What he said.
Sent from my iPhone
On Sep 3, 2016, at 6:51 PM, Henry Spencer <hspencer@xxxxxxxxxxxxx> wrote:
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
