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#3160
by
AnalogMan
on 17 Jun, 2014 18:42
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We all know about the SSME's #2#3 have a "Start" position, then once up and burning before liftoff, and then move into their "Launch" configuration.
1)I was wondering if the SSME's have a certain position for MECO(Main Engine Cut Off)?
2) If there is a certain MECO gimbal position, is it similar/same to the "Start" position?
TIA
This is the information I have about SSME gimballing at MECO:
Once MECO was confirmed the external tank separation sequence began. Various valves were opened/closed and ET umbilical plates retracted - as part of this, the SSMEs were gimballed to the "MPS dump sequence" position.
If all was well, then ET separation pyrotechnics were fired and the tank and orbiter separated. At this point there would be ~5400 lbs of cryogenic propellant trapped in the MPS (SSMEs and their feedlines). Two minutes after MECO both propellants were dumped (LO2 under helium pressure through the SSME nozzles, and LH2 through the fill/drain and topping valves without the aid of pressurants). The propellant dump took two minutes. Dumping the LO2 throught the SSMEs produced a small amount of propulsion - typically 9-11 fps of delta V.
Once the MPS dump was complete, the SSMEs were gimballed to their entry stow position, with the engine nozzles moved inward (toward one another) to reduce aerodynamic heating.
15 minutes after the MPS dump was complete, traces of remaining LO2 and LH2 were removed by venting the manifolds to space for two minutes, followed by some valve closures.
Although the there appear to be two different positions for the SSMEs in the post-MECO sequence, in practice the "MPS dump" position was set the same as the "entry stow" position (they had the possibility of being programmed differently had that ever been required).
I could not find anything about SSME gimaballing prior to MECO. About six seconds before MECO engines were throttled back to the minimum 67% in preparation for shutdown.
Hope this helps.
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#3161
by
wolfpack
on 17 Jun, 2014 19:53
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AFAIK, MECO is the end of MM103 and guidance is closed-loop. I don't see how there can be any predefined gimbal angles immediately prior to MECO. It would be whatever guidance is commanding to keep the vehicle's state and acceleration vectors where they need to be. Granted, this late in the ascent those values would probably be quite similar from flight-to-flight.
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#3162
by
Hog
on 17 Jun, 2014 20:29
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AFAIK, MECO is the end of MM103 and guidance is closed-loop. I don't see how there can be any predefined gimbal angles immediately prior to MECO. It would be whatever guidance is commanding to keep the vehicle's state and acceleration vectors where they need to be. Granted, this late in the ascent those values would probably be quite similar from flight-to-flight.
I do see your point here. Maybe the the possibility of engine bell damage isnt as great in the vacuum of space as it is at approx. sea level while sitting at the pad. The only time that the on vehicle SSME's have a commanded MECO at such low altitudes within the atmosphere is during Flight Readiness Firing and RSLS aborts.
If the engine bells were as "active" during on orbit MECO's as they were during on-pad MECO's during FRR's as seen in the 3rd video I posted, the risk of bell collision "appears" to be increased.
During on orbit MECO the SSME's are under 3'g throttleing. Not sure how 3 g throttleing to MECO while on orbit would compare to 100% to MECO while on the pad as picture during STS-1 FRR video above.
thanks alk3997, Analogman and Wolfpack for your inputs. I didnt realize that the engines were throttled back to their minimum throttle 6 seconds before MECO.
Any other input is welcome.
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#3163
by
mkirk
on 17 Jun, 2014 20:41
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We all know about the SSME's #2#3 have a "Start" position, then once up and burning before liftoff, and then move into their "Launch" configuration.
1)I was wondering if the SSME's have a certain position for MECO(Main Engine Cut Off)?
2) If there is a certain MECO gimbal position, is it similar/same to the "Start" position?
TIA
As far as I know there is no analogous “MECO shutdown position” similar to the “start position” you mentioned in your question.
Guidance does enter a trim (positioning) bias late in powered flight because steering is so dynamic going into MECO, but I am not aware of any specific MECO position requirement for managing the shutdown loads.
