Page 86 - Intro to Space Sciences Spacecraft Applications

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1 Propulsion 73
Figure 3-10. Launch sites. Launch site latitude determines the minimum
inclination that a satellite may be launched directly into.

that a spacecraft maintain a specific orbit and/or attitude with respect to
the earth or stars, then additional propulsion systems may be required.
Compared to launch and orbital transfer vehicles, station-keeping and atti-
tude control systems are made up of much smaller thrusters which mea-
sure their forces in pounds (or Newtons) or fractions thereof. An orbital
transfer vehicle may be capable of delivering a Av of several kilometers
per second in order to make major changes to a spacecraft orbit, but a sta-
tion-keeping propulsion system is designed to make fine adjustments to an
orbit, delivering Av’s measured in meters per second. Attitude control
thrusters are even smaller, designed to simply rotate the spacecraft body
and not affect the orbit significantly.
As mentioned earlier, these systems are usually custom-designed as an
integral part of a spacecraft, and the thrusters must be positioned about the
spacecraft body at precise locations to provide the correct torques and
magnitudes. If you refer back to Figure 3-5 you will notice the groups of
small thrusters placed around the body of the Apollo service module.
Associated with these thrusters, propellant tanks must be included to
store the required amount of fuel for the mission lifetime. In many cases,
the amount of onboard fuel becomes the limiting factor in useful space-
craft lifetime, and more than one perfectly-operating spacecraft has
become useless due to fuel depletion. The propellant used must be capa-
ble of on-orbit storage. If a liquid propellant is used, the spacecraft may