PROPULSION

Although Buran lacked main engines for ascent, it did have engines and thrusters for on-orbit maneuvers and attitude control functions. Buran’s propulsion system was known as the Combined Engine Installation (ODU or 17D11) and consisted of an integrated set of orbital maneuvering engines, primary thrusters, vernier thrusters, and associated plumbing.

While the overall number and general location of these engines were similar to those of the Space Shuttle Orbiter’s Orbital Maneuvering System (OMS) and Reac­tion Control System (RCS), there were some fundamental differences between the two vehicles, notably the types of propellant used. Orbital maneuvering and attitude control engines on manned spacececraft have traditionally used hypergolic propel­lants or hydrogen peroxide, which can be stored for long periods of time and do not require complex ignition and turbopump systems. The Space Shuttle Orbiter uses a hypergolic mix of nitrogen tetroxide and dimethyl hydrazine for both its OMS and RCS engines. Although Soviet designers also planned to use hypergolic propellants in their original orbiter concepts (OS-120 and OK-92), they eventually opted for a combination of liquid oxygen and a synthetic hydrocarbon fuel known as sintin. This marked the first time that such propellants were used in any type of orbital maneuvering and attitude control system. Next to the absence of main engines, this was probably the most significant difference between Buran and the Space Shuttle Orbiter.

Cryogenic propellants offered a number of advantages. They gave the orbital maneuvering engines a better performance than those of the Shuttle (although

thruster performance was virtually identical) and were safer to handle by ground personnel because of their non-toxicity. Moreover, the LOX could be cross-fed to the storage tanks of Buran’s electricity-generating fuel cells, providing extra redundancy to the power system and, indirectly, to the life support system, which drew oxygen and water from the fuel cell system. The drawbacks were that the plumbing was more complex, making the ODU 1,100 kg heavier than the Shuttle’s RCS/OMS system. Also, the mix did not ignite spontaneously on contact, such as was the case with hypergolic propellants, but required an electric ignition source. In addition to that, extra measures needed to be taken to prevent the cryogenic oxidizer from boiling off during long missions.

It is interesting to note that in the 1990s US Shuttle engineers considered a cryogenic OMS/RCS as a long-term Shuttle upgrade. This would have used a combination of LOX and ethanol and would have enabled the forward RCS, aft RCS, and OMS engines to draw propellant from common tanks, just as on Buran. It is not clear if this upgrade was in any way inspired by the design of Buran’s ODU.