1052. Measurement of the Inertial Properties of the Helios F-1 Spacecraft

Paper

Abstract

This paper presents methods of measuring moment of inertia with very high accuracy.
The moment of inertia of the Helios Spacecraft about its spin was axis determined by use of a ‘roll-fixture’
using two sets of crossed flexure pivots as elastic constraints. The test procedure entailed measurement of
system oscillation period with each of a set of added moment-of-inertia increments. The tare effect of the
fixture was determined a like process by and was subtracted from the gross value to yield the spacecraft roll
moment of inertia to an estimated accuracy of 0.2%.
‘Lateral’ moments of inertia (i.e., about each of three axes normal to the spin axis) were determined by a
gravity pendulum method that makes use of the fact that any physical pendulum has a minimal period of oscillation
determined by a particular distance from the axis of rotation to the system center of gravity. In situations where
a knife-edge support is used, this distance is equal to the system centroidal radius of gyration. In the subject
tests, the pivoting action was provided by hardened pins rolling on flat ways. The effect of the finite radius of
the pins was considered in deriving the equations of motion, from which an error analysis revealed the criterion
for maximum accuracy in determining the square of the centroidal radius of gyration.
The swing fixture provided for a number of optional pivot-pin locations giving precisely known distances between
successive axes of oscillation. This fixture, with provisions to support the spacecraft, was ballasted to bring its
vertical c.g. close to that predicted for the spacecraft. This ballasting was done not because the test method
requires an accurate foreknowledge of specimen c.g. position but, rather, to minimize errors in the parallel-axis
transfer term while removing the tare of the fixture.
Though the centroidal moment of inertia of the swing fixture was over twice that of the spacecraft, an error analysis
of measurements performed on the flight spacecraft showed that accuracies of better than 0.2% were realized for the
two lateral principal moments of inertia.
The methods presented allow all measurements test to be made with the specimen in an essentially upright position.
The vertical center-of-gravity location is derived in the process.
Comments are made on the potential importance of aerodynamic effects on measurements made in the earth’s atmosphere.
It is shown that, for the Helios configuration, these effects must be taken into account by some means to give an
assessment of spin stability in the space environment.