Space Grant Consortium - University of Wisconsin - Green Bay

B. Attenuation and Uncertainty
Shock attenuates when it propagates through a material. Attenuation, the
reduction of shock, is critical in calculating the peak levels of shock that a particular
piece of a larger structure will be exposed to. Attenuation analysis of shock is based on
the length of the dispersion path through the structure and the number and types of
structural joints in that structure. According to a NASA Technical Standards for Pyro-
Shock Requirements Document (RD 23) there is a reduction in shock magnitudes in the
realm of 20 to 75 percent from propagation through structural joints. This percentage
range depends on the material of the joint and structure, and the path of transmission
through the joint. This shows the uncertainty in quantifying shock, creating large
margins between acceptance and qualification SRS.
C. Methods to Assess Shock
There are general methods of assessing the risk of damage from shock. Coverage
of shock using random vibration loads and the two methods for structural and electrical
components from the ESA Shock Handbook are widely accepted. The 0.8f rule for
structural components is a simple comparison of a 0.8 sloped limit line, with respect to
frequency, where structures are not at risk if their SRS lie below the limit. The
6db/octave rule for electronics shares a similar concept, except it has a smaller slope of
6db/octave and tops out at 500g for 2 kHz. The other method of assessing shock allows
one to compare the shock specification to random vibration. This is the most accepted
method among the space community. This approach converts random vibration loads
into a random response spectrum or RRS that can be compared to a SRS. This is done
with the Miles formula, which is a simplified approximation of an RRS. The equation
uses Q; an amplification factor which is typically 10 or 25, fo; the frequency at which the
acceleration is calculated and W (fo); the value in g 2 /Hz of the Power Spectral Density
(PSD or random specification) at the frequency fo in the calculations. Shock will not be
an issue if the RRS covers the SRS levels. By using the formula below, the frequency
values for random vibration loads and the corresponding PSD, RRS can be compared to
SRS to asses shock and determine if direct testing is needed.
RRSMiles (fo) = 3 x SQRT [π/2 * Q * fo * W (fo)]
The James Webb Space Telescope Detectors
JWST uses new detectors that are top of the line with very low noise and darkcurrent,
and high quantum efficiency. These qualities measure the precision, accuracy
and reliability of detectors, respectively. There are a total of 18 infrared detectors
between the four instruments. Of these, 15 are HAWAII 2RG detector arrays from
Teledyne Imaging Sensors and the other three are SB-375 Si:As detector arrays from
Raytheon. HAWAII is an acronym for HgCdTe Astronomical Wide Area Infrared
Imager and 2RG denotes the size (2048x2028 pixel), and that the array has reference and
guidance pixels included. H2RG detectors are used in the Near-Infrared Camera
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