Posted: 7/8/2006 10:05:43 PM EDT
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Why are there only a few de-orbit "windows" available for the shuttle to land with? It circles the earth, so why can't it land from any particular orbit? Same with launches, what's it all about? |
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Launches: They want to put the shuttle in a specific place in space. They want to get the most bang for the buck. They'll trade fuel for cargo. That means that they need to launch during a specific time period. Landing is the same deal, they want to land at KSC, so they only have so many opportunities to de-orbit in order to get the shuttle to KSC. Why do they want to land at KSC? It cost's a lot of money to put the shuttle on the 747 for a flight back to Florida. |
I understand that part, but if the shuttle orbits once every 45 minutes, why isn't there a "window" every 45 minutes? (Or is there?) |
Gotcha. Now my question is why does it do that? And as for that space station link, why does it fly an elliptical orbit? |
The latitude is not favorable every 45 minutes. The shuttle can't make giant cross range maneuvers. |
Here's an even cooler one. 
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Kind of like my turn to final in my bugsmasher this afternoon...... |
It doesn't, but when you transpose all the info from a 3D sphere to a flat 2D representation, that is what it "looks" like. |
As a general rule, all "orbits" are elliptical. Circles are actually a special case of ellipses. Orbital mechanics are suprisingly complex for something with only two bodies and one simple physical law governing their motion, but I'll try to keep things simple. That sine wave curve of the shuttles course over the earth is due to the plane of it's orbit being tilted from the plane of the earth's equator. All orbits are ellipses centered on the earth's center of mass. So to get the shuttle to rendezvous with satellite that's not orbiting in the equatorial plane, it has to tip the plane of it's orbit. This tipping means that as it orbits, it goes above and below the equatorial plane in a very regular sine wave pattern relative to the surface of the earth as projected on a map. If the shuttles orbital period (the time it takes to complete one full orbit) exactly matched earth's rotation period, that track would be a perfect closed curve, with the shuttle running in the same "groove' every orbit. But if the orbital period is different from the earth's rotational period, the shuttle's track (the projection of it's orbit on the earth) will slide ahead/behind relative to the earth depending on the relative periods. So if you orbit every 45 minutes, the shuttle will have the right longitude every 45 minutes to hit the de-orbit window to make it to KSC, but it's latitude will be off since it's track is constantly changing in both latitude and longitude and rarely hits the same place. Hope this helps. |
Because the earth rotates and the ground track is constantly moving. |
Because the Russians must launch from a high latitude to keep their upper stages from hitting China. |
Yes, it does help. Thanks a lot. |
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This is a really cool site (yeah, i'm a nerd I guess). These are sattelites, but the principals of low earth orbit are the same. http://science.nasa.gov/Realtime/jtrack/3d/JTrack3D.html Go to "view" turn on "ground trace", then go to "options" , set timing to 1000X and change the update rate. Now click on the white dots moving closer to the earth. This visual aid will answer many of your questions about orbits. EDIT:: forgot the link 
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BINGO Heavy, the earth rotates and the shuttle orbits... They generally can hit a point in 1 or 2 orbits, but if they miss the "window" then they have to wait for 24 hours to set it up again. |
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An orbiter flys an elipse that has a more or less fixed orientation in space, with respect to the stars, but the earth rotates under that orientation. If you are north or south of the equator, say, 30 degrees, the earth's surface is rotating toward the east at about 1,000 miles per hour. If the shuttle's orbit period is 1.4 hours then the earth's surface advances 1,400 miles during that 1.4 hours. Suppose a sattelite, the shuttle in this case, wants to photograph Houston at 3:00pm from directly overhead (to get a desirable sun angle and to see between tall buildings) and it passes over Houston at 3:00pm. The next time the shuttle comes around, at 4:24pm, Houston is not under it, because Houston has moved 1,400 miles to the east and so the shuttle passes over San Diego. The next day, 24 hours later at 3:00pm, the shuttle has made 24/1.4 = 17.1 revolutions and is 0.1 revolutions or 2,400 miles past Houston, when Houston passes below the shuttles orbital track. At the time that the shuttle passed the north-south position of Houston (north of the equator) Houston was 8.4 minutes of time west of the shuttles track. (the shuttles orbital period of 1.4 hours is 60x1.4=84 minutes, 0.1x84=8.4) 8.4 minutes is 0.140 hours. (8.4/60=0.140) Since Houston is moving eastward at 1,000 miles per hour it was 140 miles west of the shuttles track when the shuttle passed Houston's west-to-east track. Assuming the shuttle is in a 150 mile high orbit...Houston was about 45 degrees off side, to the west, when the shuttle past closest to Houston. Bummer, no good 3:00 pm overhead pic on this day. To get a good, directly overhead, 3:00pm pic the camera might have to wait several days or even weeks, in this case the wait is 10 days, unless there is bad weather then. Landing is a lot like taking pictures, the track of the spacecraft has to cross the track of the runway or be close to it. The space shuttle, being a plane, can glide some distance right or left of it's ballistic track and also extend forward and backward of it's ballistic impact point but it can only land within some eliptical footprint on the earth's surface. Well, it's more complicated than that and I'm sure I've done a poor job of explaning it but, yes, those launch windows are real and not just made up. And, that is also why we cannot depend on intelligence sattelites to gather photos at some specific place and at some specific time. Orbital mechanics just won't allow it. So, we still need recon planes. Need em badly too. PS, the numbers are off some. I am speaking in generalizations and not doing rigorous calculations, but the general idea is correct. "Guns are good, democrats are bad." |
I have an old 486 that I occasionally have running STSPLUS or JPLClock, programs a guy out at JPL wrote years ago. I don't know if they're still supported; I need to go see if the ephemeris data is still available. Jim |
ding, ding ding, we have a winner. you have more patience than I for explaining things. I would have mentioned the earth's axis and been done with it. |
Holy crap! Someone's brain is in overdrive. 
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