Multistage rocket
A multistage rocket is, like any rocket, propelled by the recoil pressure of the burning gases it emits as it burns fuel. What characterizes it as "multistage" is that it successively jettisons one or more stages as they become empty. (Generally a stage consists of one or more engines, plus fuel and oxidiser tanks (for a liquid rocket) or of the casing for a solid rocket.)
Essentially, the principle is that once the fuel in a section of the rocket is expended, that section becomes dead weight. By jettisoning the dead weight, less total fuel is needed to reach a given velocity. A big rocket can be used to push a smaller rocket into space. Then the big rocket is jettisoned and the smaller rocket can accelerate, starting from the speed it attained under the propulsion of the big rocket. This can, in principle, be done any number of times.
A further advantage is that each stage can use a different type of rocket engine, with each stage/engine tuned for the conditions in which it will operate. Thus the lower stage booster can use an engine suited to use at atmospheric pressure, while the upper stages can use engines suited to near vacuum conditions. Lower stages tend to require more structure than upper as they need to bear their own weight plus that of the stages above them, optimizing the structure of each stage decreases the weight of the total vehicle and provides further advantage.
This concept was developed independently by at least three individuals:
the Russian Konstantin Tsiolkovsky and the American Robert Goddard, and the ethnically German, Transylvanian-born Hermann Oberth.
For 1.5 stage-to-orbit, see Atlas (rocket).
This term is typically contrasted with single stage to orbit.
