Dave-Platt%LADC@CISL-SERVICE-MULTICS.ARPA (Dave Platt) (02/07/86)
That would depend upon the design of the mechanism. If the hydraulic force/fight function is performed within one monolithic block (with, for example, one source of hydraulic pressure, and internal pressure arbitrarion), then you're quite correct... it would probably constitute a single point-of-failure for that portion of the shuttle system. It's quite possible, however, to design such a mechanism so that it is not a single POF. One sample design: each computer sends its signals to a completely separate hydraulic controller (independent pressure supplies, etc.). The outputs of the four controllers are combined only through the actuator itself (e.g., each one pushes the actuator, and if one unit fails then the other three will push it through the actuator, rather than directly). So... the single POF in this situation would be the actuator... which is a big, physical hunk of matter. If its hinges or couplings were to freeze up, then the actuator would be a single POF for that engine (I suppose), but then the computers would automatically swivel the actuators on the other engines to make up for any off-symmetric thrust due to the frozen actuator. I believe that the shuttle does have multiple source of hydraulic pressure.