Plasma Propulsion Equipments for Multimedia Satellites

Electric / Plasmic Propulsion is characterised by a very high specific impulse, thus reducing the propellant required on the spacecraft, leading to a reduced launch mass and an increased manoeuvre life duration. The thrust is typically in the range of 25mN to 80mN and thereby the control for North/South Station Keeping (NSSK) is kept very smooth, with low disturbing torques. Not only can the manoeuvres be performed autonomously on the satellite, the thrusters can also be used at the end of the transfer orbit for circularisation, dramatically reducing the launch mass, at the cost of a few more weeks for circularisation.

Astrium's Civil Telecommunications platforms have been designed to include Plasmic Propulsion on the generic spacecraft product, offering unique advantages for most of the usual telecommunication missions, particularly:

• Reduced launch mass by about 500 kg, allowing the launch of the satellite on high inclination launchers, and at reduced launcher cost
• Manoeuvre Life duration extended well beyond 15 years on most of the launchers available on the market
• Autonomous station keeping for North/South manoeuvres, allowing also for less frequent ranging operations and easier co-location of satellites
• Smooth North/South station keeping manoeuvres, and therefore an improved antenna beam pointing accuracy
• Efficient (low propellant requirements) and fast repositioning capabilities
• Gyroless on-station operation thanks to yaw estimation based on sun sensors

The ATMA provides;

• Structural support for the plasma thrusters during launch
• Thermal isolation between the thrusters and spacecraft (since the thrusters can operate at temperatures greater than 2000C) Pointing of the thrusters once on station

The thruster pointing is to allow redirection of the thrust vector to track the slowly changing COG of the spacecraft as propellant is consumed over the spacecraft life. The ATMA is launched in the nominal, mid-life position, with the ATMA pointing platform locked in position by a Hold Down Release Mechanism. The HDRM is released after the launch campaign by a low shock, non-explosive device. The pointing platform can then be rotated about two axes by struts actuated by nuts traversing spindle assemblies. The spindles are driven by stepper motors on the ATMA and the stepper motors are powered from the onboard control electronics.

The XRFS provides;

• Regulation of the high pressure xenon propellant to a nominal constant pressure over the mission life
• Xenon mass flow rates at the regulated pressure compatible with the plasma thruster requirements

This is achieved by a system of valves, plenum volume, pressure transducers and flow restrictors to regulate the high pressure xenon stored in the spacecraft Xenon Storage Tank.

The CDPS provides;

• A pressure sensing capability of the environment local to the interior of the spacecraft

The measurable range for the CDPS extends from 0.1mbar to 0.001mbar over a temperature range of -300C to +600C. It is within these pressure ranges that Corona events are most likely to occur. Given an accurate knowledge of the spacecraft local pressure environment the appropriate Corona preventative measures may be implemented.