[AR] Re: helium tank submerged in LOX \ Re: Re: SpaceX failure update
- From: Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Mon, 7 Nov 2016 10:41:02 -0700
On 11/6/2016 8:04 PM, David Weinshenker wrote:
So it seems like the trick to making this work would be to get the
helium tank to its final state (or at least its final pressure)
before exposing it to oxygen: start with the LOX tank dry, and
filled with helium gas, then tank the helium as a liquid. Then
purge the LOX tank with gaseous helium at LOX temperatures, venting
the helium tank as needed, until the helium is at the desired final
condition (or at least up to pressure, and warm enough not to freeze
the oxygen) before filling the LOX tank.
The problem with that is, you can't keep subcooled LOX at stable
temperature once loaded by letting some boil off. So running with
subcooled LOX means a fairly rapid propellant-loading cycle - AIUI on
the order of a half-hour from load start to engines lit. (There are
details scattered at various places around the web, but no unified
description of the process I've been able to find.)
Complicating the problem is parts of NASA having issues over having
astronauts on board during propellant loading. (More on that at
nasawatch in the last couple weeks - of note, it's the ISS Advisory
Committee pointing out that they've never done it that way in the past,
not any of the official Commercial Crew safety people.)
So, from what I have been able to find, SpaceX has been loading LOX and
helium at least partly in parallel (with LOX loading starting first.)
(And they may (this is speculative) have been tinkering to further speed
up the process under NASA pressure.)
From
http://spaceflight101.com/falcon-9-explosion-progress-toward-root-cause/
(via a nasawatch comment), "Liquid Oxygen loading picks up on Stage 2
with just 19.5 minutes on the countdown clock, followed at just over
T-13 minutes by Helium loading, creating much tougher conditions for the
hardware than the previous iteration of the Falcon 9 that used LOX at
boiling point and began fueling over three hours before launch."
So, apparently they're pre-chilling the COPV vessels with subcooled LOX
externally before commencing (presumed liquid) He loading.
One question I've been curious about: How did LOX get into the COPV
fiber overwrap? How robust a coating (if any) is there over the carbon
fiber overwrap on the F9 COPV He bottles? From the picture of a
reentered F9 He bottle at
http://i.imgur.com/K3H7mdY.jpg, there is a
coating, but it looks to be fairly thin. (It's obviously not robust
enough to fully survive reentry and ground impact, FWIW.)
Henry
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