Figure 133.- Block diagram of PAU.


In all harnesses, the connectors were potted, spare connector locations were not wired, redundant wiring did not in general exist, and 22-gage Teflon insulated wire was used and shielded only when necessary. The electrical system ground was attached to the structure via a connection to the cable trough. All pyro functions (fig. 131) for V S/C A separation were on the Centaur side of the in-flight separation interface and all VLCA/aft BS separation pyro functions were on the orbiter side of the interface. This avoided pyro firing currents going through the separation connectors. Performance margins and component derating were applied to wires and connectors to allow for the effects of the thermal and vacuum environments, such as heating by the Sun, cooling in a shadow, and wire bundle flexing during flight. Electrical cross-coupling of cabling circuits was controlled within the limits required for reliable system operation by providing isolation of incompatible circuits, by using physical separation, by twisting, and by shielding. Power was distributed to the VO subsystems and to the VLC during cruise as required. Power cables were designed and fabricated to minimize electrostatic and electromagnetic coupling with other circuits and to minimize power losses. Cabling utilized for electroexplosive device firing circuits excluded undesired electrical energy by the use of complete and continuous shielding from the pyrotechnic control unit to the device housing, including exclusive use of approved type connectors.

Physical Characteristics and Restraints

Consideration was given to the following physical factors affecting the design: insulation properties; mechanical strength; protection from heat; dissipation of heat; and accessibility during construction, rework, adjustment, and test. The cable harnesses were designed to withstand the environmental stresses encountered on Earth during testing, checkout, and launch, as well as those environments encountered in space. The effects of the space environment upon all components and materials used in the cabling subsystem were evaluated for compliance with design criteria. Wiring across articulated interfaces was given special design consideration to insure reliable operation over the required range of motion under all anticipated environments. Cables were protectively jacketed in regions of possible abrasion or stress concentration. Every effort was made to select connectors sufficiently dissimilar to prevent incorrect coupling to other connectors in the vicinity. Electronic subassembly receptacles, test receptacles, and in-flight separation receptacles of the flight orbiter employed sockets (not pins) which were recessed within the connector insulating insert. Subsystems did not share connectors. Connector shells were conduc-tively mounted to the associated mechanical element (subchassis, bracket, etc.). The connectors at the interfaces between the VO and V S/C A and between the VO and VLCA were capable of separation in flight. The connectors at the interfaces between V S/C A and Centaur and between the VLC and aft bioshield were capable of being connected and disconnected under field conditions. Direct access test circuits between the VO and the support equipment were carried by the orbiter cabling to test receptacles mounted on the lower brackets of electronic assemblies as required to be consistent with orbiter design criteria. Twisted groups of wires utilized adjacent connector contacts. Wires were also grouped within a connector to separate signals and power of differing characteristics as much as possible to minimize cross-coupling. Splicing was kept to a minimum; however, splices were allowed to save weight.

Propulsion Subsystem

Purpose and Function

The propulsion subsystem was a modular element designed to deliver over 3.87 MN-sec (870 000 lb-sec) of propulsive impulse to the V S/C. The basic purpose of the PROPS was to produce, upon command, a directed impulse to accomplish ITC, an MOT maneuver, and up to 20 MOT maneuvers. Proven concepts and hardware were integrated into the basic design to yield the highest possible reliability. The subsystem design requirements were formulated from V S/C physical and operational constraints, LV characteristics, ground and in-flight conditions, and PROPS characteristics. In summary, the PROPS was capable of providing a total velocity change of 1480 m/sec to a 3430-kg (7556 lb) spacecraft with a usable propellant mass of 1387 kg (3055 lb); providing, in a single burn, an MOI velocity increment ranging from 900 to 1325 m/sec; performing up to 4 ITC's, 1 MOI, and 20 MOT's; performing a firing within 32 hr after the preceding firing; providing a minimum impulse capability of 534 N-sec (120 lb-sec); performing an engine firing within 240 sec after termination of the last attitude orientation maneuver; producing thrust with very small swirl disturbance, that is, less than 45.2 N-cm (4.0 in-lb) of roll torque about the roll axis; and functioning without degradation in performance for 510 days after launch in a vacuum and gravity-free environment within a temperature range of -1° to 32° C (30° to 90° F) .

PROPS was a fixed thrust, multistart, pressure-fed, Earth-storable bipro-pellant system utilizing the propellants nitrogen tetroxide (N204) and mono-methy1hydrazine (CH3NHNH2) with helium for pressurization. A two-axis, gim-balled engine and electromechanical actuators provided thrust vector control in the pitch and yaw directions during engine operation. The subsystem with its structure was a mechanically defined module with eight subassemblies and is depicted in figure 134. These eight subassemblies were functionally and physically independent and extended the modular design concept into the subsystem. These subassemblies are identified in figure 134 as PTA, PCA, PTA's - fuel and oxidizer, PIA's - fuel and oxidizer, REA, and GSA. Two identical propellant tank assemblies and two identical propellant isolation assemblies were integrated into the subsystem to store and control the flow of propellants - oxidizer and fuel. Subassemblies were fabricated and tested prior to integration into a propulsion module. Brazed tubing fittings were utilized in nearly every joint to minimize leakage. Where these fittings could not be used, metallic


PTA - (lx
0 0

Post a comment