To achieve these functions, two different types of mechanisms are used one after the other, the Hold-Down and Release Mechanisms (HDRMs) and the Deployment Mechanisms (DMs).

The Hold Down and Release Mechanisms are standard components for spacecraft in order to achieve mission related critical functions. Their main functions are to secure during launch and to release once in orbit, or during descent to/on planetary surface, movable payload items, deployable appendages and separable mission elements. They can also be used in order to achieve timely synchronisation for the deployment and/or ejection of specific appendages or separable mission elements.

The Deployment Mechanisms are used to enable deployment of a released appendage from its stowed position to its operational position following a defined set of kinematics and passive-to-active controlled dynamics. Once the final position is reached, the appendage is either latched at a defined position or the DM is used as a re-pointing or trimming device in order to achieve specific mission related functions.

HDRAs usually rely on one of the following technologies:

• Pyrotechnic devices (release nuts/bolt cutter, separation nut, cutters, brazing melt, wire cutter, cable cutter),
• Split spool devices (Fusible wire, SMA wires),
• Solenoid actuated nuts,
• SMA triggered release nuts, with a temperature range of -60°C /+70°C,
• SMA actuators (pin pullers/pushers) with a range of -60°C/+70°C,
• Paraffin actuators (pin pullers/pushers) with a range of -60°C/+80°C,
• Electro-magnetic / solenoid pin puller/pusher actuators,
• Electromagnets / magnetic clamps,
• Thermal cutters/knife,
• Piezoelectric actuators.

Due to the generally external location of the HDRA, they must withstand a large temperature range, typically -100°C/+120 °C for most of the applications. SMA and paraffin based technologies do not meet this temperature range but can generate very low release shocks and can be used as triggers.

For all HDRMs, the tightening tension can be settled and checked in different ways:

• Flight torque or force sensors,
• Tooling (on ground) torque or force sensors,
• Torque controlled screwing,
• Angle controlled screwing.

In most HDRMs, the actual tightening tension (minimal guaranteed preload), once the preload is applied, can hardly be known without the use of external devices e.g. load cells or strain gauges. Users often rely on the preload versus torque relationship which cannot guarantee a preload value accurate enough for space applications associated with the required repeatability.

Up to the recent past, the high shock level generated by the HDRA's release was tolerated thanks to the following measures:

• Definition of shock areas on the spacecraft where sensitive equipments must not be located,
• Equipment qualification to high shock levels,
• Dampers implementation within the HDRM or at HDRM I/F with the appendage and/or spacecraft.

Then, as the majority of spacecraft have become more and more complex, with high need of architecture modularity and versatility, a general trend appeared both in Europe and outside to reduce the shock level generated by the HDRM itself. In addition, the trend to move away from pyrotechnic systems is growing due to the fact that either certain spacecraft do not allow pyrotechnics or that substantial cost savings can be achieved with the avoidance of safety-related costs.

Another general trend was that telecommunication satellites are becoming bigger and heavier (Eurostar, SpaceBus and AlphaBus), while some Science / Earth Observation satellites feature composite architectures and require optical terminals mechanisms operating within a cryogenic environment.

This situation maintains the need for a family of low-shock, HDRA covering the full tightening tension range [10 N - 150 kN], temperature range [-130°/+150°], and at a competitive price. It has to be noted that the full tightening and temperature ranges should not be achieved with the single hardware.

End users also require reusable HDRAs in order to lower the effort during AIT phases at S/C level.

The use of Shape Memory Alloys for HDRAs has been identified as a technology trend. However, its broad application for commercial applications can only be successful if their operating range could be increased up to 110-120 degrees C.