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#2960
by
Jim
on 14 Sep, 2013 17:19
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I am looking for a "primer" on open loop vs closed loop guidance.
Why doesn't the shuttle use closed loop from launch all the way up?
Thanks!
Because it has to ride out winds aloft. It worries about q alpha vs proper path to orbit
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#2961
by
iskyfly
on 14 Sep, 2013 18:35
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I am looking for a "primer" on open loop vs closed loop guidance.
Why doesn't the shuttle use closed loop from launch all the way up?
Thanks!
Because it has to ride out winds aloft. It worries about q alpha vs proper path to orbit
If I understand the part about the winds- if closed loop, correcting for winds might overstress the stack?
q alpha?
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#2962
by
Specifically-Impulsive
on 14 Sep, 2013 20:41
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I am looking for a "primer" on open loop vs closed loop guidance.
Why doesn't the shuttle use closed loop from launch all the way up?
Thanks!
Because it has to ride out winds aloft. It worries about q alpha vs proper path to orbit
If I understand the part about the winds- if closed loop, correcting for winds might overstress the stack?
q alpha?
Here is a really good paper explaining the shuttle day of launch trajectory design process; peruse it and you will learn all about q and alpha and much more. The author knows whereof he speaks, he was involved in the process for many shuttle missions. Read through it and come back if you still have questions.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110003654_2011001858.pdf
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#2963
by
brahmanknight
on 14 Sep, 2013 20:54
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That PDF won't load for me. Anyone else?
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#2964
by
AnalogMan
on 14 Sep, 2013 21:04
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That PDF won't load for me. Anyone else?
Worked for me. I've attached a copy - see if that's any better.
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#2965
by
brahmanknight
on 14 Sep, 2013 21:08
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Worked great, thank you AnalogMan!
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#2966
by
alk3997
on 15 Sep, 2013 14:44
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I am looking for a "primer" on open loop vs closed loop guidance.
Why doesn't the shuttle use closed loop from launch all the way up?
Thanks!
Because it has to ride out winds aloft. It worries about q alpha vs proper path to orbit
If I understand the part about the winds- if closed loop, correcting for winds might overstress the stack?
q alpha?
As you'll see from the paper, there was correction done based on the winds aloft (high altitude winds). But, those were based on balloon data taken multiple times, with the last balloon being about 3 hours before launch. The results were integrated to make sure that the trends were good, as well as the last balloon data.
The problem you have with closed loop guidance in first stage is how to determine winds in real time and then feeding that back into guidance in time. If you work out a method for measuring the winds aloft in real time while going supersonic, let me know. It's something I've pondered for years. Using a ground system was probably not workable due to the possibility of dropouts for the radio uplink.
We always considered it bad form to reach orbit without the wings attached (:-)) and that is what you are trying to prevent by managing dynamic pressure on the various surfaces. So we stayed open loop until second stage. The penalty wasn't that great since you can think of the whole purpose of first stage is just to accelerate enough for second stage flight and get out of the heavier atmosphere.
And, feel free to use past tense with Shuttle. It's taken a while but the program really is over even if there is no replacement vehicle yet.
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#2967
by
alk3997
on 15 Sep, 2013 14:57
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As a programmer I am fascinated and amazed by flight computers.
In regards to the GPC's, has there ever been a fault during a mission that was attributed to a software bug?
Thanks!
There were a few - it averaged one found every 6 years in-flight (roughly). None of them prevented the mission from being accomplished. None were classified as a safety of flight issue other than the bug escaped our very thorough process of detection on the ground. Remember we did not classify requirements issues as flight software issues since the requirements were often provided by groups outside of flight software.
Try this paper for more (you can click on the pdf link at the top of the page):
http://ntrs.nasa.gov/search.jsp?R=20110014946and these charts...
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100029536_2010030196.pdfAndy
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#2968
by
Jim
on 15 Sep, 2013 16:15
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1. The problem you have with closed loop guidance in first stage is how to determine winds in real time and then feeding that back into guidance in time. If you work out a method for measuring the winds aloft in real time while going supersonic, let me know. It's something I've pondered for years. Using a ground system was probably not workable due to the possibility of dropouts for the radio uplink.
