Why are shuttle launches in the evening?

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clinn

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With all the shuttle launch delays due to thunderstorms, I am wondering - is there some special reason beyond space tourism that NASA schedules many launches in the evening? It seems that in an area such as Florida in the summer this is the WORST time from a thunderstorm POV, as this is a typically late afternoon / early evening occurrence. Furthermore, I would think that the light would be better from a safety / imaging standpoint as well. For the past several years, 9 out of 12 launches have been between 2 pm and 9 pm, with only 3 outside this window. I am guessing that the probability of thunderstorms almost centers around this same window, e.g. around 5 pm.

Why don't launches happen in the late morning? I am not a Florida resident, but this seems to be a period of less convective activity, etc.
 
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drwayne

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Launch window for rendevous with ISS drives the time.

Wayne
 
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clinn

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Great article, explains enough for me. I knew there would be some dynamics there, but never appreciated how narrow they are. I had expected such alignments to look rather randomly distributed in Earth time, so was suspicious of the (what seemed to me) strong bias into a 7 hour period. Curious.

Thanks again for the responses!

-Chuck
 
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neilsox

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Those are interesting reasons for a tight launch window, but does not explain evening. Perhaps the visibility is better close to sun set. In Jacksonville, Florida, the rain is most likely mid afternoon, so perhaps early evening is just as good as morning. Morning is bad as the team needs to be on station hours before launch and many of them would be groggy and more likely to make an error if they started work shortly after midnight. Neil
 
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MeteorWayne

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Read it again carefully. Here's the relevant part:

"When visualizing plane window scenarios, it’s important to remember that Earth rotates at 1,035 mph, but an object’s orbit is fixed in space. That means that the orbital path of the ISS passes over a different part of Earth on each 90-minute, 17,000-mph orbit—the station’s “ground track” is always sliding to the west. The ground track of the ISS may cross near Kennedy Space Center on one orbit, but when it comes back around 90 minutes later, Earth will have rotated and the orbit will cross at a point about 1,000 miles due west. The result: There is only one plane window per day for a rendezvous mission, because it takes about 24 hours (Earth’s circumference is approximately 25,000 miles, divided by a rotation rate of 1,035 mph) for the target orbit to return."

So if you want to launch on July 15, 2009 the only time when the plane is overhead was 6:03:10 PM EDT
 
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CalliArcale

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clinn":2goy2fsb said:
Great article, explains enough for me. I knew there would be some dynamics there, but never appreciated how narrow they are. I had expected such alignments to look rather randomly distributed in Earth time, so was suspicious of the (what seemed to me) strong bias into a 7 hour period. Curious.

Thanks again for the responses!

-Chuck

If it helps, look at the launch times for other ISS missions. Not all have been evening flights. Some have been morning flights. Some have been noon flights. Some have even been middle-of-the-night flights. It's driven entirely by the plane of the ISS's orbit, and that precesses steadily over time. The windows for this flight were a bit earlier each day, you may have noticed. Wait long enough, and you can get a morning window.

Post-Columbia, night launches were temporarily banned to ensure high quality launch imagery. This made it even harder to pick a launch date, because once that window crept into the twilight hours, they'd have to wait weeks or even months for it to come around to daylight hours again. Once enough confidence was gained in the system, NASA started to permit nighttime launches again, though it still favors daylight when practical taking into account all the other factors (ISS thermal environment, other launches using the eastern range, whether or not those other launches can move, supply levels aboard the ISS, whether or not there are crewmembers to ferry to/from the ISS, what other flights to the ISS are scheduled, etc.). It's gotta be a logistical nightmare, frankly, and I greatly respect the folks who take care of it.
 
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earth_bound_misfit

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I've wondered myself why they don't "pop up" closer behind the ISS. I guess it's concern about the ET being in a similar orbital plane.
 
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rwhigh42

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The reason the shuttle (or any of the other three spacecraft that have visited the ISS) doesn't pop-up closer to the station is two-fold.

