Adaptations for long missions

For long-duration flights or flights requiring extra propellant reserves, it was possible to mount additional tanks in the payload bay. There was room for an additional fuel tank in the front of the bay and for an additional oxidizer tank (big or small) in the aft. These would have been placed such that the vehicle’s center of gravity was not disturbed. The additional tanks could have increased Buran’s overall propellant load from 7.5 tons to 14 tons, allowing the vehicle to reach an altitude of up to 1,000 km. Plans for a comparable “OMS kit’’ in the Shuttle Orbiter’s payload bay were never implemented.

In order to counter evaporation of the cryogenic oxidizer, Buran’s ODU was filled with supercooled LOX at a temperature of —210°C (with LOX having a boiling point of approximately — 180°C). This, along with the use of several layers of thermal insulation and LOX-mixing techniques, was enough to prevent any significant boil-off for 15 to 20 days. On longer missions the LOX would have been maintained at proper temperatures by circulating cooled helium through the tank’s heat exchanger and also by installing a special cryocooler using the so-called reversed Stirling cycle.

The ODU had an elaborate fault detection and identification system, consisting among other things of about 100 sensors to measure pressures, temperatures, vibra­tions, etc. The engines could be shut down in a fraction of a second if a dangerous situation developed [21].