Henry,
Thanks for your reporting on this.
Very informative, instructive and valuable
because of the effort you put into it.
You've helped move another "Who'da thunk" on to the checklist for many.
All in all, not so bad a day.
Thanks,
Ed Kelleher
At 11:38 PM 07/20/2015, Henry Vanderbilt wrote:
A more complete writeup of today's presser is at http://www.space-access.org/updates/sau145.html
Stray points that didn't make the writeup:
- there was some mention of the bolt at the base of the strut also - maybe changing the bolt material to inconel? maybe a problem in the grain structure of a bolt? - not real clear from my notes; Musk was talking about the bolt-head of the strut as the likely problem most of the time, so this may have been misspeaking, or me mishearing on a so-so cell connection. Or they may be looking at the bolt also. (Recordings have been posted online, someone ambitious enough could double-check precisely what was said.)
- the infamous "anomalous indications" turn out to be that the He pressure (measured where? in a common manifold, presumably) dropped suddenly, then came back up to nominal. Musk's speculation is that He tank motion opened a leak (at the loose bottle?) but then pinched off a tube (between the leak and the rest of the manifold?) He said not much of a leak would have been needed - He bottles were at 5500 psi - as ullage space was ~2% at that point.
- He seemed definite that there are "hundreds" of such struts in each F9, thousands flown overall, which may indicate they're used in other places than one under each He bottle. He said there are several He bottles in the upper stage, number varies with the mission, and a "whole bunch" in the first stage. Doesn't seem to add up to "hundreds", so more of these struts used elsewhere seems very likely.
- things looked fine in hi-res closeout photos, so assembly is presumed not likely the problem.
- They already load-test the primary airframe, but not auxiliary stuff with high engineering margin. Unclear from today how universally that might change - the He tanks hold-down struts and support structure was what was specifically mentioned as due for pull-test in future.
Henry
On 7/20/2015 7:31 PM, Steve Traugott wrote:
Very cool. That's the sort of thing it smelled like from here -- some
missed-NDT corner case. Folks who know my wife and I (and our
NDT-related business) will understand how gleefully I just now forwarded
this to her. ;-)
Thanks Henry,
Steve
On Mon, Jul 20, 2015 at 1:09 PM, Henry Vanderbilt
<hvanderbilt@xxxxxxxxxxxxxx <mailto:hvanderbilt@xxxxxxxxxxxxxx>> wrote:
Further detail: One steel holddown strut per He bottle, QC via
material cert from external vendor, visual inspection, and ~3x
design margin, thousands of such struts flown previously. Strut
design limit is ~10Klbf pull, max expected load ~3-3.5Klbf, actual
failure at ~2Klbf. Testing of on-hand inventory went through very
large number before finding one that failed at <2Klbf; check on that
revealed bad metallurgy. Immediate fix is a new-design strut, to be
individually pull-tested.
To emphasize, this is a preliminary conclusion. Final results will
need signoff from all interested parties and won't be for a while.
Gotta go do some less hasty writing now...
Henry
On 7/20/2015 1:02 PM, Henry Vanderbilt wrote:
And the (probable, not final) winner is, a failure in a support
strut
holding an He bottle *down* against buoyancy in the LOX. Bottle
then
apparently shot to top of tank, releasing enough He to
overpressure the
tank and cause the failure.
On 6/29/2015 7:36 AM, Henry Vanderbilt wrote:
This strongly implies there was no clear cause in the data
before the
final milliseconds, which rules out a whole class of things
that might
gradually overpressure the second stage LOX tank: Frozen
pressure-relief
valves, a stuck-on helium regulator, a heat source boiling
the LOX.
(Come to think of it, the latter two would also require
stuck-closed
pressure-relief valves.)
As for overpressure sources that split a tank within
milliseconds of
onset, the first thing that comes to mind is a failure under
flight
loads in the high-pressure helium storage bottles-plus-plumbing
submerged in the LOX tank.
A second possibility is some sort of ignition inside the
tank - EG,
something breaking loose and scraping an aluminum surface,
or a brace or
baffle cracking, so a significant area of unoxidized aluminum is
suddenly exposed to LOX.
A distant third would be detonation of just that tank's
segment of a
flight-termination linear shaped charge. (That assumes the
charges are
in separate segments; I couldn't find a detailed description
of the
system.) (Note that any such detonation would not likely
have been
commanded - I can think of no reason to design in the ability to
separately command local charge segments in such a system.)
A secondary implication of "final milliseconds" is that whatever
happened was violent enough to stop data transmission from
the stage, or
at least from the relevant parts of the stage.
Mind, a crack developing in the tank outer skin under flight
loads then
unzipping rapidly would also explain the results - it just
wouldn't
explain the statement about an "overpressure event".
On 6/29/2015 6:12 AM, Ian Woollard wrote:
*Elon Musk* â@*elonmusk* <https://twitter.com/elonmusk>
5h5 hours ago
<https://twitter.com/elonmusk/status/615431934345216001>
"Cause still unknown after several thousand
engineering-hours of review.
Now parsing data with a hex editor to recover final
milliseconds."
