European Solar Polar Orbiter Mission



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Sail Attitude Control


Prior solar sail attitude control system (ACS) studies are limited. Studies for the ST-7 sail estimated that a sail turn rate of 0.01 deg s-1 was attainable,Error: Reference source not found significantly higher than the required value of ~10-4 deg s-1 for the SPO mission. The large moment of inertia of a solar sail and the low-frequency structural dynamics present many unique attitude control challenges. It has been determined that the solar sail required for the SPO mission need not be particularly agile, this significantly simplifies sail ACS hardware design. The optimal ACS solution will likely use the sail structure and mechanisms for attitude control rather than employ a secondary system, which would incur a mass penalty.Error: Reference source not found
Much prior sail design work has base-lined the use a deployable gimbaled boom due to the many apparently attractive features of such a design. However, it has become apparent on detailed study that such a solution is less than optimal due to a lack of full redundancy and control accuracy issues.Error: Reference source not found The use of tip-vanes exclusively is also considered a sub-optimal solution for a variety of reasons.Error: Reference source not found The optimal solution will thus likely employ a combination of systems, thus the inclusion of a secondary system such as μPPT (Pulsed Plasma Thrusters) should not be completely dismissed. In this paper the use of sail tip-vanes is assumed due to the lack of prior ACS studies. Thus an appropriate mass allocation for the sail ACS is included within the sail design, while leaving open the potential for adoption of an alternative ACS.
Recall that the required time to slew ~70.5 deg is approximately 7.05 days, averaging 10 deg per day. However, since the slew maneuver will be symmetrical about the zero clock direction we can model the slew more accurately as acceleration through ~35.2 deg followed by deceleration through ~35.2 deg, each lasting ~3.5 days. If the centre-of-pressure and centre-of-mass are perfectly aligned we find that the sail can accelerate through ~35.2 deg in ~3.5 days with small tip vanes. It is however unlikely that the centre-of-pressure and centre-of-mass will be perfectly aligned due to deployment inaccuracies and sail flexing. Thus we design the tip-vanes such as to compensate for a given centre-of-pressure and centre-of-mass offset, as well as to be able to perform the required slew maneuver in the correct time. The tip vanes are assumed to be isosceles triangles as this minimizes the structural member mass assuming non-inflatable technology is used.

Solar Sail Mass Budget and Definition


Recall from Table 1 that the total sail sub-systems mass is 196 kg. The solar sail mass budget is detailed in Table 5, where it is seen that the mass of the four 14.9 m triangular tip-vanes is 11.4 kg, giving a tip-vane assembly loading of 102.1 g m-2. We note that the wires supplying power and command capability to the boom tips have a total mass of 12.7 kg and form a significant percentage of the total sail mass. While the optimal ACS solution remains to be defined the use of large tip-vanes allows a suitable mass allocation to be defined thus leaving open the potential for adoption of an alternative ACS at some point in the future. The tip-vanes are sized for a centre-of-pressure / centre-of-mass offset error of 0.25 % the sail side length, which corresponds to 0.38 m for this design point. Prior solar sail studies for ST-7 also assumed a centre-of-pressure / centre-of-mass offset error of 0.25 %.Error: Reference source not found
The square sail side length is 153 m, including a 10 × 10 m square central cut-out to allow for sensor field-of-view requirements, at an assembly loading of 8 g m-2. The main sail booms, which support the sail film, are based on a scaling from the Advanced CoilAble booms and a projected near-term solar sail technology roadmap.12, 13 It is seen in Table 5 that the Sail Stowage Box mass allocation is split into several components, including Primary and Secondary Structure, spacecraft adaptors and the sail deployment equipment. Note the sail stowage and deployment equipment is not jettisoned following sail deployment as would be ideal. This apparent omission actually provides benefits for such an early mission and technology analysis by maintaining a conservative design ideology which increases the sail technology demands above there apparent required level and allows for an increased technology margin. Furthermore, the actual method of jettisoning requires detailed study to ensure against sail film damage from jettisoned equipment. The “Spacecraft adaptor (carrier side)” detailed in Table 5 provides the mass allocation of the sail jettison mechanism (SJM) on arrival at the target orbit. The mass allocation for the SJM is defined following launch vehicle adaptor methodologies,14 due to the lack of current SJM designs. The SJM mass allocation is found as 5 % of the spacecraft mass plus a 10 % DMM. THE SJM mass is split 75:25 between the carrier (that is to say the sail) and the spacecraft respectively, thus minimizing spacecraft mass following sail jettison. The SJM, spacecraft side, is allocated within the Mechanisms & Structure sub-system and has mass 3.4 kg. The second adaptor seen in Table 5 corresponds to the launch vehicle adaptor and is defined using similar criteria to the SJM.Error: Reference source not found
The specifics of the solar sail deployment sequence and mechanisms are not defined within this paper as such detail requires specific hardware studies and trades to ensure the optimal sail deployment scenario is identified. The complete solar sail system requires significant further technology development.

Table 5 Solar sail mass breakdown, with scaling laws rounded to one decimal place.



Component


CBE Mass (kg)

DMM (%)

Total Mass
(kg)

2 μm CP-1 (Clear Plastic-1) film substrate

66.4

20.0

79.6

0.1 μm Al sail front coating

6.3

5.0

6.6

0.01 μm Cr sail rear coating

1.7

5.0

1.7

Main sail bonding

1.9

10.0

2.0

Main sail booms (65.2 g m-1)

28.1

20.0

33.8

Tip vane mass

2.2

12.0

2.6

Tip vane gimbal, motor and housing

8.0

10.0

8.8

Tip-vane control wires

12.1

5.0

12.7

Sail Stowage Box



Primary structure

19.4

10.0

21.3

Secondary structure

3.2

10.0

3.5

Deployment and aux. equipment

2.5

10.0

2.8

Spacecraft Adaptor (carrier side)

9.3

10.0

10.2

Misc. apparatus, inc. deploy mechanisms

4.3

5.0

4.5

Launch Adaptor (spacecraft side)

5.6

5.0

5.8

Total

170.7

10.3

195.9





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