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Purpose of the
ablative
Ablative
materials are fiber-reinforced organic materials used extensively to
provide sacrificial cooling through progressive endothermic
decomposition in liquid and solid propellant rocket engine applications.
The mass flow of pyrolysis gases away from the heated surface blocks
heat flux to the outer surface. The advantages of ablative cooling
include simplicity, reliability, ease of fabrication, and compatibility
with deep throttling requirements. Another major advantage is the
elimination of the need for expensive, complex, regenerative engine
cooling systems, with high pressure pumps and tanks.
The ablative
protects the structure so it can be used multiple times. The wear caused
by the launch leaves the ablative with a very complex geometry.
Maximizing the life of the structure requires optimizing the ablative
geometry so that more material can be provided in the areas that require
the greatest protection. Measuring this geometry to a high level of
accuracy plays a crucial role in the process of designing the ablative
to obtain the maximum number of launches.
The CMM measures
points one by one, which means that it takes a lot of time to measure a
complicated 3D contour like the one the ablative exhibits after a
launch. While a CMM machine can determine the probe’s position with a
very high degree of accuracy, it’s very difficult to put the probe in
exactly the right position for a measurement. In the past it took about
five days to collect enough points to define the part geometry and
convert the points into a surface model needed by the engineers working
on the launch system design. A complicating factor is that to measure
the ablative the operator needs to climb into the launcher. There is
enough space for the operator to fit into and move around but it is
fatiguing to work in a confined location for such a long period of time.
Seeking an
alternative reverse engineering method
The company’s
engineers looked for an alternative method of measuring the ablative. “I
had been aware for some time of the development of laser scanning
technology and felt that it offered significant potential for improving
this application,” said a Project Engineer for the company. Laser
scanning systems work by projecting a line of laser light onto surfaces
while cameras continuously triangulate the changing distance and profile
of the laser line as it sweeps along, enabling the object to be
accurately replicated. The laser probe computer translates the video
image of the line into 3D coordinates, providing real-time data
renderings that give the operator immediate feedback on areas that might
have been missed. Laser scanners are able to quickly measure large parts
while generating far greater numbers of data points than probes without
the need for templates or fixtures. Since there is no contact tip on a
laser scanner that must physically touch the object, the problems of
depressing soft objects, measuring small details, capturing complex free
form surfaces are eliminated.
Instead of
collecting points one by one, the laser scanner picks up tens of
thousands of points every second. This means that reverse engineering of
the most complicated parts can often be accomplished in an hour or two.
Laser scanning can reverse engineer parts that are so complex that they
would be practically impossible one point at a time. Finally, the
software provided with the scanner greatly simplifies the process of
moving from point cloud to computer aided design (CAD) model, making it
possible in minimal time to generate a CAD Model of the scanned part
that faithfully duplicates the original part. Special, but readily
available software can be used to compare original design geometry to
the actual physical part, generating an overall graduated color error
plot that shows in a glance where and by how much, surfaces deviate from
the original design. This goes far beyond the dimensional checks that
can be performed with touch probes on CMMs.
Company engineers
looked at various laser scanning systems on the market. They picked the
SLP-330 probe from Laser Design Inc., Minneapolis, Minnesota for several
reasons. This probe was easily retrofitted to their existing touch probe
so it helped to preserve the value of their CMM investment. The SLP-330
uses a dual detector approach that captures much more part geometry per
pass, up to 50,000 points per second, than single receptor lasers.
Another advantage of the laser probe is that dual detectors view the
laser line from two different angles, reducing the number of scanning
passes required to capture steep sidewalls and deep geometries. “A
particular advantage of the SLP-330 in this application is that is it
can capture geometry over a 130 degree field of view compared to a 60
degree field of view provided by most of the other probes we evaluated.
This makes it much easier to use the SLP-330 in the confined space of
the launcher because there is no need for the operator to contort his or
her body in an effort to move the probe into position,” said the Project
Engineer.
Substantial
time savings and accuracy improvement
Now, instead of
having to move the touch probe into position for each individual point
that is to be captured, operators simply move the laser probe across the
surface of the ablative as if they were spraying it with paint. The
laser probe captures the coordinate data and an interface card
translates the video image into 3D coordinates. This process captures
many more points, which increases the accuracy of the geometry, in a
much shorter period of time. When the launch system is completely
scanned, the operator exports the point cloud data and opens it in
Geomagic Studio software which is used to convert the point cloud to a
surface model. This surface model can be imported into the CAD system
used by the company’s engineers. Sometimes engineers also use Geomagic
Qualify software to compare the scan data to the original CAD model of
the ablative to quantify the exact wear that occurred during the launch.
“Laser
scanning provides substantial time savings in this application,” the
Project Engineer concluded. “We can now scan the ablative and produce a
surface model of its geometry in about two and a half days, one half of
the time that was required in the past with a CMM. We can capture many
times more points than was possible in the past which improves the
resolution of the final results. The elimination of the need to
physically touch the ablative improves accuracy, particularly in smaller
cavities where it is difficult or impossible to insert a touch probe.
The wide field of view of the new probe makes it much easier for the
operators to capture the complete geometry of the ablative within the
confined space of the launch system. Finally, the operators’ job has
been made much easier because the time they need to spend and the amount
of maneuvering they must perform in the confined space of the launch
system has been greatly reduced.”
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