Success Stories

Virtual Prototyping Saves Hundreds of Thousands in Missile Ejector Design

Virtual prototyping saved hundreds of thousands of dollars in the design of a missile ejector by reducing the need for physical testing. The advanced medium range air-to-air missile (AMRAAM) vertical eject launcher (AVEL) is designed to accelerate the missile to an end of stroke velocity that ensures safe separation of missile from the airstream of an F-22 Raptor, the Air Force's most advanced fighter aircraft. The traditional approach to designing the mechanism would be to build a test fixture to simulate the loading of the aircraft environment and instrument the launcher - the cost would be prohibitive. EDO Marine and Aircraft Systems, North Amityville, New York, pioneered a new approach in which they simulate the AVEL using virtual prototyping software. A total of 223 critical load conditions were applied to the model to account for the impact of air, inertial, and oscillatory forces under a wide range of different flight conditions. For each load case, Nicolas DeSimini, Senior Staff Engineer for EDO ran the virtual prototype through 15 time steps designed to simulate the missile launch and separation from the aircraft. "Virtual prototyping made it possible to resolve the critical design issues in much less time and at a far lower cost than if we had to rely on physical testing alone," DeSimini said.

EDO is under contract to Lockheed Martin to design and develop a high performance, lightweight missile launcher for the F-22 Advanced Tactical Fighter. The AVEL was specifically designed to maximize the F-22's punch by giving it the ability to carry six Raytheon AIM-120C missiles. The launcher will be installed in the internal weapon bay of the aircraft and will be required to hold AMRAAM missiles securely within the bay as the aircraft maneuvers through its rigorous flight envelope. Even more challenging is the requirement to safely separate the missile from the aircraft prior to rocket motor ignition. To do so, the AMRAAM missile must be controlled and ultimately ejected at velocities sufficient to penetrate the air boundary layer as it makes the transition from the relatively benign environment of the weapon bay to the adverse conditions found in the aircraft's flow field. The AVEL provides an extremely rapid launch-to-eject cycle during which the main weapons bay doors are opened and closed while the missile is ejected into the air stream. It consists of a primary structure, suspension and release mechanisms, and a self-contained, reusable energy system. EDO anticipates supplying more than 2,000 AVELs for the 339 F-22 aircraft required by the U.S. Air Force, which would keep the production line open for the next 12 to 14 years. 108 are presently on order and are being manufactured at the EDO Marine and Aircraft facility in North Amityville.

Critical design challenge

Perhaps the most critical challenge in designing the AVEL was validating its ability to propel the missile beyond the jet's airstream under a wide range of different flight conditions. The challenge is greater on the F-22 than previous generations of aircraft because in order to maintain the stealth characteristics of the aircraft the missile must be installed inside the body of the aircraft. EDO engineers created the concept design for a pneumatic/hydraulic ejector mechanism that operates through a six-bar linkage. The mechanism folds up like a scissors in the stowed position and during ejection it is stroked 9 inches by the pneumatic actuator, at which point it develops the required end of stroke velocity to safely separate the missile from the aircraft, all in a matter of milliseconds. Understanding the dynamic loading is critical to the design of the launcher. The challenge was increased by the fact that the launcher had to be designed long before the aircraft had flown. In the past, EDO would have built a test fixture to apply loads on the missile designed to simulate the load environment due to aircraft maneuvers. Then they would have instrumented the launcher at key locations in order to measure the loads. The cost of building a representative test fixture, and running a single test, would have been prohibitive. Multiple build-and-test iterations would have probably been required in order to meet the design specifications. One reason why this fixture would have been so expensive to build is the need to distribute loading over the entire surface of the missile. The accuracy of the results would still have been in question because the fixture would have been able to apply static but not dynamic loads to the missile.

Instead, DeSimini decided to use ADAMS software from Mechanical Dynamics, Inc., Ann Arbor, Michigan, to produce a functional virtual prototype of the AVEL. ADAMS is designed to realistically simulate the full-motion behavior of complex mechanical systems on a computer and quickly analyze multiple design variations until an optimal design is achieved. It reduces the number of costly physical prototypes, improves design quality, and dramatically reduces product development time. "We selected ADAMS after evaluating other simulation technologies," DeSimini said. "We were impressed with its functionality and how well it suited our development environment. We found it easy to use and were up and running with it doing production-type work in less than one week."

Simulation procedure

A CAD model of the initial concept design was imported into ADAMS, then more detailed joints, nonlinear forces, state and design variables, auxiliary differential equations and hydraulics were added. DeSimini began by modeling the actuator and mechanism under the assumption that they were entirely composed of rigid bodies. To increase the accuracy of the model, he later incorporated finite element models of the major linkage components -- including the upper beam, lower beam and four link arms -- into the virtual prototype as super-elements using a modal neutral file that describes the geometry, nodal mass and inertia, and generalized mass and inertia for each mode shape. The piston is driven pneumatically from an accumulator that is charged to 4000 PSI, using aircraft hydraulic system. DeSimini solved the gas equations to determine the pressure and forces generated during the actuator stroke and created a lookup table used by the virtual prototype as a forcing function. Lockheed Martin provided the external maneuver loads for several hundred different conditions obtained during wind tunnel testing. DeSimini validated the virtual prototype by comparing its results, such as the system-level mode shapes, natural frequencies, deflections and reaction loads, to finite element analysis results. The values obtained from the two analytical techniques correlated within 10%.

DeSimini then created a script to automatically simulate the AVEL hundreds of times, once for each of the loading conditions provided by the aircraft manufacturer. Each design iteration took about an hour and didn't require any hardware, which meant that the design process moved much more quickly than would have been possible using the old build-and-test approach. The simulations also provided much more information than could be gained from physical testing, such as displacement, acceleration, loads and torques of every node in the model throughout the time period of interest. This information played a critical role in the detailed design of the AVEL by providing the loading cases needed to design the actuator and mechanism. "Based on loads provided by the virtual prototype, we determined that there were certain areas in the initial concept design that were overloaded from a stress standpoint," DeSimini said. "Having this information prior to prototyping made it possible to incorporate the necessary design changes in the early phases of the process and avoid costly retrofits. The customer's testing showed that the new design met all of their performance requirements."

"Our aerospace customers demand products that are affordable, delivered on-time, and comply with stringent specifications," DeSimini concluded. "We are constantly driven to develop high-quality products that are lightweight, compact in size, and out-perform the competition. Typically, we are most concerned with issues such as design, weight, cost, size of the envelope, and performance. Structural integrity is an unspoken requirement. Virtual prototyping helps us address these demands by verifying mechanism integrity and performance, and generating loads and deflection data in the early stages of the design process. The software helps enhance our technical capabilities, minimize product risk, and reduce test cycle cost. As competitive and market pressures continue to increase, simulation and virtual integration efforts will become even more important."

Customer
EDO Corporation
North Amityville, NY
United States
http://www.edocorp.com

Software involved
MSC.ADAMS®,