Any technique used for transmitting a mass from one point to another in the aerospace environment is called as propulsion. From past till now, propulsion has been one of the most active fields of research for many scientists. The main reason for this is the need for speed, faster trip time, exploration and discovery.
Some of the advanced technologies in rocket and spacecraft propulsion which are used and to be used in the near future are given below:
Some fundamentally different concept was introduced for accelerating the propellant mass to overcomes the limitations of chemical thermodynamic expansion. Into this breech steps the family of electric propulsion possibilities.
Historically, conceptually and pragmatically this field has tended to subdivide into three categories:
1) Electro thermal propulsion
In this process propellant is heated by some electrical process then expanded through a suitable nozzle. The arc jet is an electro thermal rocket because it uses electrical energy to heat a propellant. In this method, an annular arc is created in the chamber and the propellant is heated to high temperatures as it interacts with the arc. After the heating, the propellant is expanded through a conventional nozzle. This type of propulsion takes advantage of using hydrogen as a propellant, and, like nuclear rockets, experiences a similar performance gain in specific impulse (up to 1,200 seconds). Unlike nuclear rockets, arc jets are small, producing little more than several pounds of thrust.
2) Electro static propulsion
These are commonly called ion rockets. Neutral propellant is converted ions and electrons and withdrawn in separate streams. The ions pass through a strong electrostatic field produced between acceleration electrodes. The ions accelerate to high speeds, and the thrust of the rocket is in reaction to the ion acceleration. It is also necessary to expel the electrons in order to prevent the vehicle from acquiring a net negative charge. Otherwise, ions would be attracted back to the vehicle and the thrust would vanish.
The excess electrons could be removed by re- injecting them back into the exhaust ion beam. Ion rockets offer very high specific impulses, but very low thrust, one-half pound being high. It has been estimated that an ion rocket employing cesium propellant would require over 2,000 kW of electrical power per pound of thrust.
The propellant for ion engines may be any substance that ionizes easily. Unlike thermodynamic expansion, the size of the molecules is not a primary factor. The most efficient elements are mercury, cesium or the noble gases.
3) Electro magnetic propulsion
In this method plasma is used with crossed electric and magnetic fields to accelerate the plasma. A plasma is an electrically conducting gas. It consists of a collection of neutral atoms, molecules, ions, and electrons. The number of ions and the number of electrons are equal so that, on the whole, the plasma is electrically neutral. Because of its ability to conduct electrons, the plasma can be subjected to electromagnetic forces in much the same way as solid conductors in electric motors.
There are three major types of electromagnetic rockets: magneto gas- dynamic, pulsed-plasma and traveling wave
a) Magneto gas dynamic drive
Strong external electric and magnetic fields direct and accelerate the plasma stream, imparting high exhaust velocity. The performance is limited due to non- perpendicular currents flowing in the plasma at high field strengths. The specific impulse is lower than ion rockets but still very high (around 10,000 seconds). The mass flow rate is restricted so the thrusts remain low.
b) Pulsed plasma
One of the disadvantages of the steady crossed-field accelerators is that they require a substantial external field and therefore, a massive electromagnet. It is possible to make an accelerator for which an electromagnet is unnecessary by using the plasma current itself to generate the magnetic field, which gives rise to the accelerating force. Whereas the crossed- field accelerator is analogous to a shunt motor (which has separate current circuits for the electric and magnetic fields), the analog of this type of accelerator is the series motor in which the magnetic field is established by the same current which interacts to establish the crossed field force.