To launch satellites into space with a rocket, you need a voluminous “combustion engine”. In the larger models the associated tanks and chemical fuel alone weigh more than 1,000 kilograms. Significantly more delicate ion drives are used to move the artificial satellites that are already in orbit. These are powered by electricity rather than chemical reactions and usually use xenon or krypton for the reaction. Messer offers these gases in high purity and with customized specifications for this application.

Satellites are subject to north-south drift in space, caused by the gravitational pull of the sun and moon. This gradually changes their orbit. To compensate for the deviation and maintain the desired orbit they have to achieve a speed change of about 45 to 50 meters per second per year. In the weightlessness of space no great forces are needed for such a correction. A small, precisely measured impulse from time to time is enough to adjust the position.

Light electric motor vs. heavy combustion engine

In contrast to the heavyweight burner of the rocket, ion thrusters – in particular so-called Hall Effect thrusters – are small and light: the thruster for a satellite weighing up to one ton weighs just 1.5 kilograms, and the gas tank holds only a few hundred kilograms of fuel, even for large communication satellites. The thruster is only ignited for a few minutes at a time for the correction maneuver and usually draws its energy from the solar collector that also supplies the space probe with electricity.

In an ion engine, a small amount of a gaseous propellant is ionized. An electric field is then used to accelerate the ions to speeds of up to 400,000 kilometers per hour and direct them into space through the outlet. The resulting recoil causes the desired change in position; after the targeted impulse, the engine is switched off again. In addition to satellites, space probes for space research also use this type of propulsion to penetrate into the depths of space. In both cases, the limited gas supply must last for many years of operation.

Ideal propellant

The noble gas xenon is particularly suitable as a propellant for ion propulsion; it is used in most of today's satellites and space probes. Krypton has been used much less frequently, but this could change in the future. Unlike some of the metals that were initially experimented with, the gases do not need to be vaporized first; they are also environmentally neutral and easy to handle. Ion propulsion is remarkably efficient: with xenon in the tank, the specific impulse by weight is more than six times higher than that of a thermal rocket engine.

The only source of both noble gases is the Earth's atmosphere. They are only present in the air we breathe as trace elements, but krypton, at around 1 ppm, is more than ten times more abundant than xenon, at 0.09 ppm. This makes it easier and cheaper to extract. Xenon, due to its higher atomic weight, enables more thrust with the same tank volume, but this difference is becoming less and less important due to the increasing efficiency of the latest engine types. For example, SpaceX's Starlink satellites are powered by krypton.

Gas quality ensures efficiency

The efficiency of an ion engine depends, among other things, on the filament, a glowing thread used to generate electrons. It reacts sensitively to various molecules; in particular, oxygen and oxygen compounds can trigger unwanted reactions on its surface. Therefore, the gas of the supporting mass must meet high quality requirements, which are specified very precisely by the manufacturers of the drives – but not at all the same. In addition to the degree of purity, the composition of the remaining impurities plays an important role. The extent to which the performance of an ion engine is affected depends crucially on the type and quantity of residual molecules.

Messer offers two gases with a reduced level of impurities, Xenon 5.0 and Krypton 5.0 for ion thrusters. They were developed specifically for this application and enable the thruster to maintain a high level of efficiency. The extraction and quality control processes are meticulously tailored to the special requirements of use in an ion drive. They go beyond the standard specification of a purity level of 5.0 – 99.999 percent by volume of the respective gas.

Quality control process for gas and cylinder

Given their minimal concentration in the atmosphere, the only way to make the production of the two noble gases economically viable is to use very large air separation units (ASUs). These can be found at steel mills, for example, where they provide a continuous supply of the large quantities of oxygen and nitrogen needed there. Messer operates a number of such plants in Asia, Europe and America. This ensures a continuous supply in all regions of the world.

The first steps of the quality process are carried out in the LZA after the separation of the noble gases with downstream cleaning processes. This further reduces the small amounts of other substances that remain, including oxygen, hydrogen, methane, CO and CO₂, as well as moisture. At the same time, the containers for the engine gases undergo a multi-stage process to ensure that the purity achieved is maintained until delivery.

The cylinders are made of particularly high-quality stainless steel with extremely smooth, electro-polished surfaces. This helps to prevent any possible buildup of water molecules from the humidity in the air. In addition, the cylinders are purged with high-purity nitrogen before filling. During the purge, the quantity and size of any solid particles present are determined by means of laser light absorption. The cylinders are only used if they comply with the strictly defined limit values. Quality control also includes a valve test to ensure that the fitting is free of grease and oil. The cylinder certificate contains the dimensions of the cylinder as well as pictures of its condition before delivery. All relevant components are recorded in the detailed documentation.

Comprehensive documentation and customized solutions

Filling after removal of the purge gas follows a documented standard operating procedure. After filling, the quality of the gas is checked again – in other words, in the state in which it reaches the user. For this purpose, Messer uses a gas chromatograph with thermal conductivity detectors that is specially calibrated for the analysis of noble gases. The values obtained are also documented in the quality certificate.

The manufacturers and users of ion thrusters have defined individual specifications for their various Hall Effect thrusters. In addition to standard products, Messer's range, therefore, also includes customized solutions with appropriate documentation in accordance with the relevant specifications. The sales organization relies on personal contact and individual advice in order to tailor the supplied gases precisely to the customer's requirements.