On Wed, 10 Aug 2005 00:40:55 -0700, Reunite Gondwanaland (Mary Shafer)
wrote:

On Wed, 10 Aug 2005 16:25:28 +1000, "DarkD"
wrote:

They don't bank just one way but both ways several times on all
landings. Each time almost the exact same force is applied. The banks
get bigger as the speed gets lower. So no.

The energy management maneuver is an S turn, so the Orbiter first rolls
one direction then back through wings-level to the same angle in the
other direction and then back to wings-level in each maneuver.

I think they alternate the direction of the starting rolls, but that's
just a vaguely remembered bit of trivia and could well be entirely
wrong.




If you look at the controls used during entry, Pitch and Yaw are fixed.
Pitch is used to adjust angle of attack to 40 degrees, and Yaw to adjust
the angle of side slip to zero. So, these are fixed due to the design of
the heat shield.

The goal of entry is to land at a runway with a given location, using only
one parameter, the Roll. The magnitude of the Roll angle is related to the
future altitude of the vehicle. Altitude to drag or in terms of the final
goal, uprange and downrange. Or, what NASA calls energy management. It is
used to bleed off energy in the right way so that the vehicle doesn't
overshoot or under shoot the runway. Well in reality the an entry point in
to Terminal Area Energy Management (TAEM).

Well what about crossrange?

Since altitude, drag and energy are only related to the magnitude of the
Roll command, that leave the sign of the Roll command free to be used to
control left and right crossrange. A binary left - right switch, that is
flipped several times during entry so that the vehicle ends up with just
the right amount of crossrange on entry into TAEM.

The more crossrange that is required, the more the binary switch (sign of
Roll) will be either positive or negative. The maximum crossrange of the
Shuttle would be attained when the binary switch doesn't switch at all,
and is either positive or negative the entire time of entry. But in
reality, this maximum crossrange will never be used, as some crossrange
must be held in reserve to cover atmospheric uncertainties. So there will
always be some turns.

So, on entry, Shuttle is always turning to the left or right. It flys the
magnitude of maximum crossrange every time. But it changes the sign of the
roll and therefore the direction of this crossrange. The initial sign of
the roll is related to direction of crossrange to the runway at the
beginning of entry. The Shuttle then uses S-Turns to fly back and forth
across the runway (in terms of crossrange). It's not random.

--
Craig Fink
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