From a structural point of view, what is holding up the orbiter on the launch pad prior to a launch? Are the tail service masts more than just a collection of conduits and plumbing?

Or, are the solid boosters supporting the whole stack?

Tail Service Mast


Prior to launch the Orbiter was mounted on the External Tank and the External Tank was mounted to the Solid Rocket Boosters. The entire assemblage was referred to as "the stack".

The entire weight of the stack was supported by four posts at the bottom of each Solid Rocket Booster (eight total). The boosters were attached to the posts by large bolts (aka "studs"). At liftoff time the nuts on the bolts were exploded apart and the bolts retracted downward.

The tail service masts did not provide structural support to the stack.

Picture from 1982 Shuttle Press Reference Manual.

enter image description here

  • $\begingroup$ Was the reason for protecting the stud after falling so that it could be inspected and such, without additional damage? $\endgroup$ – Brad Aug 6 '18 at 15:32
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    $\begingroup$ The bolt shot downwards because it was highly pre-tensioned. The "stud deceleration stand" was filled with sand to absorb the impact. I am sure that it got inspected as you say but the idea was also to prevent damage to the Mobile Launcher from flying bolts. Late in the program there was a problem with the bolts not always retracting and they may have added weights to pull them down - I'd have to research that. $\endgroup$ – Organic Marble Aug 6 '18 at 15:35

The only hold-downs are the eight SRB bolts.

This leads to the "Shuttle twang" when the main engines start. They're pushing to one side of the SRB hold downs, so they bend the stack just a bit. From "Space Shuttle Twang" by Tom Irvine in the Vibration Data newsletter (2010):

The orbiter’s SSMEs are offset from the vehicle stack’s center of gravity. The offset thrust from the Shuttle's three main engines causes the entire launch stack to pitch down about 2 meters at cockpit level, after the main engines start, but while the solid rocket boosters are still clamped to the pad.

This motion is called the "twang.” The boosters then flex back into their original shape due to internal stiffness forces. The launch stack pitches slowly back upright. This back-and-forth motion takes approximately six seconds.

At the point when the vehicle stack is perfectly vertical again, the hold-down post pyrotechnic nuts are ignited, the boosters ignite and the vehicle lifts off the pad. The twang is shown in the following video:

It's an engineering question as to why NASA didn't decide to hold the tail of Orbiter in place, rather than dealing with this motion. I haven't found that written down anywhere yet.

What this doesn't say is whether there's an static support before the twang starts: Is the shuttle sitting flat on something, without hold downs, just a surface to bear weight? This seems unlikely, because any over-twang would come back and hit any support pretty hard, but with the information I have I can't rule it out.

  • $\begingroup$ Awesome thanks... yeah, the "twang" was going to be my follow-up question... as in if some support of orbiter support was removed at main engine ignition (allowing the movement), how they re-attached it in the event of an abort. I guess all that's avoided by just skipping that part! Thanks for the info. Amazing that the boosters hold all of that up... I suppose much of those engines is just support structure? $\endgroup$ – Brad Aug 6 '18 at 15:03
  • $\begingroup$ @brad I haven’t done the calculation, but I expect that the strength required of the SRB structure & Orbiter attachments goes up after launch, I.e. the static load is the easy part because the SRBs accelerate the Orbiter and External Tank upward. $\endgroup$ – Bob Jacobsen Aug 6 '18 at 15:11
  • $\begingroup$ I don't understand what this means: "What this doesn't say is whether there's an static support before the twang starts: Is the shuttle sitting flat on something, without hold downs, just a surface to bear weight?" Your answer about the hold-down bolts is correct. The stack is sitting on the hold-down post supports. The nuts are blown at T0 and the stack lifts off. If you can explain what you are asking, I could probably answer it. $\endgroup$ – Organic Marble Aug 6 '18 at 15:11
  • $\begingroup$ @OrganicMarble Sorry to not be clear. There could (as a matter of logic) be something directly supporting the Orbiter without holding it down. The original question was about support. The Orbiter is free to move up, but does something keep it from moving down? $\endgroup$ – Bob Jacobsen Aug 6 '18 at 15:14
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    $\begingroup$ Bob is correct. The thrust of the 3 RS-25 SSME is not enough to lift the orbiter and the external tank, so the SRBs provide additional thrust. Before launch, the load path is from the SRBs to the ET to the orbiter. You can think of the ET as the spine of the entire stack. On the launch pad, the SRBs are the only part of the stack directly supported by the launch pad. The combined thrust of the 3 RS-25 rocket engines is 1,254,000 pounds while the dry weight of the orbiter is 171,000 pounds so the load between the orbiter and ET reverses after launch. $\endgroup$ – user8269 Sep 8 '19 at 20:38

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