2. We always considered it bad form to reach orbit without the wings attached (:-)) and that is what you are trying to prevent by managing dynamic pressure on the various surfaces. So we stayed open loop until second stage. The penalty wasn't that great since you can think of the whole purpose of first stage is just to accelerate enough for second stage flight and get out of the heavier atmosphere.
1. ELV's have the issues.
2. ELV wanted their fairings intact until they determine to jettison them. Same process for ELV's
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#2969
by
baldusi
on 15 Sep, 2013 21:28
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Wasn't GPS precise enough?
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#2970
by
iskyfly
on 16 Sep, 2013 01:14
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Am I incorrect in recalling that Challenger encountered high winds / shear and because of that and of the venting / igniting of the fuel leak the computers commanded corrective actions? If so, how is that possible in open loop?
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#2971
by
Jim
on 16 Sep, 2013 01:26
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Am I incorrect in recalling that Challenger encountered high winds / shear and because of that and of the venting / igniting of the fuel leak the computers commanded corrective actions? If so, how is that possible in open loop?
Still have to maneuver to maintain q alpha.
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#2972
by
alk3997
on 16 Sep, 2013 05:27
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Simply (with Shuttle) a table of yaw and pitch commands that were commanded based on velocity (reach this velocity then command this pitch and yaw or theta and psi, more correctly). The table could be uplinked with DOLILU before launch. To go much deeper would require more math than I'm 1) comfortable with in an open forum and 2) can probably remember.
It's not an easy problem and GPS doesn't help because we knew exactly where we were, just not what the winds were when flying through them.
Jim, if I understood what you wrote then does that mean EELV has not found a better solution than table driven?
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#2973
by
baldusi
on 16 Sep, 2013 13:37
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What happens with pressure transducers at supersonic speeds? That's not the information you need or you simply can't react fast enough?
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#2974
by
Jim
on 16 Sep, 2013 14:54
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What happens with pressure transducers at supersonic speeds? That's not the information you need or you simply can't react fast enough?
Pressure transducers aren't used for ascent.
Launch vehicle ascent through the atmosphere is like a boat in a storm; the boat is more concern with fighting the waves vs the actual course. And it isn't really fighting the waves, it is going with them. Once the boat breaks through the storm (vehicle leaves the atmosphere) then close loop guidance can be initiated.
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#2975
by
Specifically-Impulsive
on 16 Sep, 2013 21:52
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Am I incorrect in recalling that Challenger encountered high winds / shear and because of that and of the venting / igniting of the fuel leak the computers commanded corrective actions? If so, how is that possible in open loop?
It's the
commanded values of pitch and yaw that are open loop. The vehicle still has to maneuver in pitch and yaw to achieve the commanded state.
51-L was before DOLILU but even with DOLILU, changes in the wind after the last balloon would change the structural loads to be different from the predicted loading. There was margin built into the system based on wind history statistics to cover for this, on top of other margin.
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#2976
by
Planegazer
on 18 Sep, 2013 20:44
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Hello!
I was wondering if anyone could recommend some books that would contain information about the equipment that supported the Space Shuttle program, such as the Shuttle Carrier Aircraft, the Shuttle Training Aircraft, the Rotating Service structure, the Crawler, etc. Most of the resources I have consulted so far concentrate on the main components of the shuttle stack, with very detailed descriptions on the orbiters and boosters. I’d like to learn more about other “stuff”.
Any suggestions would be appreciated. Thanks!
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#2977
by
OV135
on 19 Sep, 2013 00:08
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There is the WorldSpaceflight News site that has stuff like this. LC-39: A history from Apollo to the shuttle is a big binder of diagrams and data on the LC-39 complex. I own a copy of this and reference it a lot for my model project.
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#2978
by
roma847
on 19 Sep, 2013 08:20
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Would you be so kind sharing a link to this source for all of us?
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#2979
by
OV135
on 19 Sep, 2013 14:03
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