Multistage rocket

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 burning 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 each other 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.

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 (linear staging), when the first stage's motor(s) fire, the entire rocket is propelled upwards. When the first stage's motor(s) run out of fuel, the first stage is detached from the rest of the rocket (usually with some kind of small explosive charge) and falls away. 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.

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

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 boosters and/or 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 or boosters 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 booster can use a motor suited 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.

Disadvantages

On the downside, staging requires the vehicle to loft 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. Since 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 cost. Since staging is expensive in terms of manpower, a new movement has concentrated on single stage to orbit designs that do not have separate stages.

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:
*the Polish Kazimierz Siemienowicz
*the Russian Konstantin Tsiolkovsky
*the American Robert Goddard
*the ethnically German, Transylvanian-born Hermann Oberth.

See also

* Single-stage to orbit