Launch windows to space stations

Sven Grahn

The figure above shows the northbound equator crossing longitude (NBECL) at the launch of a Soyuz ferry as a function of nodal period of the space station. This graph is based on launches to Salyut-1,3,5, and 6 using a one day rendezvous and docking phase. All launches to the Almaz stations fall into one distinct group, while many launches to Salyut-6 are nicely clumped together. The table below the graph summarizes the Almaz launches.

The NBECL of rev. 0 for a Soyuz is marked. All launches below this line means that the Soyuz is trailing the space station at launch. Interestingly, the very first Soyuz to dock with a space station, Soyuz-10 was launched so that it was ahead of the station. This meant that it approached Salyut-1 from in front and above. This was never done thereafter. Clearly, this was necessary, because the period of Salyut-1 at that point was so low (88.4 minutes), that it would have been diffcult to keep a lower period in order to catch up. The only alternative was to go higher and slower! Also, if one looks at the orbital period of Salyut-1 at the time of launch of Soyuz-10, one realizes that the Soyuz-10 flight was done in a great hurry. The station was decaying quickly and no maneuver had been performed to raise the orbit. that was performed just after Soyuz-10 had undocked! This drama is clearly seen in the graph below.

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