Dynamic Modelling and Simulation Study for the
Galileo Spacecraft Pulsed-mode Spinup/400N Main Engine Burn/Spindown Maneuvers

Che-Hang Charles Ih,         Elihu H. McMahon
Che-Hang.C.Ih@jpl.nasa.gov   Elihu.H.Mcmahon@jpl.nasa.gov
NASA/JPL

Abstract:
The dual-spin Galileo spacecraft was launched on October 18, 1989.  As of
December 7, 1995, it was successfully inserted into Jupiter's orbit and began its 23-month
exploration of the planet and its moons.  Normally, the spacecraft spins at 2.9 rpm.  For
critical events such as atmospheric probe release and 400N main engine firing, the
spacecraft is required to spin up to 10,5 rpm to enhance gyroscopic stability, avoid
propellant unporting, and preserve scienceimagnetometer boom integrity.  Since test results
of the Retro Propulsion Module Team showed that prolonged continuous firing of the spin
thrusters is not acceptable, spinup/spindown manouver has to be done in pulsed mode.
Examination of the dynamic interaction among the spin thruster pulsing frequency,
science/magnetometer boom flexible modes, and the propellant slosh modes was
mandatory to ensure the science/magnetometer boom structural integrity and to avoid
catastrophic propellant unporting.

To this end, a 7-body Galileo model was developed.  It consisted of a base body
(stator, dry rotor, and scan platform), four propellant slugs, science boom, and
magnetometer boom (with a 3 degrees-of-freedom spring connecting the two booms).  The
model properties of the science/magnetometer boom modes up to 2.2 Hz were reproduced
accurately by this mass-spring model, Movements of the propellant slugs can be realized by
two rotations of the imaginary link connecting the slug and the, tank center.  Thrusts are
represented as forces and torques.  SD-EXACT was used to generate equations of motion,
calculate angular momentum and kinetic energy, etc.  The main program was written in
ACSL and subroutines were written in FORTRAN.

The complete spinup/400 N engine burn/spindown maneuver sequence has been
simulated for the critical events of Orbit Deflection Maneuver (ODM), Jupiter Orbit
Insertion (JOI), and Perijove Raise Maneuver (PRM).  The prediction of safe ODM and JOI
was validated in flight.  For PRM, however, the simulation results showed that although the
science/magnetometer boom structural integrity is ensured, propellant unporting will occur
(due to low propellant level) if the current spin thruster duty cycle (1.3 sec ON 1 3.9 sec
OFF) remains unchanged, A group of new duty cycles which correspond to thruster
pulsing frequencies that are higher than the propellant slosh frequency range were then
proposed.  Simulation results demonstrated that propellant unporting problem can be
avoided if any of these duty cycles is used, The project management has selected the
proposed 0.9 sec ON / 0,9 see OFF duty cycle for PRM scheduled to be executed on
March 14,1996.