Lat Tracker Tower Comments

Re: Telecon 6/19/2002

We agree with these conclusions from the presentation:

Proceeding on with the inferior screws in the first test should not have been done - Once you do this you are testing a configuration that is not flight like and not definable.
Proceeding on with a known crack in the second test should not have been done - Here again you are testing a configuration that is not flight like and not definable.

Some things that we don’t agree with:

“The test served its purpose” - Very little was accomplished since the test configuration was wrong from the beginning.

“Most of the tower module design is sound” – This design directs the entire load from the tower inertia through the bottom tray interface. It’s probably too late now, but a better approach would be to have an independent load path that does not involve the tray.

Some other issues

The structural model should be verified and the design should be qualified (including minimum frequency requirements) without consideration of the gasket. Then the gasket can be added and that configuration qualified. A major effort should be made to find a gasket or alternate configuration that does not change properties during test.

Estimated design loads or the results of coupled loads analysis should be used to assess the vibration test responses at the CG of the tower to see if over-testing occurred. Using the responses measured during a single tray test doesn’t relate to this test since the load paths and stress through the lower tray are totally different. On future testing the input should be assessed in pre low level testing to determine if notching is warranted to maintain CG responses below predicted design loads.

Three sigma responses are generally assumed in predicting peak random vibration loads for metallic structures due to their ability to redistribute load by local yielding. Composites, however, are not as forgiving and can experience ultimate failure due to the 4 to 5 sigma peaks that are many times seen in random vibration testing. This should be considered when setting notch levels.

In the thermal cycling of the grid interface, only the bottom tray was used. It seems that the addition of the side walls and other tray elements could affect the strain distribution significantly.

It is somewhat hard to tell from the pictures, but it seems that the fix that was performed on the lower tray added additional closeout fasteners pretty much along a line with the original fasteners. This won’t help much with any heel and toe type loading that is usually a problem in highly loaded areas near the interface. If you can swing it we should probably have one of our analysis people take a look at it.