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#580
by
kneecaps
on 16 Feb, 2008 20:46
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I think may also be due to the difference in first and second stage guidance too. First stage MM102 flys a 'canned' open loop type guidance. However once MM103 is entered guidance becomes closed loop and can respond to changes in performance etc etc.
I would imagine that it's a better time to be making inputs to the 'state' of the vehicle in MM103 than in MM102. This just speculation on my part.
Finallly, maybe somebody can help out as I don't have the numbers to hand...does the stack actually get high enough during 1st stage to even safely burn the OMS engines?
Pete
P.S. Good question though...i've never though about it
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#581
by
Jorge
on 16 Feb, 2008 20:53
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kneecaps - 16/2/2008 3:46 PM
I think may also be due to the difference in first and second stage guidance too. First stage MM102 flys a 'canned' open loop type guidance. However once MM103 is entered guidance becomes closed loop and can respond to changes in performance etc etc.
I would imagine that it's a better time to be making inputs to the 'state' of the vehicle in MM103 than in MM102. This just speculation on my part.
Fairly good speculation, I would think.
Finallly, maybe somebody can help out as I don't have the numbers to hand...does the stack actually get high enough during 1st stage to even safely burn the OMS engines?
IIRC, minimum altitude for OMS is 85 kft and SRB SEP is around 150 kft.
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#582
by
Lee Jay
on 16 Feb, 2008 20:59
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Jorge - 16/2/2008 2:53 PM
kneecaps - 16/2/2008 3:46 PM
I think may also be due to the difference in first and second stage guidance too. First stage MM102 flys a 'canned' open loop type guidance. However once MM103 is entered guidance becomes closed loop and can respond to changes in performance etc etc.
I would imagine that it's a better time to be making inputs to the 'state' of the vehicle in MM103 than in MM102. This just speculation on my part.
Fairly good speculation, I would think.
Finallly, maybe somebody can help out as I don't have the numbers to hand...does the stack actually get high enough during 1st stage to even safely burn the OMS engines?
IIRC, minimum altitude for OMS is 85 kft and SRB SEP is around 150 kft.
I always *try* to answer my questions on my own before asking, and I did find another of your (Jorge) posts in this thread that suggested ~70k and up was safe for OMS ignition, which I did know was pre-SRB sep.
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#583
by
Oberon_Command
on 16 Feb, 2008 21:04
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What would happen if the OMS engines were ignited at ground level? Would they just produce very little thrust, or would something bad actually happen?
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#584
by
Lee Jay
on 16 Feb, 2008 21:38
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Oberon_Command - 16/2/2008 3:04 PM
What would happen if the OMS engines were ignited at ground level? Would they just produce very little thrust, or would something bad actually happen?
One of the rocket folks will correct me if I'm wrong, but it's a vacuum-optimized engine. In a rocket engine, you'd like to match nozzle exit pressure with absolute pressure outside. That's not possible when the outside is at zero pressure (infinite pressure ratio), but it's still very low. Low pressure inside nozzle + sea-level pressure outside nozzle = nozzle goes "squash - just like grape".
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#585
by
Jorge
on 16 Feb, 2008 21:44
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Lee Jay - 16/2/2008 3:59 PM
Jorge - 16/2/2008 2:53 PM
kneecaps - 16/2/2008 3:46 PM
I think may also be due to the difference in first and second stage guidance too. First stage MM102 flys a 'canned' open loop type guidance. However once MM103 is entered guidance becomes closed loop and can respond to changes in performance etc etc.
I would imagine that it's a better time to be making inputs to the 'state' of the vehicle in MM103 than in MM102. This just speculation on my part.
Fairly good speculation, I would think.
Finallly, maybe somebody can help out as I don't have the numbers to hand...does the stack actually get high enough during 1st stage to even safely burn the OMS engines?
IIRC, minimum altitude for OMS is 85 kft and SRB SEP is around 150 kft.
