Page 197 - Intro to Space Sciences Spacecraft Applications

184 Introduction to Space Sciences and Spacecraft Applications
decrease internal or even local temperatures. Refrigeration devices, heat
pumps, and heat pipes are examples of active spacecraft thermal control
systems. These devices have the ability to more precisely control space-
craft temperatures, but their disadvantage is their increased weight and
complexity. Special thermal systems, such as cryogenic coolers, are usu-
ally accounted for as unique payload support systems. Thermal subsystem
mass is usually estimated as a percentage of spacecraft dry mass, typical-
ly 2-5% for passive systems and 48% for active systems.

Orbital Maintenance Subsystem

This subsystem design determines the amount of propellant needed to
keep the satellite in the desired orbit over the specified mission lifetime.
Atmospheric density causes drag and is the primary force that will cause
many spacecraft to lose altitude. Atmospheric density is a function of the
solar environment, which can vary widely as a function of the expected
solar cycle activity, and satellite altitude, as shown in Table 8-7. The final
orbit altitude and mission timeline is used to select the appropriate value
of atmospheric density. The satellite ballistic cross-section, which is the
cross-sectional area of the satellite in the direction of flight, determines
the amount of drag. A mission duration Av is calculated to determine the
total propellant needed to maintain the orbit. The amount of propellant

Table 8-7
Atmospheric Density

Expected Solar Cycle Activity (kg/m2)
Altitude
(km) Low Average High
1 00 9.8E-09 9.8E-09 9.8E-09
200 1.95E- 10 2.7E-10 3.45E-10
300 8E- 12 2.23E- 1 1 3.658-11
400 1.04E- 12 4.77E-12 8.5E-12
500 1.21E- 13 1.05E- 12 1.98E-12
600 2.45E- 14 3.12D- 13 6E- 13
700 7.1E-15 9.3E-14 1.8E-13
800 3.4E- 15 3.4E-14 6.4E-14
900 2E-15 1.4E-14 2.5E-14
1,000 1.3E-15 6.7E-15 1.2E-14