Experiment

Experiment

Project ASTER aims to demonstrate a high-performance, low-cost and easy to integrate attitude control system for FFUs from sounding rockets. It shall be capable of stabilizing an FFU after ejection and subsequently performing slewing manoeuvres through the exchange of angular momentum by means of three reaction wheels. The performance of the Attitude Determination and Control System (ADCS) will be recorded throughout the mission, and will later be analysed to compare the expected performance with the flightdata

The developed system shall be able to be used in future sounding rocket experiments, which need to be attitude stabilized. Therefore, the system shall be compact to ensure sufficient space for possible payload components. Furthermore, team ASTER decided to publish the complete detailed design after the data analysis, so future experiments can utilize our technology.

Objectives

Project ASTER aims for three primary and three secondary objectives. 

The primary objectives of the experiment are:

  • OBJ 1 To develop an attitude controlled FFU to be ejected from a sounding rocket.
  • OBJ 2 To demonstrate that the ADCS is capable of stabilising the FFU.
  • OBJ 3 To recover the FFU.

The secondary objectives of the experiment are:

  • OBJ 4 To demonstrate that the ADCS can perform slewing manoeuvres of the FFU.
  • OBJ 5 To design an FFU which is able to accommodate payloads of future REXUS experiments.
  • OBJ 6 To design and build an FFU, including the ADCS, that is easy to integrate with future experiments.

Segments

The entire system consists of three distinct segments. All three operate independently, but flawless interaction between the segments is crucial for mission success

  • Free Falling Unit (FFU): carries the main experiment components and is ejected from the REXUS rocket following despin.
  • Rocket Mounted Unit (RMU): is mounted directly to the rocket during the entire mission and is responsible for the ejection of the FFU.
  • Ground Station (GS): consists of the equipment required for the communication and the successful return of the FFU.

Subsystems

The whole system is further divided into nine subsystems, of which eight belong to the FFU. The GS is defined to be part of the communication subsystem.

Rocket Mounted Unit

The RMU is the mechanical and electrical interface between the FFU and the REXUS rocket. The primary task of the RMU is the ejection of the FFU just after the despin of the rocket.

The RMU consists of the container, hatch, flange, ejection mechanism, retention mechanism and release mechanism. The main components of the RMU will be made of aluminium, with the exception of the retention cables, springs, pyro-cutter fixation brackets and the ejection spring mounting blocks. The RMU is placed inside an aluminium module with a cut-out through which the FFU is ejected.

Our design is based on a previous REXUS experiment, TUPEX-6 from TU Berlin, and it is adapted to fit our bigger FFU. The main structure is the container where the FFU is placed. The retention mechanism keeps the hatch and FFU inside the module during launch and ascent. The release mechanism, which consists of five pyro-cutters and their brackets, cuts the retention cables. The ejection mechanism first ejects the hatch and then the FFU with an ejection velocity around 3 m/s.

A camera is placed at the back of the container to record and confirm the ejection of our FFU. The video will be downlinked to the ground station in real-time.

Experiment module with RMU, isometric view

Experiment module with RMU, sectional view

Attitude Determination and Control System

The Attitude Determination and Control System is the core subsystem of our experiment. It consists of three segments, which are the Attitude Determination System, Attitude Control System, and the Reaction wheels.

The Attitude Determination System provides the input for the Attitude Control System. This data is obtained by two Inertial Measurement Units which determines the attitude with integrated accelerometer, gyroscope and magnetometer. The inertial measurement units were previously flown and approved on REXUS 19 by the MIRKA2-RX experiment from KSat e.V.

The Attitude Control System determines the revolutions per minute of the reaction wheels which is required to stabilise the system in different modes with varying complexity levels. The complexity levels reach from simple constant rotor speed control  to complex cascade control loops for slewing maneuvers.

The reaction wheel assembly is being designed by the ASTER team itself. It consists of a rotor connected to an electric motor which is controlled by a motor controller. The wheel’s efficiency increases with the inertial moment of the rotor. To maximize this inertial moment, the rotor is as large as possible and is made out of steel which has a higher density than aluminum, which is the material used for most other components. The motor is mounted inside a bracket which can be screwed onto a frame plate of the FFU. The rotor rests on the shaft of the motor on the other side of the bracket. By adding two bearings around the motor shaft which are press-fit into the bracket, loads from the rotor are transferred to the brackets instead of the motor. A plastic washer is used to preload the bearings and a lock nut prevents the rotor from sliding off the shaft. This design creates the angular momentum required to stabilize the free falling unit. Three reaction wheels will be used to achieve 3-axis stabilization and perform slewing maneuvers during the free fall.

