What is "soyuz-u"

The Soyuz-U launch vehicle is an improved version of the original Soyuz rocket. Soyuz-U is part of the R-7 family of rockets based on the R-7 Semyorka missile. Members of this rocket family were designed by the TsSKB design bureau and constructed at the Progress Factory in Samara, Russia (now a united company, TsSKB-Progress). The first Soyuz-U flight took place on 18 May 1973, carrying as its payload Kosmos 559, a Zenit military surveillance satellite. the rocket is still in service, although production has stopped since April 2015 as part of the transition process to Soyuz-2. The last flight is expected to take place on February 1, 2017, carrying Progress MS-05 to the International Space Station.

Soyuz-U has been in use continuously for 43 years, the longest lifetime of an orbital rocket worldwide. Production of R-7 derived launch vehicles peaked in the late 1970s-early 1980s at 55-60 a year. Soyuz-U holds the world record of highest launch rate in a year in 1979 with 47 flights. Over its operational lifetime, the Soyuz-U variant flew a total of 784 missions, another world record.