682 MECHANICAL DESIGN AND PACKAGING 



maintenance factors will count heavily in the location of the other units. 

 The principle should, however, be borne in mind. 



Another important factor controlling the severity of a particular type 

 of environment is its duration. For example, the temperature rise of the 

 aircraft skin may be a transient phenomenon rising to a significant magni- 

 tude only during a particularly high-speed flight condition. If this high- 

 speed flight condition exists for a sufficiently short period of time, the efi^ect 

 upon the radar may be small, because of thermal lag. Failure from vibration 

 and shock is also very much determined by the duration of exposure. 



It is necessary that the effect of environment be considered from the very 

 beginning of the packaging design effort. Initial engineering decisions on 

 configuration and many details will be directly related to this problem. 

 The design for vibration, shock, heat, and some other factors is so difficult 

 that it is often necessary to supplement engineering judgment by the 

 testing of models and prototypes at the earliest possible stage. This point 

 may seem too evident to require emphasis. In practice, however, a designer 

 is often under pressure to meet a time schedule, and it is not necessarily easy 

 to obtain a clear and consistent description of all phases of the environment. 

 Thus there exists a severe temptation to proceed without due regard to the 

 basic environment problem. Such a procedure can be expected to produce 

 equipment which will fail to pass the acceptance tests to which it is 

 ultimately subjected. Experience with radar and other airborne electronic 

 equipment shows that this chain of events happens all too frequently. Since 

 these final tests usually occur very late in a large and lengthy development 

 program, the correction of design errors then becomes costly and difficult. 



In principle, there are two methods of designing to meet the environ- 

 mental challenge. One is to obtain components and to use design factors 

 that will enable the radar to function properly in spite of any specified 

 environment. At the other extreme is the selection of components and 

 completion of all design assuming an ideal environment, that is, ignoring 

 the environmental problem. The equipment is then isolated from all 

 undesirable factors and put in the ideal environment required for its 

 operation. It is doubtful if any useful airborne radar could be designed on 

 the basis of either method alone, at this time. Practical design will produce 

 reliable equipment by a suitable combination of these two approaches. This 

 point will be expanded in the following discussions of the various types of 

 environment. 



13-2 MILITARY SPECIFICATIONS 



To obtain an overall system design that will provide optimum operation 

 while being subjected to the various categories of environment, it is 

 necessary to have design criteria and methods of testing the individual 



