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#600
by
TJL
on 21 Dec, 2006 01:59
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If I remember correctly, the weather at KSC for the "121" landing wasn't the greatest. I don't believe there was a previous shuttle landing with that much cloud cover. Is the Friday forecast calling for clouds, wind or rain?
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#601
by
DaveS
on 21 Dec, 2006 02:10
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TJL - 21/12/2006 3:42 AM
Is the Friday forecast calling for clouds, wind or rain?
All three.
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#602
by
psloss
on 21 Dec, 2006 02:12
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TJL - 20/12/2006 9:42 PM
If I remember correctly, the weather at KSC for thr "121" landing wasn't the greatest. I don't believe there was a previous shuttle landing with that much cloud cover. Is the Friday forecast calling for clouds, wind or rain?
Off the top of my head, the weather for STS-108 was probably generally better, but ended up with low clouds over the end of the runway. A couple of screen shots are attached from that; these probably make it seem more dramatic than it was, but...
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#603
by
hutchel
on 21 Dec, 2006 10:17
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Answers the question - thanx!
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#604
by
psloss
on 21 Dec, 2006 11:48
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STS-121 did have a pretty late runway "redes"; the other late one I remember was for STS-30 (that was at Edwards) and it was for winds as opposed to rain showers. (There have probably been others, though, post-deorbit burn.)
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#605
by
shuttlefan
on 21 Dec, 2006 12:54
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psloss - 21/12/2006 6:31 AM
STS-121 did have a pretty late runway "redes"; the other late one I remember was for STS-30 (that was at Edwards) and it was for winds as opposed to rain showers. (There have probably been others, though, post-deorbit burn.)
During the STS-107 re-entry, they were seriously considering changing KSC runways. Someone correct me if I'm wrong, but I think it was because they were fearing the sun would be in the CDR's eyes.
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#606
by
Joffan
on 21 Dec, 2006 14:44
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hutchel - 20/12/2006 5:28 PM
OK Landing track question -
I've noticed that all of the landing tracks come in from the west - this seems to limit the landing opportunities. Does the shuttle not have sufficient energy from the 2 orbits prior to the ones use to make it safely to the runway?
I know in the early stages of the entry, the shuttle is not powered (all the way to the ground) and is basically flying a "ballastic" trajectory, but at some point the control surfaces become effective and the shuttle starts to behave more and more like an airplane, at which point a turn more to the north and eventually west would be conceivable - this would seem to give them a couple of more landing opportunities.
The big-scale approach to landing come from the west because the orbiter is ...ah... orbiting, and that means huge ground-speed. The return to Earth needs to lose that orbital speed and that is done mostly through aerobraking, so that can't happen until the craft is in the atmosphere, so previous orbits are irrelevant. You'll recall that Columbia broke up over Texas, which was quite early in the aerobraking process approaching Florida. That's the sort of scale to bear in mind for the re-entry.
At high speeds the shuttle can curve her track a little by attitude control. Once her speed is low enough, the orbiter can start manouevering more impressively and the approach to the KSC runway at least requires a 270degree turn. So the heading of the runway is not a problem for landing.
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#607
by
mark147
on 21 Dec, 2006 17:45
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A related question then -- why not consider landing from the opposite side of the orbit? The planned approaches for STS-116 all come from an ascending orbit (approach from South West). Surely there's also a landing opportunity from a descending orbit (approach from North West), about 12 hours later?
Mark
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#608
by
Joffan
on 21 Dec, 2006 18:10
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I think that would be about 6 hours later rather than 12, but I understand there are such tracks calculated. I don't know what makes them less desirable.
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#609
by
DaveS
on 21 Dec, 2006 18:11
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mark147 - 21/12/2006 7:28 PM
A related question then -- why not consider landing from the opposite side of the orbit? The planned approaches for STS-116 all come from an ascending orbit (approach from South West). Surely there's also a landing opportunity from a descending orbit (approach from North West), about 12 hours later?
Mark
Not an option! DN approaches would overfly CONUS and present an unacceptable risk situation if something were to go wrong during entry and lead to break-up of the orbiter.
