Introduction
Spacecraft can be used as:
- Communication Satellite
- Weather Satellite
- Navigation Satellite
- Military Satellite
- Earth Observation Satellite
- Scientific Satellite
Spacecraft is generally divided into two parts
- Payload – to carry out set task for above listed operations.
- Service Modules – like attitude control, propulsion, power supply, thermal control, structure, deployable mechanisms and telemetry.
Design Process
The design is lead by the criteria – mass, stiffness, shape, orbit, cost, volume, material etc. So, the first step is to derive design specification which needs to comply with the criteria.
Based on the design specification, the development and test plan are made. Test models are made as a part of design process. Following are the test modesl:
- Structural Model
- Thermal Model
- Electrical Model
- Qualification model (qualification for the production of FM)
- Attitude Control model for attitude control system.
Test on test models may lead to change in design.
Reviews are done before production of FM
- PDR before starting production of test models. (Preliminary Design Review)
- CDR before start of production model. (Critical Design Review)
Launch Vehicle Systems
The choice of launch vehicle is dependent on the spacecraft mission. The launch vehicle sets restrictions for the spacecraft, such as possible launch mass and the available volume. There are two type of launch vehicles: Expendable Launch Vehicle(rocket is used only once) and Reusable Launch Vehicle(where parts can be used several times).
The chosen launch vehicle for the spacecraft contains the user manual outlines the following critical spacecraft requirements:
- Materials & Safety: Compliance with safety regulations and outgassing criteria.
- Physical Properties: Precise dimensioning for center of gravity and balancing.
- Environmental Testing: Specifications for qualification and acceptance tests, including static loads, sinusoidal vibrations, acoustic loads, and shocks.
Subsystems of Spacecraft
Other than the general requirement of spacecraft to be light-weight, strong, and reliable, each of the subsystem in a spacecraft has a specific demand like the operating temperature, stiffness, interaction with other subsystem etc.
Because spacecraft systems have so many variations, a short overview cannot cover them all.
Major of the subsystems of a Spacecraft are –
Power Supply
Used to deliver energy to other subsystems. This include batteries and solar cells.
Attitude Control System
Attitude control system is necessary to eliminate the influence of external disturbance moments.
Active system that require an energy source like reaction wheels.
Passive system that do not require energy like Gyro stabilization.
Data System
The information is stored temporarily into computer memory and transferred to ground station, known as telemetry. The information is generated from sensors and antennas.
Thermal Control System
The proper functioning of elements is possible within a certain temperature range. The temperature range must be regulated during operations. Two type of systems are used to maintain the temperature of the spacecraft –
Passive systems require no electricity and rely on natural materials and geometry.
Active systems require power and use mechanical parts to dynamically move heat across complex or high-heat environments
Telecommunication System
Used to maintain contact with ground station on Earth.
Signal sent by spacecraft – Telemetry
Signal sent by ground station to spacecraft – Telecommand
Structure
Spacecraft experience loads like life-off load, sine vibrations, random vibrations, shock load etc. So, spacecraft is designed to withstand those loads. Structure is usually made of sandwich panel so that it is light-weight and has high stiffness.
Propulsion System
To change the orbit during flight, propulsion system is used. A propulsion system is essential for a spacecraft to move, change direction, and travel through space.
The systems listed above interact with each other. Some of the examples are –
- Like unfolding solar panels disturbs attitude. Large solar panels flex in space and this can change satellite’s attitude control system.
- For maintaining attitude, reaction wheels or gyroscopes are used. The reaction wheels use strong electric current while gyroscopes need to be maintained at a particular temperature. So this uses power system and thermal control system.
- Heat is conducted through the structure so the shape of structure determines the type of thermal control system to be used.
- Solar panels needs a low operating temperature to work efficiently, while the attitude control system points solar panels continually in the direction of the sun. So the temperature is maintained using thermal control system by radiating dissipated energy to space. Explained better here.