Design and Stress Analysis of Extraterrestrial ... - The Black Vault

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From the design and operational points of view, these two
diagrams differ mainly in the thermal:stress of the main elements.
A comparison of the working processes for these diagrams show
the ion motor with a three-dimensional ionsource can be made
so that none of its parts will be heated above 7000C. At the same
time an ion motor with a surface ionizer, all other things being
equal, requires the ionizer temperature and the temperature of
elements connected with it to be approximately 140000. Therefore,
in the second diagram it is considerably more difficult to achieve
a high level of reliability than it is in the first. In addition,
the first type of motor is less sensitive to contamination, the
quality of material processing, and fabrication--precision for a
considerable number of parts; it does not require highly oxidationresistant
metals rand insulators.. In it can be used quite a variety
of working media, while in the ion motor with a surface ionizer
virtually only cesium and rubidium can be used. Thus, a comparison
of module diagrams gives -the designer many starting pdint~s for
solving reliability problems.
Structurally it is advisable to build an ERE with an ion motor
on the basis of modules. For the current reliability evaluations
of an ion motor module, in,-the process of experimental work, we
can, use the assumption made in calculating reliability that the
planned module, which must have a long lifetime can have two types of
failures:
- sudden failures, which can be caused by the effect of random
factors (in the presence of equipment for checking and maintaining
'reliability, there can be virtually no sudden failures);
- gradual failures, which are due to irreversible processes
(material aging, electrode erosion, various types of wear,
irremovable contamination, etc.). In this case, We can take as a
reliability criterion probability PM of failure-free work of the
ion motor module during given period of time T. In other words PM( )
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