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Innovating is positively in our DNA

Our new Bi-Axial Solar Array Drive Mechanism is designed with ingeniously clever engineering and was built in a rapid timescale.

The big ask

In 2013, SSTL identified the need to design a Bi-Axis Solar Array Drive Mechanism (BSADM) to continuously orientate solar array wings towards the Sun for satellite missions flying in an inclined orbit that, unlike standard Earth observation orbits, is not synchronous with the Sun.  The first axis of the BSADM is needed to track the Sun as the spacecraft orbits the Earth, and the second axis is needed to compensate for the slowly changing angle of the Sun with respect to the orbit plane throughout the year.

With a constraint on cost, we set our engineers off on the task of designing and building the BSADM, combining a traditional solar array sun tracking axis with a hinge which trims the tilt of the array perpendicular to the sun.  The team were on a rapid timescale – the challenge was to have an Engineering Qualification Model of the BSADM 8 months after kick-off – and so the engineers used qualified key component-families from other proven SSTL mechanisms where possible, and focussed their design activities on the mechanism features unique to the BSADM.  It’s an example of the pragmatic approach SSTL takes to engineering, allowing us to achieve low-cost and rapid-build missions for our customers.  


Identifying and focussing on the important things first

In our experience the best approach is to focus on the key design constraints first and not get side-tracked onto “nice to haves” so for the BSADM we honed in on the following key requirements:

* The SADM axis shall give a continuous 360° rotation.
* The hinge shall stow at -90° and deploy to 0°, and then have a range of +60/-70°.
* The BSADM shall be able to return from any position to a SAFE position in 5 minutes.
* The BSADM must have a modular design, such that the tracking axis could be used in its own right, without the trim axis, as a conventional tracking SADM for future missions.
* The design shall include a deployment lock for the solar array to mitigate against the high loads generated at the end of deployment.


What did we already have in our toybox?

The first step for any new development at SSTL is to perform a make-buy trade to review what is available within SSTL, and also externally within the market place.  The make-buy trade considers the prominent issues, such as development costs, recurring costs and technical compliance to requirements specification, but also includes schedule, technical and programmatic risks assessments too.  This make-buy trade identified that there was no product on the market, or able to be developed externally, that could compete with an SSTL developed product for the mix of technical, cost and programmatic criteria. Once SSTL had committed to develop the BSADM itself, the first task was to identify modules and components that we have already developed, and which have earned invaluable heritage in-orbit on our previous missions. 

This approach reduces costs and risk, and thus both rotation axis designs incorporate these standard or heritage elements:


A stepper motor generating the torque needed for the rotation axis is from the same family as our Antenna Pointing Mechanism, Imager Focus Mechanisms and Instrument Scanning Mechanism.
A planetary gear box and spur gear which transmit and amplify the motor torque is based on the design used for our Antenna Pointing Mechanism and Imager Focus Mechanism.
A redundant potentiometer which generates an analogue signal between 0V to 5V, proportional to the absolute angular position of the rotation axis: as used on a former SSTL mission, GIOVE-A. 
A slip ring used for power transfer which has been used on a bought-in SADM on a former SSTL mission, GIOVE-A.
The drive electronics which control the motion parameters of the BSADM mechanism and incorporate stepper motor drivers are from SSTL’s Antenna Pointing Mechanism, and housed in a standard module tray. 

The track/trim axis bearings are from the same family as our Antenna Pointing Mechanism bearings and a Reaction Wheel bearing type.

An ingeniously clever design – and rapid execution

SSTL's BSADM design

The BSADM includes two rotation axis assemblies.  The lower axis assembly consists of a traditional SADM and is responsible for continual tracking of the Sun.  The upper axis (hinge) is responsible for the array trimming to compensate the satellite orbit and attitude changes needed for correct payload operation. 

