This thesis examines how a recently proposed concept for a highly-truncated aerospike nozzle can be expected to perform at altitudes corresponding to ambient pressures from sea-level to full vacuum conditions, as would occur during second-stage ascent and during second-stage descent and return to Earth. Of particular importance is how the base pressure varies with ambient pressure, especially at low ambient pressures for which the resulting highly underexpanded flows exiting from discrete thrust chambers around the truncated aerospike merge to create a closed (unventilated) base flow. The objective was to develop an approximate but usefully accurate and technically rooted way of estimating conditions for which the jets issuing from adjacent thrust chambers will merge before the end of the truncated aerospike is reached. Three main factors that determine the merging distance are the chamber pressure, the altitude, and the spacing between adjacent thrust chambers. The Prandtl-Meyer expansion angle was used to approximate the initial expansion of the jet flow issuing from each thrust chamber. From this an approximate criterion was developed for the downstream distance at which the jet flows from adjacent thrust chambers merge. Variations in atmospheric gas composition, specific heat ratio, temperature, and pressure with altitude from sea-level to 600 km were accounted for. Results showed that with decreasing atmospheric pressure during vehicle ascent, the merging distance decreases as the jet flows become increasingly under-expanded. Increasing the number of thrust chambers decreases the merging distance exponentially, and increasing chamber pressure results in a decrease of the merging distance as well.