Electric Propulsion Pointing Mechanisms (EPPM)
Electric thrusters have today a fixed direction of thrust. In order to optimise electric thruster propellant resources, to limit the number of electric thrusters on a platform and, to perform advanced Attitude and Orbit manoeuvres, Electric Propulsion Pointing Mechanisms (EPPMs) are therefore required.
Electric Propulsion Pointing Mechanisms are used on the majority of spacecraft using electric thrusters with a power consumption > 1kW. Their main functions are:
- To accommodate and support electric thruster(s) during ground, launch and orbit,
- To secure during launch the electric thrusters in a stowed configuration,
- To allow on-orbit multi-axis pointing capabilities of the thrust vector(s) of the operational and redundant (when applicable) thruster(s) under the control of the attitude and orbit control systems (AOCS),
- To provide a dynamic transfer function compatible with the thruster allowable mechanical loads,
- To provide a thermal design compatible with thruster and platform thermal requirements during both thrusters’ operation and non-operation modes,
- To accommodate and route the electrical harness and pipes around each rotation axis ensuring adequate life and torque margin,
- To accommodate Electric Propulsion System ancillary equipments (e.g. HIB, XFCU).
EPPMs mainly consist of a mobile plate supporting electric thruster(s), a multi-axis pointing system (generally two) and a Hold Down and Release Mechanism (HDRM). Due to the high sensitivity of electrical thrusters to vibrations and shocks, EPPM structures are often equipped with dampers.
Typical performances of existing and qualified EPPMs in Europe are:
- Angular range: from +/-8 deg to +/- 12 deg per axis,
- Payload mass (1 or 2 Electric Thrusters) : 3.6 kg to 6 kg per thruster (low power),
- EPPM mass (1 or 2 Electric Thrusters): 9 kg to 14 kg.
EPPMs always belong to a so-called Thruster Module Assembly (TMA) which often refers to a stand alone unit accommodating the EPPM, the electric propulsion systems, the MLI and an interface structure with the platform. The TMA can be developed, qualified and acceptance tested relatively independently and/or in parallel from the Spacecraft development and AIT phases.
Generally, the EPPMs are composed of the following elements:
- A mobile plate supporting the thruster(s) and including thruster interface shims (if any),
- A multi-axis pointing assembly including drive units and a kinematics assembly,
- A sensor units which complexity depends on the AOCS control logic,
- A Hold Down and Release Mechanism,
- A Flexible thruster supply lines assembly including supports and protection from radiations (new responsibility of EPPM suppliers),
- A damping system,
- Tailored mechanical and thermal interfaces with the thruster(s) and ancillary equipments,
- Supports for EPS equipments,
- Passive and active thermal hardware.
Generally, the EPPMs shall support and interface with the following EPS equipment:
- Satellite interface adaptor;
- Thruster interface shims;
- Thruster(s) feed line(s);
- XFCU (not as a baseline for most applications);
- Active and passive TMA thermal control elements,
- Protection thruster(s) feed lines from radiations,
- MLI and MLI tent,
- Alignment mirror.
Main technical challenges in the design and development of EPPMs are:
- To minimise the mechanical environment at thruster(s) interface(s),
- To operate under a stringent thermal environment,
- To allow the routing and flexibility of thruster(s) supply lines.
EPPMs have to be considered as an enabling technology in order to maximise the performances of electric propulsion systems.
EPPMs developments in Europe were initiated in the early 1990s as the Electric Propulsion was identified as the most promising technology to provide the specific impulse needed for future Telecommunication, Science and Manned missions.
Last update: 7 May 2014