unto the very living rock upon which we now stand:
Here's a link to the 'Countdown 101' pages at NASA:
In a nutshell, the two tests are basically the same except on
different parts of the orbiter. The tests verify the range of motion
for the respective elements (rudder and flaps for the areo surfaces,
the three main engines on the latter).
NASA TV usually shows these test live when they are performed (at the
T-3 min 55 sec mark). Usually you see the rudder move left, then
centered, then right, then centered. Then the 3 main engines move
from centered to up, down left, down right, then centered again.
The very next thing checked (though they never show this for some
reason) is to verify the 'DOOR AJAR' and 'CHECK ENGINE' lights are
both off. (see http://www.hal-pc.org/~jsb/countdown.html for this and
other humorous items).
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.