Page 198 - Intro to Space Sciences Spacecraft Applications

Spacecrafr Systems
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needed is influenced by the specific impulse of the propellant (Isp). The
higher the specific impulse, the greater the energy, which results in less
propellant needed for on-orbit maintenance. Typical specific impulse val-
ues are shown in Table 8-8. Both monopropellants such as N2H4
(hydrazine), and bipropellants such as N2H4/N204 (hydrazinehitrogen
tetroxide) are used for orbital maintenance. Depending on spacecraft
operational altitude, atmospheric drag may be compensated by an electro-
static system which produces a small amount of thrust over long periods
of time.
The details of an orbital maintenance subsystem design depend upon the
specifics of the satellite orbit (LEO, SSO, or GEO). For GEO missions,
nortWsouth and eastlwest station-keeping requirements must be considered
due to perturbations produced by sun and moon gravitational effects as
well as nonuniformity of the earth’s gravitational field. The output of the
design process is the mass of the orbital maintenance fuel and associated
tankage, plumbing, and thrusters. Tankage and plumbing may be shared
with the propulsion subsystem; however, in many instances, thruster sizing
and placement is unique to the orbital maintenance subsystem.

Propulsion Subsystem

The satellite propulsion subsystem is used to take the spacecraft from a
parking orbit to a final orbit. The spacecraft design may choose a “stand-
alone” stage for this task. In most cases, modem spacecraft will use a com-

Table 8-8
Thrust System Specific Impulse

Type Propellant ISP
Liquid Monopropellant 220 220
Liquid Bipropellant 350 350
Liquid Bipropellant 425 425
Liquid Bipropellant 430 430
Liquid Bipropellant 450 450
Electrostatic Resistojet 400 400
Electrostatic Resistojet 700 700
Electrostatic Arcjet 500 500
Electrostatic Arcjet 1,000 1 ,Ooo
Electrostatic Arciet 1,500 1,500