RHESSI News Item

Dolores Beasley
Headquarters, Washington, DC May 12, 2000
(Phone: 202/358-1753)

Mark Hess
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-8982)

RELEASE: 00-80


The HESSI Mishap Board released its final report today saying 
that the High Energy Solar Spectroscopic Imager (HESSI) spacecraft 
was damaged March 21 during pre-flight vibration tests because of 
a malfunction in the vibration test system at NASA's Jet 
Propulsion Laboratory in Pasadena, CA. (The vibration tests 
simulate the stresses that the spacecraft experiences during the 
rollout, release and flight to orbit aboard the Pegasus launch 

The damage was caused when the test device, called a 
"shaker," delivered approximately 20 G's, ten times the 
appropriate level for the test, to the spacecraft. (A G is a unit 
of force equal to the gravity exerted on a body at rest.) As a 
result, the spacecraft's structure was damaged and three of the 
four solar arrays were severely damaged. Satellites are routinely 
subjected to vibration testing as part of preparing them for 

Mishap Board Chairman Denny Kross, Manager, Engineering 
Systems at NASA's Marshall Space Flight Center, Huntsville, AL, 
said a misalignment between two pieces of the test stand led to an 
abnormally high level of static friction (what engineers call 
"stiction"). The computer used to control the test then tried to 
compensate and induced too large a shock into the satellite. 

"It's similar to what happens when you are trying to close a 
sticky, wooden window that's just a little out of kilter in the 
frame," Kross said. "As soon as the window starts to stick, your 
brain says, 'push down harder.' And if you are not careful, you 
can push so hard that, when the window does break free, its slams 
down onto the bottom of the window sill." 

To prepare for the test, the satellite is mounted on a device 
called a slip table, which attaches atop a large slab of granite 
mounted to the floor. A thin layer of oil is continuously pumped 
between the slip table and the granite slab to allow the slip 
table to move freely when stimulated by the "shaker." A computer 
controls how hard the spacecraft is shaken, and accelerometers 
measure the response of the spacecraft to the shaking. 

Engineers found that the shaker mechanism had shifted on its 
mounting base, due to a failed support bearing. The problem was 
not discovered until after the accident. The broken bearing 
shifted the position of the shaker mechanism causing the 
misalignment between the slip table and the granite mass, and this 
misalignment in turn created friction between the two pieces of 
hardware. The computer, sensing this friction, calculated an 
inappropriate drive signal and the resulting pulse was 
significantly higher that expected, damaging the satellite.

Two primary factors contributed to the accident, Kross said. 
One was the absence of a scheduled maintenance program requiring 
periodic inspections of the shake table. The other was the lack 
of a procedure requiring the test team to look for any shaker 
performance problems in the pre-test data. "Had either of these 
procedures been in place, this incident could have been avoided," 
Kross said. 

The Board has made a number of recommendations in its report 
for changing processes and procedures used by NASA for vibration 
tests. "The JPL test team responded magnificently in the wake of 
this incident," he said. "They are revamping their inspection 
program so the test fixture is physically and visually inspected 
at regular intervals. They are adding steps in the testing 
procedure so that results of earlier tests are reviewed and 
analyzed to look for early indications of stiction before the 
final tests are conducted." 

Kross said other recommendations, such as refurbishing the 
shaker and implementing over-test protection methods, also are 
underway at JPL. As a result of the Board's work, new procedures 
were put in place at NASA's Goddard Space Flight Center, 
Greenbelt, MD, where similar testing is done, and alerts have been 
sent to other sites in the U.S. and overseas where satellites 
undergo vibration checks. 

The HESSI satellite will be repaired and re-assembled at the 
University of California, Berkeley, which is serving as the prime 
contractor on the project and is home to the principal 
investigator. HESSI will be returned to JPL for continued 
spacecraft testing after re-assembly. Launch plans will be 
announced when available.

NASA's development cost for the HESSI spacecraft was budgeted 
at $40 million. Development, launch vehicle and mission operations 
costs bring the total mission value to $75 million. 

HESSI is a Small Explorer mission and is managed by Goddard 
under the Explorer Program. The science team includes co-
investigators from Switzerland, Scotland, Japan, France and The 
Netherlands. More information on the mission can be found at:




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