The xenon ion propulsion system, or XIPS (pronounced "zips"), is the culmination of nearly 40 years of research into the use of electric propulsion as an alternative to conventional chemical propulsion on spacecraft.

Also used on the HS-601 and high power 601HP models as well as the HS-702 satellites, the increased efficiency offered by XIPS allows a reduction in propellant mass of up to 90% for a satellite designed for 12-15 years operation, says Hughes Space and Communications.

The on-board XIPS system is used primarily for spacecraft stationkeeping. Small thrusts are required to correct for the tug of solar or lunar gravity and to reposition the satellite in its proper orbit and attitude. A satellite's lifetime as well as its launch weight is thus determined by the amount of fuel required for its thruster system. While most current satellites use a chemical bipropellant propulsion system, a XIPS-equipped satellite uses the impulse generated by a thruster ejecting electrically charged particles at high velocities. XIPS requires only one propellant, xenon, and does its stationkeeping job using a fraction of the mass required by a chemical propellant system.

The heart of the XIPS is the ion thruster, measuring less than 25cm (9.8in) across. Two other key units include a tank containing xenon gas and a power processor. Thrust is created by accelerating the positive ions through a series of gridded electrodes at one end of the thrust chamber. The electrodes, known as an ion extraction assembly, create more than 3,000 tiny beams of thrust.

The beams are prevented from being electrically attracted back to the thruster by an external electron-emitting device called a neutraliser. Ions ejected by the XIPS travel in an invisible stream at a speed of 30km/s (18.6m/s), nearly 10 times that of its chemical counterpart.

Because ion thrusters operate at lower force levels, attitude disturbances during thruster operation are reduced, simplifying station-keeping. Chemical thrusters are limited by the amount of energy released during combustion. Ion thrusters depend on the amount of electrical power available. More power means faster-moving ions and higher thrust.

The XIPS on a HS-601 HP model, for example, uses 500W from the satellite's 8kW solar array. The HS-702 XIPS uses 4.5kW from the 10-15kW solar array. XIPS operations have no effect on broadcasting and telemetry operations. A typical satellite uses up to four XIPS thrusters (two primary, two redundant) for station-keeping, all connected to the same xenon supply. Each primary device is switched on and off by a smart power unit that monitors and diagnoses operations automatically. In normal operation, each ion thruster will operate for about 5h a day on an HS-601HP and about 30min a day on the HS-702.

The HS-601HP satellite uses the 13cm XIPS to perform all north-south stationkeeping and spacecraft momentum control in two axes. The 13cm thruster operates at a specific impulse (ISP) of 2,568s with 18mN of thrust.

The satellite flies with four 13cm xenon thrusters and two power processor units. Orbit and momentum control are accomplished through a series of two burns on each day of the stationkeeping cycle. Only two of the four thrusters are required to perform a complete mission of on-orbit manoeuvres.

The HS-702 uses its high-power capacity to take full advantage of XIPS technology with higher-power 25cm thrusters. The 25cm thruster operates at an ISP of 3,800s with 165mN. The satellite has four 25cm thrusters and two XIPS power processors. Like the HS-601HP, only two of the four thrusters are required to perform the entire on-orbit mission manoeuvres. These functions are accomplished autonomously with four daily burns providing precise orbit control. This strategy maintains a ±0.005° stationkeeping box, allowing for co-location of many satellites in a single orbital slot.

The HS-702 offers the additional option of XIPS orbit raising. Using XIPS to augment chemical system transfer orbit burns further reduces the amount of chemical propellant loaded and therefore larger payloads can be accommodated.

The satellites are normally placed in a highly elliptical geostationary transfer orbit, circularised by an on-board apogee kick motor. More recently, a new type of orbit has been reached during the launch phase. The satellite can be placed into a super-synchronous elliptical transfer orbit and pre-programmed XIPS manoeuvres are used to circularise the orbit and position the satellite in its final orbit.

In 1992, Hughes committed to XIPS technology, identifying it as the basis for next-generation satellites. The first shipment of XIPS thrusters was the culmination of a programme of testing, performance documentation and measurement. Hundreds of hours of burn-in testing were performed to assure that there would be no early wear-out problems.

Source: Flight International