The roll program occurred during a shuttle launch for the following reasons:
(...)
Orienting the shuttle more parallel toward the ground with the nose to the east.
But why east?
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2$\begingroup$ Possible duplicate of How does the roll maneuver allow more mass to be lifted into orbit? $\endgroup$– SF.Commented Dec 4, 2018 at 13:07
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$\begingroup$ (long story short: utilizing the speed of rotation of Earth as a part of orbital velocity. This choice of direction sheds some 200-300m/s off the delta-V required to orbit.) $\endgroup$– SF.Commented Dec 4, 2018 at 13:09
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3$\begingroup$ I disgree with the duplicate topic votes. That topic has some relevant information, but doesn't explain why launches were aimed to the East. $\endgroup$– HobbesCommented Dec 4, 2018 at 13:53
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1 Answer
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The phrase 'with the nose to the east' is a shorthand. The direction depended on the orbital inclination chosen for the mission.
- Missions to the ISS have to match the 51.6° inclination of the ISS
- the possible inclinations are limited by the launch location: from Cape Canaveral, you have to launch over water, so in a direction roughly between North-East and South.
You usually want to launch to the East (as opposed to launching to the West) because of the Earth's rotation to the East. If you launch to the East, you get part of your orbital speed (about 300 m/s on the equator) 'for free'.
The Shuttle needed to perform a roll program to get to the launch inclination: the launch pad is at a fixed angle that usually doesn't match the launch inclination, so you need to change course (preferably just after launch when your speed is low and the energy cost of the maneuver is lowest).
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$\begingroup$ So, this is a bit more complicated. If this was all there was to the story, you could must have a yaw program and yaw to the east. In reality, the roll program orients the stack aerodynamically with the comm antennas (on the top of the orbiter) facing down -- a "heads down" ascent. Later in the program, there were plans to do a "heads up" ascent out of Vandenberg, but it never happened. $\endgroup$– ErikCommented Dec 11, 2018 at 3:06