Ion Propulsion - How The Force Of A Sheet Of Paper Powers Cutting Edge Space Missions

The force from a typical ion thruster is roughly equivalent to the weight of a few sheets of office paper. Yet, this small force is crucial to the performance and success of the most daring and cutting edge space missions today.

For example, the types of ion thrusters that have flown or are flying today on missions such as the Gravity field and Ocean Circulation earth Explorer (GOCE), Small Missions for Advanced Research in Technology No. 1 (SMART-1) and Dawn all have thrusts in the 1 to 100 milli-Newton range. This is equivalent to a weight range of approximately 0.1 to 10 grammes.

If you consider that the average sheet of A4 office paper weighs about 10 grammes then ion propulsion devices such as these are not exerting any real force by Earth standards.

It is this very idea though that makes them so useful and often critical to satellite missions in space; because in space weight is not as important as here on Earth. In addition, fine control of satellites and probes, for long periods, is starting to replace quick and powerful chemical burns for orbit change that have been used previously.

Most people have a picture of what a space engine looks like; it is either the kind of engine seen on a rocket, launching a payload into the sky in a fiery frenzy; or it is something out of science fiction, a device with a blue glow that propels spacecraft at unfathomable speeds across the universe.

Though some individuals recognise that there are varying types of thrusters, for example, used in space on satellites or on astronaut EVA suits, not many outside of the thruster community know how many types there are, how they are actually used and in particular just how versatile ion propulsion can be.

There are, of course, many types of ion propulsion these days all with a specific niche that they have been designed to fill. Here are some examples:

• Resistor jets and colloid thrusters
• Field-emission electric propulsion (FEEP)
• Ferroelectric thrusters and pulsed plasma thrusters
• Gridded ion thrusters - Kaufman, microwave or radio-frequency
• Magneto-plasma-dynamic thrusters
• Hall-Effect thrusters

How does a force so small become crucial to cutting edge missions?

The answer to this question lies in the nature of how ions are made and used. Typically, an ion thruster is a device in which a low temperature singly-charged plasma is formed using a gas like Xenon or Argon and application of electromagnetic fields. The ions are accelerated by high voltages once produced and then 'neutralised' using a special device conveniently called a 'neutraliser' that injects electrons into the ion beam.

Even in the more simplistic devices, creation of a plasma allows for a number of parameters and characteristics to be varied to optimise the device for a mission. This may be the range of gas flow used; it could be range of electric fields applied.

Typically each device can be characterised to fit into a performance envelope that allows quite a wide range of thrust levels to be achieved without degrading the performance of the engine.

A common characteristic is that a lot of the devices can be set on a low thrust level or a high thrust level if needed with the difference being much larger in thrust range than a turbo-fan jet engine used on an aeroplane.

Turbo-jets idle at around 35 to 40% of full thrust; an ion thruster can idle as low as 5% of full thrust as in the case of the GOCE T5 ion engine.

It is this variability of performance parameters and control of the thrust range that sets ion thrusters apart form other types of space propulsion. Yet, most ion thrusters are designed to operate at nominal levels for long durations or to be operated only for a few hours each day for station-keeping.

Has any ion thruster been used to its full potential?

There is currently only one mission that I am familiar with that utilises the full capability and promise of an ion thruster. It is the European Space Agencies GOCE mission, one that I was heavily involved with, principally in the design and development of the thruster and its control.

Unlike most ion thruster missions, and in fact most space missions, measuring the gravitation field of the Earth to the resolution desired, meant taking the risk of actually flying a spacecraft in the upper atmosphere.

This meant that a propulsion system had to be developed to counteract the drag on the spacecraft so that it could maintain its altitude and perform measurements. In other words, the thrusters had to literally stop the spacecraft falling out of the sky.

The thrust range specified was from 1 to 20 milli-Newton, or approximately one tenth to 2 sheets of A4 paper. If you were to rip off a corner of a sheet, that would be the lowest thrust; if you were to place 2 sheets on top of each other that would be the highest thrust.

However the specification did not stop there. The thrusters and propulsion system were designed to be able to vary the thrust at micro-Newton resolution. The actual ratio of the highest thrust level to resolution of that thrust level was just less than 2000:1.

