Roller screws provide benefits for satellite launcher

SKF's latest generation of roller screws are described as an 'essential element' of the thrust vector control (TVC) subsystems of the new Vega satellite launcher. Developed by the Italian Space Agency and the European Space Agency (ESA) to put small satellites into orbit, the Vega launcher is just 30m high and 3m in diameter, of single-body configuration with three solid rocket stages and a liquid rocket upper module.

The TVC maintains control during the key propulsion phases. It comprises two electromechanical actuators, an integrated power and drive unit, a battery set, and a cable harness that connects the individual elements. The integrated power and drive unit receives its commands directly from the onboard computer, which is monitored from the base station.

Kristof Decoster, a design engineer at Société Anonyme Belge de Constructions Aéronautiques (SABCA), one of the organisations responsible for the development of the TVC system, explains the challenging demands placed on the actuators: "For actuators to be used in launchers, all the environmental requirements are much more severe, with the units having to withstand intense vibrations, very high temperatures and extreme temperature fluctuations, and be able to operate within a vacuum. Furthermore, prior to launch in Kourou, in French Guiana, the actuators have to be able to withstand a hot, humid and corrosive environment, exposed to dust and salt. For these reasons, we decided to use electromechanical actuators for the Vega, which is a first for ESA.

Advantages over hydraulics

"Electromechanical actuators weigh less than hydraulic actuators, and any savings that can be made on the weight of the equipment used to drive and power a launcher means a greater payload can be carried. Using electromechanical actuators will reduce maintenance, and preparing the launcher for take-off will also be easier as will be storage and testing, as the whole system is much more flexible."

Continuing a successful working partnership with SKF, SABCA chose to use SKF roller screws as an essential element in the design of these actuators. The roller screws incorporate rolling elements rather than ball bearings to combine extremely high load ratings and positional accuracy with long, near-zero-maintenance operating life, which is essential for the launcher. The devices offered the exceptional rotational speeds and acceleration required, while also being extremely cost-effective.

Candide Netchenawoe, who works in the purchasing and subcontracting department for space programmes at SABCA, comments: "What we are looking for first and foremost amongst our suppliers is competitiveness. Our big clients are always looking to reduce costs, and we must do the same with our suppliers.

"Proximity is also essential. The farther away we are from the supplier, the greater the transport problems. And of course, quality and reliability are of huge importance."

With its latest generation of roller screws, SKF has helped SABCA to meet these requirements effectively and on schedule. The Vega launcher is due to be tested at the end of 2007.