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   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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