Construction of macroblocks

The spacecraft is built in simplified macroblocks. The fundamental hypothesis assumed is that each of them consists of extrusion and union of simple figures such as cones,

parallelepipeds, spheres and cylinders. The masses include not only the structures but also all the internal subsystems. This means that mass density is considered homogeneous and this is not true. Each object has its own local geometric reference system. The origin is then

modified according to reasons of simplicity. To exploit the symmetries, the origins are inserted in the symmetry centers.

Figure 6.1.1 - Construction of blocks

72 6.1.1 Main module

The main module is built as a parallelepiped with a cylindrical cavity: the presence of the antenna and the thrusters is ignored. It is hypothesized for simplicity that the axis of the cylindrical cavity coincides with the axis of symmetry of the parallelepiped, but it is necessary to remember that the MM’s parallelepiped is not square based.

Figure 6.1.1.1 MM block

The cavity will receive the payload, but the parallelepiped structure has the dimensions sufficient to house the propellant tanks inside it.

An important simplification concerns the lack of modelling of the main antenna of the orbiter:

its mass is considered in the Main Module as uniformly distributed.

For these reasons, the centre of gravity divides the dimensions in half: as it was conceived, the MM module is geometrically symmetrical with its centre of gravity.

73 6.1.2 Solar Array

The solar arrays were built exclusively in the deployed configuration. They are simply represented by two parallelepipeds, having a thickness of orders of magnitude lower than the other two dimensions.

Figure 6.1.2.1 - Solar Array block

As can be seen, they are at equal distance from their axis of symmetry.

Once you have drawn the solar arrays, there are articulation properties added, to allow the rotation of the Solar Arrays on the longitudinal symmetry axis. As is known, the solar panels rotate to have the best alignment with the sun's rays, so the center of gravity and moments of inertia of the whole system vary accordingly: the MCI analysis must be allowed according to various angles of rotation.

After defining the solar arrays as assembly elements and not equipment elements, the assembly properties are added, that is, the rotating reference system with the origin at the centreline.

Figure 6.1.2.2 - Articulation values for MCI

In all six configurations, the panels can rotate in continuous and non-discrete intervals, according to the whole lap angle.

74 6.1.3 Propulsion Module

The Propulsion Module is built as union of a parallelepiped with a square base and a hollow cone of constant thickness, that has a height greater than that of the parallelepiped: it comes out from both the lower platform of the parallelepiped and the upper one.

Figure 6.1.3.1 - PM Structure definition

The object is symmetrical only in plan: so the centre of gravity lies on the intersection of the diagonals of the square.

Figure 6.1.3.2 PM block

75 6.1.4 CCM + RVA, ERM and OS

Everything related to the payload is enclosed in the following figure, where:

- CCM + RVA is represented by the turquoise cylinder.

- ERM consists of the most stocky cylinder, and represents the union of EEV and the external structural part of the ERM, which are geometrically indistinguishable with the ERM. In fact, what changes between these objects is mass, which for ERM consists of the sum of EEV and ERM structural unit.

- OS is a sphere contained in the EEV.

In this case, some conical truncated cone objects were approximated to an equivalent cylinder, having a diameter equal to that relative to the major base of the truncated cone.

Figure 6.1.4.1 - Payload module

Since detailed information are not available for positioning the OS and EEV, it was decided to position them with reasonable local z-axis values. Also in this case, the objects are

symmetrical respect to the central axis of the cylinders with homogeneous mass density.

76 6.1.5 Tanks

In the system structure, the tank subsystem is only present in the PM and in the MM. This is because is necessary that the relative mass of propellant not to be separated from the

structures that house it. In IDM-CIC, the shape of a tank consists of two identical hemispheres divided by a cylinder of the same radius. In the Main Module, two tanks of Xenon and two tanks of hydrazine have been dimensioned, while in the Propulsion Module there are four Xenon tanks (identical to the previous ones) and five chemical bipropellant spherical tanks: the cylinder height of the tank figure is zero. Four tanks out of five are equal and the last one is larger than all. In reality some real PM’s tanks are closer to an ellipsoid rather than a sphere, so the chosen diameter is a weighted average of the semi-major and semi-minor axis. Unlike the previous elements, in this case it is necessary to add information related to the tank capacities and fillings.

Figure 6.1.5.1 - IDM-CIC units

In this case the choice of tank thicknesses was derived from the company know-how, as the internal pressure is not yet known. For the sake of simplicity, the plant positioning of all the tanks has been made symmetrically. To all this must be added the difficulty of modelling the position of the centre of gravity of the propellant for tanks not full.

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Figure 6.1.5.2 - Tank modules

The position along the thrust axis (x axis of the whole system) of the tanks and therefore of the propellant is approximate, very close to the centre line of the structural side panels. In this case the symmetry is only in plan because the tanks do not occupy perfectly symmetrical positions respect to the global centre of gravity of all thirteen. It is opportune to remember that the bipropellent tanks, not being in the reality of a perfectly spherical shape, assume very rough position values along the X axis of the whole system.

It is mandatory to remember, when modelling a tank, whatever the shape, one must choose a geometric modelling as a "tank" otherwise it will present computational problems of insertion of the propellant. Construction of tanks with objects of an unconventional shape must always take place starting from the form of a candy.

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