Continuing Why does the Space Shuttle roll after take off..?

Question

Do you remember that recently I asked "Why does the Shuttle roll after take off?" Several people answered and some people said it was because they were still using the old Apollo launch pads facing the wrong way. One person said this was impossible since the old Saturn Vs did it too. Well...I'm still a bit unclear myself because of two reasons:

1/ I've heard a NASA astronaut say on the web on an audio file that the reason WAS that they are still using the old Apollo pads. Unfortunately I can't find the file again!

2 I've read the NASA shuttle manual online but I can't find where it gives a really specific reason for the roll...

So my conclusion is that it is something to do with the old pads...but I still don't understand the other reasons. Why would the pilot need a ground reference anyway since they are going straight up...and I don't see why the astronauts would be pushed out of their seats...

Can anyone add anything else? I wish I'd made a note of that sound file.

Answer 1

WHY DOES THE SHUTTLE ROLL JUST AFTER LIFTOFF?

The following answer and translation are provided by Ken Jenks ([e-mail]).

The "Ascent Guidance and Flight Control Training Manual," ASC G&C 2102, says:

"During the vertical rise phase, the launch pad attitude is
commanded until an I-loaded V(rel) sufficient to assure launch tower clearance is achieved. Then, the tilt maneuver (roll program) orients the vehicle to a heads down attitude required to generate a negative q-alpha, which in turn alleviates structural loading. Other advantages with this attitude are performance gain, decreased abort maneuver complexity, improved S-band look angles, and crew view of the horizon. The tilt maneuver is also required to start gaining downrange velocity to achieve the main engine cutoff (MECO) target in second stage."

This really is a good answer, but it's couched in NASA jargon. I'll try to interpret.

1) We wait until the Shuttle clears the tower before rolling.

2) Then, we roll the Shuttle around so that the angle of attack between the wind caused by passage through the atmosphere (the "relative wind") and the chord of the wings (the imaginary line between the leading edge and the trailing edge) is a slightly
negative angle ("a negative q-alpha").This causes a little bit of
"downward" force (toward the belly of the Orbiter, or the +Z
direction) and this force "alleviates structural loading." We have to be careful about those wings -- they're about the
most "delicate" part of the vehicle.

3)The new attitude (after the roll) also allows us to carry more
mass to orbit, or to achieve a higher orbit with the same mass, or to change the orbit to a higher or lower inclination than would be the case if we didn't roll ("performance gain").

4) The new attitude allows the crew to fly a less complicated
flight path if they had to execute one of the more dangerous abort
maneuvers, the Return To Launch Site ("decreased abort maneuver complexity").

5) The new attitude improves the ability for ground-based radio
antennae to have a good line-of-sight signal with the S-band radio antennae on the Orbiter ("improved S-band look angles").

6) The new attitude allows the crew to see the horizon, which is a helpful (but not mandatory) part of piloting any flying machine.

7) The new attitude orients the Shuttle so that the body is more nearly parallel with the ground, and the nose to the east (usually). This allows the thrust from the engines to add velocity in the correct direction to eventually achieve orbit. Remember: velocity is a vector quantity made of both speed and direction. The Shuttle has to have a large horizontal component to its velocity and a very small vertical component to attain orbit.

This all begs the question, "Why isn't the launch pad oriented to give this nice attitude to begin with? Why does the Shuttle need to roll to achieve that attitude?" The answer is that the pads were leftovers from the Apollo days. The Shuttle straddles two flame trenches -- one for the Solid Rocket Motor exhaust, one for the Space Shuttle Main Engine exhaust. (You can see the effects of this on any daytime launch. The SRM exhaust is dirty gray garbage, and the SSME exhaust is fluffy white steam. Watch for the difference between the "top" [Orbiter side] and the "bottom" [External Tank side] of the stack.) The access tower and other support and service structure are all oriented basically the same way they were for the Saturn V's. (A side note: the Saturn V's also had a roll program. Don't ask me why -- I'm a Shuttle guy.)

I checked with a buddy in Ascent Dynamics.He added that the "roll" maneuver" is really a maneuver in all three axes: roll, pitch and yaw.

The roll component of that maneuver is performed for the reasons stated.

The pitch component controls loading on the wings by keeping the angle of attack (q-alpha) within a tight tolerance.

The yaw component is used to determine the orbital inclination.

The total maneuver is really expressed as a "quaternion," a grad-level-math concept for combining all three rotation matrices in one four-element array.

Answer 2

There are three reasons.

First, the Shuttle reaches a point about one minute after launch when the pressure force of the atmosphere rushing past the rapidly accelerating rocket reaches a peak. The roll maneuver is performed shortly before max q is reached because this "heads-down" orientation helps alleviate the stresses that the dynamic pressure loads cause on the vehicle's structure.

The second factor we need to consider is that for each mission, the Shuttle must launch at a certain azimuth angle in order to be inserted into the correct orbital plane. Since the launch pad (and therefore the Shuttle) sits in a fixed position, the Shuttle must perform a roll maneuver during ascent in order to orient itself to achieve the desired launch azimuth angle. If it were possible to rotate the launch pad prior to launch, the pad could simply be rotated to accomodate the launch azimuth angle, and the Shuttle could launch into a heads-down orientation while gradually pitching over during ascent.

Finally, the Shuttle orbits such that its cargo bay faces towards the Earth. The heads-down position assists in communications with the ground and allows instruments within the cargo bay to be pointed back towards Earth, which is required for many of the experiments carried within the bay. There is probably also some psychological benefit to the crew since they are given spectacular views of home rather than staring into the cold darkness of the great void of space.

Answer 3

One of the big reasons is aerodynamics. When the shuttle rolls, the orbiter is placed heads-down, below the external tank. This protects it from most of the aerodynamic stress it encounters as it speeds through the atmosphere to orbit.

As a side note, the mobile launcher platforms (MLPs) used for the shuttle were indeed originally constructed for the Apollo program. The Saturn V rockets were launched in a similar manner to the shuttle. They rolled after launch, too.

Answer 4

Maybe instead of all these crazy theories about old launch pads and whatever, we could realize that it rolls to increase stability. Have you ever seen a missle lauched? It rolls too. Or a frisbee? It spins. It helps keep the shuttle stable. Like when you spin a wheel around and hold on to the axle, it's hard to turn it and get it off the center of mass. A slight roll helps keep things more stable while flying. (Yes, planes don't roll while flying, but they are using air pressure on the wings for lift. The shuttle just has a giant rocket for lift...these are two different ways of flying).