Exploded view of the reaction wheel design

Recovery System

The Recovery subsystem is responsible for ensuring that the Free Falling Unit (FFU) of our experiment survives landing and can be located afterwards. Since most of our experiment data can not be sent to the ground station in real time, but are instead stored onboard, the recovery of our experiment is critical for mission success.

To make sure the FFU lands safely, a parachute will be released at an altitude of 5-6km. This will slow the module from beyond the speed of sound to below 30 km/h. While a touchdown at this speed can not be called soft, the experiment is designed to withstand the landing impact. And considering the REXUS rocket is scheduled to launch in March from Esrange Space Center in the Arctic, there will likely be snow to cushion the landing.

After the FFU has landed, it still has to be found to allow us to analyse the experiment outcome. The landing zone of Esrange is vast, extending all the way to the Finnish-Norwegian border. To narrow down the landing location, the FFU contains a GPS module and an Iridium chip, allowing us to transmit the experiment’s coordinates to the Iridium satellite network in real time. From the Iridium network, the transmission is passed on to the ASTER ground station.

After landing, the Esrange recovery team will search for the FFU at the received landing site to return it to the ASTER Team. Since GPS coordinates are only accurate to a couple of meters, and the experiment may be buried in snow, it may not be easy to find. For this reason the FFU has RECCO tags attached to its outside. Designed to be attached to skiing gear and help locate victims of avalanches, the tags reflects radio waves much like a bike reflector reflects light. The recovery team will be equipped with a detector capable of locating these tags from their reflections, so that the FFU can be spotted from a helicopter.

Top plate ejection system

Mechanical design overview of the recovery system

Communication

As its name indicates, the Communication subsystem allows us to communicate with our experiment. This is relevant during all mission phases, and several different modes of communication are included in this subsystem.

Already before and during the assembly of the Free Falling Unit (FFU), the communication subsystem comes into play, as we need to make sure everything works as intended during flight. Therefore all aspects of the experiment have to be thoroughly tested.

For this purpose, the Test Environment Software (TES) is an integrated part of the communication system. The TES provides the us the functionality to send commands to the experiment, upload simulated sensor outputs to the On-Board Data Handling subsystem, access parts of the system which normally run autonomously, and of course log everything that happens during testing.

When assembly and testing are successfully completed, we reach the launch campaign. While our FFU is inside the REXUS rocket, we will be able to communicate with the ground station via the REXUS Service Module. Our own ASTER Ground Station will be used to send and receive data. Up until launch, the experiment can be controlled from the Ground Station via telecommand. The experiment will send telemetry via the rocket to the Ground Station, from prior to launch until the ejection of the rocket of the FFU, informing us of the general system status.

When the FFU is ejected from the rocket, the connection to the REXUS Service module is lost and no further telemetry can be sent from the experiment. However, shortly after ejection, two antennas aboard the FFU will be used to obtain its GPS coordinates and acquire a link to the Iridium satellite network. While the amount of information we can transmit via the Iridium network is very limited, it enables us to send the FFU’s coordinates to the ASTER Ground Station during free fall until after landing, to ensure the experiment can be recovered.

Ground Station Concept

Structure

The frame of the FFU is the mainframe on which all FFU components, such as the reaction wheels, the PCB, the antennas, etc. are mounted.

Additionally, it provides the structural stiffness capable of handling all the loads during launch, ejection, free-fall and landing, as well as protecting the system until recovery. The design has to be finely balanced between stiffness and weight. The six aluminium wall plates, that make up the frame, have to be thin and lightweight, but also provide enough stiffness, because we don’t want our FFU to crumble during impact or already during launch. For this, the plates contain cutouts in places, where no stiffness is required, so the thickness is either 3 or 0.5mm. Each plate will have a different design to account for the various functions.

To connect the wall plates, some of them are actually thicker in certain areas to provide the space to drill in holes from the side. We will insert Heli-coils into those holes. Heli-coils are basically springs that provide a higher stiffness and protect the aluminium from the steel bolts (Because steel is much harder than aluminium, we would destroy the thread in the aluminium after a few times screwing and unscrewing).

