APIS Experiment: Spanish SOYUZ Mission, CERVANTES

Coordinator: Ana Laverón, Victoria Lapuerta
Research domain: Classical mechanics, education


The aim of the educational experiment APIS is to perform a set of configurations to show the different torque-free rotational motions which can occur depending on the mass distribution of a body (that is, depending on the inertia tensor structure; spherical, cylindrical or ellipsoidal). Thus, the effect of mass distribution changes (deployment of booms, solar panels, antennae, etc.) can produce a change in the stability of the motion. Microgravity conditions allow obtaining those torque-free rotational motions.

Context of the experiment

The study of the torque-free rotational motion of a rigid body around its centre of mass is one of the classic problems of Mechanics. It has interest itself from the educational point of view as well as for its technological applications (for example, the study of spacecraft dynamics).
The main expectation of this experiment is to develop educational material: videos complemented with a theoretical programme including analytical and numerical models, that allow to students a better understanding of how the mass distribution lead to stable or unstable configurations.

Description of the experiment

The basic experimental set-up consists of a spherical shell, a handle to rotate the spherical shell (see Figure 1), and a video camera PD-150. The spherical shell is constituted by two hemispheres, made of polycarbonate and has a diameter of 8cm. Each hemisphere has a support glued allocated at the pole of the spherical shell where different modules with different inertial properties can be screwed.

Figure 1
The APIS handle is a cylindrical tube with a piston. Two arms stick out from the mechanism. One is joined to the cylinder and works as a handle and the other is joined to a moving part (i.e., sliding piston). This mechanism also has a spring that’s natural position corresponds to the release position. When the spring is totally compressed the sphere is fitted between the arms (support position), and held by two pins that fit inside two small cavities places at the sphere poles. These pins are supported by two small bearings. In this position the sphere can spin freely around the polar axis. Once the sphere is spinning, it is released by removing the pressure over the piston. The release should be done carefully in order to prevent the sphere drifting, which would make the filming of the sphere more complicated. In absence of gravity, and neglecting the effect of electrical or magnetic fields, we can consider the spin of the sphere as a torque-free rotational motion.
The real configuration of the experiment is shown in Figure 2. Labels of different colours were attached to the sphere to identify the rotation axis in the recording.

Figure 2
Figure 3 shows the flight hardware of the experiment: The hemispheres, the handle and three modules. All parts except the spring and the bearings are made up of aluminium alloy 7075.

Figure 3
One of the modules, see Figure 4, has cylindrical symmetry. It is constituted by a threaded main axis, which fits at the sphere pole supports, and two disks. The relative position of the disks can be changed. Note that this design allows having bodies that’s moments of inertia can be changed in a simple way. Both disks remain equidistant from the centre of the bar, to fix it as the centre of mass of the body. Three sessions with different positions of masses were performed: Adjustment of the distance between disks to the (i) maximum, (ii) intermediate and (iii) minimum position is analyzed. There is another module very similar to this one, with some additional loads which could have a little displacement.

Figure 4
A third module, see Figure 5, consists of a hollowed main axis within which there is a hinged piece, whose inclination angle with respect to the rotation axis can be varied with the central screw.

Figure 5
Three sessions with different positions of the hinged piece were performed adjusting the movable piece to the (i) perpendicular position (as shown Fig. 5), (ii) intermediate position and (iii) vertical position. Figure 3 showed this last position. The aim of the intermediate session is to study the motion that is generated when the angular moment direction does not coincide with any of the principal inertia axis.
Several videos showing both, stable and unstable configurations, were recorded in ISS, during the Spanish Soyuz Mission Cervantes. Later, in ground, those videos have been analyzed and reproduced with analytical and numerical models. All this material allows to students a global comprehension of the dynamics of solid body rotation and its dependence on the distribution of mass whiting the body (that is, on the structure of inertia tensor).


Further information

[1] APIS Experiment During the Spanish Soyuz Mission Cervantes. A. Laverón, V. Lapuerta et al. Microgravity Science and Technology, Vol. XIX, Issue 5-6, pp. 253-259 (2007)

[2] Tyrrell Thomson, W: Introduction to space dynamics, John Wiley & Sons, Inc., New York, London (1961)

[3] Goldstein, H: Classical Mechanics, Addison-Wesley Publishing Company, Inc. Reading, Massachusetts, U.S.A (1980)