"John" wrote:
WOW, I have neever seen (or perhaps noticed) the overshoot during roll
program. Does anyone know if this is common or why it happens?
Although I never gave it a lot of thought and given the precision of
guidance needed to hit the desired orbital targets, I would have
thought the guidance would be solid enough to nail the desired amount
of roll without overshooting. Is guidance in a"brute force" mode
during the early part of ascent?

Yes - it's far more concerned with managing aerodynamic loads than
hitting it's orbital parameters at that point. Roll control, while
important for managing those loads - is the least important term for
hitting your orbital target. Those parameters are *EXTREMELY*
sensitive to pitch and yaw - and virtually not at all to roll.[1]

Also, the PAO noted after SRB sep that the guidance was converging.
What process is taking place?

As I said above, during the early part of the ascent - the emphasis is
on managing aerodynamic loads on the airframe.[2] This almost
certainly deviates from the 'ideal' trajectory for reaching a desired
orbital target. Generally you want to ascend more-or-less straight up
to get out of the atmosphere (and minimize drag) during the early
portion of the launch - but obtaining orbital velocity actually means
you need to thrust more-or-less parallel to the earths surface.

Thus, once above the sensible atmosphere, you can start steering your
velocity vector[3] from where your aerodynamic steering profile left
it, towards the 'ideal' trajectory - this process is called
convergence.

[1] Though roll may effect your ability to steer in those planes - it
doesn't actually appear in the idealized ballistic equations.

[2] Even traditional (I.E. cylindrical) rockets have to worry about
this to some degree. You can here the same call during Apollo
liftoffs for example.

[3] An important concept to note: What you are managing is the
spacecrafts velocity in various axes - not the spacecrafts actual
position. The neat little diagrams on the wall of the MCC and in
various books, etc... lead you to incorrectly believe the opposite.

D.
--
Touch-twice life. Eat. Drink. Laugh.

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL