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Multi-stage)
Description
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.
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 an motor suited to 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 that launches payloads into orbit uses staging.
In more recent times the usefulness of the technique has come into question. As the costs of space launches appear to be almost entirely the operational costs of the people involved (as opposed to fuel or other costs), reducing these costs seems like the best way to lower the costs. Since staging is expensive in terms of manpower, a new movement has concentrated on single stage to orbit designs that have no stages.
Development
This concept was developed independently by at least four individuals:
Variations/Examples
There are variations on this theme, the most common of which is the rocket booster. Rather than being on the bottom, these are often strapped to the side of a rocket or an aircraft, usually in sets of two or more (to provide symmetric thrust; otherwise the rocket/aircraft would fly around in circles). When they burn out, they're released and fall away. The Space Shuttle uses a pair of rocket boosters, plus a large external fuel tank to power its internal motors.
Many rockets use linear staging, as described above, in which a number of rockets are stacked on top of each other and fire one after the other. An example of such a rocket is the Saturn V. In order to increase the efficiency of the staging, the "upper stages" are typically fueled by hydrogen, meaning there is much less mass for the lower stages to lift than if the use kerosene. For atmosphere-bound rockets like surface-to-air missiles, the first stage is often solid fuelled for high initial acceleration (boost) while the second stage might be propelled by a ramjet or a liquid fuelled motor which is better able to provide sustained thrust efficiently, for the sustain phase.
Alternatively, stages are overlapping, i.e. the "next" stage fires before the previous one is disconnected, or even right at the start. Some examples include:
- The Space Shuttle fires its SRBs and SSMEs at the start, but the SRBs can be considered the first stage, because after they are discarded the SSMEs continue to function. During the first part of the launch the SRBs provide the main thrust.
- There are some Soviet designs with "cages" between their stages, to allow the rocket exhaust from the upper stage to blow out and around the stage below it - in other words to fire before the lower stage(s) are discarded. The usefulness of this technique is questionable unless the vehicle has motors that take some time to get up and running, which was the case for the Soviet designs that used many small motors.
Many designs have been made to use parallel staging in which a number of stages fire at the same time. Early rockets in both the US and USSR used such designs. Today many rockets that formerly did not use this technique have started to via the use of "strap on" solid rocket boosters to increase the delivered load over the original design. A good example of this is the original Thor (a development of the V-2) which evolved into the Thrust Augmented Thor, and finally to the Delta.
Several attempts have been made to build completely parallel stages, in which an economy of scale could be achieved by using a large number of identical stages strapped together into a bundle. The most complete was the OTRAG project, which failed for political reasons.
See also
