ROCKET PROPULSION SYSTEMSRocket propulsion systems can be classified according to their energy source (chemical, nuclear, solar), thrust level, or type of propellant. They can also be classified according to their application (for example, a booster rocket for a large launch vehicle, a small attitude-control rocket, air-to-air missile propulsion, or a high-acceleration gun-launched rocket-assisted projectile).Table 1 shows several different types of rocket propulsion systems. The first three, called chemical propulsion systems, are the most common. They convert the chemical energy of combustion in part into directed (kinetic) energy of the high-speed exhaust gases, by means of a bowtie-shaped supersonic nozzle; and in part into thermal energy, or heat (high exhaust-gas temperature). High-thrust rocket propulsion systems are used on the first or second stages of large missiles or large space launch vehicles. They push a heavy payload or warhead over a long range or on an ambitious spaceflight trajectory (that is, high-orbit or earth-escape missions). The expansion of gases in a nozzle causes changes in the internal pressure distribution and thereby creates thrust.
These gases can be generated by combustion (chemical rockets), stored in the vehicle under high pressure (cold-gas systems using air or helium for attitude control), or evaporated from liquids and heated (liquid hydrogen heated by solar or nuclear energy).The electromagnetic and electrostatic rocket propulsion systems listed in Table 1 have very different mechanisms for creating a high-speed exhaust and do not rely on thermodynamic gas expansion in supersonic nozzles. They rely instead on the interaction of electric and/or magnetic fields with electrically charged low-pressure gases (plasmas or ions) to accelerate these dilute gases to high exhaust velocities, thus achieving very high specific-impulse values. Their thrust values are very low compared to those of other rocket types, which limits the accelerations they can impart to a flying vehicle.Table 2 relates several important applications with respect to thrust level.