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s HMM Assessment Study Report: CDF-20(A) February 2004 page 82 of 422 and further apoapsis lowering manoeuvres at the next pericentre pass. Alternatively, it might be an option to force a low-velocity escape. Failure before reaching a bound orbit: The manoeuvre fails while the spacecraft is still in a hyperbolic orbit with respect to Mars. In the case regarded in section 2.7.12.1, only the case of a complete failure was regarded. A partial failure would result in a trajectory closer to the orbit of Mars. • Incomplete execution of TEI: Here the case distinction made above also applies. 2.7.12.3 Conclusions An abort cannot be always guaranteed with no further consequences: • During the MOI and TEI manoeuvres abort is not possible • During the first part of the transfer to Mars, abort is always possible without mission mass increase • During the second part of the transfer to Mars, abort is always possible but mission mass increase is needed (either propulsion system or ERC) • From low Mars orbit, there are two possibilities: return via Venus swing-by but mass increase is needed or waiting for next return window 2.7.13 Aerobraking Aerobraking is a proven technique to remove energy from an orbit, e.g., when transferring from a highly eccentric orbit to one of low eccentricity, with minimal propellant consumption. Aerobraking involves lowering the pericentre of the initial orbit so that it grazes the upper atmosphere. At every perigee pass, the spacecraft loses some orbital energy to atmospheric friction. This lowers the apocentre radius. After a number of passes, during which the pericentre altitude must be observed and repeatedly corrected so that it does not descend too deeply into the atmosphere, the apocentre will have reached the required altitude. Then, a manoeuvre at the apocentre raises the pericentre and the aerobraking phase is terminated. The use of aerobraking rather than propulsive manoeuvres for final orbit acquisition can lead theoretically to significant savings in propellant mass. (see Mission architecture) For this reason, a preliminary estimation was performed in this study. 2.7.13.1 Requirements and design drivers Aerobraking is a lengthy process but it is relatively safe. The structural and thermal loads imposed on spacecraft components are low compared to other techniques involving atmospheric flight such as aerocapture and entry/landing. However, with the present spacecraft there were design concerns for some of the subsystems, in particular the solar arrays. If left deployed during aerobraking, they would provide the large surface area required to maximize the deceleration and minimise the manoeuvre duration but they would also be particularly vulnerable to the increased structural and thermal loads. Therefore it was necessary to perform a trade-off between the manoeuvre duration and the solar array restrictions. The constraints are summarized in Table 2-29:

s Maximum manoeuvre duration 6 months (about 180 days) Maximum dynamic pressure • Solar arrays facing flow • Solar arrays parallel to flow Maximum heat flux (Q) • Solar arrays facing flow • Solar arrays parallel to flow 0.2 N/m 2 13 N/m 2 10 kW/m 2 Uncertain Table 2-29: Aerobraking constraints HMM Assessment Study Report: CDF-20(A) February 2004 page 83 of 422 The dynamic pressure constraint comes from the structural limitations of the solar array structure. For the hinge the maximum allowable bending moment is 185 Nm and the maximum allowable shear load is 25 N and for the support beam the maximum allowable bending moment is 300 Nm (from the solar array design specifications). The force acting on the panels was evaluated from the dynamic pressure as: F = 1 PdynC D S 2 where CD is the drag coefficient of the structure (assumed to be that for a flat plate for which CD=2.0) and S is the surface area facing the flow. For a solar array area of 95 m 2 and a thickness of 0.1 m (including the thickness of the support beam), the maximum allowable dynamic pressure loads given above were derived. 2.7.13.2 Assumptions and trade-offs A trade-off was required between the aerobraking manoeuvre duration and the structural and thermal loads on the solar arrays. Three solar array configurations were considered in the tradeoff: 1. Solar arrays facing flow. 2. Solar arrays turned parallel to flow (to avoid stowage requirements). 3. Solar arrays stowed. The results of the various analyses are summarized in Table 3-31 below. The highlighted areas show values that violated the constraints outlined above. Option Qmax [kW/m 2 ] Pdyn,max [N/m 2 ] Duration Operational Issues 1 Solar arrays deployed 45.0 11.0 3 months facing flow 2 Solar arrays deployed 23.0 5.5 6 months facing flow 3 Solar arrays deployed low 0.2 about 8 yrs facing flow 4 Solar arrays deployed 60.0 13.0 About 16 Turning of arrays parallel to flow months 5 Solar arrays stowed 630.0 145.0 3 months Retraction deployment of arrays and 6 Solar arrays stowed 315.0 72.0 6 months Retraction deployment of arrays and Table 2-30: Results of aerobraking analyses

s<br />

Maximum manoeuvre duration 6 months (about 180 days)<br />

Maximum dynamic pressure<br />

• Solar arrays facing flow<br />

• Solar arrays parallel to flow<br />

Maximum heat flux (Q)<br />

• Solar arrays facing flow<br />

• Solar arrays parallel to flow<br />

0.2 N/m 2<br />

13 N/m 2<br />

10 kW/m 2<br />

Uncertain<br />

Table 2-29: Aerobraking constraints<br />

HMM<br />

Assessment Study<br />

Report: CDF-20(A)<br />

February 2004<br />

page 83 of 422<br />

The dynamic pressure constraint comes from the structural limitations of the solar array<br />

structure. For the hinge the maximum allowable bending moment is 185 Nm and the maximum<br />

allowable shear load is 25 N and for the support beam the maximum allowable bending moment<br />

is 300 Nm (from the solar array design specifications). The force acting on the panels was<br />

evaluated from the dynamic pressure as:<br />

F = 1 PdynC<br />

D S<br />

2<br />

where CD is the drag coefficient of the structure (assumed to be that for a flat plate for which<br />

CD=2.0) and S is the surface area facing the flow. For a solar array area of 95 m 2 and a thickness<br />

of 0.1 m (including the thickness of the support beam), the maximum allowable dynamic<br />

pressure loads given above were derived.<br />

2.7.13.2 Assumptions and trade-offs<br />

A trade-off was required between the aerobraking manoeuvre duration and the structural and<br />

thermal loads on the solar arrays. Three solar array configurations were considered in the tradeoff:<br />

1. Solar arrays facing flow.<br />

2. Solar arrays turned parallel to flow (to avoid stowage requirements).<br />

3. Solar arrays stowed.<br />

The results of the various analyses are summarized in Table 3-31 below. The highlighted areas<br />

show values that violated the constraints outlined above.<br />

Option Qmax<br />

[kW/m 2 ]<br />

Pdyn,max<br />

[N/m 2 ]<br />

Duration Operational Issues<br />

1 Solar arrays deployed 45.0 11.0 3 months<br />

facing flow<br />

2 Solar arrays deployed 23.0 5.5 6 months<br />

facing flow<br />

3 Solar arrays deployed low 0.2 about 8 yrs<br />

facing flow<br />

4 Solar arrays deployed 60.0 13.0 About 16 Turning of arrays<br />

parallel to flow<br />

months<br />

5 Solar arrays stowed 630.0 145.0 3 months Retraction<br />

deployment of arrays<br />

and<br />

6 Solar arrays stowed 315.0 72.0 6 months Retraction<br />

deployment of arrays<br />

and<br />

Table 2-30: Results of aerobraking analyses

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