Multistage rocket

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The second stage of a Minuteman III rocket

A multistage (or multi-stage) rocket is, like any rocket, propelled by the recoil pressure of the gases it emits as it burns fuel. What characterizes it as "multi-stage" is that it successively jettisons one or more stages as they become empty. It is effectively one or more rockets (stages) stacked on top of or attached next to each other ("parallel staging"); in order to reduce the total amount of mass which needs to be accelerated to the final speed/height. Generally each stage consists of one or more motors, plus fuel and oxidiser tanks for a liquid rocket or the casing for a solid rocket. In rocketry, this concept is known as staging.

Solid or liquid rocket Boosters are often used for parallel staging schemes and all motors are ignited at launch. These are sometimes referred to as 'stage 0'. The first stage is at the bottom and is usually the largest, the second stage above it and is usually the next largest, etc. In the typical case, the first stage's motor(s) fire and then any fitted boosters, and the entire rocket is propelled upwards. When the boosters run out of fuel, they detached from the rest of the rocket (usually with some kind of small explosive charge) and fall away. The first stage then burns to completion and falls off. This leaves a smaller rocket, with the second stage on the bottom, which then fires. This process is repeated until the final stage's motor burns out.

The Space shuttle has two large boosters and is not Single-stage-to-orbit (SSTO).

A two-stage Delta III rocket with nine rocket boosters attached (Photo courtesy Boeing)

1 Advantages
2 Disadvantages
3 Development
4 Alternatives to rockets
5 See also

[edit] Advantages

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 (in the case of liquid-fuelled rockets) 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. The thrust of the future stages is able to provide more acceleration than if the earlier stages were still attached, or than a single, large rocket would be capable of. 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.

A further 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. 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.

[edit] Disadvantages

On the downside, staging requires the vehicle to lift motors which are not being used until later, as well as making the entire rocket more complex and harder to build. Nevertheless the savings are so great that every rocket currently used to deliver a payload into orbit uses staging.

In more recent times the usefulness of the technique has come into question due to developments in technology. In the case of the Space Shuttle the costs of space launches appear to mostly composed of the operational costs of the people involved (as opposed to fuel or equipment), reducing these costs appears to be the best way to lower the overall launch costs. New technology that is mainly in the theoretical and developmental stages are being looked at to lower the costs of launch vehicles. More information can be found on single stage to orbit designs that do not have separate stages.

[edit] Development

Cutaway drawings showing three multi-stage rockets (Image courtesy NASA)

An artist's conception of the separation of the S1-B stage of a Saturn IB rocket (Image courtesy NASA)

The second stage being lowered into the first stage of a Saturn V rocket (Photo courtesy NASA)

A diagram of the second stage and how it fits into the complete rocket (Image courtesy NASA)

The earliest experiments with multistage rockets were made by Austrian Conrad Haas, the arsenal master of the town of Sibiu, Transylvania (now in Romania) in 1551.

This concept was developed independently by at least four individuals: