Minutes of the Tracker Technical Meeting
August 7, 2003
Sidewall issues: Sandro and Nanda were at Plyform, where some new coupons were being pulled.† Through some miscommunication, Plyform only made unidirectional K13D coupons (22 plies), so they were not able to test new coupons of the Tracker design laminate.† However, Plyform does have enough prepreg material to make one or two new panels.† That will be done after the 2-week vacation break, which starts tomorrow.
Sandro noted a problem with variation of the laminate thickness in the panels laid up a couple of weeks ago.† For example, one panel with aluminum was only 1.15mm thick, while another panel WITHOUT aluminum was 1.34mm thick.† He noted that the latter value is actually consistent with COI predictions, given that the panels has only a single ply of the YS90 fabric on each side.† Furthermore, Sandro said that the 22-ply unidirectional K13d laminates come out at 86 micron thickness per ply, which is about 20% less than the COI prediction.† Sandro said that the panels were made too quickly, without proper controls in place.† They will repeat the procedure with remaining material, using careful procedures, after the vacation break.
Plyform is cutting coupons from the thicker laminate (1.34mm) for the thermal testing.† Robert and Tom requested that samples of the thick and thin laminate be sent to SLAC to be analyzed to try to understand the thickness and resin-content issues.†
There was some discussion to try to understand the Plyform process. †Sandro said that the relevant engineer was not present, so they will have to wait until after the vacation break to get detailed information.† Apparently the laminates are cured with a Teflon-like foil on both sides.† There is no evidence of resin coming off with those foils.† Only a small amount of resin is lost around the edges.† A 5mm thick aluminum plate is put on top of the stack under the vacuum bag during curing.† There was some discussion of what its purpose might be, but nobody really knew.
Sandro relayed a few of the pull test measurements during the meeting.† The average tension strength was 1200MPa, whereas 1800MPa was expected (this is for the unidirectional K13d laminate).† (For reference, the Tracker allowable is 165MPa, but that is for the design laminate with fibers in all directions and with YS90 fabric on both surfaces.)† Sandro also said that there was still a problem with some of the pulls not breaking uniformly.
During the meeting Sandro also relayed one compression measurement (i.e. very preliminary) that was 300MPa, whereas 900MPa was expected.
Robert asked Erik about work on understanding the bottom-tray repair.† He needs Steve to do analysis, and Steve will return next week.† They plan to study the effect of different patch sizes.† Robert requested a written report on the analysis and the proposed repair plan and procedure.
There was a discussion of how to proof test the side flexures under the GSFC specified shear and axial loads.† The plan is for Hytec to analyze the static test with the corner flexures detached.† If the desired loads can be achieved in the side flexures that way, with still sufficient margins in the tracker structure, then this will be the easiest way to qualify the flexure design.
Robert summarized the situation with Teledyne MCM production.† Teledyne received a PO from SLAC to do the preliminary work needed on process revisions.† This will involve partially assembling 6 MCMs.† The will start by attaching flex circuits to 2 existing MCMs boards from the previous run, which were reworked at SLAC to correct the dimensions on the raised right-angle-interconnect area.† This will verify the alignment of the flex attach.† They will then continue with those parts by loading capacitors and ICs, doing wire bonds, and then encapsulating, to test out the process revisions in encapsulation and staking of the large capacitors.† In parallel they will design and prototype a cover for protecting wire bonds and will add ground screw holes to the assembly fixtures.† Meanwhile SLAC will machine at least 4 of the new boards from DDI, to put in the radius and cut the boards to the precise height.† SLAC will also get some flex-attach epoxy mixed with about 1.5% by weight of Cab-o-Sil (treated fumed silica), which should improve the bondline, squeezout, and peel strength properties of the flex attach.† Those boards and adhesive will then be used by Teledyne to do 4 more flex attaches.† They will also put capacitors on those boards and vibrate all 6 boards to verify that the capacitors donít fall off.
SLAC also expects to receive a proposal from Teledyne for the flight build by tomorrow.† Also, an order should be going out today for the flex circuits (pitch adapters), with a 2-week delivery for the first 50 parts.† Therefore, the earliest time to get the flight build started would be about August 25, but this depends on getting the preliminary work done, as described above, and getting a contract finalized with Teledyne.