Page 201 - Intro to Space Sciences Spacecraft Applications

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Introduction to Space Sciences and Spacecraft Applications
Thruster and tankage mass are calculated using accepted thruster com-
ponent and tankage values as a function of the total fuel mass, the number
of thrusters required, or other factors. Qpical values are 10% of the
required fuel mass. After the mission and payload input has been com-
pleted, the designer estimates a spacecraft dry weight (no fuel) and then,
using the above method and fuel calculations, makes an estimate of the
spacecraft wet (parking orbit) and on-orbit (final orbit) mass estimates.
These values are then used to make new estimates for the propulsion and
orbital maintenance subsystems. The process is repeated iteratively until
an accessible estimate is obtained.

Data Handling Subsystem

Most spacecraft are engaged in the collection or transfer of some form
of information, even if the specific mission performed by the spacecraft is
not involved with communications. The data handling subsystem collects
data from the payload and sends the data to the ground. The collected data
may be transmitted as it is collected or recorded for later transmission.
Data transmission is either direct to a ground station and/or to a ground
station via a communication relay satellite, such as the tracking and datu
relay satellite (TDRSS). The designer must select the downlink method
and frequencies and indicate if onboard recorders are used. A design will
calculate the downlink rate (Kbps) and present the downlink options for
users to select from. There are several communication bands normally
used to send the data to the ground, such as:

S-Band (2-4 GHz) Low data rate (could be shared with TT&C)
X-Band (8-10 GHz) Payload specific high data rate
K-Band (13-15 GHz) Payload specific high data rate
After the communication band(s) are selected, the designer calculates
the subsystem envelope, mass, and power requirements using a database
of transponder, antenna, and recorder information. Conventional designs
assume that multiple transponders, antennas, and recorders are always
used for redundancy. The result of the data handling subsystem analysis is
the determination of subsystem mass and power.

TT&C Subsystem

Associated with most spacecraft is the telemetry signal which contains
information on the status of the spacecraft and its systems and through
which a controlling ground station can modify the spacecraft’s instruc-