Space Shuttle Liftoff

[dseagrav]

You might be right, I seem to be pretty gullible when it comes to sarcasm on this site.

If it's any consolation, I was certain your response was sarcasm, and reread the post a few times looking for the punch line...
(And it's really funny that everyone skipped over "killing more brown people for Jesus" and focused on crediting the French for ISS as the give-away... ^_^)

 

[mzaharis]

I like how after people post a reliable source for the reason (from the NASA website or so)

People will still post some opinion that dosn't fully agree with the reliable source. Guess the NASA guys aren;t as smart as us pilots eh?

To EagleRJ's credit, the antenna issue is probably a secondary issue for ascending "upside down", although if the configuration settled on favored going out "rightside up" dynamically, NASA probably would have worked out some method to communicate in this orientation.

As for his other comment, it referred to in-orbit orientation, and didn't contradict the Space Shuttle News reference. Of course, mission requirements sometimes necessitate an attitude other than upside down, but that's the default orientation in which the Shuttle flies, at least in part for the reasons he states.

 

[USMCmech]

Does anyone on here know why the Space Shuttle rolls on it's back during liftoff?

Idealy it wouldn't have to. On liftoff the shuttle needs to be faceing towards the ocean so that it will quickly begin to arc downrange rather than straight up.

However the shuttle launch pads are leftover from the Saturn rockets and there wasn't any way to manuver the shuttle in to it's ideal position. So right after liftoff it rolls 180 into it's proper attitude.

If it didn't roll, it would head West across Florida, the Gulf of Mexico, and have to land in DFW or IAH.

 

[EagleRJ]

Idealy it wouldn't have to. On liftoff the shuttle needs to be faceing towards the ocean so that it will quickly begin to arc downrange rather than straight up.

However the shuttle launch pads are leftover from the Saturn rockets and there wasn't any way to manuver the shuttle in to it's ideal position. So right after liftoff it rolls 180 into it's proper attitude.

If it didn't roll, it would head West across Florida, the Gulf of Mexico, and have to land in DFW or IAH.

It's true that the Shuttle quickly begins to pitch over so that most of its accelleration is lateral instead of vertical. The exact course on which it departs the pad is critical, since it sets the orbit's inclination, or the angle between the orbit and the equator. The inclination required for a trip to the ISS is different than that needed for satellite launches, radar mapping, etc.

Orbits are always Eastbound, since the Earth's rotation works with the launch vehicle instead of against it (think of throwing a ball while running- the ball will be going faster if it's thrown in the same direction you are running).
Rockets are never launched to the West- it would require too much fuel.

The Shuttle would be capable of just pitching over and accellerating after launch, without rolling. The heads-down configuration was decided on for the reasons already mentioned- lower aerodynamic loads, it makes the horizon visible to the crew, and it points the antennas toward the ground (although this requirement is probably no longer needed, since the voice/data link has been handed off to the TDRS satellite network shortly after launch since the late '90s).

 

[mzaharis]

Orbits are always Eastbound, since the Earth's rotation works with the launch vehicle instead of against it (think of throwing a ball while running- the ball will be going faster if it's thrown in the same direction you are running).
Rockets are never launched to the West- it would require too much fuel.

Nitpicky exception - polar orbit satellites are launched predominantly North or South, but with a slight Western component to cancel out the Earth's rotation.

But, as you imply, it's a definite performance hit. Interestingly, that's one of the reasons why Ariane claims an advantage over Kennedy-launched satellite launchers - French Guyana is closer to the equator, and picks up more velocity from the Earth's rotation.

 

[big_al]

why would the crew need to see land? they dont do anything manually, right?

 

[CutEmUp]

I think that the reason launching from the equator is more efficient is not because it gets "thrown" off of the earths surface faster. It is because there is less gravity at the equater.

Since the earth is a slightly squashed sphere, the center of the earth (center of mass) is furthest from the launch site when that sight is at the equater. And since the force of gravity varies inversley with the distance from the center of mass squared, there is less gravity at the equater, or on top of a mountain for that matter.

 

[mzaharis]

I think that the reason launching from the equator is more efficient is not because it gets "thrown" off of the earths surface faster. It is because there is less gravity at the equater.

Since the earth is a slightly squashed sphere, the center of the earth (center of mass) is furthest from the launch site when that sight is at the equater. And since the force of gravity varies inversley with the distance from the center of mass squared, there is less gravity at the equater, or on top of a mountain for that matter.

The Earth's surface is a gravitational equipotential surface. What that means is that the (mean sea level) surface of the earth is at a level where the gravitational force + centrifugal force is equal all over the surface. This is the reason why the Earth is squashed like an oblate spheroid. So, two locations on the earth, both at MSL, both experience the same perceived gravity. Otherwise, the water would "slosh" to the place with greater gravity. So, in effect, the water "self-levels" to match a gravitational equipotential surface. Most launch complexes are near-sea-level (because being next to the ocean gives you a good place to dup stages), so they are all at pretty much at the bottom of the same gravitational well (Baikanour, on the Kazakh steppes, being the possible exception, but I still think you won't see much benefit, as the Kinetic Energy you must impart to accelerate a spacecraft to 17,600 mph is MUCH greater than the energy saved by starting it out a few thousand feet out of Earth's gravitational well).

Read up on the WGS84 ellipsoid that GPS systems use as a reference surface for the Earth. It is an equipotential surface, more or less.

Now, consider the fact that, to orbit something, I must accelerate it to 17,600 mph. Getting the first 1,037 mph from the earth's rotation (its rotational speed at the equator) is no small matter.

EDIT - in the case of the earth's gravitational equipotential surface, it is usually referred to as a geopotential surface, or geoid. It is actually NOT a perfect ellipsoid, due to mass concentrations, but the sea level does correspond to it.

http://en.wikipedia.org/wiki/Geopotential
http://en.wikipedia.org/wiki/Geoid