# Propulsion

Propulsion means to add speed or acceleration to an object, by an engine or other similar device. The word 'propulsion' can be used with many other words (such as jet, rocket, spacecraft) to become-'jet propulsion', 'rocket propulsion', or 'space craft propulsion' etc.
Spacecraft propulsion is used to change the velocity of spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. Most spacecraft today are propelled by heating a reaction mass to high temperatures and exhausting it from the back/rear of the vehicle at very high speed. This sort of engine is called a rocket engine.
All current spacecraft use chemical rockets (bipropellant or solid-fuel) for launch, though some (such as the Pegasus rocket and SpaceShipOne) have used air-breathing engines on their first stage. Most satellites have simple reliable chemical rockets (often monopropellant rockets) or resistojet rockets to keep their station, although some use momentum wheels for attitude control. Newer geo-orbiting spacecraft are starting to use electric propulsion for north-south stationkeeping. Interplanetary vehicles mostly use chemical rockets as well, although a few have experimentally used ion thrusters (a form of electric propulsion) with some success.

## Necessity

Artificial satellites must be launched into orbit, and once there they must be placed in their nominal orbit. Once in the desired orbit, they often need some form of attitude control so that they are correctly pointed with respect to the Earth, the Sun, and possibly some astronomical object of interest. They are also subject to drag from the thin atmosphere, so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections (orbital stationkeeping). Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion. When a satellite has exhausted its ability to adjust its orbit, its useful life is over.
Spacecraft designed to travel further also need propulsion methods. They need to be launched out of the Earth's atmosphere just as satellites do. Once there, they need to leave orbit and move around.
For interplanetary travel, a spacecraft must use its engines to leave Earth orbit. Once it has done so, it must somehow make its way to its destination. Current interplanetary spacecraft do this with a series of short-term orbital adjustments. In between these adjustments, the spacecraft simply falls freely along its orbit. The simplest fuel-efficient means to move from one circular orbit to another is with a Hohmann transfer orbit: the spacecraft begins in a roughly circular orbit around the Sun. A short period of thrust in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to the orbit of its destination. The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination. Special methods such as aerobraking are sometimes used for this final orbital adjustment.
Spacecraft for interstellar travel also need propulsion methods. No such spacecraft has yet been built, but many designs have been discussed. Since interstellar distances are very great, a tremendous velocity is needed to get a spacecraft to its destination in a reasonable amount of time. Acquiring such a velocity on launch and getting rid of it on arrival will be a formidable challenge for spacecraft designers.

## Coursework

In relation to aerospace engineering the study of propulsion is usually undertaken in the following listed avenues:
* The basics of Newtonian mechanics which are essential for the understanding of any objects which move.
* A historical overview of the various propulsion systems which were used in various times and their beginnings and consequent applications.
* The basic theory of gas turbines, which usually describes the physics behind the propulsion system.
* A general introduction to the components of gas turbines, which include the compressor, combustor, turbines.
* A detailed study on the basic cycles which are the basics of the gas turbine namely the Brayton cycle and some other supplemantary cycles like the Rankine cycle. This portion also contains material on intercooling and reheat processes. A study of the effeciency of various cycles and ways to enhance the effeciency are studied.
* A detailed study on compressors , the physics , types, performance characteristics, effeciencies, steady - state relations , and design
* A detailed study on turbines.
* A detailed study of the combustor.
* A study of thrust augmentation methods.
* Performance calculation of turbojets, turbofans , and turboprops.
* A study of rocket propulsion, unconventional methods of propulsion,specific impulse calculation for various thrusters.

## Standard Text Books

* "Gas Turbine Technology" - HIH Saravanmuttoo, Cohen & Rogers
* "Gas Turbine Engineering Handbook" - P Boyce
* "Elements of Aircraft Propulsion" - Jack D Mattingly
* "Gas Turbine Performance" - Philip Walsh and Paul Fletcher
* "Mechanics and Thermodynamics of Propulsion " - Philip G. Hill and C.R.Peterson
* "Rocket Propulsion Elements" - George P. Sutton and Oscar Biblarz

## External Links

* Spacecraft Propulsion on Wikipedia
* The status of warp drive (faster than light travel)