(MSu1049321) wrote in
:

Now wait a sec, I think I read somewhere that while gear deployment is
done by a manual switch, there IS a tie-in to the computer or to a
radar altimeter such that in the last critical seconds, a pyro will
BLOW the doors open and the gear down and locked, if the regular
system fails.

You may well have read it somewhere, but this is what NASA says about how
the system works - no links to computers or altimeters, just a simple time-
activated backup to the crew pressing the button:

http://spaceflight.nasa.gov/shuttle/.../lgear/overvie
w.html

For deployment of the landing gear, the uplock hook for each gear is
activated by the flight crew initiating a gear-down command. The uplock
hook is hydraulically unlocked by hydraulic system 1 pressure applied to
release it from the roller on the strut to allow the gear, assisted by
springs and hydraulic actuators, to rotate down and aft. Mechanical linkage
released by each gear actuates the respective doors to the open position.
The landing gear reach the full-down and extended position within 10
seconds and are locked in the down position by spring-loaded downlock
bungees. If hydraulic system 1 pressure is not available to release the
uplock hook, a pyrotechnic initiator at each landing gear uplock hook
automatically releases the uplock hook on each gear one second after the
flight crew has commanded gear down.
--
JRF

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