It Really is Rocket Science (Cont.)

Two time periods come in to play during this selection process – launch period and launch window. Launch period is a broader time span, which refers to the days in the year during which it is possible to launch the payload into its proper orbit or trajectory. The launch window is a specific time period on the day of launch when a rocket can lift the satellite into its desired orbit. The time period can range from a few seconds to typically one or two hours for commercial geostationary satellites. Launch windows can change on a day-to-day basis throughout the launch period.

Khrunichev uses the launch windows provided by the SC contractor and designs a mission profile that meets all of the mission objectives. Proton does not require its own launch window but rather, mission designers will select a launch time within the specified window and perform the mission that meets all of the objectives.

In addition to meeting the mission performance requirement, the primary constraints on the Proton as the SC makes its journey from parking orbit to geostationary transfer orbit (GTO) are to ensure SC battery power is at an acceptable level by pointing the SC’s solar arrays toward the sun, and to maintain thermal control – the SC cannot get too hot or cold on either side. These restrictions are known as sun-angle constraints. In order to satisfy these requirements, the orbital unit (which consists of the satellite and Breeze M upper stage) must orient its attitude relative to the sun’s direction for a specific time period and then reorient itself again for the opposite side.

Once in the elliptical orbit of GTO, the satellite must perform a series of orbital maneuvers to position itself into Geostationary Orbit (GSO), the desired orbital slot in the equatorial plane – approximately 35,786 kilometers above the earth. These maneuvers, called “delta-V maneuvers” occur at apogee, when the satellite is at its furthest position from the earth, helping to circularize the orbit.

Final selection of the launch window is made by working backwards from these factors and the flight profile of the Proton. What starts with a series of calculations to determine the optimal time ends once the Breeze M delivers the satellite to orbit, completing another successful Proton launch.
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