Measurements of positive ion composition in the payload bay of the Space Shuttle confirm that the vernier reaction control system rocket engines cause major changes in the shuttle plasma environment. Previously reported measurements of increases and decreases in ambient and contaminant ion fluxes are corroborated and are shown to depend on the attitude of the shuttle, the direction of exhaust plume flow, and the pointing direction of the mass spectrometer sampling orifice. The column density of exhaust neutrals and the length of the effective column are both roughly estimated from the magnitude and time response of the reduction in ambient O+ flux. The measurements are consistent with control of the contaminant ion motions by the Earth's magnetic field. Contaminant ions are detected only if the mass spectrometer is approximately aligned with the plane of ion motion perpendicular to the magnetic field vector, and only if the plume gases expand into a region of space from which the pickup trajectories are directed back into the instrument sampling orifice. The degree to which contaminant ions are picked up by the magnetic field depends on the chemical dynamics of the ion-molecule reaction that formed them. The appearance of large fluxes of ions at mass 28 cannot be explained by the chemistry of O+ reacting with the principal exhaust neutral species. Electron impact ionization of neutral N2 to give N+2 is proposed as a possible source for this mass 28 ion.