Fueling tests

It was not until 14 March 1985 that the 4M stack was once again erected on the UKSS for the long-awaited fueling tests of the core stage’s oxygen and hydrogen tanks. While the Russians had plenty of experience with loading liquid oxygen tanks, they were newcomers to fueling big hydrogen tanks, the more so because they used a special type of subcooled liquid hydrogen. After an initial series of tests in which the hydrogen tank was conditioned with nitrogen and hydrogen gas, the core stage was declared ready for the fueling tests. Between mid-April and late September 1985 the 4M core stage underwent nine fueling cycles. These included both partial and full loads of the hydrogen and oxygen tanks separately as well as one complete load of the entire core stage. After each test, engineers carefully checked the condition of the core stage’s outer insulation layer. While the insulation remained intact during and after fueling, some debonding was observed during draining of the tanks.

The 1985 pad tests were rounded out in the first days of October with several other tests of the core stage, including a nitrogen purge of the stage’s tail section and two test firings of the hydrogen igniters, needed to burn off any excess hydrogen gas accumulating on the launch pad prior to engine ignition. A derailing incident during the roll-back of the 4M stack on 5 October did not result in any damage to the vehicle [8].