3.5 Mission Analysis and Results
3.5.2 Case study #1: results
De-orbiting by means of drag sail only
As it was expected, the employment of a drag sail only is feasible in a limited number of cases, where the debris mass and the initial altitude are quite low. In our case the only debris that respected these two conditions was the #1 (see Table 3.1. For this debris, a drag sail of 20 m2 could allow to de-orbit
from the initial altitude of 717 km within 18 years, while a sail with an area of 30 m2 reduces the or-
bital lifetime to 12.5 years. These values are mean values obtained through the Equation 3.20; a more accurate estimation of the de-orbiting time could be obtained through the software DAS, which takes into account the solar flux variation during the de-orbiting manoeuvre, for a given epoch. The results of these simulations are reported in the last column of Table 3.3. As it can be seen, the analytical model underestimates the orbital lifetime respect to the values obtained with DAS. This is mainly due to the solar flux variations that are not considered in the model. A safety factor should be then considered and an increase in the sail size could be needed in case of de-orbiting through drag sail only. Anyway, from these results it emeregs that drag sails could be not efficient for the debris population considered in our study.
Debris #1
A
sailm
remm
rem\m
deb∆T
deorbit∆T
DAS[m
2]
[kg]
%
[y]
[y]
20
53.3
7.17
18.7
[26 - 35]
30
54
7.27
12.5
[20 - 16]
Table 3.3: De-orbiting by means of drag sail only: results. The table shows the remover configuration, the remover mass fraction (%) and the total de-orbiting time for a given sail area.
De-orbiting by means of electric propulsion only The main results are shown in Figure 3.13 and 3.14.
The advantage of a propulsive only scenario is that the propulsive phase could be employed to lower the orbit of the debris at an altitude where atmospheric drag is more effective and allows a faster re-entry of the debris. In such way, a significant reduction in the orbital lifetime derives from the propulsive phase, respect to a drag only scenario, and mass savings are possible since the debris could continue the descending phase without any additional device. Obviously, the total orbital lifetime is influenced by the intermediate orbit at which the propulsive phase ends, as well as the A/m ratio of the debris. If this parameter becomes too high and the probability of collisions during the second phase becomes significant, it is sufficient to increase the number of assembled units in order to reduce the manoeuvre time and, hence, the collision risk. And this is one of the main advantages of the employment of the modular architecture. Anyway, it can be noted that the intermediate altitude reached with the lowest number of propulsive units, which correspond to the minimum number of units needed to lower the orbit at a level where the re-entry of the debris occurs within 25 years, is in all the cases around 600 km. The most critical orbits for the collision risk assessment are at higher altitudes, so it is reasonable to conclude that propulsion could be a promising solution for ADR. It can be observed, for the less massive objects (mass ≤ 2000 kg), one or two propulsive units are sufficient to lower the orbit enough to guarantee a fast re-entry of the debris: the orbital lifetime is almost always less than 4 years (well below the limit of 25 years). For more massive objects, the number of propulsive units varies between 4 and 6, for the same orbital lifetime; a numer of assembled units np ≥ 6 appears to be useless and hence the remover
Chapter 3
3.5. Mission Analysis and Results
Figure 3.13: Intermediate altitude that allows a natural re-entry of the debris within 25 years, reached after the propulsive phase, in function of the number of propulsive units (propulsion only scenario).
Chapter 3
3.5. Mission Analysis and Results
De-orbiting by means of electric propulsion and drag sail combined
A combined manoeuvre gathers the advantages of both the propulsion only and drag only scenarios. In this case it can be determined the better remover configuration that allows to optimize both the total remover mass and the orbital lifetime. In this analysis, the total de-orbiting kit mass and orbital lifetimes obtained in case of combined manoeuvre are compared to the mass and orbital lifetime determined in the previous analysis, where only electric propulsion was implemented, considering for each debris, only the values correspondent to the minimm number of propulsive units required to allow the natural re-entry of the object within 25 years.
