Ion propulsion

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Test run of a xenon ion engine from NASA
NSTAR ion engine of the Deep Space 1 space probe
Japanese space probe Hayabusa with ion thrusters (left)

Ion propulsion is a propulsion method for spacecraft ; an ion thruster uses the recoil of a generated (neutralized) ion beam to move. Depending on the energy source used, a distinction is made between solar-electric (English Solar Electric Propulsion , SEP) and nuclear-electric propulsion (English Nuclear Electric Propulsion , NEP).

Ion engines generate too little thrust for a rocket launch directly from Earth , but use less support mass than chemical engines. That is why they are suitable as secondary engines for energy-efficient continuous operation, especially for the long trajectories of interplanetary probes.

function

The ion beam is generated by first ionizing gas particles (e.g. xenon ) or tiny droplets (e.g. mercury ) through a cathode . They are then accelerated in an electric field . After passing through the so-called neutralizer , which feeds electrons back into the beam and thus makes it electrically neutral, the particles are ejected in the form of a beam.

The neutralizer is an important part of the system. Without it, it would become charged and the beam would diffuse and return to the spacecraft in an arc. The force of attraction between the ions and the missile would consume the thrust.

The drive power is not bound in the reacting fuel components, as is the case with chemically operating rockets, but comes from the applied electromagnetic field. The energy to generate the fields has so far mostly been obtained with the help of solar cells . A fuel in the conventional sense does not exist, but the support mass is lost.

In radio frequency ion thrusters (RIT), the thruster usually uses the noble gas xenon as a supporting mass. The working gas is ionized by electron impact ionization in that free electrons are accelerated to energies of 3 to 10 electron volts by an electrical vortex field generated by an induction coil wound around the engine. The resulting plasma discharge belongs to the class of low-temperature plasmas, which is used in many technological areas (including for fluorescent tubes). The ions created by the ionization (positively charged in the case of xenon) are extracted from the engine by means of an electrostatic field through a grid arrangement, which, according to the law of conservation of momentum, causes thrust in the opposite direction of the escaping ions.

For the successful commissioning of an RIT, some peripheral devices such as gas flow regulators and energy sources are required, which, for example, provide the high voltages required for the extraction. The feeding of the powerful high frequency is typically achieved with a half-bridge topology in a series resonance converter, as this enables high electrical efficiencies that further promote the thermal management of the satellite.

Both the plasma-physical processes and the engineering of an engine system (assembly) are currently the subject of many space-related institutions and companies worldwide. RIT technology is represented commercially by the company ArianeGroup . In Germany, in addition to ArianeGroup ( Lampoldshausen ), the universities in Giessen ( Justus Liebig University in Giessen and the Technical University of Central Hesse ) and the German Aerospace Center in Göttingen are working on this technology.

comparison

Compared to conventional chemical rocket engines, previous ion propulsion systems have far less thrust, in the case of probe propulsion systems roughly comparable to the weight of a postcard (70 millinewtons ), but with a significantly increased exit speed of the gas (10 to 130 km / s, prototypes up to 210 km / s) and a significantly longer duration of action. The total mass of the spacecraft must nevertheless be kept as small as possible in order to achieve sufficient accelerations and thus acceptable thrust times for operation. The SMART-1 probe weighs e.g. B. 367 kilograms and carried 84 kg of xenon as a supporting mass.

One problem with ion thrusters is their power requirements (with SMART-1 1300 W just for the thruster). Only the latest triple junction GaInP2 / GaAs / Ge solar cells provide sufficient power per surface (with SMART-1 approx. 370  watts / m², efficiency 27%) to supply usable ion drives with a reasonable solar panel size.

Doubling the exit speed of a certain mass requires four times the energy. The aim in the construction of an ion drive is to keep the required supporting mass as low as possible. According to the basic rocket equation, this requires a maximum outflow velocity. The construction of an ion drive is therefore always a compromise between energy and supporting mass requirements.

The advantage of ion propulsion over chemical propulsion is that with the same total impulse delivered (i.e. achieved change in speed), less support mass is consumed because the speed of the emerging particles is much greater. The specific impulse normalized to the acceleration due to gravity is here with over 3000 s about six times higher than with chemical engines with 470 s.

Conventional ion drives only worked in a vacuum. The force exerted by normal air movements is usually greater than the thrust. In November 2018, scientists at MIT presented the development of an ion engine that works in the atmosphere.

Ion thrusters have input powers in the watt to kilowatt range and thrusts below 1 N. Therefore, ion thrusters are only suitable for transporting larger masses if they can work for a long time (weeks, months or years).

history

The principle of ion propulsion was introduced by space pioneer Hermann Oberth in his most famous work "The Rocket to Planetary Spaces" as early as 1923, in which he first described the ion propulsion system he designed.

