A rocket needs enormous power to escape from the Earth’s gravity. The velocity required to achieve this is called the escape velocity, which is about 49,000km/h (29,000mph). The rocket’s power comes from burning liquid hydrogen and oxygen. Each stage of a rocket is a fuel tank, which is jettisoned when its fuel is used up. After all, carrying an empty fuel tank will only use up more fuel? Only the top stage of the rocket, called the payload, makes the whole journey and brings the crew back to Earth.
The study of rockets is an excellent way for students to learn the basics of forces and the response of an object to external forces. All rockets use the thrust generated by a propulsion system to overcome the weight of the rocket. For full scale satellite launchers, the weight of the payload is only a small portion of the lift-off weight. Most of the weight of the rocket is the weight of the propellants. As the propellants are burned off during powered ascent, a larger proportion of the weight of the vehicle becomes the near-empty tankage and structure that was required when the vehicle was fully loaded. In order to lighten the weight of the vehicle to achieve orbital velocity, most launchers discard a portion of the vehicle in a process called staging. There are two types of rocket staging, serial and parallel.
In serial staging, there is a small, second stage rocket that is placed on top of a larger first stage rocket. The first stage is ignited at launch and burns through the powered ascent until its propellants are exhausted. The first stage engine is then extinguished, the second stage separates from the first stage, and the second stage engine is ignited. The payload is carried atop the second stage into orbit. Serial staging was used on the Saturn V moon rockets. The Saturn V was a three stage rocket, which performed two staging maneuvers on its way to earth orbit. The discarded stages of the Saturn V were never retrieved. The other type of staging is called parallel staging.
In parallel staging, several small first stages are strapped onto to a central sustainer rocket. At launch, all of the engines are ignited. When the propellants in the strap-ons are extinguished, the strap-on rockets are discarded. The sustainer engine continues burning and the payload is carried atop the sustainer rocket into orbit. Parallel staging is used on the Space Shuttle. The discarded solid rocket boosters are retrieved from the ocean, re-filled with propellant, and used again on the Shuttle.
Some launchers, like the Titan III’s and Delta II’s, use both serial and parallel staging. The Titan III has a liquid-powered; two stage Titan II for a sustainer and two solid rocket strap-ons at launch. After the solids are discarded, the sustainer engine of the Titan II burns until its fuel is exhausted. Then the second stage of the Titan II is burned, carrying the payload to orbit. The Titan III is another example of a three stage rocket. “Serial” brings the rocket back to its original serial configuration and “Parallel” brings back a parallel configuration.
While they cannot fly all the way to orbit, there are two stage model rocket kits available. You can study the flight characteristics of a two stage model rocket by using the Rocket-Modeler II simulation program. And you can use the Circular Orbit simulation program to investigate the velocity and altitude requirements for specific orbit.
