2008 Issue 2 - Raytheon

Figure 1. Estimated spectrum for aggregate film based on only
50 percent of all organic materials used in a seeker. The red line
indicates the system-level allowance for molecular film derived from
the PIDS.
film accumulation. Its name is derived from
the three analysis legs that support it: materials
characterization, aggregate film estimation
and sensor performance prediction.
This type of interdisciplinary analysis, if used
during the design process, eliminates molecular
film contamination as a limiting factor
of service life.
Figure 1 shows TRIPOD’s estimate of an
aggregate film based on an examination of
50 percent of the materials in an optical
seeker. The red line indicates the systemlevel
allocation for film absorbance derived
Figure 2. Example map of calibration
error due to a specific contaminant as a
function of single-surface-layer thickness
(left y-axis) , optical-layer thickness
(right y-axis) and calibration-source
temperature ( x-axis).
from the prime item development
specification (PIDS).
While this estimate appears to exceed the
allocation, the true significance of the film’s
effect can only be evaluated after this effect
is propagated through a system performance
model that includes guidance algorithms
and missile response to guidance signals.
Figure 2 is an example of a signal response
map. Such a map can be interpreted by
discrimination and guidance experts to evaluate
the aggregate film’s effect on overall
system performance.
If the film’s effect is deemed negligible, then
the system is considered robust. Otherwise,
the TRIPOD tool is used to locate the largest
contributors to the film’s absorbance. Steps
can then be taken to reduce the load of
outgassing products.
Figure 3 shows that most of the film’s
absorption in the region of interest results
from three materials. The total load of outgassing
products from these materials can
then be reduced by performing additional
materials processing or by using an alternate
material.
The patented (USPTO 7,319,942) TRIPOD
tool gives Raytheon a strategic competitive
advantage by enabling us to use established
physics-based system performance models
YESTERDAY…TODAY…TOMORROW
MATERIALS & STRUCTURES
Figure 3. Contributions to aggregate film from the three largest
sources. Space wire is a dominant source of contamination despite its
extremely low outgassing.
to provide the customer with a conservative,
quantitative assessment of long-term
missile storage risks.
TRIPOD requires validation. The best validation
technique would be to measure system
performance changes after 10 or 20 years
of fielded service. This, however, is not practical.
Accelerated testing is another validation
option, but interpreting such test
results is controversial, particularly in this
case: the fundamental physics, though
known, are so complex that they have not
yet been combined into a single predictive
model. Nonetheless, we are developing
validation strategies.
TRIPOD is built and maintained by an interdisciplinary
team of engineers and scientists
ranging from chemists and physicists to
production engineers. It has been used to
define the cleanliness production requirements
of several programs, and it serves as
the electro-optical community’s design
guide on molecular film contamination.
TRIPOD is designed to evolve as a repository
of data that is available to all programs —
each contributing to its growth and utility.
This tool bridges a gap that has existed for
many decades.
D. Brooke Hatfield, Ph.D.
dbhatfield@raytheon.com
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