I always *try* to answer my questions on my own before asking, and I did find another of your (Jorge) posts in this thread that suggested ~70k and up was safe for OMS ignition, which I did know was pre-SRB sep.
You're right, I misremembered 85k.
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#586
by
janmb
on 16 Feb, 2008 22:10
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Lee Jay - 16/2/2008 3:38 AM
One of the rocket folks will correct me if I'm wrong, but it's a vacuum-optimized engine. In a rocket engine, you'd like to match nozzle exit pressure with absolute pressure outside. That's not possible when the outside is at zero pressure (infinite pressure ratio), but it's still very low. Low pressure inside nozzle + sea-level pressure outside nozzle = nozzle goes "squash - just like grape".
And even if the effect wasn't bad enough to actually destroy the engines, they would definitely be very ineffective since the nozzle shape and pressures are optimized for little or no ambient pressure.
If I'm not way off here, this is the entire reason for wanting to get rid of external "bell" type nozzles in the first place - you cannot make it both efficient at sea pressure AND in vacuum, it's one or the other - or something in between. Still puzzled the "spike" type engine studies were scrapped, but I'm sure there were good reasons for that. It was a far better engine type on paper at least, losing a large part of the inefficiency of operating a nozzle outside it's optimal ambient pressure range.
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#587
by
janmb
on 16 Feb, 2008 22:15
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Jim - 10/2/2008 10:26 PM
Also, not every satellite is assigned to the shuttle/ISS, only 3 or so.
Structure still applies
Another small follow-up on this...
On the Houston MCC world map with the ground track... The ZOE label is placed approximately over Iran IIRC. This is obviously only the label and has nothing to do with the actual location of the zone, so the question is, to what graphics on that map does the label belong? I see no circle or other area anywhere nearby that could fit to have ownership of this label. Is it the brackets on the ground tracks themselves? Starting shortly after north max and stopping shortly before south max? Or are those indicating something entirely different?
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#588
by
Jim
on 16 Feb, 2008 23:39
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janmb - 16/2/2008 6:15 PM
Jim - 10/2/2008 10:26 PM
Also, not every satellite is assigned to the shuttle/ISS, only 3 or so.
Structure still applies
Another small follow-up on this...
On the Houston MCC world map with the ground track... The ZOE label is placed approximately over Iran IIRC. This is obviously only the label and has nothing to do with the actual location of the zone, so the question is, to what graphics on that map does the label belong? I see no circle or other area anywhere nearby that could fit to have ownership of this label. Is it the brackets on the ground tracks themselves? Starting shortly after north max and stopping shortly before south max? Or are those indicating something entirely different?
The ZOE isn't a circle. It is defined by the intersection of the two TDRSS LOS "circles". It looks like the standard intersection of two circles
The brackets on the ground tracks indicate entering and exiting sunlight for that orbit.
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#589
by
spaceshuttle
on 17 Feb, 2008 04:28
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Oberon_Command asks:
Does anyone know when the last time a shuttle stack rolled out to the pad while another was in flight?
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#590
by
Analyst
on 17 Feb, 2008 06:45
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Lee Jay - 16/2/2008 7:54 PM
The OMS assist performed during launch happens after SRB separation. Why isn't this performed as soon as possible after throttle up, once high enough in the atmosphere to safely fire the OMS engines?
Why is OMS assist performed during powered flight anyway? The rocket equation indicates it would be more efficient after ET sep (you only have the Orbiter then and not the ET dead mass). Why not a OMS-1 or longer OMS-2 burn instead of OMS assist?
Analyst
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#591
by
ChrisGebhardt
on 17 Feb, 2008 07:20
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I thought OMS assist during second stage was used for direct insertion trajectories. Correct?
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#592
by
GLS
on 17 Feb, 2008 09:55
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Trekkie07 - 17/2/2008 8:20 AM
I thought OMS assist during second stage was used for direct insertion trajectories. Correct?
