Multistage rocket

A multi-stage rocket, or multi-stage rocket consists of several, often mounted one over the other rocket stages of decreasing size, wherein the fuel tank empty and no longer required engines are discarded so that they do not have to be further accelerated along with the payload. In this manner, higher speed and thus higher orbits achieved than with single-stage rockets.

History

First descriptions and illustrations of multi-stage rockets appeared in the 14th century in the Huo Lung Ching, a Chinese manual on firearms on. The Austrian military engineer Conrad Haas described 1529-1556 in his art book ( found in 1961 ) a variety of types of missiles, including the multistage rocket. The Polish artillery officer Casimir Simienowicz described in 1650 three-stage rockets. The Russian teacher and self-taught Konstantin Tsiolkovsky presented the multi-stage principle with his Rocket Equation on a scientific basis. This states that the terminal velocity of a single-stage rocket in severe free space only by the exhaust velocity of the engine gases and the ratio of take-off mass to final mass - dependent ( off mass fuel).

A9/A10 The term was started by German engineers during the Second World War on the basis of the A4 rocket the development of a multi-stage rocket, but not completed. The development of multi-stage rockets Great was particularly promoted for military use in the United States and the Soviet Union from the late forties of the 20th century. In this case, different configurations were examined, of which the stacked rockets and rocket boosters are used today.

Designation

Simple rockets consist of only one stage with a motor. In stacked rocket stages are stacked and fired one by one. Theoretically, this process could be repeated any number of times, with more than four levels of effort for the engines but is usually greater than the mass gain and problems arise, to achieve the required reliability. Become levels simultaneously ignited, but burn different lengths, one often counts them as half steps. If this principle is used when starting, it is called boosters. For example, Ariane 5 is a 2.5 -stage rocket: You blasts from their booster before the first stage is burned out. The second stage carrying a payload that often has its own motor. Actually there is a third stage, which is not often counted in launchers. It does not include the driving of the actual payload to reach the planned path, since this is not part of the carrier rocket.

Implementation

The technical implementation is challenging and requires precise timing. In particular, the hitherto mechanically highly loaded connection of the stages must be mechanically separated, as a rule pyrotechnic or strap systems. In this case, a collision must be avoided, whereby the separated stages often still has a residual thrust. Therefore, small auxiliary power units on the upper level are used that cause them from the lower level to move away ( " ullage " ) and so on liquid rocket also has a defined position of the fuels. Finally, the correct ignition sequence of the upper level must be passed. In later phases of flight unpowered ( " coasting " ) phases are possible.

Situation today

The Saturn V, the booster rocket of the Apollo lunar missions, consisted of three rocket stages. The Indian PSLV consists of five stages (if you count the booster as a separate step).

Most of the steps used in the aerospace missiles now have three levels, but there are also two, four - and five-level systems. The higher the maximum speed required, the more levels are usually employed. So often two levels for a low Earth orbit (LEO, v> 7.9 km / s, eg Gemini), three levels of geostationary orbits or moon flights (v at 11 km / s) and four levels for interplanetary orbits ( v> 11 2 km / s ) are common. The last stage is sometimes also referred to as a kick level or kick motor, as these kicks the satellite from the current orbit to a higher or interplanetary web.

Many missiles can be equipped with various upper stages. Then usually has the upper and lower levels each have their own control system ( system of gyroscopes and other sensors). Other rockets, such as Ariane basically have only a control system that sits at the top step, and all stages controls (also in versions with various upper stages such as Ariane 5 GS and Ariane 5 ECA). The upper is usually separated by a small explosive charge of their lower level before they ignite.

The terminal velocity is composed of the speeds achieved individually by each stage, apart from the negative acceleration by the Earth's gravity field:

Where c1 the exhaust velocity of the gases from the engine of the first stage (a few kilometers per second), and ln m for the logarithm of the respective mass ratio of take-off weight is to curb weight. It should be noted that m1 has to bear as curb weight, the second and third stages, etc., but m3 only the payload - the or satellite.

The curb weight is not only determined by the rocket shell (which must not be too thin for the sake of stability ), but also of the Laval nozzle and the fuel pump and auxiliaries. In low -built rocket is the mass ratio of about 5, so that engines with nozzle velocities to c = 3 km / s, about 5 km / s to achieve (3 km / s · ln 5 = 4.8 km / s). De facto, however, reduced gravity, the speed of the lower rocket stages significantly (depending on initial acceleration typically around 1 to 2 km / s at three minutes burning time ), which must be considered in the system design.

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