While it is true that keeping the engine bells from colliding is an important consideration, the “start position” (as it was explained to me) is really about loads management of the entire stack (i.e. twang) during the thrust buildup. The other day I just happened to have a discussion over lunch about thrust buildup with some of the folks involved in evaluating/quantifying the “Twang” pre and post STS-1. As it turns out, the timing of those events did not go quite as expected or planned for STS-1.
Mark Kirkman
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#3164
by
Hog
on 17 Jun, 2014 20:53
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In the STS-93 mission control video where the booster and its backroom loop can be heard, what are the controllers looking for on their data screens for them to make the call "ignition" ?
Thanks
As in the call being made here at 1:01 of this video.
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#3165
by
mkirk
on 17 Jun, 2014 21:22
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In the STS-93 mission control video where the booster and its backroom loop can be heard, what are the controllers looking for on their data screens for them to make the call "ignition" ?
Thanks
It’s really a combination of cues. Among them are the MCC (main combustion chamber) pressures, discharge pressures and temperature for all the turbopumps, valve positions, engine status word (aka feedback from the main engine controllers mounted on the engines), etc.
I've attached screen grabs of a couple of the primary engine related displays in the Mission Control Center to give you an idea of what the data looks like coming into the room.
Mark Kirkman
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#3166
by
alk3997
on 18 Jun, 2014 04:27
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I read through the responses and would like to add a few more points,
1) There was always a chance that an engine or even all three could shutdown during ascent at any point. Therefore relying on moving the engines at MECO to avoid transient movement of the bells or of the bell shape to avoid impacting another bell would be a problem during an unexpected shutdown. The natural position of the bells for performance was such that the bells would not flex into each other at shutdown.
2) We only had a few seconds of margin for reaching nominal orbit, so anything that would cause a loss of performance (such as moving the bells away from cg thrusting) would probably not have been good for getting payload into orbit. Of course since anything impacting the bells (see STS-95) could cause a hydrogen leak, a performance impact would have been preferable to the leak. But, it wasn't necessary.
3) Pre-launch gimbaling was done by KSC using ground LPS commands. The end of the T-4 minute ground sequence would then leave the engines in their desired start-up position. The launch position was then commanded by the GPCs after ignition.
All SSME movement post-landing was planned movements initiated by the crew using their onboard displays (SPEC 104 maybe? - you would think I could remember that after all those landings in the MER waiting for that sequence to complete).
Andy
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#3167
by
Hog
on 18 Jun, 2014 12:10
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All great input M Kirk and alk3997.
I dont know where I gained the working assumption that engine bell collison was a "primary" reason for a "start" engine position that transitions into a "flight" position after ignition. Controlling application of forces during the "twang" movement didnt even factor. This control of forces no doubt explains the engine startup sequence and shutdown sequence that was used during FRR's. Engine 3 (Edit-#1) being furthest away from the CG of the stack would induce a greater moment of inertia than engines 1(Edit-#3) and 2 each on their own, all else equal. Light up 2 and 3 in a position vectored from the vertical thrust centerline, then light up #1, then gimbal 2 and 3 towards each other, then reverse the process upon termination of a successful FRR(Edit-Flight Readiness FIRING-not Review). Having 2 and 3 splayed outwards slightly during "ignition" would add a measure of lateral stability. Such a dynamic event, even though the bottom portion of the SRB's remain completeley static on their mounts.
Also makes great sense that in the event of an engine cut during entry, that the engine would be shutdown rapidly regardless of its "flight" gimbal position.
Here is the anomaly showing the drag chute door contacting engine #1 bell during main engine startup. (note:the Mission Elapsed Time was Zero, trouble before the mission officially started)
SSVEO IFA List Date:02/27/2003
STS - 95, OV - 103, Discovery ( 25 ) Time:03:56:PM
Tracking No Time Classification Documentation Subsystem
MER - 1
MMACS-01
MET: 000:00:00:00
GMT: 302:19:19
Problem FIAR
SPR
IPR 96V-0001
IFA STS-95-V-01
UA
PR
STR,MECH
Manager: Mike Porter
562-922-3887
Engineer: Jeff Goodmark
281-483-0347
Title: Loss of Drag Chute Door (ORB)
Summary: During Main Engine ignition at approximately T-5 seconds, ground-based photography showed the drag chute door detach from the Orbiter and impact the rim
of SSME bell #1 during its downward descent. In-flight evaluation of the condition led to a decision to not deploy the chute for landing. Initial postlanding inspection
showed that the drag chute remained in place undisturbed throughout the flight. Follow-on inspection to determine the condition of the chute and other components in the
drag chute compartment is in work, as is an investigation and failure analysis to determine the cause of the door failure.
|
All discoveries of the mind lead to further exploration, hence a further question.