In the 1960s, cesium or mercury was used as a fuel in initial experiments , but the metal components used to generate ions quickly corroded . The biggest problem was the corrosion of a razor-sharp edge on which the necessary ions were generated by means of droplet ionization. Only with the use of the noble gas xenon as fuel was this problem better managed. Further advantages of xenon are that, unlike metals, it does not have to be vaporized, is non-toxic and can easily be transported into the engine from a pressurized gas tank. The extraction of the normally solid cesium was particularly difficult in practice. The lower atomic mass is seen as a disadvantage compared to mercury . In addition, the xenon requires higher ionization energies than the two metals.

In the RIT engine ( Radiofrequency Ion Thruster ), the ions are generated by inductive coupling of a high-frequency signal, while in the electrostatic Kaufman engine the gas is ionized by a direct current discharge. The HET-engine (engl. Hall Effect thrusters , Hall Drive ) ionizes the drive gas with electrons, which are guided on a circular path. A prototype of an RIT engine first worked on the European EURECA satellite in 1992 . SMART-1 was equipped with an HET engine.

Today's ion thrusters are suitable for two main applications due to the limited amount of electrical energy available:

  • Marching engine for interplanetary probes to the near-sun planets Venus and Mercury, as solar energy can still be used here during long thrust times.
  • Orbit control thrusters for large satellites in high orbits of the earth, since here the disruptive forces and the necessary correction pulses to compensate for them are very small.

Use in space travel

The Deep Space 1 space probe launched in 1998 with the NSTAR ion thruster , which is based on the Kaufman type. In 2001 ESA launched the Artemis satellite , on which two new types of ion thrusters have been installed on a test basis, which differ in the way the xenon ions are produced. The satellite covered the last 5000 km to the planned geostationary orbit with the help of the RIT-10 ion engine , which was originally only intended for orbit correction because the upper stage of its Ariane 5 brought it into a geotransfer orbit (GTO) with too low an apogee . It took him 18 months for this route.

The ion thruster has now established itself on many commercial communications satellites . There it does not serve as the primary drive to reach the orbit, but as a path control engine for the north-south drift, since the satellite has to generate around 45 to 50 m / s of speed change ( delta v ) per year due to the gravitational influences of the sun and moon . The use of ion thrusters to regulate the orbit increases the service life of the satellites because less fuel is required because the specific impulse is higher than that of chemical thrusters.

Implementation in the atmosphere

In November 2018, MIT succeeded for the first time in moving a missile in the atmosphere using an ion drive. For this purpose, an airplane-like body with a span of 5 meters was constructed. There were electrodes under the wings, to which a voltage of +20,000 volts was applied. The nitrogen in the air ionized at the electrodes . The ions were accelerated by an applied voltage of −20,000 volts on the wings. The flight time was 10 seconds and bridged around 60 meters in a sports hall. According to the scientists involved, the distance was limited solely by the size of the hall. It is currently not possible to transport people or goods. As possible areas of application, the researchers name z B. quieter drones.

Further developments

  • Some projects aim to increase the speed of the ions. The ESA's DS4G uses z. B. an acceleration voltage of 30 kV.
  • With the magnetoplasmic dynamic drive and the related VASIMR , on the other hand, attempts are made to achieve higher efficiency through an electrically generated magnetic field .
  • The Magnetic Field Oscillating Amplified Thruster (engl. Magnetic Field Oscillating Amplified thruster or MOA) used Alfvén waves , a physical principle of MHD , which varying magnetic fields in the electrically conductive media such as plasma density waves can produce, the particles can accelerate in the medium to very high velocities.
  • Bismuth is being investigated as a substitute for supporting xenon.

See also

literature

  • Heinz Mielke : Space flight technology - an introduction . Transpress VEB publishing house for transport, Berlin 1974.
  • Dan M. Goebel et al .: Fundamentals of electric propulsion - Ion and Hall thrusters. Wiley, Hoboken 2008, ISBN 978-0-470-42927-3 .

Web links

Commons : ion propulsion  - collection of images, videos and audio files
Wiktionary: ion drive  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Super-powerful new ion engine revealed . New Scientist, Jan. 18, 2006.
  2. ↑ a b First ion-powered aircraft completes test flight. In: wired.de. November 22, 2018. Retrieved November 27, 2018 .
  3. ^ Ion drive: The first flight. In: nature video ( Youtube ). November 21, 2018, accessed November 27, 2018 .