WMartin@SIMTEL20.ARPA (04/14/84)
From: William G. Martin <WMartin@SIMTEL20.ARPA> The discussion in the news article regarding the use of fuses in Solar Max was completely mystifying. Why put fuses in a satellite's circuitry anyway? I could understand using circuit breakers which could be reset by solenoid under ground command, or auto-reset after some time elapsed, but why use FUSES? They whole concept of a fuse is that it is a cheap and easily replaceable unit whose self-destruction protects more valuable components in cases of failure or abnormal conditions. But when the| circuit is in orbit, and any repair is done by replacing the entire module, as we saw done in this case (not a repair technique that could be relied upon to be available, anyway, in the general case of satellite circuits!), fuses have no rational justification as far as I can determine. If they had been replaced by bus bars, or other conductive jumpers, and the active components had burned out due to the situation that caused the fuses to blow (itself doubtful, according to the article), so what? If the satellite could have been reached for fixing, the entire assembly/module would be replaced in any case. So the fuses protected nothing, and only caused problems. Can anyone offer any excuse for those fuses to have existed, or was this just poor design? Will -------
karn@mouton.UUCP (05/10/84)
Fuses are common practice in spacecraft. Normally, they are sized such that only a catastrophic failure of the associated subsystem will blow the fuse. This prevents a failure of a (possibly replicated) subsystem from dragging down the common power supply bus and possibly losing the entire spacecraft. Subsystems which could reasonably be expected to draw occasional overloads are indeed often protected instead with resettable (by command or by timer) "circuit breakers". But there are still many failure points in a spacecraft which are hard to protect against without total redundancy of each and every subsystem, which is often just not practical. You also have to weigh the (normally high) reliability of fuses against more complex (and hence more likely to be unreliable) auto-resetting circuit breakers. Phil