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  STS-1 Gaseous Vent Hood Retraction

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Author Topic:   STS-1 Gaseous Vent Hood Retraction
Lunar_module_5
unregistered
posted 06-23-2009 06:16 PM           Edit/Delete Message   Reply w/Quote
Can someone please tell me why, during the countdown of STS-1, the Vent Hood was retracted from the ET at the T-9 minute and holding point in the count?

heng44
Member

Posts: 3387
From: Netherlands
Registered: Nov 2001

posted 06-24-2009 12:26 AM     Click Here to See the Profile for heng44   Click Here to Email heng44     Edit/Delete Message   Reply w/Quote
Don't have the answer handy here at work, but I can say that there were more 'anomalies' in the STS-1 countdown. From memory: the SSMEs ignited at T-zero, then the clock started counting and the SRBs ignited a few seconds later. At that point the clock was recycled back to zero and the Shuttle lifted off.
It was probably determined after STS-1 that retracting the 'beanie cap' closer to liftoff would be better.

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

posted 06-25-2009 04:55 AM     Click Here to See the Profile for Jay Chladek   Click Here to Email Jay Chladek     Edit/Delete Message   Reply w/Quote
The main thing about STS-1 is it was the first time they had done a full count, as such they did find out some things that needed modification for later missions, the LOX vent line retraction would have been a big one and as I recall for even STS-2, they didn't do vent line retraction until about T minus 4 minutes.

SRB ignition and liftoff today typically still takes place about a second or two after the countdown clock reaches zero, so that hasn't really changed since STS-1. Look at the countdown clock if you have the opportunity and you see these days it sort of hangs or counts up until liftoff, then it recycles back to zero and begins to count up again.

The firing of the mains and the resulting twang of the stack forward of vertical and back has to time just right so that the shuttle stack is at pure vertical when the SRBs fire, otherwise there will be some bending forces that could damage the orbiter. I believe this is probably the main factor that dictates why a shuttle doesn't necessarily fire its SRBs right at the zero second mark. The countdown clock tends to be more of a guideline rather then an absolute unless we are talking about a VERY tight launch window of only a second or two (I don't recall a shuttle flight ever having that tight a launch window).

Indeed there was a slight mis-timing on STS-1 as engineers didn't fully take into account that the electrical resistance of the wire used to send the signal to ignite the SRBs would cause a slight delay in transmission of the signal, so the SRBs did fire a milli-second late and bending forces did cause some hidden damage inside the orbiter that wasn't found until preparations for STS-2 were well underway.

All times are CT (US)

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