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Rockets

A. A rocket is a vehicle which obtains thrust by the reaction to the ejection of fast moving fluid from within a rocket engine.

B. Rockets are used for fireworks and weaponry, as launch vehicles for artificial satellites, and for human spaceflight and exploration of other planets. While they are inefficient for low speed use, they are, compared to other propulsion systems, very lightweight, powerful and can achieve extremely high speeds.

C. In 1903, high school mathematics teacher Konstantin Tsiolkovsky (1857-1935) published the first serious scientific work on space travel. The Tsiolkovsky rocket equation—the principle that governs rocket propulsion—is named in his honor. Tsiolkovsky proposed to use liquid oxygen and liquid hydrogen as a nearly optimal propellant pair and determined that building staged and clustered rockets to increase the overall mass efficiency would dramatically increase range.

D. Most current rockets are chemically powered rockets. A chemical rocket engine can use gas propellant, solid propellant, liquid propellant, or a hybrid mixture of both solid and liquid. A chemical reaction is initiated between the fuel and the oxidizer in the combustion chamber, and the resultant hot gases accelerate out of a nozzle (or nozzles) at the rear end of the rocket. The acceleration of these gases through the engine exerts force (thrust) on the combustion chamber and nozzle, propelling the vehicle.

E. Due to their high exhaust velocity (Mach ~10), rockets are particularly useful when very high speeds are required, such as orbital speed (Mach 25). Rockets remain the only way to launch spacecraft into orbit. They are also used to rapidly accelerate spacecraft when they change orbits or de-orbit for landing. There are many different types of rockets.

F. A multistage rocket is the most popular, it uses two or more stages, each of which contains its own engines and propellant. A stacked stage is mounted on top of another stage; a parallel stage is attached next to another stage. The result is effectively two or more rockets stacked on top of or attached next to each other. Two stage rockets are quite common, but rockets with as many as five separate stages have been successfully launched.

G. By jettisoning stages when they run out of propellant, the mass of the remaining rocket is decreased. This staging allows the thrust of the remaining stages to more easily accelerate the rocket to its final speed and height.

H. In stacked staging schemes, the first stage is at the bottom and is usually the largest, the second stage is above it and is usually the next largest. Subsequent upper stages are above those. In parallel staging schemes solid or liquid rocket boosters are used to assist with lift-off.

I. The main reason for multi-stage rockets and boosters is that once the fuel is burnt, the space and structure which contained it and the motors themselves are useless and only add weight to the vehicle which slows down its future acceleration. By dropping the stages which are no longer useful, the rocket lightens itself. When a stage drops off, the rest of the rocket is still travelling near to the speed that the whole assembly reached at burn-out time. This means that it needs less total fuel to reach a given velocity and/or altitude.

J. An advantage is that each stage can use a different type of rocket motor, with each stage/motor tuned for the conditions in which it will operate. Thus the lower stage motors are designed for use at atmospheric pressure, while the upper stages can use motors suited to near vacuum conditions.

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