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108
promising, had unfavorable characteristics in space, such as unstable, weakly
constricted arc discharge; unstable globular transfer; and increased weld
porosity.
During experimental retrofitting in simulation facilities-chiefly in
space simulation chambers placed in flying laboratories-the difficulties
related to these characteristics were successfully resolved. Specialized
welding equipment and techniques also were developed for this purpose, and
the required welding consumables often were selected from those used in the
aerospace industry.
However, it was clear to space system developers that almost all
maintenance and repair of long-term flying vehicles - for which neither the
scope of work needed nor the components to be repaired and restored are
known in advance-had to be performed manually with only partial
mechanization. This increased specialists' interest in studying the possibility
of manual welding in space, which led them to consider which of the existing
welding processes to use.
Welding processes such as electron beam, consumable and
nonconsumable electrode arc in vacuum, flash-butt, hollow cathode, and
helio welding were tested in vacuum chambers and in flying laboratories at
different stages of experimental studies in the 1970s and 1980s.
Technology and material versatility and minimal power consumption
ultimately were deciding factors that led them to choose the electron beam
process. This process allowed
technicians to perform operations
that could be required to produce a
permanent joint in open space:
heating, brazing, welding, cutting,
and coating deposition.
But selecting this process
didn't solve all the problems. As
investigations progressed, the
number of problems, technical and
psychological, increased. An
opinion existed that this process, which involves high-accelerating voltage,
the possibility of X-ray radiation from the weld pool, and manipulation of a
sharply focused electron beam, couldn't be done manually.
A series of experiments in a ground-based, manned space simulation
chamber enabled the engineers to solve the key technological and hardware
issues and develop a flight sample of an onboard electron beam hand tool. In
1984 and 1986 this tool was successfully tried out on the outer surface of the
Salyut 7 orbital complex (see Figure above).
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