Layout

Because of the absence of main engines, the layout of the aft part of Buran’s on-orbit propulsion system differed from that of the Shuttle Orbiter. The Shuttle’s OMS and aft RCS engines are concentrated in separate “OMS pods’’ on either side of the vertical stabilizer and are each divided into two compartments, one for the OMS and one for the aft RCS. Buran had a single pod (“Base Unit’’ or BB) for both DOM engines under the vertical stabilizer, with two Reaction Control System units (“Con­trol Engine Unit’’ or BDU) attached to either side of the aft fuselage. The left BDU (BDU-L) and right BDU (BDU-P) each had twelve primary thrusters and four verniers.

The pod housed one big LOX and one big sintin tank, two auxiliary sintin tanks (exclusively used for the Reaction Control System), and gaseous oxygen tanks (solely used for the primary thrusters). if needed, sintin could be transferred from the main tank to the auxiliary ones with a turbopump driven by gaseous oxygen. The helium tanks were immersed in the main LOX tank in order to save space and cool the gas.

The forward reaction control system unit (BDU-N), situated in the nose of the vehicle, had 14 primary thrusters. Unlike the Shuttle Orbiter’s forward RCS, it carried no verniers. Also installed in the BDU-N were one gaseous oxygen tank and one auxiliary sintin tank. The BDU-N was connected to the aft engine pod via several interconnect lines that allowed gaseous oxygen, sintin, and helium gas to be trans­ferred from aft to front. After the orbital phase of the mission was completed, any remaining sintin from the front auxiliary tank was transferred back to the aft main tank to satisfy center-of-gravity requirements for landing. Although the Shuttle

Orbiter has always had the capability of cross-feeding propellant between the two OMS pods, it cannot transfer propellant between the OMS pods and the forward RCS. Such an interconnect system was proposed as one of many Shuttle upgrades, but the idea was eventually shelved.