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vehicle designed for spaceflight is called a spacecraft. When the spacecraft travels on a sub-orbital flight, the vehicle travels to the outer-space and returns to Earth without taking a full orbit.
On the other hand, a spacecraft taking an orbital space flight completes the orbiting of the Earth before returning to the planetary surface.
Spacecrafts may be of two types, one that carries people as its crews or passengers, and the other is the robotic spacecraft which is controlled by either autonomously or tele-robotically.
The use of robotic spacecrafts are of two types, the one which leaves just vicinity of the Earth are called space probes, and the other which remain in orbit of the Earth are called artificial satellites.
There are quite a few uses of spacecraft that goes into the outer space. They are used for several purposes, which would include communication, meteorological assessments, warfare, navigation, planetary exploration, and many more.
Depending upon the purpose for which a spacecraft has been built, the vehicle may consist of several sub-systems, which are applicable to the mission profile of the spacecraft. Few of the important sub-assemblies are briefly discussed below.
Attitude determination and control
This is otherwise termed as ADAC, ADC or ACS.
This sub-system is used for the spacecraft to remain balanced in space considering its orientation, and also remain under control of external torques and forces in a proper way.
The sub-system consists of sensors and actuators with proper software controlling the working of the mechanism. The control system also facilitates the spacecraft’s correct pointing of its relevant components, such as, pointing the solar arrays towards the sun, earth pointing for communication, and many more.
This sub-system is used for guidance of the spacecraft as per the commands issued. It does the necessary calculations to transmit them to the sensors in order to guide the spacecraft to the direction or position that is required.
This way the spacecraft is retained in the correct orbital path, and the flight path is also adjusted through this sub-system.
Command and data handling
This sub-system is used for decoding the commands that it received from communication sub-system, and after decoding, it sends them to the various control sub-systems of the spacecraft.
The sub-system, called CDH in short, also receives other data from various sub-systems, and stores the data on solid state recorder.
The power sub-assembly plays an important part in spacecraft flights. Every electrical and electronic element requires power to operate satisfactorily. When the spacecraft is near the Sun, the solar panels are used to generate this power and charge the on-board batteries.
The generation of power is achieved with the electrical power sent to power conditioning system, which essentially corrects the power to its right specification, removing any electrical noise generated, and then distributed over the other sub-assembled. When the spacecraft reaches a dark area, where the sun light is not available, the battery power is switched on for the spacecraft elements to perform.
PropulsionIt is not necessary that all spacecrafts will have a propulsion sub-system. This would depend upon the mission purpose of the vehicle, for example, the spacecrafts requiring altitude and inclination adjustments would include this sub-system.
For any momentum management maneuvers of a spacecraft the propulsion sub-system is also required. The different elements of a propulsion sub-system include fuel, tank, valves, pipes, and thrusters.
Besides the above, a spacecraft has sub-systems such as, the communication system, pyrotechnic control, thermal control, computing and software, and structure and mechanisms.

Source: .thinkengineering

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