During sea-level transients, such as engine start-up or shut-down, ramping chamber pressure causes the nozzle exit pressure to fall below ambient conditions, resulting in flow over-expansion. Internal shockwaves form that incite unsteady boundary layer separation producing an asymmetric internal pressure field that manifests as dynamic off-axis loads. These side-loads reduce the safe-life of the vehicle and have also be known to cause sudden catastrophic failure. As a result, rocket nozzle area ratio is purposely limited to ensure that flow separation does not occur elsewhere in the mission profile and as such, the vacuum performance of the vehicle is reduced by as much as 20%. An experimental study comparing the side-load distributions of four conical nozzles with wall angles, 8.3°, 10.4°, 12.6° and 14.8° a truncated ideal contour and thrust optimised parabolic nozzle has been carried out. Direct side-load measurements taken using a strain tube have shown that conical nozzle wall angle has very little affect on the side-load magnitude. The truncated ideal contour nozzle displayed the lowest side-load distribution and was found to be approximately 50% lower than the magnitudes produced by the thrust optimised parabolic. An analytical model has been developed to simulate the side-load distribution across geometries which only produce free-shock separation. A universal method of replicating the pressure distribution across a free-shock separated nozzle was first developed, validated with a high level of confidence against three nozzle geometries. This was used in conjunction with a shock excursion model, whereby the oblique shock relation was perturbed to first order in order to generate internal pressure field asymmetry. Comparisons made to experimental results have shown the side-load model can predict distributions with errors as low as 3.58% for truncated ideal contour nozzles.