2017 Apr 30
Sunday, Day 120
zarya.info article at the Space Policy Online website
Launch Windows for Rendezvous Missions
China has flown nine missions using its Shenzhou piloted spacecraft and a clear picture has developed of what determines the time of day of any particular launch. With missions involving visits to the Tiangong orbital laboratory, it also becomes possible to determine, in advance, likely launch dates.
It's a case of doing the same calculations as Chinese mission planners.
By the time the Shenzhou 8 mission was completed, flights seemed to drop into two distinct clusters, those that landed a couple of hours either side of 10:00 UTC and another set that landed around 21:00 UTC. Shenzhou 9 seemed to throw things a little off balance by setting down in the early hours of the morning but it actually belongs to the second of the two groups.
There are two touchdown windows for Shenzhou.
One thing became clear, it is conditions around re-entry and touchdown that determine the dates for a flight. Once it is decided when a vehicle will be ready to fly, a time of day for launch will be chosen to ensure conditions are at their best for landing on the last day of the mission.
Shenzhou's operating orbit is such that the ground track passes over the landing site every two days but each time it is nearly 57 minutes earlier. For example, a mission lasting nine days would need to lift off 57 minutes later than a seven day one in order to bring it back into the standard landing zone at the same time of day at the end of the flight.
The Tiangong 1 Factor
Tiangong 1 added a complicating factor that made it easier for analysts to work out when launches might occur. Before the need for on-orbit rendezvous, a flight could take off on any day of the year at a time determined only by the mission duration and the need to meet the touchdown requirements. A spacecraft in orbit complicates matters in that there is only one time on any particular day when a rendezvous launch can occur because the visiting spacecraft needs to get into the same orbit plane.
For rendezvous, choice of a launch date is constrained by the Tiangong 1 orbit. There are recurring periods of 4-5 days when a vehicle returning from Tiangong 1 can re-enter and land under the right conditions. The launch date for a mission is then the appropriate number of days earlier. Mid December 2012 the ideal landing time in one of the two window types occurs at 11:12 UTC. For a mission to fly for 13 days, launch would have to be at 15:09 UTC. In order to rendezvous with Tiangong 1 and launch at that time, a Shenzhou would have to lift off on December 15 or 16. For a 21 day mission it would be December 8.
The Two Groupings
Shenzhou landings are set to occur about one day after orbit precession aligns the Sun with the orbit plane, In a simple analogy, an observer on the surface of the Sun would see the orbit edge-on, as a straight line, and Shenzhou would look as though it was simply moving forward and back along the line.
There are two configurations that allow this. First, where Shenzhou is heading south to north as it orbits the Earth (Ascending) and the other when it is heading north to south (Descending). It is illustrated in these two diagrams showing segments of the ground tracks of Shenzhou 6 and 7 on their final day in orbit. In each case the sub-solar point is shown. Shenzhou 7 is an example of the sub-solar alignment occuring on the ascending ground track. Shenzhou 6's track is descending.
The two sets of Shenzhou touchdowns are created by the two types of solar alignment. The Ascending (Type 1) alignment produces the grouping around 09:00 UTC, and the Descending (Type 2) version sets Shenzhou down around 21:00 UTC.
Throughout the year, the sub-solar latitude varies between 23°.5 South and 23°.5 North. The effect of this is to make the actual time of day of landing vary with the date as illustrated here. The time of landing on the horizontal axis is plotted against the astronomical Declination of the Sun, equivalent to geographical latitude. The really helpful data point was Shenzhou 9 as the only launch not to occur in the winter half of the year.
The data relating landing time to Declination/latitude show that there is a slight spread either side of a distinct line so there is obviously some leeway in the relationship between ground track and sub-solar point. It equates to margins of approximately ±1 hour.
On this plot, derived from the one above, the red lines cover Type 1 (ascending) windows and the blue lines are for Type 2 (descending). Without Shenzhou 9's mid-year mission, there would be much more uncertainty.
Use for Forecasting
Because the windows for Shenzhou are controlled by constraints on the landing, determination in advance of launch dates is possible only if the intended mission duration is known. As yet, there is no obvious aspect of the first nine missions that points to an absolute means of working out the launch date.
There is an example analyis here looking at potential dates for the Shenzhou 10 launch.
Page date: 2012 Jul 19
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