The results of the comparison are shown in Figures 3.15 - 3.20. The blue bars in the charts refers to propulsive mission scenario, while bar in light blu refer to the combined manoeuvre. The number of units in the latter case is the sum of the propulsive units and the drag sail unit, recalling that only one drag sail unit is always employed, regardless the sail area. Debris #1 can be excluded from this analysis: as emerged from the previous analysis, the debris can be de-orbited employing only a drag sail or elecric propulsion. In the former case the mass of the removal system is very low, being only the mass of the main bus plus the drag sail unit, but the de-orbiting time is quite high, and proper risk assessment evaluations should be performed to exclude any possible failure due to orbital impacts during the de-orbiting phase. On the other hand, electric propulsion allows a faster de-orbiting, although the mass of the de-orbiting kit is almost doubled. Figure 3.15 reveals that a combined manoeuvre is not useful, being the remover mass and the total de-orbiting time the same in both de-orbiting scenarios. The main effect of the imple- mentation of a combined manoeuvre can be observed in Figures 3.16, 3.18, 3.19 and 3.20: simply adding a drag sail unit to the de-orbiting kit determined in the only propulsion scenario, with a minimal increase of the total removal system mass, the de-orbiting time can be significantly reduced. The decrease is more evident for debris with mass lower than 4000 kg, like debris #2, #4 and #5, where the change in time is between 28% and 60% respect to the de-orbiting time obtained considering a natural re-entry after the propulsive phase. For more massive debris, such as debris #6, the reduction of the orbital lifetime is less effective, being around only 16%. In this case, an increase in the number of propulsive units could be considered, both if natural re-entry or drag sails are implemented. Another effect can be observed from the results obtained for debris #3, Figure 3.17: in this case, two propulsive units were needed to lower the orbit enough so as naturally re-entry would have occured within 25 years. If a drag sail is employed, the number of propulsive units can be reduced to one, and the overall system mass, with the darg sail unit, cqan be reduced of 28%. Although the employement of the largest sail considered in this mission analysis (30 m2), the de-orbiting time in this case increases respect to the propulsion only scenario. Risk
assessment evaluations could be performed to identify the better de-orbiting scenario for debris #3. In conclusion, drag - propulsion combined scenarios resulted advantageous for debris with mass lower than 4000 kg, depending on their initial orbit: faster de-orbiting maneouvres could be performed with a mini- mal increase of the de-orbiting kit mass. For more massive debris, increases in the number of propulsive units or sail area should be considered to significantly accelerate the de-orbiting phase. Risk assessment evaluation are anyway needed to identify the better solution resulting as the compromise between the better combination of remover mass and orbital lifetime.
To conclude the analysis, the total de-orbiting kit mass, in case of a combined maneouvre, was compared to the debris mass. As it can be observed from Figure 3.21, the de-orbiting kit mass is a small percentage of the debris mass, lower than 15% in most of the examined cases. Furthermore, it emerged that this percentage decreases as the debris mass increases.
Chapter 3
3.5. Mission Analysis and Results
Figure 3.15: Debris #1. Total mass of the de-orbiting kits and total de-orbiting time: comparison between de-orbiting by means of propulsion only and combined maneouvre drag sail + electric propulsion (Asail = 10 m2).
Figure 3.16: Debris #2. Total mass of the de-orbiting kits and total de-orbiting time: comparison between de-orbiting by means of propulsion only and combined maneouvre Drag sail + electric propulsion (Asail = 10 m2).
Chapter 3
3.5. Mission Analysis and Results
Figure 3.17: Debris #3. Total mass of the de-orbiting kits and total de-orbiting time: comparison between de-orbiting by means of propulsion only and combined maneouvre Drag sail + electric propulsion (Asail = 30 m2).
Figure 3.18: Debris #4. Total mass of the de-orbiting kits and total de-orbiting time: comparison between de-orbiting by means of propulsion only and combined maneouvre Drag sail + electric propulsion (Asail = 30 m2).
Chapter 3
3.5. Mission Analysis and Results
Figure 3.19: Debris #5. Total mass of the de-orbiting kits and total de-orbiting time: comparison between de-orbiting by means of propulsion only and combined maneouvre Drag sail + electric propulsion (Asail = 30 m2).
Figure 3.20: Debris #6. Total mass of the de-orbiting kits and total de-orbiting time: comparison between de-orbiting by means of propulsion only and combined maneouvre Drag sail + electric propulsion (Asail = 30 m2).
Chapter 3
3.5. Mission Analysis and Results
Figure 3.21: Remover mass fraction, in percentage, respect to the debris mass. As it can be observed, the mass of the remover is, in most of the cases, less then 15% of the debris mass
Controlled re-entry
The mass of the hybrid module was determined considering the final mass of the de-orbited system (debris + de-orbiting kit). As example, it was selected the most massive case among the previous anal- ysed, i.e. the maximum number of propulsive units - maximum drag sail area. The results of such analysis are listed in table 3.4. As it can be observed, the hybrid unit required for the controlled re-entry is always a small percentage, around 1.3%, of the entire system mass.
Debris
m
HP Um
HP U\m
sys[kg]
[%]
#1
11
1.31
#2
14.6
1.29
#3
20.5
1.31
#4
35.1
1.43
#5
46.6
1.35
#6
110
1.29
Table 3.4: Mass of the hybrid unit required to perform a controlled re-entry from an altitude of 250 km, in function of the final mass of the de-orbited system. In the second column it is reported the percentage of the hybrid unit mass respect to the final system mass.