The Construction of Space Shuttle Launch Complex 39-B
A very personal and technical written and photographic history, by James MacLaren.
The PGHM: A Mechanical Astoundment.
Wish me luck with this one, ok?
I'm gonna need it.
The
Payload
Ground
Handling
Mechanism.
Everybody out on the Pad pronounced it "piggum."
Or maybe "pigg'm", with the accent on the first syllable.
I have set foot upon it, at Pad A, more than once, even, but I never "worked" on it.
Beyond the Lift and emplacement of the large Bridge Beam which it hangs beneath, my experience with it is woefully inadequate, and after all these years of searching, I still have not found the first trace of any of the 79K Drawing Package(s) from which it was originally fabricated and erected.
I'm flying blind with this one, and am going to try and steer us to a safe landing in the black of night, without radar, without an altimeter, without lights, without anyone in the tower go help guide me in, without... anything.
And I shall begin by asking, "Why?"
Why PGHM?
Why did they even
need a thing like this six-story tall, ridiculously over-the-top, precision-monstrosity in the first place?
And to start feeling our way along with this stuff, we need to temporarily forget all about the
Payloads which the PGHM handled, and instead give the free-standing Space Shuttle, sitting there at the Launch Pad on top of the MLP, a closer look.
The Space Shuttle is the literal size of a high-rise hotel.
It is
tall, and it is also
wide.
And it
doesn't stay put!
The damn thing
moves around, even while it's just
sitting there, unfueled, not planning on going anywhere any time soon.
And we ask ourselves, "How?"
"How might it
move, even though it's just sitting there?"
And that is the question that opens up the hidden entrance to a Rabbit Hole which is going to take us deep underground, into a labyrinth of pathways and side-chambers that turns out to be
much larger, and
much more complicated, than we might have imagined it to be, before we entered it.
And oh by the way, the Orbiter also
moves while it's
moving and by that I mean that when it's on its way uphill, during powered ascent, it flexes and twists. Not much, but... enough. Enough that it has to be accounted for by the system which holds the Payload in place, inside of the Payload Bay, or otherwise the flexing and twisting might
break that Payload in some way, and this we do not want. But let's not go there just yet. For now, we're firmly attached to the ground, just sitting there, doing nothing, minding our own business.
All structures move.
From
birdhouses to
buildings to
bridges, and a lot of other places too, things we imagine to just be "sitting there" are in fact,
constantly moving, changing size and shape.
And, although your eyes will never pick it up, they move
enough, that when they get large enough, or complicated enough, or made out of differing materials enough, we will have to
account for the mind-boggling amount of different ways that different parts of them, in addition to the whole thing itself,
will be moving around,
and transmitting and passing-along developed loads. Even while it's just
sitting there, minding its own business, nevermind what starts happening when we go from purly static loads to any kind of
dynamic loads. And if we
don't incorporate these ineluctable truths into our design...
...
Bad Things will happen.
These invisible (to our everyday eyes, anyway) motions will cause things to
tear themselves apart, where the
developed loads are
always seeking
the weakest part of the overall
system, and oftentimes that weakest part turns out to be quite small and otherwise utterly insignificant at first glance. A bolt. A weld. The mortar between bricks in a load-bearing wall. The list is endless. And this stuff
never sleeps. This occurs slowly, or maybe not so slowly, and either way, we need to
account for it, so as we don't tear ourselves, or perhaps something nearby or even in direct contact with us, to pieces.
We like and expect our
structures to stay put, right where we built them, and for that to happen,
allowances must be made.
In most cases, with large-enough and homogeneous-enough structures, our main two areas of concern are the size and shape alterations caused by
thermal expansion and contraction, and
wind loads. Often enough, the
weight of whatever our structure might be holding up will also enter into the picture, and we can never forget that end of things too, for stuff that's going to be carrying weight which can
change.
A bridge, at 3:00am local time, when traffic is essentially non-existent,
has a different size and shape than it does at 5:30pm local time, during rush-hour, when it's pretty much carpeted with cars and trucks. The load changes, and the bridge flexes as that load increases and decreases over time, and the size and shape of the whole bridge itself, as well as the components that make it up,
changes in lockstep with the flexing. And then of course if you live in
California, or perhaps the New Madrid Seismic Zone, you're gonna have to plan for the ground you're sitting on top of to occasionally start
dancing around, but that's taking us farther away from our present discussion than we need to be going, so we get to ignore it for now, along with a surprising amount of other things too, and I'm not gonna be getting into any of it, and if you want to research the sorts of things that can cause "fixed" objects to become...
unfixed, in any number of strange and wonderful, and occasionally
lethal ways... be my guest. Have at it. But not here. Not with me. And not now, ok?
