EHARNDEN@AUVM.BITNET (Eric Harnden) (02/13/89)
In response to the query as to why a light bulb, a seemingly
inconsequential device, should 'bring down' half of a satellite system...
The aforementioned light bulb was the illuminator for the mechanical
shaft encoder, a graduated cylinder whose outer markings are read by an
optical device. The rotational position of the encoder indicated to the
craft's controls the current attitude of it's sensors. It turns out that
bulbs of the type used are subject to crystallization of the filament,
which of course breaks under any appreciable shear force. Far from being
a lousy design, however, it must be remembered that the system actually
operated some months (nine? I think?) beyond it's five year design spec.
Anyway, when that bulb goes, the spacecraft is for all intents and purposes
finished, because even if the rest of the systems are still operational, you
can't point the sensors.
Eric Harnden (Ronin)