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#2260
by
vsrinivas
on 05 Sep, 2011 19:33
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What sort of DSPs were used in AHMS?
What was the system looking for? Changes in vibrational frequencies of the pumps?
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#2261
by
sivodave
on 05 Sep, 2011 23:18
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What was the system looking for? Changes in vibrational frequencies of the pumps?
From the interview reported with George Hopson reported in the JSC Oral history project this is the explanation:
The AHMS [advanced health management system], the big difference between it and the first vibration shut down system was that the first system used composite vibration. In other words the old system used the whole spectrum of frequencies, so any noise was considered along with real vibration. The AHMS only considered synchronous vibration. The pump turns X number of rpm [revolutions per minute]. If you have a vibration that matches the speed that the pump is turning, then that’s synchronous. And if it’s synchronous it’s real.
hope this help in answering your question
Davide
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#2262
by
sivodave
on 06 Sep, 2011 11:43
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Hi all.
I was reading that the guidance during the first part of the ascent (up to SRM sep) is open loop. The guidance has in its memory a pre loaded set of attitude positions the orbiter has to have when it reaches a given velocity.
I was wondering why is the guidance control logic up to SRM sep, open loop type? This exclude the possibility to correct any large deviations from a nominal attitude due to some for example some strong gust of wind. Since up to SRB sep the shuttle is still in the lower portion of the atmosphere where the winds are stronger, it would have made sense to use a closed-loop control logic. What have I missed?
thanks
Davide
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#2263
by
Danny Dot
on 06 Sep, 2011 12:08
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Hi all.
I was reading that the guidance during the first part of the ascent (up to SRM sep) is open loop. The guidance has in its memory a pre loaded set of attitude positions the orbiter has to have when it reaches a given velocity.
I was wondering why is the guidance control logic up to SRM sep, open loop type? This exclude the possibility to correct any large deviations from a nominal attitude due to some for example some strong gust of wind. Since up to SRB sep the shuttle is still in the lower portion of the atmosphere where the winds are stronger, it would have made sense to use a closed-loop control logic. What have I missed?
thanks
Davide
In first stage there is also feedback from the lateral accelerometers that react to off nominal winds. So there is a response to winds in first stage.
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#2264
by
sivodave
on 06 Sep, 2011 13:14
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In first stage there is also feedback from the lateral accelerometers that react to off nominal winds. So there is a response to winds in first stage
but how this information is put into the open loop? if you have this kind of feedback the guidance is not anymore open loop.
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#2265
by
Jim
on 06 Sep, 2011 13:23
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This exclude the possibility to correct any large deviations from a nominal attitude due to some for example some strong gust of wind.
yes
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#2266
by
sivodave
on 06 Sep, 2011 13:37
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But it is safe not being able to make corrections due to winds up to SRB sep? which is the reason for having chosen to adopt this kind of guidance?
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#2267
by
alk3997
on 06 Sep, 2011 13:37
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Hi all.
I was reading that the guidance during the first part of the ascent (up to SRM sep) is open loop. The guidance has in its memory a pre loaded set of attitude positions the orbiter has to have when it reaches a given velocity.
I was wondering why is the guidance control logic up to SRM sep, open loop type? This exclude the possibility to correct any large deviations from a nominal attitude due to some for example some strong gust of wind. Since up to SRB sep the shuttle is still in the lower portion of the atmosphere where the winds are stronger, it would have made sense to use a closed-loop control logic. What have I missed?
thanks
Davide
First, here's a publicly available paper on first stage guidance,
http://www.allmanpc.com/papers_presentations/SpaceShuttleSimplex.pdf1st stage is open loop. Corrections for wind were made pre-launch using the DOLILU II system.
Andy
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#2268
by
wolfpack
on 06 Sep, 2011 14:02
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But it is safe not being able to make corrections due to winds up to SRB sep? which is the reason for having chosen to adopt this kind of guidance?
I would think fine control using the rock and tilt actuators in the SRBs would be impossible. The stack is doing its best to just fly a compass heading and proper alpha angle so as not to tear the wings off during first stage. Second stage is another 6 minutes of flight well out of the thick air - plenty of time to fine tune where the vehicle is headed.
