 September 2014, URL: https://aerospace.org/article/propelling-electric-propulsion-forward
 


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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