A multistage (or multi-stage) rocket is a rocket that uses two or more stages, each of which contains its own engines and propellant. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or attached next to each other. Taken together these are sometimes called a launch vehicle. Two stage rockets are quite common, but rockets with as many as five separate stages have been successfully launched. 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.
In serial or tandem staging schemes, the first stage is at the bottom and is usually the largest, the second stage and subsequent upper stages are above it, usually decreasing in size. In parallel staging schemes solid or liquid rocket boosters are used to assist with lift-off. These are sometimes referred to as 'stage 0'. In the typical case, the first stage and booster engines fire to propel the entire rocket upwards. When the boosters run out of fuel, they are 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. Known in rocketry circles as staging, this process is repeated until the final stage's motor burns to completion.
In some cases with serial staging, the upper stage ignites before the separation- the interstage ring is designed with this in mind, and the thrust is used to help positively separate the two vehicles.
The Taurus rocket is unusual in that its 'stage 1' ignites in flight; this designation is used because its upper three stages are identical to those of the Pegasus rocket, with the 'stage 0' booster replacing the Pegasus' carrier aircraft.
The main reason for multi-stage rockets and boosters is that once the fuel is exhausted, 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. The thrust of future stages is able to provide more acceleration than if the earlier stage were still attached, or a single, large rocket would be capable of. When a stage drops off, the rest of the rocket is still traveling near 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 each tuned for its particular operating conditions. 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.
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. In addition, each staging event is a significant point of failure during a launch, with the possibility of separation failure, ignition failure, and stage collision. Nevertheless the savings are so great that every rocket ever used to deliver a payload into orbit has had staging of some sort.
An upper stage is designed to operate at high altitude, with little or no atmospheric pressure. This allows the use of lower pressure combustion chambers and engine nozzles with optimal vacuum expansion ratios. Some upper stages, especially those using hypergolic propellants like Delta-K or Ariane 5 ES second stage, are pressure fed which eliminates the need for complex turbomachinery. Other upper stages, such as the Centaur or DCSS, use liquid hydrogen expander cycle engines, or gas generator cycle engines like the Ariane 5 ECA's HM-7B or the S-IVB's J-2. These stages are usually tasked with completing orbital injection and accelerating payloads into higher energy orbits such as GTO or onto escape velocity. Upper stages such as Fregat used primarily to bring payloads from low Earth orbit to GTO or beyond are sometimes referred to as space tugs.
Passivation and space debris
Upper stages of launch vehicles are a significant source of space debris from spent boosters remaining in orbit in a non-operational state for many years after use, and occasionally, large debris fields created from the breakup of a single upper stage while in orbit.
After the 1990s, spent upper stages are generally passivated after their use as a launch vehicle is complete in order to minimize risks while the stage remains derelict in orbit. Passivation means removing any sources of stored energy remaining on the vehicle, as by dumping fuel or discharging batteries.
Many early upper stages, in both the Soviet and U.S. space programs, were not passivated after mission completion. During the initial attempts to characterize the space debris problem, it became evident that a good proportion of all debris was due to the breaking up of rocket upper stages, particularly unpassivated upper stage propulsion units.
History and development
From an illustration and description in the 14th century Chinese Huolongjing of Jiao Yu is the oldest known multistage rocket; this was the 'fire-dragon issuing from the water' (火龙出水, huo long chu shui), used mostly by the Chinese navy. It was a two-stage rocket that had carrier or booster rockets that would eventually burn out, yet before they did they automatically ignited a number of smaller rocket arrows that were shot out of the front end of the missile, which was shaped like a dragon's head with an open mouth. This multi-stage rocket may be considered the ancestor to the modern YingJi-62 ASCM. The historian Joseph Needham points out that the written material and depicted illustration of this rocket come from the oldest stratum of the Huolongjing, which can be dated roughly 1300-1350 AD (from the book's part 1, chapter 3, page 23).
Another example of an early multistaged rocket is the Juhwa(走火) of Korean development. It was proposed by Choe Museon and developed by the Firearms Bureau (火㷁道監) during the 14th century. The rocket had the length of 15 cm and 13 cm; the diameter was 2.2 cm. It was attached to an arrow 110 cm long; experimental records show that the first results were around 200m in range. There are records that show Korea kept developing this technology until it came to produce the Singijeon, or 'magical machine arrows' in the 16th century. The earliest experiments with multistage rockets in Europe were made in 1551 by Austrian Conrad Haas (1509–1576), the arsenal master of the town of Hermannstadt, Transylvania (now Sibiu/Hermannstadt, Romania). This concept was developed independently by at least four individuals:
- Kazimierz Siemienowicz of the Polish–Lithuanian Commonwealth (1600–1651)
- the Russian Konstantin Tsiolkovsky (1857–1935)
- the American Robert Goddard (1882–1945)
- the German Hermann Oberth (1894–1989), born in Hermannstadt, Transylvania
In 1947, Mikhail Tikhonravov developed a theory of parallel stages, which he called "packet rockets". In his scheme, three parallel stages were fired from lift-off, but all three engines were fueled from the outer two stages, until they are empty and could be ejected. This is more efficient than sequential staging, because the second stage engine is never just dead weight. In 1951, Dmitry Okhotsimsky carried out a pioneering engineering study of general sequential and parallel staging, with and without the pumping of fuel between stages. The design of the R-7 Semyorka emerged from that study. The trio of rocket engines used in the first stage of the American Atlas I and Atlas II launch vehicles, arranged in a "row", used parallel staging in a similar way: the outer pair of engines existed as a jettisonable pair which would, after they shut down, drop away with the lowermost outer "skirt" structure of the booster, leaving the central "sustainer" engine to complete the first stage's engine burn towards apogee or orbit.
Separation of each portion of a multistage rocket introduces additional risk into the success of the launch mission. Reducing the number of separation events results in a reduction in complexity. Separation events occur when stages or strap-on boosters separate after use, when the payload fairing separates prior to orbital insertion, or when the launch escape system—used in many early human spaceflight missions—separates after the early phase of the launch. Pyrotechnic fasteners are sometimes used to separate rocket stages.
Where is the effective exhaust velocity, the initial mass, and the mass of the rocket at the point of burnout of each stage.
When the and mass ratios are the same for all the stages, this simplifies to:
and it can be seen that the delta-v is limited only by the n, the number of stages.
Alternatives to rockets
- Multistage rocket
- Reusable launch system
- Space tug
- Conrad Haas
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