by Launchspace Staff Writers
Bethesda MD (SPX) Jul 31, 2013
There are dozens of launch vehicle configurations available around the world. Most space professionals are familiar with at least some of these. The major producers of launch vehicles include the United States, Europe, Russia, China and the Ukraine. Other countries are also trying to compete in the launch marketplace, including India and Japan. Still others are in earlier stages of developing competitive space launch systems and these include Israel and Brazil.
Overall, there is a great deal of interest and activity regarding the launch of satellites into orbits around the Earth. However, few in the satellite business seem to be aware of the subtle demarcation between different launch vehicle families and their areas of application. For example, many of the launch systems with limited payload capability, such as the long-retired Scout and the currently available Pegasus, offer to place small satellites into low orbits.
Most of these are able to place payloads into their final-desired orbits through a process referred to as "direct injection." This simply means the satellite is placed into its operational orbit at the moment of upper-stage-burnout.
In other words, no further propulsion burns are needed to achieve the final orbit. Typical achieved altitudes are from roughly 300 km to 1200 km for circular orbits. One of the major advantages of the direct injection technique is that the satellite need not carry a maneuvering propulsion system. In fact, the majority of all satellite launches are for direct injections into low orbits.
When a higher orbit, say beyond about 1200 km, is required, very different ascent techniques come into play. In such cases the most popular approach is one in which the launch vehicle injects a satellite into a "transfer orbit."
These orbits are elliptical and usually designed such that their apogee altitudes correspond to the altitudes of the final desired orbits. An excellent example is the ascent of a geostationary communications satellite. Almost all such launches place the satellite in a geostationary transfer orbit (GTO) with a perigee at about 200-300 km and an apogee at 35,786 km.
This high altitude corresponds to the circular orbit altitude of an apparently stationary satellite over the equator. In order to achieve its final orbit, the satellite must carry sufficient propellant to make the transition from elliptical orbit to circular orbit.
Even though the mass of propellant needed may exceed 40% of the injected spacecraft's mass, this approach is still more efficient than using the launch vehicle's upper stage to perform the final injection.
Although not always the case, launch vehicles used for low orbit injections tend to be small when compared to those used for geostationary satellites. One primary reason: most low-orbiting satellites are small when compared to geostationary and other high altitude spacecraft.
Furthermore, the energy requirement for GTO injection can be much higher than that for a low circular orbit. Because of this phenomenon, launch vehicles such as Atlas V, Sea Launch and Ariane 5 are rarely used for low-orbit satellite launches.
There is a great deal to learn about launch operations and Launchspace offers the best training opportunities for space professionals to become knowledgeable in this field. Contact us to learn more.
Launch Pad at Space-Travel.com
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