November-December 1981:Soyuz landing times

The use of Soyuz as a man-carrying space vehicle began in April 1967 with Vladimir Komarov's one day flight. To date, around forty crews have been carried to or from orbit by various versions of the craft, and more than twenty flights have taken place under various parts of its development programme. The large number of manned missions has provided a wealth of statistics for the outside observer which, taken together with information provided directly by the Soviet Union, allows a fairly close study of the application of basic mission rules.

There has always been a tendency for Soyuz landings to occur in the second quarter of the day (GMT), a fact which has been noticeable to observers from a very early date. It was taken as an indication of the Soviets' desire to ensure daylight landings, with a preference for local afternoon over the morning in the landing area. Other than this type of casual observation, no definitive explanation of how mission planners chose recovery and launch dates was available until 1973,when technical details of the Apollo-Soyuz Test Project flight profile began to become available.

One document in particular contained details of the basic conditions which govern the choice of Soytiz missions' launch dates and times. Retrospective analysis of successful manned Soyuz flights reveals that the conditions have been applied fairly rigidly to flights over the whole period that Soyuz has been operating, but starting after Soyuz 1.

Two constraints are applied to the conditions under which a Soyuz is to return from orbit. They must be obeyed on the day of recovery for one of the orbital passes over the recovery area. The constraints are:

“1. The landing must take place at least one hour before actual sunset in the landing area.

2.. Prior to the firing of the retro-rocket, the requirements of the manual orientation system must be fulfilled, i.e. the time from crossing the Earth terminator to the retro-fire must be at least eight minutes" (sic).

Normally, these requirements are required to apply to the first pass over the landing area on the chosen landing day and on the following (reserve) day. However, in the case of ASTP, the Apollo and Soyuz safety conditions were mutually exclusive, so the Soviet side agreed for that mission only to change the application of the safety constraints to the third orbit over the landing area.

Soyuz Landings

Owing to the rotation of the Earth beneath a satellite orbit,the ground tracks of successive orbits move progressively westwards; in the case of Soyuz by about 23 degrees each pass. The majority of Soyuz landing locations have clustered around a point near to 50 degrees north, 70 degrees east which is near to the northern apex of the 51.6 degree inclination Soyuz orbit. Consequently, it is possible for a Soyuz to set down in that area from any particular orbital pass which has a northbound equator crossing between 20 degrees east longitude and 48 degrees west, over a period of four and a half hours as the Earth rotates. The first pass over the area, as described in the landing constraints, would be a 20 degrees east equator crossing. The third pass, as defined for ASTP, would be an equator crossing around 25 degrees west longitude. To date. Soyuz 11 holds the record for the furthest west reentry track, setting down in the normal area from an equator crossing at 42 degrees west.

Interpretation of the Constraints

The effect of the second constraint on a Soyuz landing time depends on the height from which a Soyuz is returning to Earth. The lower the orbit, the shorter the period between retro-fire and landing, hence the further north along the final ground track is the point of retro-fire. In the case of a Soyuz returning from around 350 km (e.g. from Salyut 6) retro-fire occurs near to latitude 43 degrees south. The early test flights of Soyuz (e.g. Soyuz 4, 5. 6 etc.) orbited at altitudes just above 220 km and retro-fire took place near to latitude 28 degrees south.

Although the constraints are applied to the first possible landing orbit each day. there is not a strict requirement for the Soyuz actually to return to Earth on such an orbit. Of the two.the first constraint is flexible; for example, although Soyuz 28 would have landed just one hour before sunset from a 20 degrees east ground track, it actually landed in darkness after crossing the equator at 5 degrees west, one orbit later. Because of the Earth's rotation from west to east, it follows for the Soyuz orbit that as long as both constraints are fulfilled on the first orbit of each day. the second constraint will be obeyed on all possible landing orbits on that day, even though the actual landing might take place in darkness. The constraints are designed to ensure that in the even! of a manually navigated reentry, the crew have time to make necessary sightings and computations, and the recovery crews on the ground have time to locate the returning craft visually before the sun sets.

The Soyuz orbit completes one rotation with respect to the Earth-Sun line every eight weeks, and during that period the constraints are obeyed, on average, for about one week. Consequently, it is possible to look critically at the Soyuz flights which are known not to have completed their planned flights, and project them forward into the landing “window". The results of doing this are in reasonable agreement with those derived empirically by Phillip Clark. There is only Soyuz 1, of the 51.6 degrees inclination flights,for which it is not possible to do this and arrive at an acceptable answer. The location of Soyuz 1’s orbit plane may have been chosen to maximise the power available from the solar panels,while ensuring a landing around dawn, local time. One consequence of such a choice would be that the whole recovery sequence would have to take place in total darkness without a visible horizon. The outcome of enquiries into the Soyuz 1 accident probably led to the introduction of the two constraints. Evidence for this conclusion lies in the fact that the next manned flight,Soyuz 3,was the first to obey them.

Salyut 6

Four of the flights returning from Salyut 6 during 1979 and 1980 obeyed the retro-fire safety requirement but fell outside the first constraint. They were Soyuz 33 (projected forward to an eight day flight), Soyuz 35, Soyuz 36 and Soyuz 38. The reasons for these are obviously open to debate. That for Soyuz 35 was certainly the immediately following flight of the new Soyuz T-2. Soyuz 38's early landing was probably dictated by the need to fly Progress 11 up to Salyut and to follow it with the recovery of the long stay crew (Popov and Ryumin) in the October 1980 “window". The thinking behind the other two remains obscure at present, though there is no suggestion that they returned early as a result of their missions being curtailed. Both were carrying International crews back from a standard eight day flight.

One feature of the Salyut 6 missions has been the occasional transfer of a Soyuz from the rear docking unit to the forward one in order to allow a Progress supply craft to dock and transfer fuel (only the rear unit on Salyut 6 was equipped with the necessary fuel pipe connections). All such transfers have taken place during periods when the landing "window" was open, apparently to ensure that should the Soyuz be unable to redock it would be able to navigate its reentry under ideal conditions.

Salyut 6 was originally intended to operate for eighteen months from its launch in September 1977. taking it up to the end.of the first quarter of 1974. All missions since then have been a bonus in that its continued operation saved the launch costs of its successor. Starting with the February 1979 launch of Soyuz 32, all subsequent main crew launches have been timed so that they actually went into orbit during a landing “window", again ensuring that a shortened mission due to docking problems or inability to enter the station would be able to stage a well-managed recovery.