Evolution of ion propulsion systems for deep space exploration

Citation

(2004), "Evolution of ion propulsion systems for deep space exploration", Aircraft Engineering and Aerospace Technology, Vol. 76 No. 1. https://doi.org/10.1108/aeat.2004.12776aaf.007

Evolution of ion propulsion systems for deep space exploration

Evolution of ion propulsion systems for deep space exploration

Keywords: Ions, Propulsion, Space

An in-depth understanding of material properties and applications engineering has enabled Morgan Advanced Ceramics to make a significant contribution to the evolution of Europe's deep space exploration programme.

Morgan's materials experts at Erlangen, Germany, are working closely with EADS Space Transportation to improve the efficiency of ion propulsion systems. Their role is to optimise the design and material properties for the ceramic thruster chamber in which the propellant is ionised. This will include the development of suitable production methods to achieve the fine tolerances required.

How it works

Ion propulsion technology does not burn fuel as chemical rockets do. Instead, it uses electricity to charge heavy gas atoms, which accelerate from the spacecraft at high velocity and push it forwards. The gas of choice is Xenon, the same substance that is commonly used in photo flashlights and in some modem car headlights.

There are three main categories of ion engines, classified according to their basic physical method of operation: electrothermal, electrostatic and electromagnetic. The design on which Morgan is engaged is a gridded ion engine, which falls under the second category.

The ceramic chamber is central to the ionisation process. It is used to contain the propellant while it is subjected to an electrical current that charges the gas atoms to create ions. There are a number of ways in which the electrical charge can be applied. In a gridded ion engine – or radio-frequency, ion thruster (RIT) – a metallic coil around the outside of the vessel is used to induce a radio frequency (RF) field inside the chamber. This field is sufficiently powerful to accelerate electrons which then collide with the Xenon gas, creating positively charged ions. These ions are then accelerated through grid system carrying a negative charge and neutralised directly after leaving the vessel at high speed. It is this stream of Xenon plasma, with its characteristic blue colour, that creates the thrust to push the spacecraft forwards.

Ion propulsion is said to offer enormous potential benefits for deep space travel. For example, an ion engine can run on a few hundred grams of propellant per day, which makes it favourably lightweight. Less weight means less cost to launch. This is an extremely important factor for any space programme's budget: it currently costs around 10,000 Euros per kilogram of weight to launch a craft. It could also mean we arrive at our destination more quickly, or go farther than before with the same payload: ion propulsion can reportedly push a spacecraft about ten times as fast as chemical propulsion per kilogram of fuel.

Ions of history

Despite the many benefits claimed, ion propulsion has not been that readily adopted. It is 40 years since the first ion engine was built. One reason for this may be that ion propulsion, although highly efficient, has a very gentle thrust. It is therefore not suitable for applications where rapid acceleration is required. The technology is also limited by the storage capacity of the electrical power source. Another barrier to adoption is a common problem for new technologies: perceived risk.

Although scientists had conducted many laboratory tests and some limited testing in space, no mission had been willing to use the technology as a primary propulsion system. The risk of failure, which would include the loss of precious scientific data as well as the financial cost, was simply too high

However, the need to explore space less expensively and with more capable craft has persuaded international space agencies such as NASA and ESA to continue investing in unproven, but potentially important technologies. Ion propulsion is one technology that has attracted more investment. In the early 1990s, agreements were reached between the Russian, American and European space industries to pursue the development and commercialisation of ion thrusters.

Recent developments

Events within the last 5 years have proved that this decision was correct. Ion propulsion technology has become a serious proposition. In 1998, NASA launched the Deep Space 1 mission to validate 12 advanced technologies including ion propulsion. Not only did the prototype ion propulsion system perform throughout the 11-month mission, it continued to work perfectly until the DS1 spacecraft was retired in December 2001.

A second ion engine was run under laboratory conditions in parallel with the DS1 mission. The purpose was to perform a life test and learn more about the wear characteristics of this system of propulsion. By August 2002, the laboratory engine had processed 200kg of Xenon propellant. This is precisely the fuel load that NASA has calculated and it will take Dawn, an ion propelled spacecraft due to launch in 2006, on its mission to study the structure and composition of the Ceres and Vesta asteroids.

In Europe too, ion propulsion technology is proving its worth. In January 2003, the ESA satellite Artemis finally reached its assigned geostationary orbit, following a recovery operation that had lasted for 18 months. Due to a malfunction in the Ariane 5 rocket that carried it, Artemis had been launched into a lower than intended elliptical orbit. In fact, at its apogee the satellite was achieving only 17,487 km: less than half its intended maximum distance from Earth.

