[AR] S-IVB-503 explosion (was Re: SpaceX)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 3 Sep 2016 18:51:55 -0400 (EDT)
On Sat, 3 Sep 2016, Michael S. Kelly wrote:
The S-IVB explosion was due to a design flaw of major proportions. The
helium pressure tanks were mounted inside the LH2 tank, to allow more
helium to be stored. The helium tanks were made of titanium, and the
explosion was due to failure of the girth welds from hydrogen
embrittlement.
I regret that I have to contradict Mike on some of the details here. :-)
The best account of the S-IVB-503 explosion is Alan Lawrie's article in
the Dec. 2007 issue of Spaceflight (p. 470); in particular, he talked
extensively with Don Brincka, who was the Douglas test director in the
blockhouse when the stage went up, and was heavily involved in the
investigation.
The S-IVB had both "cold" helium tanks inside the LH2 tank, and "ambient"
helium tanks around the engine on the base of the stage (easily visible in
many photos). It was in fact one of the ambient tanks that ruptured to
cause the accident. (This is confirmed by other shorter accounts,
including p. 186 of "Stages to Saturn" [Bilstein, NASA SP-4206].)
The examination of the debris found that three of the helium tanks (all
ambient tanks) had split into their hemispheres -- in fact, only one
hemisphere of each tank was found. (The test area was an old gold-mining
site which had been reworked extensively to recover gold residues from the
tailings, so the ground was uneven, with ponds and lakes everywhere.
Various bits of missing debris, including the missing hemispheres, almost
certainly were underwater; Navy divers, hampered by bad weather and thick
mud, unfortunately didn't find much.) Two of the found hemispheres, from
tanks 65 and 66, were blackened on the exterior, but the hemisphere of
tank 69 was bright and shiny. 69's rupture had come first, so its
hemispheres had departed the vicinity *before* the fire and explosion,
hence the clean surface. Its lower hemisphere, the one found, had gone
down into the flame trench; its upper hemisphere had gone clear through
the stage and out the top. 65 and 66 had come apart during the blast.
The problem wasn't that the tanks were titanium; the problem was use of
the wrong welding wire for the girth welds (combining two hemispheres into
a single spherical tank), and general neglect of quality control.
The tanks were titanium 6Al 4V, a fairly standard alloy, and the weld wire
should have been the same alloy. In fact, the shipping container said
that the shipment of three spools of weld wire was all 6Al 4V. However,
if you read the accompanying test certificates carefully, two of the three
spools were Ti 55A -- straight titanium -- not 6Al 4V. The result was
welds that were 30-40% under strength to start with; all the tanks were
successfully proof-tested at 1.6x operating pressure, but some of them
were probably very close to failure then. Later, as the tanks were
pressure-cycled -- and those tanks were cycled many times -- needles of
titanium hydride gradually formed at the interface between the two metals,
making the interface layer brittle and further weakening the tanks.
Aggravating the problem, the tank subcontractor's QC paperwork wasn't
being done properly. Worst of all, Douglas wasn't paying attention.
Each batch of new tank welds was accompanied by a weld test ring, which
went out to an independent test lab. Test ring #1 had been fine and had
been used to qualify the subcontractor. Test ring #2 (for tanks 04-60)
had *failed* testing; Douglas claimed ignorance of this until the
investigators pointed out that they had received and paid an invoice for
the testing! Test ring #3, for tanks 61-75, also failed testing, and
again Douglas hadn't noticed.
There might have been more of a public fuss about this, had the explosion
not been totally overshadowed by the Apollo fire a week later.
The fix was to move tank manufacture in-house at Douglas, tighten up
quality control in general, and inspect existing tanks to weed out the
ones with bad welds. Plus repairing the test stand and replacing the
S-IVB stage. Plus repairing assorted damaged cars -- the explosion had
happened in late afternoon, just as the day shift was leaving the test
site, and there had been a number of minor car crashes due to distracted
drivers!
Henry
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