example embodiments as illustrated in the figures and is not intended to limit the claimed invention beyond the limits expressly included in the claims for purposes of simplicity and clarity detailed descriptions of well known devices circuits and methods are omitted so as not to obscure the description of the present invention with unnecessary detail fig1 a 1b illustrate an example spacecraft comprising a stack of modules in accordance with this invention the stack of modules includes an anchor module 110 an upper module 120 and a plurality of intermediate modules 200 a d between the anchor and upper modules typically the stack of modules will include a communications module an attitude determination module an attitude control module a power supply module and a solar panel module preferably alternative configurations of each of these modules are available and the spacecraft system designer selects from among these configurations to satisfy the given mission objective for example the attitude determination and control modules may be a momentum biased earth pointer a three axis stabilized star pointer and so on each having a particular accuracy and resolution and each consuming different power the solar panel and power supply modules may each have different capacity power voltage and so on the spacecraft system designer selects the appropriate combination of modules to satisfy the mission objectives knowing that the modules are designed to be compatible with each other each of the intermediate modules 200 has a common cross section profile so that they.