[155] During the closing days of 1968, the
engineers at Langley, in consultation with specialists at JPL and
NASA Headquarters, completed a Viking spacecraft design. Viking
would have two major systems&emdash;an orbiter and a lander. While
the lander would provide the means for safely delivering the
scientific instruments to the surface, house, and provide the
necessary power source and communications links for those
experiments, the orbiter had a series of equally important
functions in the Viking mission. The orbiter would transport the
lander to Mars, provide a platform for the Viking imaging system
so that proposed landing sites could be surveyed and certified,
relay lander science information (pictures and other data in an
electronic format) to Earth, and conduct scientific observations
in its own right.
Despite early debates among NASA managers,
it was only logical that the design and development of the Viking
orbiter system he carried out at the Jet Propulsion Laboratory,
where the engineering team already had an expertise in the design
of planetary spacecraft. After building the Ranger lunar probes
and the early Venus and Mars Mariner flyby spacecraft, the
California engineers had gone on to build the Mariner Mars 69
flyby craft and were working on the Mariner Mars 71 orbiter when
Viking was initiated. The Viking orbiter would borrow heavily from
Mariner technology, with such specialized functions as the project
demanded being added to the basic chassis.
Early plans for the Viking orbiter called
for only a few modifications of the Mariner 71 craft. However,
structural changes that permitted mating the lander to the orbiter
and enlarging the solar panels led to significant alterations of
the basic 1971 orbiter. During the long flight to Mars, the
orbiter would have to provide power to the lander, especially
during the periodic checkups on the lander's health and during
occasional updates of the landers computerized memory. These
additional energy requirements made it necessary to increase
significantly the solar panels, from 7.7 square meters to
15.4.
[156] The decision to build a large
soft-landing craft instead of a small hard-lander led to the
requirement for a large orbiter. The orbiter would not only have
to transport the lander. it could also have to carry an increased
supply of propellant for longer engine firings during Mars orbit
insertion, longer than those planned for the 1971 Mariner mission.
1 And an upgraded attitude control system with
greater impulse, plus a larger supply of attitude control
propellant, would be required to control the combined spacecraft.
Table 26 categorizes the Viking orbiter subsystems as compared to
Mariner 71, listing subsystems from Mariner requiring only minor
changes, subsystems from Mariner requiring extensive
modifications, and completely new subsystems designed for
Viking.
Table 26
Sources of Viking Orbiter
Subsystems
Mariner
Mariner Adaptations
New
Radio
Structure
Computer/command
X-band transmitter
Attitude control
Data storage
Pyro control
Propulsion
Relay link
Omni antenna
Scan platlorm
High-gain antenna
Temperature control
Science instruments
Packaging
Data system
A brief review of the Mariner 69 and
Mariner 71 spacecraft will provide a better understanding of the
technological relationships between the Mariner and Viking
projects.
