1. A processor-based method of determining a conditional computational plan for a data dependent constraint network for use in modeling an engineering system, the data dependent constraint network represented by a bipartite graph containing a plurality of variable nodes, and relation nodes, the variable nodes and the relation nodes being interconnected by arcs, the variable nodes representing variables in the data dependent constraint network, the relation nodes defining constraints between possible values of the variable nodes, and conditions on relation nodes defining applicability of different constraints during at least one of design analysis and design optimization of the engineering system, the method comprising the steps of: specifying, using a variable node specifier on a computer, a set of input variable nodes and one or more output variable nodes of the plurality of variable nodes for which a conditional computational plan is desired, wherein the conditional computational plan is a partially-ordered sequence of computational plan steps, each plan step is associated with a condition under which that plan step is executed during an execution phase of the conditional computational plan, the condition on a plan step is defined by a world set, the condition evaluates to a truth value during plan execution, the truth value of the condition is dependent on data computed in previous plan steps and is either true or false, the plan steps are processed during plan execution in an order that the plan steps are stored in the conditional computational plan, a plan step is executed during plan execution if and only if the condition associated with the plan step evaluates to a truth value of true; determining, using a plan determiner on the computer, the conditional computational plan from the input variable nodes to the one or more output variable nodes using a backward chaining search of the bipartite graph, wherein the plan determiner maintains an evolving world set condition for each search path during the backward chaining search, the plan determiner further associates each plan step with a world set condition under which that plan step is executed during plan execution, the plan determiner further includes in the plan step a minimal set of variables whose values are needed during plan execution, the plan determiner further ensures that only relevant non-superfluous variables are chosen as iteration variables in plan steps requiring iterative solution; and using the conditional computational plan to evaluate candidate design concepts to determine an optimal design of at least one of a system configuration and a vehicle configuration of an engineering system during at least one of design analysis and design optimization of the engineering system, wherein evaluation of the candidate design concepts includes executing a series of computations specified in the conditional computational plan that calculate values of the one or more output variable nodes from values of the input variable nodes using the minimal set of variable values required at each plan step, and selecting only relevant, non -superfluous variables as iteration variables in plan steps requiring iterative solution, thereby improving efficiency of at least one of design analysis and design optimization of the engineering system.