TWOBODY - IMPROVED IMPLEMENTATION OF LAGRANGE MULTIPLIER

The Lagrange multiplier method has been used for many years by engineers to
develop equations of motion in multibody problems. Use of the Lagrange
multiplier has an advantage over some other formulations in that it
preserves the original simple form of the equations of motion for each
body. However, in typical timevarying dynamics simulations that use the
Lagrange multiplier, computational instability can occur because
integration error causes the two (or more) attached bodies to drift apart
and violate the constraints. The computational instability usually happens
suddenly, and the computed values "blow up" within a few time steps.

TWOBODY, from NASA Marshall Space Center, utilizes a new programming
technique which eliminates computational instability in multibody
simulations that use the Lagrange multiplier. This new technique uses the
constraint equations, instead of integration, to determine the coordinates
that are not independent. TWOBODY employs a simple model, consisting of a
solid rocket booster and a parachute connected by a frictionless swivel,
to illustrate the new technique.

TWOBODY, MFS-28774, is written in FORTRAN 77 for IBM PC series and
compatible computers running MS-DOS. It requires 100K of RAM for
execution. A sample executable is included with the distribution media.
The standard distribution medium for this program is one 5.25 inch 360K
MS-DOS format diskette. Program $100; documentation $13.

COSMIC, The University of Georgia
382 East Broad St, Athens, GA 30602-4272
(706) 542-3265;  Fax: (706) 542-4807

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