Our bridge is also
shorter at 3:00am when it has cooled off overnight, than it is at 5:00pm when the sun has been pouring heat into it all day. Going from summer to winter and back to summer again only adds to the fun. Things
move. And if the main components in our bridge are not designed and constructed to permit them to
move, in relationship to one another, Bad Things will happen.
And as it flexes, the bridge pushes and pulls against itself, and its foundations, and if it's not allowed to
move, those pushing and pulling forces can easily become way more than enough to cause something to fail. To break. To come apart. To buckle. To fracture. To... you name it, and none of it is good, and bridges occasionally
come down as a result of this stuff, and people get killed. And it's Big Deal when it happens, and formal investigations are undertaken. And whatever it was that brought the bridge down, whether it was poor maintenance, poor construction, poor design, stupendously bad luck in the form of some concatenation of events that nobody in their right mind would
ever believe might one day happen, or some combination of these things, it needs to be FOUND, and it needs to be
accounted for, in all subsequent cases where similar conditions might be
found. Unless of course you don't mind killing people every so often as part of your ongoing efforts to save money on construction or operation, and the history of
that kind of thinking will keep you wide awake into the wee hours of the morning, once you start researching it.
Businessguy will fucking
kill you without the slightest remorse about it, as part of his never-ending efforts to stuff more cash into his own pockets, despite the fact that those pockets are
already overflowing with enough cash to operate a medium-sized nation-state. The sociopathic motherfucker can
never have enough, and you can
never let down your guard when you're dealing with him, or that which he causes to be made. People bereft of empathy will
hurt you. Make a note of it, ok?
But with the Space Shuttle, we've got Good People, and enough budget, so we're not gonna be cutting any goddamned
corners with any of it.
Or at least
this time we're not.
And the Space Shuttle is still
moving even as it sits there minding its own business, so we're gonna need to get to the bottom of that shit, or otherwise we might
break it, and nobody wants that to happen, right?
The amount that the Stack, sitting on top of the MLP, out at the Pad, can move around, side-to-side (wingtip-to-wingtip from the vertically-standing Orbiter's point of view), along the axis defined by the centerlines of the two SRB's that are holding the whole works up, is essentially negligible. The two SRB's are pinned down by the four separate hold-down bolts per Solid, and the Solids are separated by a wide enough distance, that the whole assembly forms a quite-rigid structure in that side-to-side direction. So we ignore that part of it, ok?
Furthermore, I do not have any figures at hand for how much the stack
lengthens and shortens vertically, because of thermal expansion and contraction, but I do know that it's enough that they've gotta pay attention to it, and I may or may not return to that end of things later, but for now the thing that
really needs to be dealt with, by the PGHM, is flexion (with the center of rotation for the "joint" being down where the SRB's are bolted to the MLP) of the whole Stack,
along the top-to-bottom of the fuselage axis, and since the Orbiter is standing on its tail while out at the Pad, this top-to-bottom of the fuselage direction is expressed as a side-to-side movement of things, where the Orbiter meets the RSS, and of course the PGHM which dwells within it.
The Orbiter sways in the breeze, and it moves first closer to the RSS/PGHM and then farther away from the RSS/PGHM, and then back again, endlessly cycling, in a
front-to-back sense when looked at from the PGHM's point of view, and there's a resonance there, and it has a set of characteristic distance-of-sway and frequency of oscillation modes for differing winds which will be impinging upon the widest aspect of the Orbiter, across the breadth of its wings, which of course make up most of the Orbiter's
Sail Area, so that's where we're going, ok?
Sail area.
We've been here before.
More than once, in fact.
And this is not a graduate-level course on how to design for this stuff, so above and beyond informing you that, if
you wind up desiging for this stuff, somewhere, somehow, then
you had best be on top of this shit, or otherwise people are going to be getting
killed when your structure
collapses, and if you manage to kill a bunch of people because of your own ignorance and negligence, you will very much regret having done so, and you will be given lots and lots and lots of time in the Graybar Hotel to do so.
And I don't have a
lot by way of proper citation materials for where we're about to go, but I think I've got
just enough.
I've got Bemis and Rado.
We've already been introduced to them, but I'll introduce them to you again, here and now, because we're going to be using them
a lot, and that's not so much because of how
wonderful they are, but because...
...there's nothing else out there, really, and we're...
...stuck with them, like it or not.
Bemis more or less gives us the General Overview of the PGHM.
And Rado zeros in on that part of the PGHM which makes it so very PGHM'y, and which is the Special Sauce of the whole damn thing...
...but he's done such a fuck-all lousy job of it that we
still don't quite understand how it works
exactly, but we at least get the overall
sense of the thing, and I've got a feeling that we should be most very thankful for such crumbs as we've been able to scrape together...
...because otherwise, this is one hell of a
bleak landscape.
A vast wasteland.
Containing nothing of use and nothing of interest.
Dammit!