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#760
by
Greg Hullender
on 05 Mar, 2017 18:01
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What might help your understanding is to remember that all these objects are in elliptical orbits around the Earth (counting a circle as a special ellipse). At perigee (closest to Earth), an object in an elliptical orbit moves faster than an object in a circular orbit at the same height. At apogee (furthest from Earth) an object moves slower than an object in a circular orbit.
If you change velocity at perigee that changes the altitude of apogee (highest point), which is clear on the other side of the planet. And vice versa. If you were in a circular orbit and you sped up, you'd now be in an elliptical orbit, at the perigee.
What all this means is that when you accelerate toward the space station, you are now in a different orbit. If you miss it, you'll gradually move higher above the Earth, moving slower, and it'll pass under you and leave you behind. By the time of your next perigee, it'll be way ahead of you.
Since the orbit time of the space station is 90 minutes, anything that happens in a small fraction of that time can generally ignore the orbital effects. Five minutes is probably fairly safe, assuming you do some course correcting when you get close. But 25 minutes is probably not safe. (It could be a fun problem to work out the exact amounts of error involved.)
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#761
by
yokem55
on 05 Mar, 2017 18:57
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What might help your understanding is to remember that all these objects are in elliptical orbits around the Earth (counting a circle as a special ellipse). At perigee (closest to Earth), an object in an elliptical orbit moves faster than an object in a circular orbit at the same height. At apogee (furthest from Earth) an object moves slower than an object in a circular orbit.
If you change velocity at perigee that changes the altitude of apogee (highest point), which is clear on the other side of the planet. And vice versa. If you were in a circular orbit and you sped up, you'd now be in an elliptical orbit, at the perigee.
What all this means is that when you accelerate toward the space station, you are now in a different orbit. If you miss it, you'll gradually move higher above the Earth, moving slower, and it'll pass under you and leave you behind. By the time of your next perigee, it'll be way ahead of you.
Since the orbit time of the space station is 90 minutes, anything that happens in a small fraction of that time can generally ignore the orbital effects. Five minutes is probably fairly safe, assuming you do some course correcting when you get close. But 25 minutes is probably not safe. (It could be a fun problem to work out the exact amounts of error involved.)
One of the most counter intuitive things about orbital maneuvers is that if you do a prograde burn forward, you actually will be moving more slowly relative to an object in the same orbit that didn't do that burn. So to catch up to an object ahead of you, you born retrograde, to slow down to allow an object behind you to catch up, you burn prograde. Then once you've rendezvoused to within a few hundred meters, you null out your relative velocities, and then more or less you can translate to docking in a more intuitive manner.
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#762
by
Greg Hullender
on 06 Mar, 2017 19:52
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One of the most counter intuitive things about orbital maneuvers is that if you do a prograde burn forward, you actually will be moving more slowly relative to an object in the same orbit that didn't do that burn. So to catch up to an object ahead of you, you born retrograde, to slow down to allow an object behind you to catch up, you burn prograde. Then once you've rendezvoused to within a few hundred meters, you null out your relative velocities, and then more or less you can translate to docking in a more intuitive manner.
On average, yes, you'll be moving slower, but immediately after your burn, you'll be moving faster because you are at the perigee of your new orbit (assuming the original orbit was circular). This is why you can ignore the orbital effects on a time scale that's small compared to your orbital period.
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#763
by
sewebster
on 07 Mar, 2017 20:39
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We're we able to see if they kept the rocket horizontal as it climbed the launch mount hill or did they let it go slightly inverted?
If it was kept horizontal they must have modified the late load vehicle because it would be much higher above the tracks.
I don't know if we got an answer to the above question in this thread, but I found the following discussion from a couple years ago that says the vehicle is kept roughly level.
http://forum.nasaspaceflight.com/index.php?topic=36100.msg1390980#msg1390980
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#764
by
Comga
on 07 Mar, 2017 21:20
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We're we able to see if they kept the rocket horizontal as it climbed the launch mount hill or did they let it go slightly inverted?
If it was kept horizontal they must have modified the late load vehicle because it would be much higher above the tracks.
