**Lesson Info**

-   **Lesson Number:** FPPD-LL-0051

-   **Lesson Date:** 11-Jan-2001

-   **Submitting Organization:** GSFC (543)

-   **Submitted by:** Daniel Helfrich

**Subject/Title/Topic(s):**

Landsat Solar Array Deployment Testing

**Description of Driving Event:**

The Landsat 7 (L7) solar array (S/A) is an assembly of four rigid
aluminum honeycomb panels and an aluminum yoke with a total length of
8.5 m (335 in). The g-negation deployment ground support equipment (GSE)
included: 1.) a deployment surface, which in the beginning, at least,
was a poured epoxy floor, 2.) four solar array supports stands, each on
a set of four air pads, for a total of 16 air pads, and 3.) a solar
array mounting interface simulator mounted on a three-axis positioner.
When GSE problems arose during deployment qualification tests, what
originally was a test effort planned for 6 months became twice that in
duration.

Testing of the GSE with solar array mockups in the second half of CY1997
gave the first indications that the GSE was not 100% ready. Some simple
adjustments were attempted at that time. The first full deployment of
the S/A assembly on 6 Nov 97---a manual operation before its
installation on the spacecraft---showed problems with the uneven epoxy
floor and the square air pads. GSE repairs were made, and on 28 Apr 98,
the first electrically-initiated deployment including the firing of live
cable cutters was attempted. The S/A released only partially due to
inadequate extraction of some of the restraint cables. Improper cable
pre-load was later found to be a root cause, along with a lack of
control of friction in the device. Subsequent manual deployment attempts
demonstrated the epoxy floor was still a problem.

**Lesson(s) Learned:**

The preload of the restraint cable was a critical element of the design
that was not fully appreciated nor documented. Minor design
modifications and seemingly trivial discrepancies in key dimensions of
the restraint system parts caused the unnoticed loss of preload in the
initial test. Direct measurement of the preload through the use of load
washers or from strain-gaged hardware elements is essential when the
system preload is critical such as with a solar array release/restraint
system.

Judicious testing of deployable devices early in satellite development
is a cost effective tactic of developing complex and critical mechanism
subsystems. The mechanism function is often independent of the primary
operational function of the subsystem, allowing testing of the mechanism
function using low cost mockups representing only the properties
relevant to test objectives. These mockups often allow additional cost
savings in handling environments. Subtle details in the implementation
of the design of the deployable\--and in the GSE with which it is
tested\--often lead to unexpected results in the initial functional
tests of the mechanism, and often only the test itself is revealing. If
the choice had been made to defer the Landsat 7 S/A deployment test
until later in the flow, shipment could have easily been shifted out in
time, ultimately leading to a slipped launch date and severe cost
impacts.

Using a rigid granite deployment surface (even if it is comprised of
several large granite blocks with seams) beats a cheap epoxy floor.
Precision honeycomb panels supported precisely can also provide superior
performance. Elevated surfaces are easier to clean and stay cleaner.

**Recommendation(s):**

Load washers or strain-gages can be a very helpful element for assurance
of cable preload when this is a critical element of a restraint/release
system. Don't rely on nut torque alone when preload is critical, as it
often is.

Test g-negation GSE ahead of time, and even so, be ready for problems,
especially when taking a new approach or implementing modifications to a
proven approach. Don't ignore or brush off seemingly minor checkout
issues with GSE unless there is more than enough time to correct
unforeseen problems when it's used with the flight hardware for the
first time. For GSE checked out far in advance, make adequate plans to
fully protect the GSE while it's in storage.

When testing large deployable devices on air pads, be sure the pads are
round, the air flow is strong, and the air supply tubing is flexible yet
strong. Also be sure that the table is flat and level within acceptable
tolerances that depend primarily on the available torque of the
deployment hinges.

**Evidence of Recurrence Control Effectiveness:**

With many issues from the April deployment test failure resolved, on 24
Aug 98, the S/A was successfully deployed off the spacecraft with only
minor issues found with the restraint system. A third pyrotechnic
deployment on 8 Dec 98 was completely nominal, and on 15 Apr 99, the S/A
deployed perfectly on-orbit.

**Applicable NASA Enterprise(s):**

Earth Science

Human Exploration and Development of Space

Space Science

**Applicable Crosscutting Process(es):**

Provide Aerospace Prods & Capabilities: Implementation

**Additional Key Phrase(s):**

Facilities

Ground Equipment

Test Facility

Test & Verification

**Can this lesson be included in the International Safety Lessons
Learned (ISLL) Database?** No

**Submitted by:** Daniel Helfrich

**Mail Stop/Code:** GSFC 543

**Lesson Author (if different):**

**Phone Number:** 301-286-8481

**E-Mail Address:** helfrich@gsfc.nasa.gov

**Lesson Date:** 01 / 11 / 2001