Laser Scanner Cuts Time to Measure Rocket Launcher Ablative by 50 Percent

The gas management system of a vertical launch weapon system used on surface ships is lined with ablative to protect the structure. Efforts to improve the design of the gas management system rely upon measuring the ablative material after a launch so that, for example, areas with heavy wear can be beefed up and areas with light wear can be reduced. In the past, it required a week using a touch probe coordinate measuring system (CMM) for an operator to measure the profile of the ablative. Recently, BAE Systems (Minneapolis, MN) switched to a laser scanning system that makes it possible to measure the ablative in only two and a half days. The accuracy of the laser scanned surface is considerably higher because the scanner captures far more points than the touch probe.

BAE Systems is a defense and aerospace company delivering products and services for air, land and naval forces, as well as electronics, information technology solutions and customer support services. The company develops and produces major caliber gun systems, innovative minor caliber gun systems, missile launching systems and canisters, advanced naval munitions technologies, electronic communication and identification systems and sensor technologies for naval ships and aircraft. BAE Systems' key naval programs include: Advanced Gun System and Long Range Land Attack Projectile; Mk 38 Mod 2 Naval Gun; Mk 41 Vertical Launching System and Canisters; Mk 45 Mod 0 Naval Gun; Mk 57 Vertical Launch System; Mk 110 (57 mm) Naval Gun; and Semi-Active Laser Passive IR (SALPIR).

Purpose of the Ablative

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

Instead of collecting points one by one, the laser scanner picks up tens of thousands of points every second. This means that reverse engineering 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.

BAE looked at various laser scanning systems on the market and picked the SLP-330 probe from Laser Design Inc. (Minneapolis, MN) 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 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 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.

Substantial Time Savings and Accuracy Improvement

Laser scanning provides substantial time savings in this application. The ablative can be scanned and a surface model of its geometry produced in about two and a half days - one-half of the time that was required in the past with a CMM. BAE 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 have been greatly reduced.