Why do rockets tilt after launch?

Answer 1

Because its wings are tired. But seriously; Keep reading you will get the laymans answer as well. The following answer and translation are provided by Ken Jenks

(kjenks@gothamcity.jsc.NASA.gov).

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.)