Page 204 - Intro to Space Sciences Spacecraft Applications

Spacecraft Systems
191
heavy load of the earth’s gravitational pull, survive the violent process of
launch and positioning, be able to deploy systems into an operational con-
figuration from the folded, compact configuration of launch, and finally, to
maintain spacecraft integrity during station-keeping and attitude changes.
The mass of the satellite structure may be estimated as a percentage of
the satellite dry mass, typically around 20%. However, the structure mass
is also influenced by payload and payload support dynamics, such as
scanning antennas and magnetic tape recorder transport, and operating
power levels and profiles.
An alternate estimation of the mass of the structure subsystem is simi-
lar in calculation to the previously described thermal subsystem calcula-
tion, with some important exceptions based on uncompensated momen-
tum mass imbalance created by scanning payloads. The calculation is
iterative beginning with the default values obtained for the mass of each
of the spacecraft subsystems, recalculating at the end of the design
process. An example calculation might proceed as follows:
For each element of the payload the mass multiplier is 0.1 if the pay-
load is passive and 0.15 if the payload is active. The payload mass for this
calculation is the total mass or the sum of the electronics and sensor mass.
If the payload has a scanning antenna that is uncompensated, a compen-
sating mass is added to the structure weight.
For the ARC and power subsystems structure, the mass multiplier is 0.1
if all of the payloads are passive and 0.15 if any of the payloads are active.
The mass multiplier for each of the remaining spacecraft subsystems is 0.1.
Each of these contributors are summed to produce the structure sub-
systems mass which is allocated to the contributing elements. Results
using this type of an approach are consistent with representative designs.

Design Iteration

Since the determination of the mass of a spacecraft subsystem depends
upon the mass of each of the other subsystems (for example, ARC mass will
depend upon payload requirements as well as the mass of all other subsys-
tems), a design must employ an iterative approach for establishing subsys-
tem masses. In other words, as each subsystem is designed, all known mass-
es are taken into account. Upon completion of the design of all subsystems,
an iteration process will be initiated whereby the mass of each subsystem is
recalculated, taking into account knowledge of the masses of all other sub-
systems. This cycle is repeated until the mass computations stabilize from