13-15] RELIABILITY 715 



13-14 POTENTIAL GROWTH 



The overall design criteria for an airborne radar must take into account 

 its future growth possibilities. Rapidly changing electrical requirements 

 and environmental conditions tend to make equipment obsolete before it 

 reaches the final design stage. Therefore, equipment must be made flexible 

 in order to receive the maximum return for the engineering and manufac- 

 turing effort and cost. The limits on the potential growth of a system 

 are directly dependent upon the flexibility designed and built into its 

 component units. A flexible system will be able to absorb the latest types 

 of electrical components and circuit designs. Advanced techniques in 

 structural and mechanical design, fabrication methods, and new materials 

 can be utilized in the system any time during its life. 



Plug-in subassemblies add to flexibility, especially if they are modular in 

 design, because one plug-in unit can be easily replaced with an improved 

 model. High flexibility is difficult for systems that include items that 

 cannot be easily divided into small units. Thus, major packaging problems 

 exist with radar antennas, high-voltage equipment requiring large power 

 supplies, modulators and transmitters with critical interconnecting wiring, 

 and mechanical linkages between units. 



In attaining a high degree of flexibility in a system, it is important that 

 environmental considerations are not ignored. That is, the technique 

 employed in the mounting of components within a specific package should 

 provide for maximum structural strength and stiffness to carry the imposed 

 dynamic loads, allow proper cooling, exclude moisture and dust, and so on. 

 It is often a successful technique to utilize the covers of the packaged units 

 as structural members in order to increase the structural natural frequency 

 of the unit, thus eliminating, in some cases, the need for vibration isolators. 

 The use of printed circuits, plug-in units, and potted or hermetically sealed 

 units should be considered. For major electronic assemblies, proper location 

 of mounting or installation lugs in order to carry acceleration loads should 

 be included in the packaging design criteria. 



The design of electrical equipment that depends very closely on particular 

 performance characteristics of an airplane or missile cannot be considered to 

 possess maximum potential growth capabilities as far as interchangeability 

 of equipment between aircraft is concerned. However, this equipment can 

 still be made flexible as far as its relationship to spare or improved compo- 

 nent part replacement and maintenance is concerned. 



13-15 RELIABILITY 



A major problem facing the designers of any military weapons system is 

 ensuring that the equipment will be reliable in field operation. Reliability is 

 a system or equipment trait (or capability) which has been defined as the 



