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  STS-26: atmospheric winds launch waiver

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Author Topic:   STS-26: atmospheric winds launch waiver
moorouge
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Posts: 2454
From: U.K.
Registered: Jul 2009

posted 09-18-2009 06:48 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
Has anyone an explanation please.
The launch was delayed 1 hour, 38 minutes to replace fuses in the cooling system of two of the crew's flight pressure suits, and due to lighter than expected upper atmospheric winds. The suit repairs were successful and the countdown continued after a waiver of wind condition constraint was issued. [Source]
Why would light winds cause a violation of launch rules? Strong winds I can understand.

PowerCat
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From: Herington, KS, USA
Registered: Feb 2006

posted 09-18-2009 07:56 AM     Click Here to See the Profile for PowerCat   Click Here to Email PowerCat     Edit/Delete Message   Reply w/Quote
I just happened to run on a old VHS tape I did of the CBS coverage of the STS-26 launch the other day. From the sounds of it, the computers on board Discovery were programmed for higher profile winds, and these lighter winds would affect the stack's performance. I was surprised of lighter winds possibly affecting a launch. STS-51L had the worse case of wind shear prior to vehicle breakup.

Jay Chladek
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From: Bellevue, NE, USA
Registered: Aug 2007

posted 09-18-2009 10:45 AM     Click Here to See the Profile for Jay Chladek   Click Here to Email Jay Chladek     Edit/Delete Message   Reply w/Quote
The flight profile for STS-26 was programmed with seasonal upper level winds at a certain intensity in mind. On the day of the launch, they were lighter then forecasted, so they waited until a little later in the day to fly when the upper level winds picked up in speed to where it was favorable.

Given how the shuttle's onboard computers use 1970s technology and their own programming code, I imagine a programmer needs to jump through a few hoops to program an ascent trajectory into the software, so it would be difficult to do on the fly.

As for the winds, I would liken it to a navigator dead reckoning from one place to another in an airplane over water. If he anticipates the winds going a certain direction into his calculations and then the winds shift or die down, the airplane might end up in a very different spot at the end of the trip.

moorouge
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From: U.K.
Registered: Jul 2009

posted 09-19-2009 05:38 AM     Click Here to See the Profile for moorouge   Click Here to Email moorouge     Edit/Delete Message   Reply w/Quote
Thanks for the replies, but I'm still mystified.

Are you saying that once a trajectory is programmed in, the onboard guidance systems are unable to correct any deviations? Didn't they find that on 51L there were indications that the engines were gimballed to try to compensate for the loss of thrust from the SRB? So what then is the balance between the two and what level of deviation is correctable? Again, a strong wind I can understand, but too little wind?

Jay Chladek
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Posts: 2272
From: Bellevue, NE, USA
Registered: Aug 2007

posted 09-19-2009 11:51 AM     Click Here to See the Profile for Jay Chladek   Click Here to Email Jay Chladek     Edit/Delete Message   Reply w/Quote
I would say the guidance system probably could compensate just fine at liftoff. But knowing how NASA tends to like things in the middle of their tolerance bands and have backups operational before a launch, that was probably the reason for the concerns as opposed to whether or not the launch stack could fly that way.

Given that STS-26R was the first flight after 51L and how trying to operate a system outside the temperature range got them into a bit of trouble on that launch (as we all saw), the launch control team was going to make darn sure they were going to do things by the book this time. Could the mission have flown just fine with lower then anticipated winds? Probably. But they weren't taking any chances just in case something might have caused a problem.

All times are CT (US)

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