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#2269
by
Danny Dot
on 06 Sep, 2011 14:08
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In first stage there is also feedback from the lateral accelerometers that react to off nominal winds. So there is a response to winds in first stage
but how this information is put into the open loop? if you have this kind of feedback the guidance is not anymore open loop.
Then I guess guidance is not 100% open loop.
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#2270
by
alk3997
on 06 Sep, 2011 14:19
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In first stage there is also feedback from the lateral accelerometers that react to off nominal winds. So there is a response to winds in first stage
but how this information is put into the open loop? if you have this kind of feedback the guidance is not anymore open loop.
Then I guess guidance is not 100% open loop.
...or 100% open loop, unless "the loop" includes the prelaunch wind calculations.
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#2271
by
mmeijeri
on 06 Sep, 2011 14:24
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From that paper it looks as if open-loop means they are sticking to a fixed velocity-attitude profile that is unaffected by previous deviations, not that there is no feedback at all. Inside that open-loop profile there is apparently another control system that does use feedback. Otherwise you wouldn't need sensors and they'd be playing back actuator commands. As I understand it, PEG does find the best way to get to the desired final conditions based on the current conditions, not on the conditions in the reference profile at the same point in time. In other words open-loop guidance uses feedback to steer the vehicle towards a reference velocity-attitude profile whereas PEG uses feedback to steer it towards the desired final conditions.
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#2272
by
alk3997
on 06 Sep, 2011 14:31
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#2273
by
Jim
on 06 Sep, 2011 15:34
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But it is safe not being able to make corrections due to winds up to SRB sep? which is the reason for having chosen to adopt this kind of guidance?
Most launch vehicles use open loop for their lower stages.
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#2274
by
DansSLK
on 06 Sep, 2011 21:00
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From that paper it looks as if open-loop means they are sticking to a fixed velocity-attitude profile that is unaffected by previous deviations, not that there is no feedback at all. Inside that open-loop profile there is apparently another control system that does use feedback. Otherwise you wouldn't need sensors and they'd be playing back actuator commands. As I understand it, PEG does find the best way to get to the desired final conditions based on the current conditions, not on the conditions in the reference profile at the same point in time. In other words open-loop guidance uses feedback to steer the vehicle towards a reference velocity-attitude profile whereas PEG uses feedback to steer it towards the desired final conditions.
That is my understanding of ascent guidance.
First stage guidance is closed-loop in the sense that the vehicle will try its best to meet the I-Loaded targets while minimizing stress on the airframe and 'making the most of what it's got' profile wise and open-loop in that the targets are initalized loaded (DOLILU) and not computed on-the-fly with an end goal in mind (that would be PEG)
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#2275
by
Specifically-Impulsive
on 06 Sep, 2011 21:05
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Most launch vehicles use open loop for their lower stages.
Jim's correct. The DOLILU team held fairly regular working group meetings with teams that performed similar functions for other launch vehicles. The process of "analyze the wind of the day, then design the trajectory based on that and the vehicle's load constraints" seemed to me to be consistent (at a high level) across all the vehicles. The EELV folks had the advantage of better technology in their analysis tools but the basic idea was the same.
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#2276
by
alk3997
on 06 Sep, 2011 21:21
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The bottom line to all of this is what would you use to "look ahead" at the wind in the upcoming atmosphere in real-time that would fit on a vehicle and be enough of a look-ahead that the vehicle could compensate effectively? Much easier to do that on the ground and the winds aloft don't change that rapidly (normally) compared to using balloon data. And, if the winds are changing that rapidly you probably don't want to launch. I always thought it was best to keep the wings attached to the vehicle...
Andy
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#2277
by
sivodave
on 06 Sep, 2011 22:56
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Hi all.
thanks very much for your answers and for the links...very interesting....i'm now understing.
I've now another question for you. Always during the first part of the ascent, I read the elevons are moved for stress relief during Max q. So I'm wondering in which way are they moved? is again something set in pre-launch loaded tables or is based on a closed loop? I would guess it's the first one (the preloaded table).
Thanks
Davide
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#2278
by
alk3997
on 06 Sep, 2011 23:11
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Pre-loaded and purely for load relief (no steering) and prior to max-q.
Andy
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#2279
by
sivodave
on 07 Sep, 2011 10:17
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but exactly how are the elevons moved? My guess is that they are moved in order to reduce the lift created on the wings but I'm not sure.
thanks
Davide