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Part II – Design in Thales Alenia Space

8.5 ERO Attitude Definition

8.5.6 Attitude verification



The following image, instead, shows the ERO in propulsive attitude during the BC14 Outbound phase: the X axis coincides with the thrust direction and for this reason the two vectors can’t be distinguished. It is useful to note the preferential alignment of the Sun

position vector with the Y axis. The S/C traverses a trajectory around the target planet and for this the vectors in question rotate with the passage of time. In short, the Y axis chases the Sun wherever it is. It is reiterated that this choice of arrangement is still conservative because it presupposes the perennial necessity of having to have the aforementioned attitude. During the Outbound phase, the thrust direction must be towards the outside as the semi-major axis of the ERO must be increased. As time passes, the S/C makes a trajectory arc and the thrust

direction rotates so as to bring the ERO closer to Mars. Before the end of the phase there is also a slowing maneuver in order to correct the orbit. In more detail, it is necessary to realize that in the long run a small thrust component out of the plane is necessary because the launch orbit is not on the ecliptic plane and it varies according to the inclination. These

considerations can be appreciated in the following figure, but what immediately jumps to the eye is that the non-planar component of thrust is an order of magnitude smaller than the components in the orbital plane.

Figure - BC14 Outbound attitude thrust phase

For the BC14 Outbound phase there is a continuous correction that brings the ERO into a Martian orbit by aiming the thrust on average in the velocity vector; when the propulsion is activated, unfortunately the attitude can’t be changed and therefore the definitive trend will be that seen in the previous figure.


The following figure shows the BC14 Spiral In phase: since the ERO must reduce the apoares, the thrust is in the opposite direction to the speed, and consequently the ERO is aligned with the thrust direction.

Figure BC14 Spiral In phase attitude

Figure - BC14 Spiral In final approach


In the subsequent phases the attitude is reversed because the thrust direction is opposite to the previous cases. The following figures clarify this aspect; moreover, during spiral up, there is no division in temporal arcs of the attitude strategies, since it has been modeled as an entirely propulsive phase. This is a very conservative approach because the duty cycle has not been considered in addition to the omission of the fact that during the eclipse phases there is no primary propulsion and the ERO would reach the orbit tag in more time.

Figure - BC14 Spiral Out phase attitude A

Figure - BC14 Spiral Up phase attitude B


During the Spiral Up phase the thrust is aligned with the velocity vector because the upper half-axis of the elliptical orbit must be increased. Small thrust components leave the orbital plane as a change of inclination of the orbit is required.

The following figures concern the BC14 Inbound phase. The non-propulsive phase provides alignment with the speed, the propulsive phase provides alignment with the thrust. During this last time segment there is a thrust component outside the plane as the inbound trajectory is not planar but corrects the strongly inclined orbit of the Spiral Up. For the planar thrust

components, there is a direct push towards the Earth in order to slow down the ERO.

Figure - BC14 Inbound attitude no thrust phase

Figure - BC14 Inbound attitude no thrust phase

The S/C rotates in order to turn the Solar Arrays towards the Sun. This happens because thrust has the major component direct towards the center of the solar system.


The following figure shows a particular case: Sun and Earth in phase opposition with respect to the ERO: also in this case the attitude does not change: the priority remains the alignment with the direction of the Sun. During the BC16 Outbound phase, there is a large time segment in which there is no active propulsion; this is due to the possibility of carrying out a fly-by of the Earth in order to save fuel. During this time frame the ERO is aligned with the speed.

During the propulsive segment, the thrust vector is directed towards the outside as the semi-major axis must be increased.

Figure - BC16 Outbound attitude no thrust phase

Figure - BC16 Outbound attitude thrust phase


The Inbound BC16 phase is divided into a non-propelled segment in which there is alignment with the velocity and a propelled segment in which the ERO is aligned with the thrust. The latter is directed towards the inside of the solar system in order to reduce the semi-major axis.

There are small thrust components outside the orbital plane because there are some inclination corrections to be made. The alignment of the Sun with Solar Arrays is always intrinsic in the model.

Figure - BC16 Outbound attitude no thrust phase

Figure - BC16 Inbound attitude thrust phase


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