Ion thruster

An ion drive is a drive for space vehicles, in which is used according to the reaction principle of the discharge of one ( neutralized) ion beam for locomotion. It can also be used depending on the energy source, the terms solar- electric propulsion (English Solar Electric Propulsion, SEP) and nuclear electric propulsion (English Nuclear Electric Propulsion, NEP).

Function

Is generated, the ion beam by gas molecules (such as xenon ), or micro- droplets are first ionized (e.g., mercury). They are then accelerated in an electric field. After the passage of the so-called neutralizer, which again supplies electrons to the beam and makes it thus electrically neutral, the particles are ejected in the form of a beam.

The neutralizer is an important part of the system. Without him, this would be charged and diffuse the beam and return to the spacecraft in an arc. The attraction between ions and missiles would negate the thrust.

The drive power is not bound as in chemically working missiles in the reacting fuel components, but comes from the applied electromagnetic field. The energy used to generate the fields is usually part obtained with the help of solar cells. A fuel in the traditional sense does not exist, but the supporting mass is lost ( ionized gases or micro- droplets which are ejected; are used eg xenon or mercury).

Comparison

Previous ion drives have advantages over conventional chemical rocket engines, a low thrust, in special drives roughly equivalent to the weight of a postcard (70 milli Newton ), but at a significantly higher exit velocity of the gas ( 10 to 130 km / s) and a significantly longer duration of action. The total mass of the spacecraft must still be kept as small as possible to achieve sufficient for decent acceleration and thrust durations for the operation. The SMART-1 probe weighs 367 kg and resulted, for example, 84 kg xenon as supporting mass.

A problem of ion thrusters is their energy needs (for SMART -1 1300 W alone for the engine ). Only the latest Triple-Junction-GaInP2/GaAs/Ge-Solarzellen provide sufficient power per unit area (for SMART -1 370 Watt / m², 27% ​​efficiency ) to provide useful ion drives at reasonable solar panel size.

Doubling the exit speed requires four times the amount of energy. Goal in the design of an ion drive, is to keep the supporting mass required as low as possible. This requires following the Rocket Equation a maximum exhaust velocity. The construction of an ion drive is therefore always a compromise between energy and mass support needed.

The advantage of the ion thruster with respect to the chemical propulsion is that supplied with the same total momentum (i.e. reached the speed change ) is consumed less support mass, because the velocity of the escaping particles is much larger. The weight specific impulse is here about six times higher than that of chemical engines with 470 s with over 3,000 s

A disadvantage of the ion drive to the chemical drive is that he (as well as all other electrical rocket propulsion systems ) only works in a vacuum, and thus only in space.

Today's ion engines have performance in watt to kilowatt range. For transporting large masses ion thrusters are therefore suitable only if they can work for a long time (weeks, months or years). This can eventually create fusion engines remedy, which, moreover, does not have the disadvantage of significantly lower thrust of the ion drive to the chemical drive in the future. You are currently but only a hypothetical alternative.

The possibility of accelerating the ions to very high speeds also provide particle accelerator, in particular linear accelerator. However, these are unsuitable due to their inefficiency and size.

History

The principle of the ion drive was from space pioneer Hermann Oberth in his most famous work " The Rocket into Interplanetary Space " already introduced in 1923, in which he describes the first time he designed ion thruster.

In the 1960s, cesium or mercury was used as a fuel in the first try, with the metallic components but rapidly corroded for ion production. The biggest problem was the corrosion of a razor sharp cutting edge, at the means of Tröpfchenionisation the necessary ions were generated. Only with the use of the noble gas xenon as fuel you got a better grip on this problem. Other advantages of xenon are that it must not be evaporated in contrast to metals, is non-toxic and can be easily transported from a compressed gas tank into the engine. In particular, the promotion of normally solid cesium was very difficult in practice. As a disadvantage compared to mercury, the lower atomic mass is seen. Furthermore, the xenon required compared to the two metals higher ionization.

When RIT engine (English Radio Frequency Ion Thruster ), the ions generated by inductive coupling of a high-frequency signal while the gas is ionized by a DC discharge in the electrostatic merchant engine. The HET thruster ( engl. Hall Effect Thruster ) ionizes the propellant gas with electrons that are performed on a circular path. A prototype of RIT - engine worked in 1992 on the European satellite EURECA. Smart -1 was equipped with a HET engine.

The space probe Deep Space 1 is equipped with the NSTAR ion thruster, which is based on the Kaufmann- type. 2001 ESA launched the satellite Artemis, on the two new ion engine types are installed for testing purposes, which differ in the mode of production of xenon ions. The last 5000 km to the planned geostationary orbit lay back of the satellite with the help of the ion thruster RIT -10, which was originally intended only for path correction, because the upper stage of its Ariane 5 it into a Geotransfer orbit ( GTO) brought to a low apogee. For this route it took 18 months.

Meanwhile, the ion thruster on many commercial communications satellite has prevailed. There, it does not serve as the primary drive for reaching orbit, but as a web control engine for the North-South drift, because the satellite must pass through the gravitational influences of the sun and moon in the year to raise s to velocity change (Delta V) from about 45 to 50 m /. The use of ion thrusters on the path regulating increases the service life of the satellite, because there is less fuel is required, because the specific impulse is higher than that of chemical thrusters.

Today's ion engines are useful because of the limited available electrical energy, for two main applications:

• Cruise engine for Interplanetarsonden to the sun near planets Venus and Mercury, because here at long shear times more solar energy can be used.
• Orbit control engines for large satellites in high Earth orbits, since the disturbance forces and they compensating required correction pulses are very low.

Developments

• Some projects aim to increase the velocity of the ions. The ESA DS4G used, for example, an acceleration voltage of 30 kV.
• The magneto- plasma- dynamic drive and the related VASIMR one tries, however, to achieve higher efficiency by an electrically generated magnetic field.
• The Magnetfeldoszillationsantrieb (English Magnetic Field Oscillating Amplified Thruster or MOA) uses Alfvén waves, a physical principle of magnetohydrodynamics, after which varying magnetic fields in electrically conductive fluids such as plasma can generate density waves can accelerate particles in the medium to very high velocities.
• Bismuth is investigated as a supporting mass replacement for xenon.