If you consider your average motor car and its speed range in miles per hour, say 0 to 160 miles per hour, this would be like having a cruise control system that could keep you at 160 miles an hour with a variation less than 0.1 miles an hour. That is quite a control system by any standards.

Even then, there is an extra factor in that this control is achieved at a rate of 100 times per second, which arguably makes the propulsion system the fastest and most intricate system flying today.

It is not hard to see then that ion thrusters may be the vital tool to realise more intricate cutting- edge space missions as the nature of the process of ion production is such that devices can be built to meet very demanding specifications.

Yet, ion propulsion has not really been exploited to its full potential on every space mission it has been used on, mainly because it is typically tailored to each mission.

This tailoring reflects the demand; no-one truly needs a multi-functional thruster yet more projects demand a greater level of performance in lots of different aspects. Ion propulsion is seen as a major enabler for space missions to come but it is constantly pushed at the limit.

Perhaps this is a limit that is unsustainable for development. Hopefully this will mean that missions begin to be developed with thrusters in mind so that a successful marriage of performance, complexity, functionality and cost can be achieved, otherwise the technology may not be chosen due to bad experience with implementing it.

Brown Engineer Nathanial Cooper ’12 Finishes Third at AIChE Competition

Students in the Brown chapter of the American Institute of Chemical Engineers (AIChE) traveled to Minneapolis, Minnesota, to attend the national AiChE meeting and compete in a student poster competition.

In the environmental category, Nathanial Cooper ’12 won a third place award. His poster was entitled, “Agricultural Waste Based Bio-Char Sorption Potential”. Last year, Cooper finished second in the poster competition.

Four students represented Brown at this year’s competition, including: Henry Mattingly ’12 (supervisor Robert Hurt), William Trinh ’12 (supervisor Indrek Kulaots), Cooper (supervisor Indrek Kulaots), and Ellison Kandler ’13 (supervisors Steve Greenbaum of the City University of New York and Eric Suuberg).

“As I've attended these student conferences over many years, and I do carefully review most of these posters presented, I must say that this year was even beyond what I have seen before,” said Kulaots. “The competition has gotten more and more competitive every year, and the level of science presented by undergraduates is remarkable.”

Ancient Lamps, Earrings Yield Their Secrets Under Neutron Imaging

“Neutron imaging gives researchers new tools for exploring artifacts and ancient technology”

Brown University School of Engineering Professor Brian Sheldon is the co-principal investigator on an exciting colloborative project that also includes Brown's Joukowsky Institute for Archaeology

For the first time at Oak Ridge National Laboratory (ORNL), neutron images in three dimensions (3-D) have been taken of rare archaeological artifacts. Bronze and brass artifacts excavated at the ancient city of Petra, in present day Jordan, were recently imaged in 3-D using neutrons at the High Flux Isotope Reactor’s CG-1D neutron imaging instrument.

The neutron imaging technique gives eager archeologists and ancient historians significant, and otherwise wholly inaccessible, insight into the manufacturing and lives of cultures that once occupied settlements within the Roman Empire, Middle East, and Colonial-Period New England.  

The samples imaged in 3D in August came from the collections of the Joukowsky Institute for Archaeology and the Ancient World at Brown University. They include an elaborate hanging bronze oil lamp, a large Roman coin, and—most charmingly—a standing dog figure, which might have been either a religious dedication or perhaps a toy. Although their original provenance is unknown, they are all excellent examples of common metal finds from antiquity.

Principal investigator (PI) Krysta Ryzewski, an assistant professor of anthropology at Wayne State University, and her co-PI Brian W. Sheldon, professor of engineering at Brown University, were loaned the artifacts for study from professor Susan E. Alcock, director of Brown’s Joukowsky Institute. 

In earlier work, the team conducted two-dimensional imaging of copper alloy (bronze and brass) artifacts both from Petra and from Greene Farm, a colonial-period plantation in Rhode Island. The samples include artifacts from daily life: a clothing buckle, a knife, and some building hardware.

Top: photo of ancient Greek lamp. Bottom: neutron radiograph of the same lamp.

One circular object from Petra was so corroded that it was unidentifiable. But when it was imaged with neutrons, underneath was a piece of jewelry, probably an earring. Petra is most famous as a trading center in ancient times, connecting the Mediterranean world with places as far away as India and China. It was the capital of an independent kingdom of the Nabataeans, until the emperor Trajan incorporated it into the Roman Empire in the early second century A.D.

