L3B leakage currents
With the L3B leakage currents growing in FM1 and
FM2, it's pretty obvious that they'll fail in flight. I had done calculations
earlier (months ago) which showed that we could still do science with the loss
of the L3B's by accepting smaller energy ranges. I gave those calculations to
Dick, who used them in discussions with the project management. The failing
L3B's both have dual oxide layers; the good L3A's have single oxide layers, so
the problem seems associated with dual oxide
layers.
Last week, we had a number of
meetings through Friday in which we tossed about the possibility of replacing
the flight L3B's with some spare L3's we have here which should be single-oxide.
Tycho, Mark, Sandy, and Branislav favored replacing the detectors if schedule
permits, with Branislav being most forceful in support of replacing the
detectors. Ed, Dick, Alan, and Rick C. opposed replacing the detectors, citing
the fact that we can still get science without the L3B's and the risks in
missing the schedule (and being left off the spacecraft), the ~160 screws
(epoxied onto the LET instruments), the flexi-cables, etc. Branislav replied
that we had the skill on hand (notably himself and Sandy) to keep the physical
risks to the detectors minimal. Those opposing the replacement would have to be
the ones writing up a proposed replacement schedule, and they didn't feel they
could write it up convincingly. By late Thursday, the majority vote (Hiromasa
and I didn't speak up; lack of seniority) seemed to favor just accepting the
crippled instrument, but by Friday, Lil (our rep with the project) e-mailed us
and opened the possibility of our proposing a repair. Branislav jumped all over
the opportunity presented, and we had another round of meetings this
week.
Yesterday, I managed to attend
the second half of the STEREO engineering meeting (Rick C., Dick, Mark,
Hiromasa, Branislav, and Alan already in attendance), and when I arrived, they
started coming up with a possible timeline for doing the replacements. They
figured a 3 week minimum time to replace one L3 on one flight unit, barring
accidents, scheduling conflicts (e.g. in getting the thermal vac chamber
available for some CPTs afterward), and so on -- about 5 and a half weeks if we
include contingency for snafus. Later yesterday, Alan sent an e-mail to Lil
detailing our discussions. I was worried (and I noted this in the meeting)
that, while the minimum time is probably solid, detailing the contingency time
would open us up to further
criticism.
Anyway, it looks like the
LETs can come off the spacecraft around 4/29 or maybe a bit earlier, but they'd
have to be back on by around 5/12. That's not enough time for us. So, for now,
we stick with the crippled instruments and hope that some other delay on the
project opens us up some more time.
In
the meanwhile, Dick asked me to do some more simulations concerning energy
ranges with the lost L3Bs. I'm still working on reading the macsys binary
output files -- Macsys itself seems to be reading histograms in various inputs
as read by jam; my problem is just in the binary data files. Plus, I still have
to contribute to the LET instrument paper, and I have to finish my charge states
paper, which my career currently depends on. (I've been embarrassed that Luke
got his own charge states paper out in draft form recently, while mine has
languished for years.)
Posted: Fri - March 24, 2006 at 02:36 PM