[en] MHD equilibrium and stability analyses have been performed in support of the design and initial experimental operation of the National Spherical Torus Experiment (NSTX). Free-boundary equilibria have been generated to determine several aspects of the anticipated plasma configurations including the stability and shape domain. Boundary shape determination has been studied by considering random variations of modeled signals from the NSTX magnetic diagnostic set. The impact of equilibrium profile variations on the ideal low-n kink and ballooning stability of high-β configurations was also determined to assess the robustness of the stable operating space of NSTX. The profiles used included local perturbations from previously reported optimization studies and experimental pressure profiles from the START spherical torus, the DIII-D tokamak, and TRANSP modeling. The largest β-limits occur in plasmas with broader pressure profiles and are set by the n = 1 kink/ballooning mode. Values of β = 38% are achieved in the presence of a conformal conducting wall at b/a = 0.25. The corresponding calculated no-wall β-limit is 22%. The lowest limits (β = 19%) were generated in plasmas with greater pressure peaking, and are set by high-n ballooning modes