The earlier imaging and analysis resolved some questions of object identity and raised many new ones about the techniques and materials that crafts people in the past used to make these objects. “We can also examine certain objects (such as the knife or the bronze lamp) to look for trace residues of the oil once burned in the lamp or what the knife was used to cut,” says Ryzewski.

“I first learned of the developing neutron imaging instruments at Oak Ridge in my conversations with Hassina Bilheux (lead instrument scientist for CG-1D). At the time I was a postdoctoral fellow in archaeology and engineering at Brown. I attended a neutron imaging workshop at SNS in November 2008, and became the only archaeologist to be part of the VENUS instrument development team. Brian Sheldon at Brown also joined then. We have been collaborating on all of the experiments with Hassina at SNS and HFIR,” she says.

The neutron imaging beam line is a huge step forward for these scholars. “Archaeologists and scientists can obtain relatively little information about the manufacture of archaeomaterials, ancient objects, and the materials from which they are constructed from external surfaces alone,” says Ryzewski. “Very few historical accounts describe the construction of such objects and archaeomaterials, ancient bronzes, or ceramic vessels. The only source of information about how these objects were constructed comes from their material properties and composition.”

Archaeological objects are reviewed as unique cultural resources. Earlier analysis often entailed extracting a sample from such an object, which meant damage and sometimes even wholesale destruction of an artifact so it could be mounted effectively for analysis. Analysts’ necessarily conservative treatment of archaeomaterials left many questions unanswered.

Imaging archaeological objects comprehensively and systematically with neutrons only became possible with the development of the CG-1D prototype beam line. Neutron activation analysis and neutron imaging at Oak Ridge means scholars can now conduct detailed, nondestructive analysis of samples. “There currently exist a vast array of archaeological objects and research questions about ancient and historical technological development that can now be posed,” says Ryzewski. “The CG-1D beam line has offered us an invaluable alternative for performing nondestructive, noninvasive analysis.”

CG-1D data can reveal the raw materials used, the manufacturing techniques, the historical development of alloys and composite materials and the geological origins of ores and clay. On the cultural side, researchers can learn about the activities of ancient people’s daily lives that such objects served.

“Archaeologists can now begin to precisely reconstruct past networks and patterns of resource extraction, trade and exchange, environmental impacts of industrial activities on ancient landscapes, and the transmission of craft production traditions over time,” Ryzewski says. “These are some of the sorts of questions that our current research and experiments are designed to address.“

The 3-D neutron imaging and quantitative analysis occurs at an instrument that is a time-of-flight beam line, with a chopper for producing pulses of neutrons to take noninvasive images. Neutrons, rather than x-rays, do the work.

“Part of our early work was to test the parameters of the instrument and how we might need to adjust the instrumentation to suit the artifacts, which tend to vary in composition, size, and density,” Ryzewski says. 

“We anticipated that we would be able to see beneath the surface and find evidence of manufacturing steps (mold seams), impurities or other organic inclusions in the metals, residue from the objects’ use, and microstructural or compositional elements,” she says.

Their data are still being processed, but preliminary results from the bronze lamp suggest that they will be able to see and examine aspects of all of these areas of interest once the 3-D data are compiled. 

“Our work is still in its early stages. We hope to reexamine these objects in further rounds of testing in 2012. We will expand our sample base to other types of metal artifacts, perhaps some excavated from shipwrecks. We hope to examine ceramic artifacts as well, Ryzewski says.

More broadly, the scholars may be in a position to offer information to scientists who specialize in the conservation and stabilization of museum collections. Other findings may provide insights into materials behavior of interest to materials science. “Each round of experiments raises many more questions about the materials in the object and about the instrumentation itself,” Ryzewski says.

This fall the researchers will return to HFIR to image some of the bronze objects for Bragg-edge peaks in the materials. Collaborating with Ryzewski and Sheldon are Bilheux and Lakeisha Walker of SNS and Susan Herringer, a doctoral student in materials science engineering at Brown and the Joukowsky Institute.

The group will publish their results in both archaeological and neutron sciences academic publications. In addition, they will present their initial findings at the annual Society for American Archaeology meetings in Memphis in April 2012.

Courtesy of Oak Ridge National Laboratory/Written by Agatha Bardoel