Posts: 96
From: Santa Clara, CA, USA
Registered: Nov 2009

Posts: 3811
From: Midwest, USA
Registered: Jul 2005

posted 06-07-2010 02:52 AM               

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Nothing necessarily stops the tilt (or "twang"). It's just that the timing of the SRB ignition is designed to take place when the stack returns to vertical. If the SRBs didn't ignite (which has happened for on-pad aborts that have occurred after main engine start), the stack would continue to twang back and forth until it dampened out.

Here is the flight readiness firing of Endeavour's main engines prior to her first flight. You can see the twang in action throughout the firing (and for a time afterwards). It's also clearly visible in this STS-26 test firing as well.

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

posted 06-07-2010 02:14 PM               

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The shuttle sideways movement (aka the "twang") is sort of akin to a pendulum swing on a clock. So it swings one way, then eventually comes back to vertical. The SRB ignition has to be timed to go off right when the SRBs are pure vertical since when they pressurize, they snap vertical anyway. Otherwise the resulting twist loads from a pair of SRBs igniting and snapping the stack vertical could damage the shuttle internally. Worst case scenario, the damage could be bad enough to destroy the vehicle.

On STS-1, there was a slight mis-timing and it resulted in a delay to process Columbia for STS-1 STS-2 since hidden damage was located in the Forward RCS module. If Columbia had tried to fly with the damage, the FRCS likely would have ruptured a line, sprayed the nose with flaming hypergolic propellant and the result would have been the crew having to test the ejection seats.

Posts: 96
From: Santa Clara, CA, USA
Registered: Nov 2009

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

posted 06-08-2010 02:11 AM               

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Complex structures are usually designed to have a little flex in them to take the extreme forces better then a rigid structure (plus a structure with a little flex can be built lighter weight, a big plus in a rocket design).

In the SRBs case, I don't know for sure if it is the whole SRB flexing or just the joints, but since they are long hollow segmented steel tubes filled with solid fuel which have a large side load on them (the orbiter firing its engines), they can flex a bit.

Based on what I have read, it is the shuttle structure flexing, NOT the pad supports. The entire stack is supported by the SRBs via the eight hold down posts (four per booster) on the MLP. When the SRBs ignite, the explosive frangible nuts that make up the hold down posts fire, severing the stack's connection with the pad. At SSME ignition, there are three support parts that are still attached to the shuttle. Two of them are the tail service masts at the base of the orbiter near the OMS pods (these are used to fuel the launch stack with LHX and LOX) and the gaseous hydrogen vent arm mid-way up the ET. These have flexible connections and they come away at first motion of the stack at liftoff. They don't support or dampen any of the side forces of the shuttle during the twang.

As to what brings the stack back vertical, it is simple physics. If you were to twang a door spring (the kind used in houses to keep a door from banging into a wall) you'll see it vibrate back and fourth as it dampens the forces. The shuttle stack almost acts the same way, but its speed is much slower. Watch footage of a flight readiness firing and you can see the stack move back and fourth a couple times during the engine thrust phase. Eventually when the engines are cut, the stack moves back and fourth a couple more times with less motion before it comes to rest back at pure vertical.

Posts: 49
From: West Sussex, United Kingdom
Registered: Jul 2010

posted 08-01-2010 06:47 AM            

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Jay, I'm interested on your STS-1 comments. Do you have a reference for this "mis-timing"?

Did it occur during the FRF on 20 February 1981 (in which case, surely there was no SRB ignition to mis-time)? Or did the event occur during the actual 12 April 1981 launch (in which case I presume Columbia flew with a badly damaged FRCS module? Confused.

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

posted 08-02-2010 08:05 PM               

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Actually, that is a typo on my part (I didn't catch it until now and have corrected it) as I meant to say "STS-2". The SRB mis-timing occurred on STS-1 and the damage didn't get detected to the FRCS module until it was getting ready for installation on Columbia for STS-2. The story was told in the book "Memories of Columbia" by Randy Avera (pages 200 to 207). Randy was a NASA engineer who worked at KSC from about 1978 until 1991.

While pains were taken to make sure the SRB firing was timed perfectly, apparently somebody didn't take into account that all the electrical wire resistance from the orbiter's GPC to the igniters on the SRBs. The delay was about 250 milliseconds, meaning the stack was ever so slightly off vertical. It caused at least one of the titanium support braces in the FRCS to buckle and that caused the oxidizer tank to rotate a little. This rotation caused a flexible "goose neck" shaped drain line to flex to its physical limit. If the line had rotated any more, it likely would have ruptured since it could no longer flex, spilling oxidizer into the nose of the orbiter and doing some very bad things to it. The line didn't fail on STS-1 of course, but add vibration loads from a second launch and the results could have been catastrophic.