The 37KE was designated Kvant and was equipped with an astrophysics payload. It also used the Salyut-5B digital flight control computer and Gyrodyne flywheel orientation system developed for Almaz. As the module neared completion Salyut 7 experienced numerous technical problems and Kvant was retargeted for docking with Mir. But at that time Mir was planned to be in a 65-degree orbit, and Kvant was 800 kg too heavy for the Proton launch vehicle to place in such an orbit. In January 1985 Mir was changed to a 51.6-degree orbit, which solved one problem. But now it was planned that Kvant would dock with the rear port of Mir, requiring the addition of lines to conduct rocket propellant from the Progress tanker spacecraft to Mir's storage tanks. This increased weight again, forcing the FGB to have its propellant load reduced to 60% in the high-pressure tanks and empty low-pressure tanks. With a reported total launch weight varying between 20,600 and 22,797 kilograms (45,415 and 50,259 lb), Kvant-1 was supposedly at that time the heaviest payload lifted by Proton, requiring special custom modifications to its launch vehicle.