In an age of shrinking budgets, everyone is looking for ways to do more with less. The Johns Hopkins University Applied Physics Laboratory (APL) is developing an alternative, low-cost way of fabricating scale models that will make aerodynamic wind tunnel tests a more affordable way for air defense programs to collect high-quality data on conceptual missile designs.
Traditionally, wind tunnel models are made of metal and are very expensive, in part because of the intricacies involved when machining the parts to obtain the best simulation possible, which makes it too costly for most programs to produce more than one or two models.
APL's research team, led by Richard Heisler and Clifford Ratliff, is investigating alternative ways to create models from nonmetallic materials, such as engineering polymers including thermoplastic (similar to that used to make a computer's housing), and thermoset resins (like those found in tennis rackets and golf clubs). "Program managers can now afford to take some of their early ideas off the drawing board and obtain high-quality data for a fraction of the cost of building metal models," says Ratliff.
Creating Alternative Models
The Lab's low-cost manufacturing approach stems from rapid prototyping of models that helps designers quickly visualize and test early concept designs.
The nonmetallic prototypes are designed using CAD (computer-aided design) tools and a rapid prototyping, or fused-deposition machine, which builds up a prototype part, layer by layer, in polymer threads.
Under the right test conditions, a rapid prototype part could be tested in a wind tunnel. But in most cases, the part is used to create a mold for stronger models. The polymer model is then placed over a steel "strongback," or backbone, to prevent a model from bending or flexing during a test -- a tool that is in the process of being patented.
The materials used to create non-metal models depend on the required test conditions. With hundreds of materials to choose from, the team is developing a "recipe" that defines which nonmetallic materials they should use to create models for specific test conditions.
The Lab's alternative method won't replace testing a more complete design with a higher fidelity metal model at later stages in a program. But Ratliff notes that applying this technology early in a program's development cycle is a smarter way of doing business. "It gives the test community a way to explore a lot of different ideas and concepts early in a program and not constrain themselves to think that they can only test one design because of budget limitations," says Ratliff. "It's a way to weed out candidate designs by helping engineers select the optimal configuration."
Due to the high costs of building a model, program managers often rely heavily on analytical tools, such as CFD (computational fluid dynamics), to predict how a missile system might perform. Although CFD can provide valuable data, it typically requires more time to produce final results and has limitations providing data over a full range of flight conditions. "There are lots of ways our technique can serve our customers," says Ratliff. "It can be used in place of CFD or to verify CFD codes. A combination of testing and CFD can be used to acquire a more complete data set."
Validating the concept against data obtained from a 1997 APL wind tunnel test using a metal model, the APL team demonstrated that nonmetal models could be created and tested at subsonic and transonic speeds (just under the speed of sound). The group continues to make improvements to better meet customers' requirements by creating stronger models that will withstand supersonic conditions (up to nearly five times the speed of sound).
Tomahawk Program Applications
In addition to their research, the Navy recently tasked the APL team to apply its rapid prototype technology to the Tactical Tomahawk -- the Navy's next-generation tactical cruise missile. In a series of low-speed wind tunnel tests, APL team members built and instrumented a nonmetal booster and tail assembly, which they mounted to an existing metal Tomahawk model, and successfully determined the aerodynamic torque required to deploy the missile's tails after launch.
The Applied Physics Laboratory (APL) is a not-for-profit laboratory and division of The Johns Hopkins University. APL conducts research and development primarily for national security and for non-defense projects of national and global significance. APL is located midway between Baltimore and Washington, D.C., in Laurel, Md.