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#610
by
Jim
on 21 Dec, 2006 18:36
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There are other issues with DN also. mkirk can enlighten us. Way back there use to be a density shear in the upper atmosphere
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#611
by
elmarko
on 21 Dec, 2006 20:12
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I have a question that is probably very, VERY stupid, and I apologise for it in advance. The excuse I'm giving is that I just got back from my work Christmas party, so I'm a bit tipsy.
Let's talk about OMS assists. Rather than burn the OMS engines for however long is needed, why not just fuel the OMS engine pods with enough OMS fuel for a mission MINUS what would be burned for the OMS assist if it was carried out.
I mean, if it's a question of weight, then surely not carrying that fuel would increase performance?
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#612
by
Joffan
on 21 Dec, 2006 20:12
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DaveS - 21/12/2006 11:54 AM
mark147 - 21/12/2006 7:28 PM
A related question then -- why not consider landing from the opposite side of the orbit? The planned approaches for STS-116 all come from an ascending orbit (approach from South West). Surely there's also a landing opportunity from a descending orbit (approach from North West), about 12 hours later?
Mark
Not an option! DN approaches would overfly CONUS and present an unacceptable risk situation if something were to go wrong during entry and lead to break-up of the orbiter.
"Unacceptable risk"... sounds like a gut reaction rather than one with any real numbers behind it. I couldn't see the
additional risk amounting to more than 10e-5 ground fatalities per flight, which is effectively zero for most purposes and especially for the Shuttle given limited life. Even in "bad publicity" terms, the tragedy of shuttle breakup will not be noticeably more if a handful of people are killed on the ground (eg. Lockerbie). It must be something else - hopefully, as Jim says, mkirk will have more information.
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#613
by
Joffan
on 21 Dec, 2006 20:18
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elmarko - 21/12/2006 1:55 PM
I have a question that is probably very, VERY stupid, and I apologise for it in advance. The excuse I'm giving is that I just got back from my work Christmas party, so I'm a bit tipsy.
Let's talk about OMS assists. Rather than burn the OMS engines for however long is needed, why not just fuel the OMS engine pods with enough OMS fuel for a mission MINUS what would be burned for the OMS assist if it was carried out.
I mean, if it's a question of weight, then surely not carrying that fuel would increase performance?
What a great idea! In fact maybe we should remove ALL the fuel from the whole shuttle! Think how much lighter it would be then, and performance would be... would be...
OK, enough sarcasm, sorry. The weight of the fuel is an issue but overall they wouldn't carry it if it wasn't contributing more than it was expending.
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#614
by
elmarko
on 21 Dec, 2006 20:39
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lol, cheeky bugger
That was my question, really. If the thrust to weight ratio was less than 1, then there would be no point carrying it?
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#615
by
mkirk
on 21 Dec, 2006 21:31
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psloss - 20/12/2006 8:55 PM
TJL - 20/12/2006 9:42 PM
If I remember correctly, the weather at KSC for thr "121" landing wasn't the greatest. I don't believe there was a previous shuttle landing with that much cloud cover. Is the Friday forecast calling for clouds, wind or rain?
Off the top of my head, the weather for STS-108 was probably generally better, but ended up with low clouds over the end of the runway. A couple of screen shots are attached from that; these probably make it seem more dramatic than it was, but...
The weather rules are pretty clearly defined, the hard part is predicting what will happen during the usually dynamic weather scenarios encountered in Florida from the time of the De-Orbit Burn (when weather has been evaluated by the Flight Control Team) and the time the shuttle arrives on final approach for the runway.
So when the go for De-Orbit is (was) given the expectation is (was) that the approach path would be clear from at least 8,000 feet down to the runway…I really don’t recall how many times (if ever) that the ceiling rule was violated when the orbiter actually arrived at the landing site.
The ceiling limits for EOM (end of mission) are 8,000 feet. This is of course more constraining than the RTLS rule of 5,000 feet. The difference is based on the freshness of the crew. On launch day they had just finished several days of simulator and STA (shuttle training aircraft) runs which gave them plenty of landing practice. Now that they have been in Zero G for several days it is assumed they are not quite as fresh and the weather rules are packed with additional safety margin.