The really clever engineering design focus was needed for four new elements:

The Angular Range Lock

During launch the solar array will be folded and the BSADM hinge will be orientated perpendicular to the satellite surface panel as shown below: 

Angular Range Lock
Angular Range Lock

Once the solar array has been deployed, the hinge will be rotated towards its nominal operation range which is between +60°/-70°.  An angular range lock has been implemented on the hinge rotation axis to prevent the hinge and the solar array, on exceeding this operation range.  This is particularly important because without it, the solar array could collide with other instruments on board the satellite.

Deployment Lock

Under conventional circumstances, without damping, the solar array wing used for the satellite would bounce back after deployment, and come to rest at an unknown position.  A back-driving torque of 50Nm is needed during the deployment to reasonably limit the solar array wing from back-bouncing, and so an additional locking mechanism has been incorporated into the hinge assembly.  It works like this:
1. During the solar array deployment, the hinge rotation is blocked through a pin which is in contact with an end-stop on the hinge static housing. The translational displacement of this pin is prevented through an add-on feature of the gear, which forces the pin to remain in its position. The pin-carrier is mounted to the hinge shaft, onto which the solar array bracket is also attached. The rotation of the solar array is thus prohibited, and the required high back-driving torque resistance is provided through this locked pin. 

2. Once the solar array has been deployed and settled, the hinge motor is actuated and the gear begins to rotate. Since at this point the hinge shaft and the gear are still disengaged, the gear rotates, whilst the pin’s position remains static until it reaches the gear opening allowing the pin to push through. 

3. The pin pushes through into a cavity in the gear add-on feature, forming thus a rigid connection between the gear and the hinge shaft (on which the solar array is attached). The hinge drive is now engaged; the rotation of the gear is transmitted.

Solar Array Deployment Lock Operation Method - Click to enlarge

Track Axis Rear Bearing and Membrane

The track axis shaft is mainly supported by its front duplex bearing. These bearings will take most of the axial and radial loads during launch. An additional single row bearing has been implemented at the rear end to further restrict radial displacements and guarantee that the shaft (especially the slip ring shaft) remains properly aligned with respect to its stationary counterpart.

The rear bearing is supported by a flexible membrane which allows translation along the rotation axis. This membrane compensates thus for shaft elongation/retractions due to temperature gradients between the shaft and the housing, and hence prevents significant variations of the bearing load.

Rear Bearing Support

Flexible Membrane Deformation

Slip Ring

The slip ring allows the transmission of power and electrical signals from the stationary to rotating structure of the track axis. Its core consists of 60 current transfer rings made from gold plated brass, each of them having the capability to transfer 1.6A. The moulding of the rings within a space qualified epoxy provides a very high electrical insulation between the tracks. The counterparts for these rings are gold brushes, wiping over the gold rings and thus providing electrical connection between the rotating and the stationary part of the track axis. Due to the criticality of the gold-on-gold contact between the brushes and the gold rings, the slip ring was bought-in in order to benefit from existing heritage of such a sophisticated element. The slip ring was none the less completely re-qualified within the BSADM as its performance significantly depends on the method of how it’s supported.


A full range of qualification tests has been carried out on the BSADM to prove the mechanism’s performance during ground testing, the launch and whole of its orbital lifetime.  The tests included:


  • A bench test to quantify the functional performance of both rotation axes.
  • Vibration tests to demonstrate that the BSADM can sustain launch loads.
  • A deployment test to show that the deployment torque generated by the solar array wing will not damage the mechanism or the deployment lock pin.
  • A thermal test to verify the BSADM’s robustness to temperature changes.
  • A life test performed with temperature changes in vacuum, to prove that the BSADM will perform over the whole orbital lifetime.
  • End of life test functional performance tests to prove that the BSADM performance has not deteriorated over its intended life.

Job done

The BSADM design approach – based on the use of heritage components where possible and focussing resource on key design requirements – led to the rapid design, manufacture and test of the new mechanism with a qualification model.  A job not only well done, but done efficiently – the SSTL way.  

BSADM with drive elecronics






16 July 20150 Comments1 Comment

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