HYDRAULICS

Buran’s hydraulic system provided hydraulic pressure for positioning actuators needed to move the aerodynamic surfaces (elevons, body flap, rudder/speed brake), deploy the landing gear, operate the main landing gear brakes, and conduct nose wheel steering. Three independent hydraulic circuits were available to provide the necessary redundancy, with one being enough to safely land Buran. A four-circuit system was considered (as it was for the Space Shuttle Orbiter), but rejected due to weight considerations. Each circuit had a hydraulic pump and reservoir, containing

a hydraulic fluid. The hydraulic system was designed to operate in temperatures ranging from —60°C to +175°C. In order to keep the system warm enough in orbit, the hydraulic fluid was circulated periodically by an electric-motor-driven circulation pump to absorb heat from heat exchangers in each hydraulic circuit. To prevent the system from overheating during re-entry, each circuit was equipped with a water spray boiler.

Whereas airplanes use their engines to power the hydraulic pumps, gliders such as the Shuttle and Buran need Auxiliary Power Units (APUs) to perform the same function. Just like the Shuttle, Buran had three Auxiliary Power Units (Russian acronym VSU) in the aft fuselage. The VSUs were developed and built by NPO Molniya. They were fueled by hydrazine, which was decomposed in a gas turbine to produce a hot gas that powered a turbine that in turn ran a hydraulic pump. Engineers looked at several fuel combinations (tsiklin + an oxide, ammonia + nitrous oxide, hydrogen peroxide + hydrazine), but in the end settled for a hydrazine mono­propellant system, as on the Orbiter. The Russians probably made this decision before NASA realized that hydrazine-fueled APUs were not the best of choices. Aside from being a toxic fluid that requires special handling provisions, hydrazine is also a highly flammable chemical. This became all too apparent on the STS-9 mission in 1983, when a hydrazine leak caused a potentially catastrophic fire in Columbia’s aft fuselage only minutes before landing. The replacement of a hydra­zine-fueled APU by an electric APU was high on NASA’s priority list of Shuttle upgrades before the 2003 Columbia accident.

Having a dry mass of 235 kg, each VSU consisted of a fuel unit, the power unit itself, and a system controller. The fuel unit and power unit were built as one integrated system, two located on the left inner wall of the aft fuselage and one on the right inner wall. The system controllers were installed in an equipment bay at the base of the aft fuselage.

The fuel unit contained a single tank with 180 kg of hydrazine and several gaseous nitrogen tanks. Nitrogen was stored in these tanks at a pressure of 32 megapascals (MPa) and first passed through a pressure regulator where the pressure was reduced to 3.5 MPa before it entered the fuel tank to push the hydrazine to the power unit. Each fuel unit was hermetically sealed to prevent any hydrazine leakage into the aft fuselage of Buran. To minimize the fire hazard, the enclosure was purged with nitrogen during re-entry beginning at an altitude of 30 km.

The main elements of the power unit itself were the gas generator, the turbine, and an oil tank. In the gas generator the hydrazine passed over a catalyst bed, which decomposed it into a hot gas that drove a single-stage turbine. While the gas was vented overboard via an exhaust duct, a double-reduction gear reduced the turbine speed from 55,000 rpm to 4,500 rpm before the mechanical drive was imparted to the hydraulic pump. Oil was circulated through the system to lubricate and cool the gear-box and the turbine bearings.

The main difference between the Shuttle’s APUs and Buran’s VSUs is that the latter used a pressure-fed system rather than a pump to deliver the hydrazine to the gas generator. While the pressure-fed system consumes a slightly larger amount of fuel, it is less prone to fires and other serious malfunctions. Also, in the Shuttle the

fuel tanks are in different locations than the power units, complicating the plumbing and increasing the fire hazard.

The VSUs were designed to operate continuously for a maximum of 75 minutes. Because of the absence of main engines on Buran, the VSUs were not needed for gimbaling the main engine nozzles as was the case for the Shuttle’s APUs. However, the VSUs were still started shortly before launch to enable the vehicle to make an emergency landing in certain abort scenarios. They were shut down about 200 seconds into the launch because at that point Buran had enough energy to reach orbit if one or more core stage engines failed. After an in-orbit check-out the VSUs were not reactivated until after the deorbit burn at an altitude of about 100 km. On Shuttle missions one of the three APUs is activated before the deorbit burn, with the other two following afterwards [12].