Flight Rules define a ceiling as more than 50% coverage. You can have lower clouds but generally they have to be 2/8th coverage or less and they can not obstruct the visibility of the runway on final approach.
The ceiling rule ensures the Crew can visually acquire the runway after breaking out of the cloud deck and make any corrections to the flight path needed in cases where the Guidance and Navigation System were not as accurate as they should be. From 5,000 the Commander would only have about 18 seconds to make those corrections before beginning the Pre-Flare. So the EOM rule of 8,000 feet provides a little cushion in the amount of time available.
Mark Kirkman
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#616
by
mkirk
on 21 Dec, 2006 21:43
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Jim - 20/12/2006 6:48 PM
Cross range is the distance on either side of orbit track the orbiter can reach. The Shuttles cross range was to be 1000nmi and is 800nmi (mkirk can correct this). The distance between orbit tracks is much greater than this. The orbiter doesn't become an "real" actual aircraft until less than 100,000 feet (mkirk, help). the shutttle is still using the RCS sometimes in view of the landing sites.
I don’t recall the exact numbers for cross range and there really are too many variables to give an exact number. Jim’s answer is a good ball park explanation. The cross range limits are also a little more constrained for TAL (Transoceanic Abort Landing) than for EOM (end of mission) and are based on orbital inclination and the particular landing site.
As for RCS (reaction control system) jet usage; the various jets are phased out over the course of Entry. The Roll Jets are deactivated at about 5 minutes after Entry Interface (at a dynamic pressure of 10 pounds per square foot). The Pitch jets are deactivated about 3 or 4 minutes later when dynamic pressure is up to 40 psf and finally the yaw jets are deactivated last at Mach 1 (which is generally at an altitude of 50,000 – 55,000 feet).
Mark Kirkman
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#617
by
STS-500Cmdr
on 21 Dec, 2006 22:03
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elmarko - 21/12/2006 2:55 PM
I have a question that is probably very, VERY stupid, and I apologise for it in advance. The excuse I'm giving is that I just got back from my work Christmas party, so I'm a bit tipsy.
Let's talk about OMS assists. Rather than burn the OMS engines for however long is needed, why not just fuel the OMS engine pods with enough OMS fuel for a mission MINUS what would be burned for the OMS assist if it was carried out.
I mean, if it's a question of weight, then surely not carrying that fuel would increase performance?
To take a bit of a crack at elmarko's question. I kinda learned a little bit about OMS assist watching-and taping for later watching-a briefing with John Shannon during STS-105-he was answering a Q from Harwood.
I understand what your saying--we used to load up the OMS for what we needed for the mission. OMS assist was put on as a performance-enhancement where they loaded up the OMS tanks for MORE than was needed for the mission. You burn that just after SRB sep-that gives you about 250 lbs so you can keep that 250 lbs in the External Tank. You have that 250 lbs in th ET for contingencies, maybe if you had an engine problem-not necessarily and engine out but an engine problem like we saw on STS-93 where we had the small hole in the nozzle leaking LH2 into the plume-starved the engine a bit-the engine throttled up used more LO2 to compensate--left Columbia short by about 15 fps. This is me tryin to remember John Shannon's explanation one time. Does this help at all? Im not a NASA or USA person--i kinda wish i was--i might sound a bit like one here
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#618
by
elmarko
on 21 Dec, 2006 22:45
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Well, I kind of see your point, but the OMS assist wasn't put on for a contingency supply of fuel reason. It was put on to help carry more payload to the ISS. So my point was, assuming the thrust-to-weight of the OMS engines was less than 1, surely it would make more sense to not carry that fuel in the pods?
Or am I approaching this completely wrongly.
PS: I'm sobering up, so hit me with your best answers
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#619
by
Radioheaded
on 21 Dec, 2006 23:16
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edit: as usual, my question was basically answered if I'd just look a bit harder......