Well, yeah.......
OMS assist is a PE (Performance Enhancement), that was developed to increase the shuttle's cargo capability to the ISS. Other PEs are the SLWT, small changes in the nozell positions during flight to increase performance, the diet the OVs underwent in the late 90s, trajectory changes, etc...
The DI trajectory was first flown in 1984, and basically you use the ET prop (SSMEs) to hit your apogee height target on MECO instead of using the OMS prop (in the OMS-1 burn) after MECO. With DI you save OMS prop (mass), and mass=cargo. DIs are standard since like the early 90s.
So, DIs are the standard, and they added the OMS assist. They are not really related... and I think you could use the OMS assist in a SI flight no problemo.
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#593
by
GLS
on 17 Feb, 2008 10:12
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spaceshuttle - 17/2/2008 5:28 AM
Oberon_Command asks:
Does anyone know when the last time a shuttle stack rolled out to the pad while another was in flight?
I think it was rollout on STS 102 during STS 98. But the rollout on STS 105 happend while STS 104 was on the pad, so STS 105 rolled out even before the previous flight was launched!!!
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#594
by
janmb
on 17 Feb, 2008 10:55
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Jim - 16/2/2008 5:39 AM
The ZOE isn't a circle. It is defined by the intersection of the two TDRSS LOS "circles". It looks like the standard intersection of two circles
That wasn't really the question. The ZOE being the union of the individual LOS for the relevant satellites is pretty obvious and not the issue.
The question is identifying it on the Houston MCC maps. I didn't expect to see the union of the coverage LOS displayed as a separate object, but I have a hard time identifying the individual TDRS LOS areas i the first place (again, on that particular map). Which in turn makes identifying the union between the two impossible.
Are they the huge areas spanning more or less pole to pole, drawn in green and yellow?
(never mind, got it; one large area displayed with yellow line, spanning from the east end of the Mediterranean, almost all the way to the US west coast, and another, smaller area (green line) covering most of the atlantic areas and spanning a bit more south and less north)
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#595
by
DaveS
on 17 Feb, 2008 16:51
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Just a quick OBSS question: How is the sensor packages on the OBSS controlled? Is it from a PGSC or does those use controls on one of the aft flightdeck panels, like the SRMS?
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#596
by
ntschke
on 17 Feb, 2008 16:57
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Probably answered somewhere but my search reults didnt return anything...so here goes.
Why the long time (long IMO anyway) from the crew exiting the ISS to un-docking? I know there must be leak checks performed etc, but can't that be done in a few hours time? Or is there some other reason to lock the doors today and un-dock tomorrow?
Thanks
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#597
by
Jim
on 17 Feb, 2008 17:27
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DaveS - 17/2/2008 12:51 PM
Just a quick OBSS question: How is the sensor packages on the OBSS controlled? Is it from a PGSC or does those use controls on one of the aft flightdeck panels, like the SRMS?
I believe PGSC
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#598
by
kneecaps
on 17 Feb, 2008 18:53
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GLS - 17/2/2008 10:55 AM
basically you use the ET prop (SSMEs) to hit your apogee height target on MECO instead of using the OMS prop (in the OMS-1 burn) after MECO.
Also as far as I understood you are more burning OMS to 'dump' the prop and hence mass, which permits more Delta V from MPS.
I beleive it's also been stated in a previous thread that the OMS assist is preferred from a processing standpoint (tanks full) rather than just putting less prop in the tanks to begin with.
Pete
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#599
by
DaveS
on 17 Feb, 2008 19:22
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Jim - 17/2/2008 7:27 PM
DaveS - 17/2/2008 12:51 PM
Just a quick OBSS question: How is the sensor packages on the OBSS controlled? Is it from a PGSC or does those use controls on one of the aft flightdeck panels, like the SRMS?
I believe PGSC
OK, thanks. Any body else that could confirm or deny this?
