Hold-Down and Separation Systems

Most spacecraft have appendages (Solar Arrays, Antenna Reflectors, Radiators, Instruments, Doors, Sensors, Booms etc) that are held stowed during launch, in order to fit into the available launcher volume and survive launch loads, then later deployed in orbit into their operating position.

Other equipment like scanning/refocusing mechanisms, Electric Propulsion Pointing Mechanisms or Coarse Pointing Mechanisms shall be stowed during launch and released once in orbit in order to allow on station operations without any specific deployment.

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.

In some cases, HDRMs are not used in conjunction with DMs. This is for instance the case for scanning and refocusing mechanisms, Electric Propulsion Pointing Mechanisms and Ejection mechanisms.

Concerning HDRMs for Spacecraft applications, they are generally composed of three functional elements:

  • Hold Down Preloading Assembly (HDPA) (e.g. bolt, nut, threaded rod, tie-rod, cam/lever, cable, rope) which provides the required preload to be applied via manual operation or the use of specific ground support equipment in order to secure the equipment in stowed configuration during launch,
  • Hold Down Release Actuator (HDRA) which achieves the release of the preload upon the command of a drive electronic. The release actuator is generally mounted on the fixed part of the separable interfaces. Its also frees and secures the separable interfaces from any mechanical links and, prevents any interferences upon deployment or operation of the appendage. For some applications, the bolt or threaded rod is ejected upon release and secured into a so called bolt catcher via a dedicated spring and/or thanks to the stored strain energy,
  • Hold Down Load Carrying Structure (HDLCS) which guarantees the launch loads transmissibility between the fixed part and the part to be released. This element completes the HDRM assembly and is always adapted to each appendage and spacecraft I/F.

Hold-Down and Release Mechanisms (HDRMs)

Other types of HDRMs are based on classical actuator components technologies (using electrical motors, gears or ball bearings) but are not addressed here due to their very specific applications.

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.

The HDRA can be located in the load path or in a remote position via a cam/lever assembly. In this case, it is used as a trigger to initiate the release.

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.

HDRA underlying technologies can be grouped according to their level of reusability, which is a key feature with respect to their implementation in space systems:

HDRA technologies
(need for refurbishment)
(manually resettable)
(self resetting)
Pyro cutters Initiators
Pyrotechnic bolt
wire cutters and pyro-cutters.
Pyro nuts
Fusible wire actuated nuts
SMA direct actuators
Spool based devices
separation nut
Thermal cutters
Solenoid actuated nuts
SMA actuated nuts
Paraffin actuators
SMA actuators
Wire triggers
Thermal cutters
Electro-magnetic actuators and triggers
Magnetic clamps

It has to be mentioned that reusability is sometimes associated with lower reliability as the number of parts of reusable devices is significantly higher than those of partially- or non-reusable ones.

In order to classify the different technologies, the HDRA could be divided within five categories with respect to their Shock Response Spectrum peak upon operation:

  • High (>3000g),
  • Medium (between 1000g and 3000g),
  • Low (between 300g and 1000g),
  • Ultra-low (<300g),
  • No-shocks (barely measurable).

The evaluation of release shocks requires approved and standard test procedures and facilities in order to provide consistent, comparable and recognised test data.

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.

Force sensors are mainly of two types, load cells or strain gauges and are deemed essential for the knowledge of the actual preload. Usually, the actual preload versus torque relationship is not well mastered by most HDRM users.

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.

Despite the fact that a detailed evaluation of cost linked to the implementation of high shock HDRMs has never been performed, it is assumed that their impact is significant.

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.

The pdf document in the right navigation shows the list of European entities involved in HDRA technologies.

Last update: 7 May 2014

Copyright 2000 - 2017 © European Space Agency. All rights reserved.