The studies documented in this report expand the experience gained by the Syracuse University Research Corporation SURC under previous U. S. Army Advanced Ballistic Missile Defense Agency-sponsored studies. The earlier ballistic missile defense system studies have indicated the desirability of having radar systems with fully adaptive array apertures. These apertures typically contain a large number of elements. The previous studies suggested that partially adaptive apertures would provide satisfactory performance. If this were possible, the complexity of the arrays could be reduced, which would, consequently, reduce their cost. However, the results obtained at that time were only preliminary indications since the simulation used in the tests was fairly elementary. In the recent studies, the feasibility of using partially adaptive apertures was investigated using a more detailed simulation. Several configurations of partially adaptive systems were considered. Emphasis was placed on evaluating the effects of beam forming errors and relative transient response times. Related effects of main beam constraints were also considered.