Air Force Uses VisSim For 6DOF & Weapons System Design

Using VisSim, the Air Force Research Laboratory (AFRL) at Eglin Air Force Base has developed a high-fidelity 6-degree-of-freedom (6-DOF) modeling system that simulates the flight dynamics and performance of a state-of-the-art weapons system. The modeling system was built under the Munition Simulation Tools and Resources (MSTARS) project at AFRL by a joint government/ contractor team led by Larry Lewis, Munition Flyout Team Leader.

Designed for the rapid prototyping of new guided bomb and missile concepts, as well as the evaluation of new technology performance, MSTARS includes a library of VisSim-based munitions subsystems representing the accelerometer, rate gyro, autopilot, seeker, inertial navigation system, control surfaces, and air vehicle, with complete 6-DOF flight dynamics. Simplified models of a launch aircraft and threat target are also incorporated into the component library. Nearly 70 VisSim models and DLLs are currently in the library, and many more are being added monthly. Several guided bomb and missile 6-DOF simulations have been built using these components.

The current MSTARS system is the culmination of work which began about a year ago when Lewis first created a prototype 6-DOF simulation using VisSim. "We had been using another modeling system which simply was not meeting our objectives. After doing a detailed comparison of several outstanding products, my team decided to go with VisSim for MSTARS. It was a scary prospect to re-do our previous work, but we've accomplished more in one year than we had in the previous three years."

According to Lewis, numerous VisSim features were essential in building the 6-DOF system. "Embed blocks allowed us to build a truly reusable library of components," said Lewis. "And path aliases tied to the embed blocks let us specify a complete simulation configuration, eliminating the use of lengthy file specifications."

Lewis emphasized that this modular approach to system design not only saved valuable time during the design and debugging phases, but also allowed engineers to quickly test the effectiveness of new subsystem component designs. "Whenever we came up with a new design, we just changed a single name to swap the new component into the simulation."

Reuse of legacy code written in Fortran and Ada was facilitated by VisSim's DLL capability, as was the creation of new models and special utilities. "This ability to reuse existing code and create special-purpose code via DLLs shaved months off our development schedule," said Lewis. New DLLs are constantly being added, written in C++, FORTRAN, and Ada 95.

In large model design, the dynamics of the system generally demands multirate simulation. In the 6-DOF system, discrete transfer functions, unit delays, and automatic DLLs were used to achieve different update rates. "This allowed us to dramatically reduce computation time and speed up the simulation," explained Lewis.

Within the Eglin AFB community, VisSim and the MSTARS modeling system have received favorable reviews. Lewis plans to present and demonstrate the VisSim-based MSTARS system at the Joint Avionics, Weapons, and Systems Symposium in Las Vegas in June of this year.

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