Aside from the changes to the thrust buildup portion made before and the actual STS-1 mission, were there any changes made to the FRR(Edit Flight Readiness FIRING) procedures as they occured during the STS program?
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#3168
by
alk3997
on 18 Jun, 2014 13:42
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Actually we lit the engines in 3, 2, 1 sequence. I did some of the STS-26 and STS-49 FRF flight software testing and I used the countdown as a way to remember the sequence (it still has stuck). BTW, they were 120 mSec apart in start time.
Engine #1 was in the top position much like bowling pins are numbered.
FRF = Flight Readiness Firing - Testing the MPS/SSMEs
FRR = Flight Readiness Review - a meeting to make sure the vehicle was ready for flight
The shutdown positions were not modified for the FRF. We simply shutdown in 1,2,3 order as if lift-off hadn't occurred (which it hadn't).
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#3169
by
iskyfly
on 18 Jun, 2014 15:00
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Thank you Mark re: ignition.
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#3170
by
Hog
on 18 Jun, 2014 16:01
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Actually we lit the engines in 3, 2, 1 sequence. I did some of the STS-26 and STS-49 FRF flight software testing and I used the countdown as a way to remember the sequence (it still has stuck). BTW, they were 120 mSec apart in start time.
Engine #1 was in the top position much like bowling pins are numbered.
FRF = Flight Readiness Firing - Testing the MPS/SSMEs
FRR = Flight Readiness Review - a meeting to make sure the vehicle was ready for flight
The shutdown positions were not modified for the FRF. We simply shutdown in 1,2,3 order as if lift-off hadn't occurred (which it hadn't).
Apologies on the FRF/FRR and SSME 1,2 3 positions confusion, inconsistent numbering at best(edit-on my part). Thanks for the corrections.
Shuttle when viewed from the rear, wheels on the ground #1 on top, #2 bottom left, #3 bottom right, OK great.
I like your mneumonic device, the actual coundown to remember engine ignition sequence, that will help.
1)So whenever the SSME's were shutdown in a nominal fashion as a triplet, the sequence will always be engines #1, #2 and #3 no matter if its a FRF on the pad or MECO sequence on orbit?
1 b) In other words, anytime the SSME's are nominally started and shutdown as a triplet, SSME in position #1 will always have the shortest burn time of the three?
2)Comparing the STS-26 RTF FRF's for Discovery, and the STS-26 FRF for Endeavours maiden flight, from your perspective, were there any differences or improvements between the two?
Thanks again, and as always anyone with any sort of input, please feel free to contribute.
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#3171
by
psloss
on 18 Jun, 2014 16:23
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1)So whenever the SSME's were shutdown in a nominal fashion as a triplet, the sequence will always be engines #1, #2 and #3 no matter if its a FRF on the pad or MECO sequence on orbit?
1 b) In other words, anytime the SSME's are nominally started and shutdown as a triplet, SSME in position #1 will always have the shortest burn time of the three?
Same thing as the assumption before with engine gimballing -- shutdown for an abort on the pad isn't the same thing as shutdown at the end of powered flight. As noted before, there's a different set of conditions and criteria. The engines were shutdown together in flight -- with the exception of STS-125, there's video of all the MECOs beginning with STS-114 from the ET camera.
The pad abort shutdown sequence changed at some point in the 80s, probably during the first RTF; originally it was 1, then 2-3 essentially at the same time. After that, it was 1, then 2, then 3.
2)Comparing the STS-26 RTF FRF's for Discovery, and the STS-26 FRF for Endeavours maiden flight, from your perspective, were there any differences or improvements between the two?
STS-26 was a mission/flight designation -- there was no "STS-26 FRF" for Endeavour. As alk3997 noted, Endeavour's first flight was STS-49; the FRF was conducted during that launch campaign.