The daring manoeuvres executed to rescue the satellite proved successful. Artemis managed to escape its erroneously low elliptical orbit and reach a circular “parking” orbit at an altitude of 31,000 km.

Although this meant that the hardware was safe from damage by space debris and the earth's radiation belt, the satellite was still far short of its targeted geostationary transfer orbit with an apogee of 35,853 km. In addition, the first stage of the rescue had consumed much of the chemical propellant available to the craft, although there was sufficient fuel left for the craft to maintain N/S station keeping for 7 years.

However, Artemis was equipped with two experimental ion propulsion systems designed by EADS Space Transportation: an electron bombardment system and a Radio frequency Ion thruster Assembly (RITA). The ion engines had been designed specifically to control the satellite's inclination by generating thrust perpendicular to the orbital plane. The rescue operation required thrust in the orbital plane to push the satellite to final geostationary orbit. Achieving this would involve rotating the satellite by 908. The procedure required a new concept for steering the ion propulsion engines, as well as new control modes, telecommand, telemetry and other interfaces.

The attempt was successful. The four ion engines successfully maintained a slow, but steady progress during the orbit-raising process. The gentle thrust they provided was just 15mN, which meant that the satellite climbed an average of 15 km per day. This was the first orbital transfer of a European satellite to geostationary orbit using ion propulsion.

These experiences, together with continuing technical developments, have positively influenced attitudes towards ion propulsion. ESA is currently preparing to use spacecraft that rely on ion engines as their primary means of propulsion.

Improving ion technology

Morgan Advanced Ceramics, Erlangen, became associated with the European development programme around 8 years back. Its expertise in technical ceramics has ensured a continued involvement. The company first worked with Daimler-Benz Aerospace in 1995 and is now working with successor organisation EADS Space Transportation on the development of the RIT-22 ion propulsion system.

During this long association, Morgan Advanced Ceramics has delivered discharge vessels and ceramic isolators for the RIT-10, the thrusters that saved ARTEMIS, and the FSA-XX. During early development tests, the discharge vessels were made of quartz. In order to make the RIT-10 stable against the vibrations induced by the launcher's rocket engines for the ARTEMIS mission, a material 91 with the same dielectric properties as quartz but with higher structural stability was required. Alumina was the material of choice.

There were a number of potential benefits to be gained from this change. Alumina is said to be easier to fabricate and claims superior mechanical properties. Good thermal shock resistance ensures that the chamber can contain extremes of temperature, particularly during plasma ignition. Alumina is also lighter – the weight of the larger ESA-XX discharge vessel has been reduced to 7 kg using alumina. The ESA-XX ion thruster, a laboratory demonstration model, is actually a synergy of two European ion propulsion technologies. It combines the discharge chamber of a radio frequency ionisation thruster (RIT-35) with the extraction of the UK-25 electron bombardment thruster.

Morgan's early involvement in the project enabled them to take the initial drawings and convert the chamber design into a manufacturing drawing. The alumina discharge vessel was manufactured to its basic dimensions and then hardened in a sintering furnace. However, the complex shape of the green part meant that the vessel was subject to some distortion during sintering. Morgan Advanced Ceramics designed a number of tools to support the shape of the piece so that the required accuracy in the shape of the part was achievable. Final quality was achieved using a combination of grinding, drilling and machining.

Morgan's contribution has already enabled the vessel to grow from the 100 mm diameter of the ARTEMIS thruster in the first phase of development to a 220 mm diameter vessel in the third phase. This vessel is capable of generating 200 m-Ne of thrust.

The material used in the extraction grid system has also been changed on Morgan's recommendation. Early prototypes used stainless steel, but Morgan advised that molybdenum could be used instead because it exhibits virtually the same thermal coefficient as the ceramic. This minimises the difference in thermal expansion between the grid and the alumina vessel and eliminates the likelihood of cracking during operation.

Ion – the future

The technologies involved in ion propulsion continue to be developed and refined. Concerns over the limitations of the electrical power source used to ionise the gas are being overcome using improved solar arrays, with a nuclear source taking over as the craft moves further into the darkness of deep space.

Morgan is continuing to develop the beneficial properties of the advanced materials it uses and is currently engaged upon ways of improving the mechanical properties of the alumina ceramic for ion engines still further. This will enable even larger thruster vessels to be manufactured in future. Propulsion system incorporating a Morgan alumina ceramic discharge chamber is a promising candidate to be the primary power source on the BepiColombo, due to be launched in November 2009.

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