Clyde Space

SOHLA-2 Power System

Project Name: SOHLA-2

Customer:   http://www.sohla.com/

Astro-Technology SOHLA (SOHLA stands for Space Oriented Higashiosaka Leading Association), Osaka, Japan. 

The Task:

The SOHLA-2 mission represents a highly novel approach to small satellite engineering.  The spacecraft is a 50 kg microsatellite and is the first demonstrator of Panel ExTension SATellite (PETSAT) which was first proposed by the Nakasuka Laboratory of University of Tokyo.  PETSAT is unique in that it consists of a combination of standardised subsystem panels that are hinged together and deploy/unfold once the spacecraft is in orbit.

The idea behind the PETSAT concept is to provide the ability to quickly configure the spacecraft in order to meet the needs of a specific mission.  This would enable mission designers to launch their own PETSAT for a significantly lower cost and shorter development time by providing the ability to select and assemble off-the-shelf panel subsystems as required.  The main payload on SOHLA-2 will be a lightning monitoring experiment and the mission will also demonstrate end-of-life de-orbiting through the use of a low-cost on-board propulsion system.  SOHLA are currently investigating launch opportunities for SOHLA-2.

"The service provided by Clyde Space Ltd. on our microsatellite SOHLA-2 demonstrated their total commitment to the project, which meant they became an integral and trusted part of our design team, ensuring that the spacecraft was completed within a demanding schedule." Customer Quote...

The concept of PETSAT represents a highly capable spacecraft platform that has numerous applications.  However, there were several challenges in the design of the spacecraft .  These included effective and scalable communications between the Panels, distributed On-Board Data Handling and, importantly, a scalable approach to the power system design.

The Clyde Space Solution

Given the modular nature of the PETSAT concept, the design of the power system had to be equally modular.  The end design had to allow for a spacecraft design that increases in power generation and storage capacity with each Panel that is added to the platform.  Also, the design had to be such that the power generated and stored on each panel is readily available for all of the Panels in the platform. 

The figure opposite shows a simple block diagram of the PETSAT power system in its final configuration.  As can be seen, there are TWO Battery Charge Regulators (BCRs) on each Panel.  This is for the purpose of connecting the power system to solar arrays that are on opposite sides of the Panel Module (i.e. top and bottom of the module).  In addition, each Panel Module houses a 15Whr, 12V lithium polymer battery, a voltage regulator that supplies +5V and +12V, and a number of commandable over-current protection switches in the Power Distribution Module (PDM).  Each power system also has an I2C digital interface to provide telecommand and telemetry.

The spacecraft is held in an OFF state when on the launch vehicle by a relay that is connected in series with the power to the Voltage Regulator and the PDM.  This relay is held in the OPEN state when a plunger switch on the launch interface of the spacecraft is depressed.  Once deployed from the launch vehicle, the plunger switch closes and the relays on each of the Panel Modules are commanded to a CLOSED state.  This function is performed completely in analogue electronics for simplicity.

As can be seen in the block diagram, the Battery strings in each of the Panel Modules are directly connected together.  By doing this, we treat the battery on the spacecraft a single battery pack; again for simplicity and ease of power sharing between Panel Modules.  The only potential draw back with doing this was that it may be possible that the spacecraft could experience a significant thermal gradient from one end of the spacecraft to the other once deployed.  Generally, batteries do not like to have large thermal gradients across them as this may result in capacity imbalance. 

In the case of SOHLA-2, we wanted to use the Clyde Space Lithium Polymer battery.  From tests, it was known that the battery could handle a thermal gradient of 15°C or less without significantly impacting the life or performance of the battery.  Therefore, it was then necessary to perform a thermal analysis of the spacecraft to determine whether it was likely that the spacecraft would see such gradients.  Given the nature of the spacecraft design, this was not a simple task, however, it was determined that it was unlikely that the difference in temperature between the farthest Panels in the platform would exceed 15°C.

As an extra precaution, each battery in each Panel Module was designed with a built in, thermostatically controlled heater that would maintain a battery temperature of greater than 0°C.  An image of the PETSAT battery is shown opposite, integrated in situ with the power system electronics.

The ability to connect the batteries directly together on each Panel Module was critical to the success of this power system architecture.  As a result of being able to do this, the power system becomes highly modular.  A number of Panel Modules can be connected together and, as a result, the power generation and storage capacity of the spacecraft grows.  The power system within each Panel Module was sized such that the increments by which the solar array area and battery increase the overall spacecraft capability was slightly more than the power required by a typically Panel.  In addition, the Power system was designed such that it could operate when connected to n Panel Modules, or in isolation with no additional operational constraints or requirements.

The image above is of the solar panels that were designed and built by Clyde Space for the SOHLA-2 project.  For budget and schedule reasons, the solar arrays consist of both Triple junction GaAs solar cells and also space grade Silicon solar cells.  Each of the solar arrays provides 16W of power when fully illuminated. 

Four of the final Six FM power modules are shown opposite.  The housing shown in the image is integrated into the panel Module and provides mechanical stiffness, thermal stabilisation, radiation shielding and EMC screening from the rest of the Panel Module.  All of the power system electronics are located on the PCB underneath the lithium polymer battery board.  The lithium polymer battery is designed as a daughter board to the power system electronics and connects directly onto the power system electronics PCB; Battery telemetry is also routed through the power system I2C node.

The nature of the power system electronics design means that it is possible to have solar panels with different characteristics connecting onto the same bus, because each solar panel is regulated independently.

In Summary

The PETSAT spacecraft concept presents a highly innovative and practical design approach to providing a low-cost, modular and responsive spacecraft bus for many types of mission scenario.  Clyde Space was delighted to be part of the PETSAT design team and to provide a highly modular and highly efficient power system that can both operate independently of other Panel Modules or when connected to an unlimited number of Panel Modules.

The highly modular nature of the PETSAT spacecraft bus called for the power system to reflect this requirement entirely.  Clyde Space have designed and produced the Power System Electronics, Battery and Solar Arrays in line with this design requirement and have also proven that it is possible to meet the requirement and still maintain a simple power system architecture, which is essential to reduce the risk of power system failure.

The figure opposite is the SOHLA-2 Spacecraft in Flight Configuration with FOUR Panel Modules.

The SOHLA-2 power system forms the basis of our Modal Power System and Modal Power Battery.  See also our Solar Panel Manufacturing section.