China's Piloted Space Programme
2017 Apr 30
Sunday, Day 120
Tiangong 1 and Shenzhou 9
A thorough review of the Shenzhou 8 flight was undertaken prior to making the decision that Shenzhou 9 was to be piloted. The major mission goal was rendezvous and docking, testing of the hatches and transfer tunnel and gaining experience for when China's future space station is lanched.
Evolution of the Launch Date
Here is a record of how Tiangong 1 was steered back to the Shenzhou operating height. For any particular date, there is a forecast that is based on the decay rate remaining at the current level. The windows shown are for launch into a 14-day mission. The ones drawn in blue and marked "AM" lead to a landing in the morning (around 10:15) UTC. The ones in green ("PM") lead to landing in the evening (around 21:30) UTC.
In order to understand the plot, it is necessary to look at it in terms of the average position of the points plotted over about 3-4 weeks and take into account the trend in the space weather index.
The right hand line shows the level of solar radiation at 10.7 centimetres wavelength as provided by NOAA. Although there is not a continuous connection between the shapes of the two curves, noticeable peaks and troughs in the weather plot are echoed 3-4 days later by the Tiangong forecast date. In order to see the relationship between Solar Flux and drag, the two date axes have been shifted relatively to take account of the lag between a change in Flux and the resulting effect in air density at orbital altitude.
Drag and the ensuing decay rate are also influenced by the cross-sectional area that Tiangong 1 presents to its direction of travel. On at least two occasions, Tiangong's orientation was changed to counteract short-term variations in upper air density.
The plot stops at June 7, the point at which the priority was no longer to 'steer' Tiangong 1 into the launch window, pre-rendezvous manoeuvres had commenced.
2011 Nov 26 - Tiangong 1 had settled down to a steady rate of decay in its new, high, orbit established after Shenzhou 8 departed. It looked as though it would be back to a suitable height for further Shenzhou operations (330 km, 91.15 min period) during April 2012. Assuming that controllers planned to minimise propellant use, it was reasonable to take it as an indication of when Shenzhou 9 was planned to be ready for launch. It also fitted China's statement that both Shenzhou 9 and Shenzhou 10 would be orbited during 2012.
2011 Dec 15 - Chinese news agencies announced that a change had been made in Tiangong 1's stabilised attitude. It resulted in a reduced rate of decay that meant it would be an extra four weeks or so before it got down to the Shenzhou operating height. The change reduced the cross-section of Tiangong 1 as presented to its direction of motion, using it as a means of controlling the rate of orbital decay. It meant that it would be the mid-late May at the earliest before Tiangong 1 got back down to the Shenzhou operating height. December may be when China set a planning date for the Shenzhou 9 mission.
2012 Jan 15 - What appeared to be a small orientation change resulted in decreased drag, sending Tiangong 1 towards a rendezvous window late June.
This would make Shenzhou 9 the first to fly during the summer. The previous eight flights were all in the autumn/winter half of the year, and this one was originally heading for launch during that period too.
2012 Feb 15 - China announced that the launch is planned for June and will carry a biological payload including test animals. Another orientation change by Tiangong 1 shifted the focus towards a late-May launch with a mission extending into June.
2012 Feb 16 - Xinhua, China's state-run news agency announced that Shenzhou 9 will be launched 2012 between June and August. It will carry a crew of three that will undertake a manually-controlled docking with Tiangong 1, open the hatches and go inside it. At the same time, mention of Shenzhou 10 being a mission for launch during 2012 disappeared both from press releases and a published commentary on launch vehicle usage plans for 2012.
2012 Mar 23 - A decision seems to have been made to move the Shenzhou 9 launch date from May to July. The orbit was raised, meaning that Tiangong 1 would now have to make a significant propellant-wasting downward orbital adjustment in order to meet a Shenzhou launch in the earlier window.
2012 May 12 - What was probably another drag-influencing orientation change set up Tiangong 1 for a June-July mission where a small orbit change could be used to steer launch date into either of the available windows around that time.
2012 May 26 - Reducing apogee by 10 kilometres set Tiangong 1 up to meet a launch in the window that opens around Jun 17. A further orbit 'tweak' May 29 indicated that Tiangong 1 had entered a phase where manoeuvres were being directed to setting up precise orbital heights and ground track location for the Shenzhou 9 mission. There was a distinct change in drag, suggesting that Tiangong 1 was now aligned horizontally with respect to the Earth and that its long axis is pointing in the direction of travel.
2012 Jun 16 - LAUNCH!
Tiangong 1's orbit adjustment of May 26 had the effect of causing its orbit to settle down close to the operational one established by previous Shenzhou missions. The standard orbit employed by Shenzhou has a period of 91.2 minutes. Plotting the ground tracks across the Earth's surface reveals that they repeat themselves every two days.
One way of describing where an orbit lies with respect to the Earth's surface is to look at the longitude at which it crosses the Earth's equator. In Tiangong 1's case, the longitude associated with the launch of a Shenzhou that is going to chase it down is a good place to start. There is only one precedent - Shenzhou 8.
Shortly before Shenzhou 8 was launched, Tiangong 1 crossed the equator one degree east of the Greenwich meridian. The next day, the equivalent crossing was about ten degrees west longitude. Twenty four hours later it was one degree east again in line with the two day repeating pattern.
Tiangong 1's May 26 manoeuvre set up a similar pattern with the one degree east equator crossing occurring on even dates during June. Shenzhou 9's move to the launch pad June 9, combined with a historical wait of at least five days to launch tells us that lift off will occur June 14 at the earliest, or any date at two day intervals after that, ie - June 16, June 18, June 20, etc.
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