I don't know if we got an answer to the above question in this thread, but I found the following discussion from a couple years ago that says the vehicle is kept roughly level.
http://forum.nasaspaceflight.com/index.php?topic=36100.msg1390980#msg1390980
The text on that link includes
There is a hydraulic system to keep the vehicle "roughly level" as it goes up the ramp to the pad. This is not intrinsically necessary, but they raise the payload end to keep it from hitting the ground as the rocket transitions from level to the ramp.
The 39A TE will retract all the way back to horizontal prior to launch. This is "a big benefit" with the enhanced Merlins running at 100%. This will be for both the Heavies and the F9 "single stick".
We know that the second statement is not what happened with the TEL at LC-39A for CRS-10 and won't be true for future launches. Both details, the timing and angle, are at best modified since that June 2015 post.
That says we cannot rely on the first statement.
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#765
by
cscott
on 07 Mar, 2017 23:39
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Well, if we do the geometry we can figure out what angle is necessary to keep the fairing from hitting the ground. It's probably a safe bet the TEL hydraulic system allows at least that amount of levelling.
Of course, if the math shows that 0 degrees is sufficient to keep the fairing from scraping the ground, then perhaps the whole levelling system was omitted. But if not, then there definitely has to be *some* truth to the first statement.
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#766
by
Jim
on 08 Mar, 2017 01:40
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Lights at LC-39A are on. Did SpaceX keep the xenon lights that lit up the Shuttle?
the range owned the lights
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#767
by
manoweb
on 08 Mar, 2017 01:52
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Lights at LC-39A are on. Did SpaceX keep the xenon lights that lit up the Shuttle?
xenon lights? Interesting. Xenon lights are typically not used as floodlights but in applications that require a high CRI... I did some research but I cannot find more details why xenon lights were (are?) used
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#768
by
CameronD
on 08 Mar, 2017 02:18
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Lights at LC-39A are on. Did SpaceX keep the xenon lights that lit up the Shuttle?
xenon lights? Interesting. Xenon lights are typically not used as floodlights but in applications that require a high CRI... I did some research but I cannot find more details why xenon lights were (are?) used
Given the age and application, I would expect they'd be fairly standard industrial Mercury Vapour lamps - not Xenon.. Just FWIW.
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#769
by
RonM
on 08 Mar, 2017 03:50
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Lights at LC-39A are on. Did SpaceX keep the xenon lights that lit up the Shuttle?
xenon lights? Interesting. Xenon lights are typically not used as floodlights but in applications that require a high CRI... I did some research but I cannot find more details why xenon lights were (are?) used
Given the age and application, I would expect they'd be fairly standard industrial Mercury Vapour lamps - not Xenon.. Just FWIW.
C'mon, guys. A quick Google search will confirm NASA used xenon lights.
Now back to our SpX-10 discussion.
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#770
by
launchwatcher
on 13 Mar, 2017 22:56
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#771
by
Brian45
on 18 Mar, 2017 11:50
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A quick return cargo question -- I read that when Dragon leaves the ISS there will be some material placed in the un-pressurized trunk. How does that work? Isn't the trunk jettisoned before entry exposing the heat shield? Is that just a way of disposing junk?
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#772
by
NASAGeek
on 18 Mar, 2017 12:09
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A quick return cargo question -- I read that when Dragon leaves the ISS there will be some material placed in the un-pressurized trunk. How does that work? Isn't the trunk jettisoned before entry exposing the heat shield? Is that just a way of disposing junk?
Yeah the trunk is jettisoned before re entry. The items placed in the trunk are meant to be disposed by burning up in the atmosphere.
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#773
by
mvpel
on 20 Mar, 2017 13:12
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Per a spaceflightnow story, 104 kg of EVA equipment was brought home on Dragon. Does anyone (hi, Pete?) know what that was? Substantial - sounds like an EMU or at least a PLSS or HUT or something
Would it have been the leaky Parmitano / Kopra suit equipment, maybe? Or would that have come down on CRS-9?
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#774
by
Norm38
on 18 Jun, 2017 23:05
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I hadn't seen any of these "Flight Club" videos before, don't recall them being mentioned here, though there are a lot of them. The animations are fun to watch, especially for RTLS missions like this one.
Being able to watch the various burns occur along the trajectory, their timing with respect to velocity, the RTLS maneuver makes so much more sense now, seeing it like this. It really is quite elegant.
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#775
by
deruch
on 19 Jun, 2017 00:01
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