The importance of reliable and efficient launch vehicles pervades the many industries which rely on space based hardware. An experimental investigation has been performed into the effects of nozzle length and exhaust plume presence on a FESTIP axisymmetric base flow at Mach 0.7. Three different nozzle lengths (L/D = 0.6, 1.2, 1.8) were tested to account for the effects of fluidic, hybrid, and solid reattachment of the shear layer. Additionally, an underexpanded exhaust plume (NPR = 100) was simulated using compressed air driven through the model. Tomographic PIV was used in combination with a momentum based pressure reconstruction technique to study the effect on the mean pressure coefficient. This approach was validated using a similar, sting mounted model equipped with pressure transducers. Excellent agreement within 10% (less than 0.02 Cp) is found between the transducers and the PIV derived wall pressure value. Performance of the technique is found to largely rely on the in-plane, both streamwise and radial, Reynolds stress terms. For this reason, it is concluded that the pressure reconstruction technique would perform similarly well using a simpler, planar PIV arrangement. Statistically, the hybrid nozzle length with exhaust plume was the least stable and for the fluidic reattachment case, the exhaust presence had a stabilizing effect. Only in cases of fluidic reattachment does the exhaust plume affect the mean pressure field; this is attributed to a `jet-suction’ effect, which imparts a consistent decrease in wall Cp. It is recommended that future researchers also image the near-wake as this may show additional instabilities affected by an exhaust plume.