Launch Vehicle actuators
Electro Mechanical Actuators for Thrust Vector Control (TVC) of Launchers
The TVC is a subsystem which controls the direction of gimballed nozzles of rocket engines following the request/command from the launcher trajectory and attitude control system.
The subsystem comprises three major items :
- control unit with a dedicated controller and drive electronics to command and control the mechanical actuator linear displacement
linear actuators ( in pairs, acting on 90 degrees planes ) which can be operated either with an hydraulic (Ariane 5 main engine) or with an electrical motor (VEGA four stages); in the latter case the actuator is called Electro Mechanical Actuator (EMA).
( all three items are connected by cable harness for the transmission of power and signals )
EMAs have become attractive candidates to replace hydraulic actuators for TVC, thanks to easier implementation and lower maintenance requirements and are currently under ESA development for the VEGA launcher and studied in the frame of the Future Launchers Preparatory Programme for Next Generation Launchers (NGL).
The EMA is a mechanism composed by an electrical motor, either co-axial or parallel to the piston, connected via a gear box which drives a roller screw. The rotational motion of electrical motor is transformed in the linear motion of a piston via the roller screw. The linear position of the piston is measured via a resolver connected to the electrical motor and an LVDT (Linear Variable Differential Transformer) sensor, one part of which slides inside the roller screw and is rigidly connected to the piston of which follows the motion.
Due to high dynamic transients loads acting on the EMA, the roller screw is subjected to very high rotational speed which represents a physical limitation together with the electrical power to drive the motor since it can lead to not acceptable EMA dimensions and mass. The EMA function is not only to act as a directional device for the launcher but may also be used to absorb shock loads generated by rocket liquid fuel engines at their ignition coming from initial combustion instability.
In either case the performance of the EMA is strongly coupled to the launcher dynamics and its control system since the EMA operations ultimately influence the loads transmitted to the payload by the launcher.
The EMA typical parameters are :
- actuation force
- power (electrical)
- pin to pin length (in retracted position)
GSTP4 SLEA (Side Load Electromechanical Actuator) activity
The GSTP4 SLEA (Side Load Electromechanical Actuator) activity concentrates on the optimisation of the electro-mechanical actuator (EMA) design and of the control laws with respect to the transient side loads (up to 600 kN) that can be experienced on a large cryogenic engine like Ariane 5 VULCAIN2 during engine ignition.
The Preliminary Design Review (PDR) took place in January 2010, concluding that:
- The EMA technology is suitable with respect to side loads phenomenon, but, with the current requirements (ECSS sizing etc), it is too constraining at System level, in terms of size and mass.
- The test feasibility has been demonstrated; full size test rig proposals are available for the next phase; a German company was selected as the best candidate for the full size test rig design and manufacturing.
- The mathematical models and data are representative, in order to design the SLEA and assess the feasibility, and also to be used at system level.
In March 2010, the PDR steering board approved the logic proposed by the PDR report for the study continuation; the technology development should proceed with the Phase 2A taking into account the following recommendations:
The aim of the study continuation is to prepare the technology for future launchers (beyond Ariane 5 ME) such as Ariane 6 and NGL; however, the Ariane 5 requirements will be maintained for the study, in order to allow the comparison with the existing hydraulic technology, therefore:
- the ASTRIUM availability loads will be the design inputs,
- the Ariane 5 General Specifications will be applied (instead of the ECSS’s), - the recurring costs will be assessed.
- A contractual phase after CDR will be implemented within phase 2A prior to fully engage in the procurement of the test rig of the full scale actuator.
- Additional development testing at component level will be considered within phase 2A in order to consolidate the feasibility of the new actuator design prior to moving to the actuator manufacturing.
In June 2010, SABCA submitted its proposal for the phase 2A to ESA, based on above PDR considerations.
SABCA proposed to perform component testing during phase 2A in following areas:
- Tests on an electrical motor – braking function
- Test on dedicated roller screw (ZLRS), on an existing test rig in France.
Last update: 7 May 2014