Trust No One® wrote:

Before anyone flames me I'm not asking WHY the shuttle roll is performed
but rather HOW the manoeuvre is executed.

Been through the FAQ for the newsgroup, Google searches and a couple of
books and while the WHY is explained, there is precious little on how it
is actually done.

I did find an reference in the book "Voyage into History" by James
Harwood to the effect:

"The shuttle's computers moved the booster nozzles in opposite directions
to roll Challenger about its vertical axis so as the ship climbed toward
space, the shuttle would be positioned below the external tank with the
crew upside down relative to Earth and the shuttle's wings level in
relation to the horizon"

Is the author referring to the SRB nozzles?

Would very much appreciate if the one of the many experts on the newsgroup
could take time out to explain the HOW of the manoeuvre or point me to a
reference.

One of those questions I've always wanted to ask in my months of lurking,
but have only now plucked up the courage. Bit of a break from the "Maxson
wars" LOL

Thanks & Best Wishes



Hi Trust No One,

Shuttle first stage guidance is basically a table lookup. A table of roll,
pitch and yaw vs velocity. When the Shuttle lifts off from the launch pad,
it hold that attitude, until it crosses the next velocity point in the
table. The values of roll, pitch and yaw at the tower clear velocity is the
attitude that the Shuttle needs to be at the end of the Single Axis
Rotation (SAR). This attitude is very different from it's current attitude
that it has been at since liftoff. It's like having a huge attitude error,
so flight control computes the necessary rotation that the vehicle needs to
perform to get from it's post SAR attitude that guidance commanded. Flight
control also limits the rate of change of the attitude during first stage,
so the vehicle rolls, pitches and yaws at it's maximum rates to get to the
new guidance commanded attitude.

There really is no "Roll Program" in the flight software other that the
vehicle commands having a step change in the vehicle's attitude at tower
clear. And, flight control just figures out how to move the engines to
roll, pitch and yaw the vehicle to this new attitude.

I hope this helps,

Craig Fink