Introduction to Mechanical Subsystem
The mechanical subsystem is responsible for designing and manufacturing the satellite structure while ensuring that the satellite can bear all the structural and thermal loads.
This makes three major subdivisions in the mechanical subsystem:
- Structures: Ensures the structural stability of the satellite while launch and during the operations of the satellite.
- Thermals: Ensures that the satellite functions in varied thermal cycles presented by the harsh space conditions in the orbit.
- Integration: Manufacturing and the integration of the various components of the satellite to ensure proper functioning of the satellite within the mass budget.
Keeping in mind the end goal of the fully functional satellite for the whole period of mission life, the mechanical subsystem contributes by making sure that the journey to the orbit and the harsh environment of space doesn’t pose a problem to the other subsystems. The Structures division provides the basic structure of the satellite known as the bus of the satellite. Also, the various mechanisms such as the deployable solar panels are designed by the structures team. The Thermals team makes sure that all the components of the satellite are within their operational temperature throughout the mission life. Finally, the Integration team gets to do the coolest part of actually ‘building’ the satellite. They put together the whole satellite ensuring that there are no components on board which interfere with each other.
The design of satellite structure is dependent on many factors. Some of the most important factors are placement of components and the material properties of components. The design approach that is followed generally is the following: first system engineering team prepares a Configuration Layout and Mass Budget and structures team analyses it and decides the parameters like the material of satellite body and thickness of the material. The analysis is given to Systems engineering team and if there is some flaw in the system, Systems engineering team prepares a modified configuration layout. This iterative process is followed till a design satisfying both system engineering and structural requirements is obtained. The various parameters that can be changed by structural subsystem are material properties of structure, geometric parameters like thickness of the structure and joining mechanisms.
The structures team also designs the various mechanisms of the satellite and ensures reliability of the mechanisms in space conditions. The analysis of various loads encountered is done through simulations performed in Finite Element Modeling Softwares. There are many such software available in the market. To validate the results of the software, theoretical calculations are done and compared.
Requirements on Structures
Launch Vehicle Placement Requirements
The satellite is to be launched on a launch vehicle. Each launch vehicle has some specifications for different classes of satellites. The satellite structure should be able to interface with the launch vehicle and hence meet the specifications of the launch vehicle.
Launch Loading Requirements
The satellite is carried to its orbit by a launch vehicle in a flight lasting about 17 minutes for Low Earth Orbits. During this period, the vehicle experiences high levels of acceleration, vibrations and shocks which are transmitted to the payloads attached to the flight decks of the vehicle. Launch loads experienced include static loads, vibration loads, acoustic loads and shocks and impose certain strict requirements on the structure of the satellite. Satellite structure should be able to withstand these loads during launch. All the components should be safe and working after the launch. The loading specification for which the launch vehicle interface is tested is assumed to be the loading data for the satellite during launch.
Many satellites have various deployable components such as antennas. The structures team has to ensure that the components are deployed when required.
Transportation and Handling Requirements
The satellite, once integrated, needs to be handled and transported to the launch site. For this purpose the structures team needs to design a handle for the satellite and a satellite box for transportation. Many factors such as electrostatic charge, vibration, contamination from humidity, pressure management, thermal control needs to be looked at.
Apart from this transportation boxes for individual components to be sent for integration at the clean room may be required.
The atmosphere above the height of 500 km above Earth comprises of too thin to moderate ambient thermal conditions and consequently, the environment above such a height is prone to extremes of temperature, being directly exposed to solar radiation and deep space. So, the Thermals subsystem needs to ensure all the electrical components of the satellite are well within their operating range.
Various different methods are employed to ensure the thermal stability. The major two divisions being Active and Passive thermal control. By employing these methods iteratively and analysing the results for different initial conditions, the Thermals team tries to come up with the best possible solution, with minimum increase in mass or power requirements.
Requirements on Thermals
- Ensure that the temperature within the satellite is within the operating range of different electrical components.
- Remove excessive heat from heat producing components.
The team works for the goal of launch of the satellite. The final Flight Model (FM) is the one which is launched into the space and undergoes the mission of the satellite. Integration team’s work comes into the picture once all the subsystems are ready with their designs and the individual components are manufactured. Then the critical part of integrating the components comes up. Integration team makes an Integration Sequence, making sure than integration can be done in a fast and effective manner.
Apart from the Flight Model (FM), the team also integrates a Qualification Model (QM) which is used for qualifying the satellite design, before the flight model is made. It is generally a replica of the flight model, which is passed through more rigorous tests than the flight model. It is generally mandated by space agencies to ensure that the satellite design is fit to go into their launch vehicle, along with giving a confidence to the team. The Flight Model is also tested in similar conditions as Qualification Model but the loads applied in qualified model are higher.
Importance of Integration
At first glance, Integration looks like a simple task, which may not require a separate subsystem team working on it. However, a faulty integration can blow up the entire mission. Not drafting a proper integration sequence and not testing it rigorously will often cause last minute panics if you are lucky, and indefinite delays in the mission if you are not. Also, as much as 10% of the mass can come from the various integration related components such as connectors, screws etc. So neglecting integration when preparing the mass budget is a bad idea. Also mass being a critical factor, the wire routing is to be done to ensure that minimum mass of wires is added, all the while ensuring no tangling between them. Also mass being a critical factor, the wire routing is to be done to ensure that minimum mass of wires is added, all the while ensuring no tangling between them.
Requirements on Integration
- Complete all connections between electrical packages and route wires between them.
- Assemble the satellite (both QM and FM).
Requirements imposed by Mechanical Subsystem on other Subsystems
Volume and Mass requirements
The satellite’s mass and volume are constrained by the maximum payload the launch vehicle can carry. To ensure that these constraints set for the satellite are met, mechanical subsystem poses mass and volume requirements on all the subsystems.
To ensure the launch loading requirements listed earlier, mechanical subsystem poses stiffness requirements on the other subsystems. The other subsystems need to ensure that PCBs and other components comply with these requirements so that they have natural frequency well above natural frequency of the launch vehicle.
Note: All the requirements listed are the generic set of requirements for a satellite. There could be more requirements sure other constraints from a particular mission.
Below is a list of softwares that can be used for the various modeling and analyses. Please note the list is not exhaustive there may be many other softwares available.
Modeling and FEM Softwares
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