There's a lot of reference material online -- for example, Mark (mkirk) has written several posts in this thread and the previous four Q&A threads. He also wrote a nice main engine overview here:
http://www.interspacenews.com/FeatureArticle/tabid/130/Default.aspx?id=2130
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#3172
by
psloss
on 18 Jun, 2014 16:48
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#3173
by
Specifically-Impulsive
on 18 Jun, 2014 17:36
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At a nominal MECO the shutdown enable/shutdown command pairs went to all 3 engines simo.
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#3174
by
wolfpack
on 18 Jun, 2014 18:20
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At a nominal MECO the shutdown enable/shutdown command pairs went to all 3 engines simo.
Would make for some interesting torque moments at MECO if they didn't!
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#3175
by
wolfpack
on 18 Jun, 2014 19:23
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#3176
by
alk3997
on 18 Jun, 2014 23:52
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As mentioned previously, all inflight planned MECOs shutdown simultaneously. Trying to hit a specific MECO target while shutting down at different times would have been more difficult but do-able but really wasn't necessary. Again, performance being important you would have given up performance with a staggered shutdown as well.
STS-26 FRF was done for return to flight validation while STS-49 FRF was done for the first flight of OV-105. The only difference I remember is that prior to STS-26, the FRFs involved special flight software patches of the code. With STS-26 we changed the data (I-Loads) instead that went into the flight software. It was subtle from the outside but was important from a testing standpoint since we didn't modify code.
I seem to remember the total time of engine firing was slightly different but I have no data to support that memory.
Andy
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#3177
by
Hog
on 22 Jun, 2014 01:30
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1)So whenever the SSME's were shutdown in a nominal fashion as a triplet, the sequence will always be engines #1, #2 and #3 no matter if its a FRF on the pad or MECO sequence on orbit?
1 b) In other words, anytime the SSME's are nominally started and shutdown as a triplet, SSME in position #1 will always have the shortest burn time of the three?
Same thing as the assumption before with engine gimballing -- shutdown for an abort on the pad isn't the same thing as shutdown at the end of powered flight. As noted before, there's a different set of conditions and criteria. The engines were shutdown together in flight -- with the exception of STS-125, there's video of all the MECOs beginning with STS-114 from the ET camera.
The pad abort shutdown sequence changed at some point in the 80s, probably during the first RTF; originally it was 1, then 2-3 essentially at the same time. After that, it was 1, then 2, then 3.
2)Comparing the STS-26 RTF FRF's for Discovery, and the STS-26 FRF for Endeavours maiden flight, from your perspective, were there any differences or improvements between the two?
STS-26 was a mission/flight designation -- there was no "STS-26 FRF" for Endeavour. As alk3997 noted, Endeavour's first flight was STS-49; the FRF was conducted during that launch campaign.
There's a lot of reference material online -- for example, Mark (mkirk) has written several posts in this thread and the previous four Q&A threads. He also wrote a nice main engine overview here:
http://www.interspacenews.com/FeatureArticle/tabid/130/Default.aspx?id=2130
I dont remember pad aborts being discussed, but that is another mode/method /config of shutdown. Thanks for the info. And the links. Almost done the previous threads, I have seen some MKirk entries.
Your 2nd point, should have read STS 26 FRR and STS 49 FRR, in the manner which alk3997 had listed. Of course there was no STS-26 "ENDEAVOUR FRF", tough to do without such an orbiter in existence. My apologies.
Thanks for the discussion everyone, a nominal on orbit MECO is commanded simutaneously. Excellent!
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#3178
by
Hog
on 22 Jun, 2014 01:34
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At a nominal MECO the shutdown enable/shutdown command pairs went to all 3 engines simo.
Would make for some interesting torque moments at MECO if they didn't!
Different than a single engine out during powered flight?
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#3179
by
alk3997
on 22 Jun, 2014 01:41
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At a nominal MECO the shutdown enable/shutdown command pairs went to all 3 engines simo.
Would make for some interesting torque moments at MECO if they didn't!
Different than a single engine out during powered flight?
Yes, because you would still have two SSMEs operating during powered flight. With a staggered shutdown you would have had (at some point) only one engine left controlling. There were special modes for control with a single SSME.
At nominal MECO, it would have still be controllable but again, not necessary.