Part of the Frame

Thermal

The thermal subsystem is responsible for maintaining the experiment within operating temperatures during all stages of the mission. This includes the pre-liftoff phase until the experiment is located after landing. The environmental conditions can be extreme with temperatures down to -30 degrees during March when the experiment is planned to launch compared to up to +70 degrees during the launch phase until ejection. The internal heating of the FFU will reach high temperatures, mainly around the motors and battery compartment. These are the two thermal critical component which must be taken into consideration when designing and testing the system.

To confirm the thermal heating of the FFU in earlier stages, simulations will be carried out. These will be done for both internal and external heating. We will use Fusion 360 to find out how the internal heating of our system is distributed and to find out the environmental impact on the FFU we will be using CFD (computational fluid dynamics). From these analyses we will get a rough estimate how the experiment will heat up and if we need to take precautions because of it. Some components which are more exposed to high temperatures are the batteries, motors and motor controllers. For the batteries a battery department will be implemented, which includes a 3D-printed plastic containment inside an aluminum box. This is to reduce the heating of the batteries but also to avoid a fire hazard which is a potential risk for the recovery personnel. For the motor and motor controllers an option is to be using heat sinks, cooling past and heat bridges.

Testing is a vital part for the thermal subsystem. We will need to do testing in both thermal and in vacuum conditions. This is important to establish the functionality of the different parts of the FFU under low gravity conditions, which will occur the first minutes after ejection from the REXUS rocket.

Heat distribution from batteries showing the batteries with the surrounding PET plastic compartment for steady state.

Electrical Power System

The Electrical Power System (EPS) purpose is to deliver power to each one of the experiment’s subsystems in a safe, and regulated manner. The EPS consists mainly of the Energy Storage Unit (ESU), the Power Distribution Unit (PDU), and the charging unit. The purpose of the charging unit is to ensure that the power received from the REXUS Service Module is regulated to a constant voltage, so that it can be used to charge the batteries aboard the FFU. These batteries form the core of the ESU which is intended to provide power to the FFU and it’s various subsystems throughout the duration of the flight. Lastly, the PDU is intended to step the power of the ESU to the required voltages for the various components aboard the FFU while simultaneously regulating the current drawn from said components to ensure that the total available power is not consumed unnecessarily.

Power consumption of the FFU from the ESU vs time, in seconds, since lift-off

On-Board Data Handling

The On-Board Data Handling subsystem (OBDH) can be named as the brain of our experiment. It consists of a variety of components, namely, the STM32F722 microcontroller acting as the On-Board Computer (OBC), external data storage, and it provides the interfaces to all sensors and the various other subsystems.

With all on-board software running on the OBC, the OBDH controls the majority of the Free Falling Unit (FFU)’s  functionality. Sensor data, including altitude, angular velocity of the FFU and the rotors, attitude, and temperature are read and stored by the OBC. A large portion of the OBC’s processing power will be used by the Attitude Determination and Control System, which forms the heart of our experiment. In order to adjust the FFU’s attitude, real-time sensor data will be computed to provide the desired change in output of the reaction wheels.

Furthermore, the software that governs both the Communication and Recovery subsystems runs on the OBC, enabling telemetry and telecommand to be sent via the REXUS service module, and the FFU’s GPS position to be reported to the ASTER ground station. The OBC also provides the trigger to deploy the parachute, ensuring a safe landing of the experiment.

Last but not least, the experiment is required to operate completely autonomously from launch up until it is recovered, and communication during the experiment phase is very limited. Therefore it is vital that all experiment data is recorded, as only post-flight analysis of recovered data will enable us to assess experiment success. Because of this, the OBDH does not only store all data and an event log, but also provides a backup, utilizing two redundant SD cards.

Main PCB on FFU

Payload

As a demonstration the FFU will carry a payload consisting of a Raspberry Pi, a camera and several temperature sensors. The camera will take pictures that can later be used to verify the performance of the ADCS and as promotional material. Temperature sensors allow for measuring the environment inside the FFU as reference for future projects using this free-falling system

FFU with payload area (orange box)

Experiment Timeline

All these segments and subsystems aim to fulfil the experiment objectives within the following experiment timeline which can be split into six experiment phases:

  1. Launch and flight prior to ejection
  2. Ejection of the FFU before apogee of the REXUS rocket
  3. Stabilisation of the ejected FFU using reaction wheels in reduced gravity
  4. Slewing manoeuvres of the ejected FFU using reaction wheels
  5. Parachute deployment and location transmission
  6. Recovery of the FFU