Elon's comment that this couldn't have been fully tested on the ground
because you can't do the g-loading on the ground got me thinking--too bad
you can't do a test flight first (with a payload that can tolerate the
risk). That occurred to me as a great reason for reusability even if cost
models say it may not be "worth it." Of course, some things are harder to
reuse than others (second stage vs. first stage), but think of the benefit
of reusing as much as you can to minimize the risk of infant mortality.
Or another way to look at it, imagine you're settling into your seat on an
airliner and the captain comes on the intercom and says, "great news,
folks, we're going to be flying a brand new airplane today--they just towed
it over from the assembly hangar. It passed all its ground checks and we
even fired up the engines for a few seconds to make sure they work!" Would
YOU want to fly on that? That's basically what happens on each expendable
rocket (if they even do an engine firing before each flight--I think SpaceX
is the only one who does that).
Some people have mentioned how NASA or other customers won't want to fly on
a "refurbished" rocket. I think it may turn out that no one will want to be
on that first flight--let that be a test flight and put student or
small-business payloads on it for cheap. The "big boys" will want to fly on
it AFTER it's had its shakedown flight.
Of course, there will be the issue of monitoring the status of the
vehicle/stage as it is reused many times--it will be nice to have to worry
about that, to get to more aircraft-type operations including "phase"
inspections, etc.
On Mon, Jul 20, 2015 at 8:31 PM, Steve Traugott <stevegt@xxxxxxx> 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> 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* @*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."
