In Section 10 it was calculated that for the propulsion system chosen (i.e. STAR 30E solid boost motor) to give the probe the desired V_hyp of 5 km/s the total probe mass should not exceed 140 kg. The probe mass budget for the final design study configuration, based on the mass estimates for each subsystem, is given at Table 18.1.

The final total mass is thus some 17 % within the limit for the probe defined by the trajectory requirement and the capability of the chosen boost motor. Some of this mass margin will be used by attachment fittings between the probe and boost motor together with any other ancillary equipment required. However, it does leave some margin for mass growth or if more detailed design analysis indicated that the masses of certain systems had been underestimated. Many of these masses are probably conservative overestimates though; for example, the side array mass was based on a model [Larson92] that included the array structure, even though the side wall structure was specified separately. The attitude control system mass is also based on a pessimistic delta-V budget. However the overall indication is that a probe of the required specification can be built for within the allowable mass.

Moment of Inertia. During the design of the attitude control system the probe's mass moment of inertia I_probe was roughly estimated as 20 kgm². To check the validity of this I_probe was recalculated for the final probe mass and configuration. Given the probe structure it was modelled in two sections:

• A uniform central cylinder of radius 0.35 m and mass 82.16 kg (i.e. the thrust cylinder and all systems within it).
• A thin-walled shell of radius 0.5 m and mass 34.2 kg (i.e. the side wall, front and rear faces, support braces, all solar arrays, thrusters and star trackers).

Via the standard formulae for the mass moment of inertia of a solid and thin-walled cylinder, i.e.

(18-1)

(18-2)

the moments of inertia for these were found as 8.55 kgm² and 5.03 kgm² respectively, giving a total I_probe of 13.58 kgm². This is lower than the originally estimated value and thus the attitude control torque requirement will be within the amount budgeted for.