First it's due to the previously mentioned problem with having to time the launch to within a certain time limit of when the orbital plane gets within the capabilities of the launch vehicle to maneuver to it.

As for the amount of time it takes to approach the station, the time used is close to the best compromise between time spent on approach, and the fuel expended between reaching a stable orbit and reaching the station.

BTW, the ET has never been placed in orbit, re-entering the atmosphere primarily over the Indian Ocean.
 
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MikeWiggins

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Actually there are two opportunities per day to intercept the ISS. Once when the ISS is heading northeast relative to Cape Canaveral and once when it is heading southeast (relative).

But while that may be technically correct, I believe operational considerations make launching to the southeast undesirable for an ISS intercept. While launching to the southeast is still over open water, there aren't (I believe) as many usable emergency landing sites with two-three mile long runways for the shuttle in Africa except maybe South Africa. When you fly over Europe during the ascent you literally have a plethora of these landing sites to choose from. During this last launch you could hear Mission Control mentioning Moron (Spain).

Anyway, that's just my two cents worth.
 
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MeteorWayne

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The other factor is that launching southeast removes some of the benefit of the 28 degree north location of the cape.
I could be wrong about that, but it seems to me launching southeast would remove some of the earth's rotation speed benefit, reducing payload capacity.
 
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shuttle_guy

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rwhigh42":5gkc1sxu said:
...........................BTW, the ET has never been placed in orbit, re-entering the atmosphere primarily over the Indian Ocean.


Not correct we do not drop the ET in the Indian Ocean any more. That was stopped at least 20 years ago. The ET is targeted to drop into the South Pacific.
 
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shuttle_guy

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MeteorWayne":2doztiiw said:
The other factor is that launching southeast removes some of the benefit of the 28 degree north location of the cape.
I could be wrong about that, but it seems to me launching southeast would remove some of the earth's rotation speed benefit, reducing payload capacity.

That is true however the launch MUST be into the oribit of the target. Launching due east to get the full advantage of the Earth's rotation the target must be in an orbit inclined at 28.5 deg to the equator. (KSC is at 28.5 deg north). We can not launch to the ISS in a descending node (to the south east) because we would drop the SRBs on or near populated areas!
 
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Mighty

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I've wondered myself why they don't "pop up" closer behind the ISS.

There are a few reasons.

First, as others have pointed out, they don't really have a choice. There's a five minute window when the orbit of the station passes over KSC, and they have to launch right then. The station will be wherever it happens to be at that time.

Also, the crew is already pretty stressed and worn out from the launch. The 8 minute ride is just the most spectacular part of a very long day. And the day isn't over when they reach orbit. There are several tasks that need to be performed once achieving orbit and then prior to docking.

For example, this article on NASASpaceFlight http://www.nasaspaceflight.com/2009/07/ ... spections/ mentions that detailed inspection of the right wing must be performed prior to docking. The shuttle arm is attached on the left side of the payload bay and the inspection device cannot reach the right wing while the shuttle is docked.

So they can't dock for a couple of days, anyway.

Once in orbit, it takes longer to catch up or slow down to the station than I thought. Using http://home.att.net/~ntdoug/UCM2.html it looks like the difference in speed between a 250 km (150 mile) orbit vs 400 km (240 mile) orbit is 0.087 km/sec which is about 195 miles per hour. If the station is too far around the Earth at launch it would take several days (more than 5 days in this example, worst case) to catch it, even at these sorts of outrageous orbital speed differences.
 
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CalliArcale

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shuttle_guy":3t9pas7m said:
rwhigh42":3t9pas7m said:
...........................BTW, the ET has never been placed in orbit, re-entering the atmosphere primarily over the Indian Ocean.


Not correct we do not drop the ET in the Indian Ocean any more. That was stopped at least 20 years ago. The ET is targeted to drop into the South Pacific.

I did not know that. Thank you! *makes mental note to stop telling people it goes into the Indian Ocean*
 
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