European Space Agency

EPJT - Electric Propulsion Mechanism Joint Technology


The goal of the EPJT project was to fully qualify the three key-elements needed to realize an electric propulsion mechanism:

1. Geared Actuator SA15

The geared actuator SA15 is designed from scratch by RSA. The actuator shall be built and qualification tested against typical EP specifications.

2. Spherical High Load Ball Joints

The ball joints shall be procured by a commercial company with space heritage. The ball joints shall be procured and qualification tested against typical EP specifications.

3. Flexbox

Leveraging on the RSA extensive heritage to route the mechanism and thruster harness over one moveable joint, the new helix modular design shall be realized and qualification tested against typical EP specifications.

4. Hybrid Stepper Motor

Development of a competitive hybrid stepper motor. Testing of key parameters shall be performed.


In first step the main challenge has been to summarize all available data to compile typical EP mechanism specification enveloping all needs of the targeted business cases.

For the SA15 actuator new components have been used (like the torque limiter) which has not been tested under TV conditions in the past.

For the ball joints it was unclear if a supplier can be found which can offer the needed low friction torque and low play values.

For the flexbox the challenge was to provide enough guidance for the cables by maintaining low spring and friction torque values.

The hybrid stepper motor is a complete new stepper motor with a different electromechanical behaviour.


EPJT has been the technical backbone enabling RSA to successfully bid with their electric propulsion mechanisms to the ongoing Telecommunication ARTES programs, such as ARTES-14 (SpaceBus NEOsat) and ARTES-33 (Electra)

A robust actuator system is always the main key-element of an electric propulsion mechanism. The actuator SA15 features high detent (holding) torque, low weight and small size. The torque limiter provides a perfect balance between external load capabilities and maximum torque (blocked shaft condition). As all components are designed and built by RSA from scratch the product is completely ITAR and EAR free. Due to its high torque capabilities the actuator can be used on many different applications.

The ball joints are the main elements to carry the loads. This allows unloading of the actuator ball bearings which allows usage of smaller bearing sizes (thus decreasing the actuator weight).

The flexbox provides guidance for the mechanism and thruster harness by keeping the friction torque low.

The hybrid stepper motor maximises competitiveness with respect to  the permanent magnet stepper motor  by implementing modified industrial components.


1. Geared Actuator SA15

The geared actuator SA15 features a permanent magnet stepper motor followed by a 3 stage planetary gear set (which includes a torque limiter) and a large planetary output gear. Loads can be transmitted by angular contact type ball bearings. Additional a potentiometer and end-switches are attached.

2. Spherical High Load Ball Joints

The ball joints main features are low friction torque, low play, high load capabilities and low price.

3. Flexbox

The harness is separated in a low voltage section (for the mechanism harness) and a high voltage section (for the thruster harness). The design is modular so that it can be used for various types of space mechanism.

4. Hybrid Stepper Motor

The hybrid stepper motor features a higher number of full steps per revolution which and high torque which allows to use a gear with lower transmission ratio by maintaining torque and step size. This leads to lower prices as less gear stages are needed.

System Architecture

In the frame of the EPJT project components (high load bearings) or sub-level assemblies (like the actuator or flexbox) have been developed. For the sub

Actuator SA15:

Permanent magnet stepper motor followed by a 4-stage planetary gear set (PG26) and on large output planetary gear (LPG). After the first planetary gear a torque limiter is introduced to limit the maximum output torque during end-stop hitting. I/F parts are supported by angular contact ball bearings.


Helical arrangement of harness and piping. Separation of low voltage and high voltage harness. Harness is guided by an inner and an outer can as well as by separation fins. Harness clamping is done via metallic clamps and viton inlays.

Hybrid stepper motor:

The hybrid stepper motor follows a different electromagnetical approach - stacked sheet metal plates are used on the rotor which are magnetized by ring magnet. One major development is the usage of modified industrial components and a simplified design.


After Kick-Off the development activities for all three components (actuator, ball joints, flexbox) started and dedicated documents have been created (e.g. design reports, analysis reports, requirement specifications, test plans, etc.). All these documents have been submitted and reviewed at the BDR. In the timeframe from BDR to MTR all development activities continued and the design and the test approach has been refined. Documents have been updated and re-submitted for MTR.

Between BDR and MTR, the ARTES-14 and ARTES-33 business cases were secured, with their dedicated qualification program. As a consequence, the scope of EPJT project was re-modulated (EPJT-RSA-CN-0001) to remove duplications . The hybrid stepper motor was introduced to the project and presented at FR.

Current status

The project is completed. The final report has been submitted and reviewed by ESA.

The first spacecraft embedding the EPJT components has been  launched in January 2020.

Status date

Monday, October 5, 2020 - 10:22